Engineering Geological Characterization of Soils and Rocks for Urban Planning: A Case Study from Wolaita Sodo Town, Southern Ethiopia

Abstract

This study was conducted to characterize and classify soils and rocks and to produce an engineering geological map that is beneficial for overall urban planning. The soils’ moisture content and specific gravity values range from 23.47% to 44.21% and 2.68 to 2.81, respectively. The activity of soils varies from 0.34 to 0.78 (inactive to normal). The shrinkage limit and shrinkage index values of soils range from 5% to 11.43% and 14.29% to 26.9%, respectively. Free swell value varies from 5 to 23% (low expansive). The unconfined compressive strength of soils ranges from 215.8 to 333.5 kPa (very stiff). According to USCS (Unified Soil Classification System), soils are classified into lean clay, lean clay with sand, fat clay with sand, and clayey silt with slight plasticity. According to BSCS (British Soil Classification SystemS), soils are classified into clay of intermediate plasticity, clay of high plasticity, and silt of intermediate plasticity. Rocks were classified into four categories based on their mass strength: very low mass strength, low mass strength, medium mass strength, and high mass strength. The RQD Rock Quality Designatione) value ranges from 47.48% to 98.25%, indicating a quality range from poor to excellent. The RMR Rock Mass Ratinge) values range from 44 to 90%, indicating that the rocks of the study area fall into three major classes: Class I (very good), Class II (good), and Class III (fair).

1. Introduction

Investigating the suitability of ground conditions for various infrastructure constructions and addressing the geohazards and geotechnical challenges of rapidly growing towns are crucial for the proper development of a country [1,2,3,4,5,6]. Engineering geological characterization of soils and rocks is essential for designing and constructing engineering structures [7,8,9,10,11]. Detailed geotechnical and engineering geological studies and mapping are essential for understanding the interactions between geological environments and engineering projects [7]. These studies focus on the nature and relationships among geological components, active geodynamic processes, and the potential outcomes of changes induced by engineering activities. By examining these interrelationships, professionals can assess risks, predict future geological behaviours, and develop strategies to mitigate adverse effects on infrastructure and the environment.

The failure of engineering structures and an increase in construction costs resulted from a lack of understanding of soil types and their spatial variations, as well as inadequate assessment of environmental-related natural hazards [12,13,14,15]. Likewise, the geological environment, groundwater condition, and flow direction should be understood before designing a foundation for any engineering structure [16]. The damage to engineering structures due to problematic soils, improper drainage conditions, and geodynamic processes can be minimized based on scientific studies [17]. The performance of engineering structures is primarily governed by the nature of construction materials, the environment, and our knowledge related to both the construction materials and the structure itself [18].

In a developing country like Ethiopia, engineering structure construction is occasionally increased without detailed study and proper design [19,20]. Wolaita Sodo town is expanding in different directions to accommodate the increasing population from the city’s rural areas. As a result, many infrastructures have been constructed in the past and continue to be built. However, no detailed investigation of the geotechnical and geomechanical characteristics of soils and rock masses about geoenvironmental processes has been conducted from both geological and engineering perspectives. Due to this, roads, buildings, and drainage networks collapse before their expected lifespan without providing service. Building cracks (0.5 cm to 2 cm opening), heaving or bulging out of roads, asphalt distress, and pothole problems were observed as engineering structure problems in the study area. In addition, no engineering geological map was produced for the town as a requirement for planning, designing, and constructing engineering structures.

Therefore, characterizing soils and rocks from geological, geotechnical, and engineering geological perspectives and assessing geodynamic processes helps reduce problems with engineering structures [21,22]. The presence of the aforementioned engineering problems, the absence of engineering geological maps, and the lack of previous research from an engineering geological perspective are the primary motivations for this research work in the area. The study’s objectives were to determine the geotechnical properties of soils and their classifications, to select the geomechanical properties of rock masses and their engineering classification, to assess surface geodynamic processes and their influence on engineering structures and overall urban planning, and to describe the geological setup of the area. Subsequently, an engineering geological map was produced based on the results of the study.

2. Materials and Methods

2.1. Study Area

Wolaita Sodo town is the capital of Wolaita Zone, located in southern Ethiopia, approximately 380 km South of Addis Ababa, the capital city of Ethiopia, and 120 km North of Arba Minch Town. It is accessible via roads connecting Addis Ababa to Sodo, Sodo to Arba Minch, and Hawassa to Sodo, as well as Sawla. An asphalt, gravel, and cobblestone road is also accessible. It is located within a UTM coordinate system, specifically in zone 37, with coordinates ranging from 754,000 to 760,000 m in latitude and 360,000 to 366,000 m in longitude. The area varies in elevation from 1840 m to 2340 m above mean sea level (msl). The topography is flat in the south direction around the university (Wolaita Sodo University) and undulated in the town’s north, east, and west directions (Figure 1).

The drainage pattern of the area is nearly dendritic, and it is controlled by the highest mountain (mount Damota) located on the north of the town. The study area is situated in a subtropical climate at a high altitude, characterized by a pronounced pattern of wet summers and dry winters. It has an annual average maximum temperature of 25.51 °C and an annual average minimum temperature of 14.73 °C. The rainfall distribution pattern is bimodal, varying in intensity from year to year and month to month. The primary rainy season begins in mid-June and lasts until the end of September, whereas the short rainy season occurs from April to May. It has an annual average rainfall of 1314.5 mm/year.

2.2. Materials

Field equipment used for this research includes a Schmidt hammer, core cutter, measuring tape, GPS, Compass, geological hammer, Shovels, and diggers. Among these, the Schmidt hammer was used to measure the strength of rocks by recording rebound value. The core cutter collected soil samples. During the scanline survey, a measuring tape was used to count the number of discontinuities intersecting the scanline and to determine the fracture frequency. It is also used to measure the depth of test pits and the spacing between joints.

2.3. Methods

A literature review, field description, and in situ tests of soils and rocks, as well as laboratory testing of soils, were conducted to achieve the research successfully. Generally, the methodology comprises four main phases: secondary data collection and desk study, field data collection, laboratory analysis, data processing, and interpretation. Figure 2 shows the methodology flow chart of this present study.

2.3.1. Secondary Data Collection

Before fieldwork or under-desk study, a literature review and secondary data collection were performed to achieve the research goal. Secondary data, including meteorological data (rainfall and temperature) from the National Meteorological Service Agency (NMSA), as well as six borehole data sets or water well data from the Wolaita Sodo Town Water Supply office, were collected. The town’s administration map showing the master plan, and the current distribution of buildings and infrastructures was observed. This helps assess the town’s potential for expansion. Studies conducted in Wolaita Sodo town and its surrounding areas, focusing on geology, hydrogeology, climate, and geodynamic processes, were reviewed to gather background information.

2.3.2. Field Data Collection

Fieldwork was the main phase in achieving the research objectives. Primary data that helps for the research was collected in the field. Field observations and town assessments were conducted to select traverses and identify suitable locations for soil sampling from test pits, and in situ tests on soils and rocks were conducted. Field visual identification, description, and simple field tests of various soil properties were performed following ASTM D2488-00 standards [23]. Recording of geological features, including rock outcrops, topographic changes, surface geodynamic processes, and drainage conditions of the area, was carried out through traversing. Disturbed and undisturbed soil samples were collected at a minimum depth of 1 m and a maximum depth of 3 m (Figure 3). The disturbed soil samples were manually collected from 15 test pits using shovels and diggers to determine index properties in the laboratory. One test pit reveals two distinct layers, each taken as a separate sample. So, the total number of disturbed soil specimens was 16. Undisturbed soil samples were collected using the core cutter to determine unconfined compressive strength. The soil sample collection depends on variations in soil colour, topography, drainage conditions, and visible surface geodynamic phenomena.

The description and in situ tests of rocks were performed based on Reference [24] standards. Field data that help determine RMR, such as discontinuity data and groundwater conditions, were collected from exposed rock masses during the field investigation. To estimate uniaxial compressive strengths of rock masses, 18 Schmidt hammer values were recorded based on Reference [24] the recommended Schmidt hammer test procedure. The recorded Schmidt hammer values were normalized using the correction method described by Schmidt [25]. Subsequently, uniaxial compressive strengths of the rocks were determined from the recorded Schmidt hammer values of 18 sampling points by using Reference [26], as shown below in Equation (1). Besides, the classification of rock masses based on their uniaxial compressive strength values was carried out following the method suggested by Reference [27].

$${\mathrm{U}\mathrm{C}\mathrm{S}=0.0137\ast \mathrm{S}\mathrm{H}}^{2.2721}$$

(1)

UCS refers to uniaxial compressive strength, and SH refers to the Schmidt hammer value.

RQDs of rock masses were estimated by Reference [28] scanline sampling technique and calculated by following Equation (2). Several 16 scanline points were recorded in different areas, ranging from a minimum of 4 m to a maximum of 30 m in length. Finally, the classification of rock masses was conducted.

$$\mathrm{R}\mathrm{Q}\mathrm{D}=100{\mathrm{e}}^{-\mathsf{\lambda }\mathrm{t}}(\mathsf{\lambda }\mathrm{t}+1)$$

(2)

where RQD is Rock quality designation, λ stands for mean discontinuity frequency expressed in a unit of per meter (m⁻¹), and t represents threshold value. This method was preferred over the qualitative estimations because it provides a mathematical model that relates RQD to the physical parameters of the rock mass, offering a more precise, data-driven approach.

Rock mass rating values of each rock mass were determined by adding the values of uniaxial compressive strength, rock quality designation, spacing of discontinuities, condition of discontinuities (persistence, separation, smoothness, infilling, alteration/weathering), and groundwater conditions. It was calculated using the following Equation (3). Furthermore, the classification was done via [29] classification mechanism.

$${\mathrm{R}\mathrm{M}\mathrm{R}}_{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{i}\mathrm{c}}=\sum \mathrm{R}1+\mathrm{R}2+\mathrm{R}3+\mathrm{R}4+\mathrm{R}5$$

(3)

where RMR_basic stands for basic rock mass rating, and R1, R2, R3, R4, and R5 are the five parameters listed above, respectively.

2.3.3. Laboratory Analysis

After fieldwork, laboratory tests on soil samples were conducted using ASTM standard laboratory procedures to determine their index and engineering properties. The soil tests were conducted in the Soil Mechanics Laboratory at Arba Minch University. Oven-dried for 24 h at 105 °C and air-dried conditions were used for drying the soil specimen. Elemental analysis of soil samples was conducted in the geology department laboratory of Arba Minch University using XRF to identify the parent material of the red soil. Laboratory procedures for determining soil index and engineering properties are described below.

Natural Moisture Content

In this study, the moisture content of soil samples was determined using the oven-drying method. The water content of 16 soil samples was determined usNatural Moisture Contenting the ASTM D 2216 standard from disturbed soil specimens collected in the field. The soil specimen (moist soil) was first weighed before it lost its moisture and then dried in an oven at a temperature of 105 °C to 110 °C for 24 h. After one day, the mass of the dry soil was measured. The mass that is lost after the sample has been dried in an oven is water. Finally, the moisture or water content is calculated using the following Equation (4).

$$\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{C}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t} \left(\mathrm{W}\right)={\displaystyle \frac{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r} \left(\mathrm{W}\mathrm{w}\right)}{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{d}\mathrm{r}\mathrm{y} \mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l} \left(\mathrm{W}\mathrm{d}\mathrm{s}\right)}}\ast 100$$

(4)

Specific Gravity

The specific gravity of soil solids in the study area was determined using a pycnometer following the ASTM Standard. It was determined by placing a known weight of oven-dried soil sample that passed through a 4.75 mm (No. 4) sieve into a Pycnometer and then half-filled with distilled water. The air entrapped in the soil sample was removed by gently shaking the specimen for at least 10 min to remove the air. The Pycnometer was filled with distilled water up to the calibration mark or the neck of the Pycnometer. By cleaning the outside part of the Pycnometer and drying it with a clean, dry cloth, the mass of the Pycnometer filled with soil and water was recorded. Fill the Pycnometer with distilled water to the calibration mark and record the mass of the Pycnometer and water. The temperature was recorded by inserting a thermometer into the water. Then, the specific gravity of each soil sample was determined by dividing the mass of soil solids by the mass of an equal volume of gas-free distilled water (Equation (5)).

$$\mathrm{G}\mathrm{s}={\displaystyle \frac{\mathrm{W}\mathrm{d}\mathrm{s}}{(\left(\mathrm{W}\mathrm{p}+\mathrm{w}-\mathrm{W}\mathrm{p}\right)-\left(\mathrm{W}\mathrm{p}+\mathrm{d}\mathrm{s}+\mathrm{w}-\mathrm{W}\mathrm{p}+\mathrm{d}\mathrm{s}\right))}}$$

(5)

where Gs = Specific gravity, Wds = Weight of oven-dry soil, Wp = Weight of Pycnometer, Wp + ds = Weight of Pycnometer with oven-dry soil, Wp + w = Weight of Pycnometer filled with water, and Wp + ds + w = Weight of Pycnometer filled with water and soil.

Grain Size Analysis

The combination of sieve analysis for coarse-sized soils and hydrometer analysis for fine-sized soils determines the grain size distribution of soils. It was determined based on wet sieve analysis due to the flocculation of soil samples. A 1000 gm soil material was soaked for over one hour or until the soil dispersed and then washed using #200 (0.075 mm). The retained soil was dried in an oven and then sieved through a set of sieves. Sieves with openings ranging from 9.5 mm to 0.075 mm were used, and the shaking process lasted 10 min. The soil that passed through #200 during washing was used for hydrometer analysis by sedimentation utilizing a hydrometer. It was determined based on ASTM D 422–63 standard [30] test methods. After 24 h, the soil had settled, and the clean water was oven-dried again to obtain the dried mass. Next, 50 g of dried soil and 5 g of dispersing agent (sodium hexametaphosphate) were soaked in 125 mL of water and stirred using a high-speed mechanical stirrer for approximately 10 min. After 10-min stir, the slurry soil was transferred into a 1000 mL volume sedimentation cylinder, and then the soil settlement was recorded at a series of time intervals by carefully inserting 151H stem into the solution. The interval was recorded from 30 s to 24 h or one full day. Finally, the grain size distribution of soils was determined.

Atterberg Limit

This research determined the Atterberg limits of 16 soil samples based on the ASTM D 4318-98 standard test method. The soils are oven-dried, passing through sieve number 40 (425 μm). A 150- to 200-g sample of soil was taken and mixed with water in a dish for 24 h. The liquid limit was determined by adding the mixed soil to the liquid limit apparatus, also known as a Casagrande. Groove was used to cut the smooth soil on the cup into two at 10 mm height. The number of drops was counted until the two halves of the soil pat came in contact with a distance of 13 mm. It was conducted by taking the moist soil into contact and measuring its mass before and after moisture was loosened through oven drying. The test was performed by four trials for each soil specimen to plot the graph and take the moisture content value corresponding to 25 blows as the liquid limit value. The plastic limit was determined by rolling the soil with distilled water at sufficient pressure until it reached a diameter of 3.2 mm. The pieces were added to the moisture can, and the moist soil mass was measured and inserted into the oven to measure the dry mass. Three trials were conducted on it. Then, the average moisture content of the three trials was taken as a plastic limit. The plasticity index, liquidity index, and consistency index values of the soils in the study area were determined using Equations (6)–(8), respectively. Finally, the soils are classified based on their liquid limit and plasticity index values using the classification mechanisms outlined in [31,32].

$$\mathrm{P}\mathrm{l}\mathrm{a}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{I}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x} \left(\mathrm{P}\mathrm{I}\right)=\mathrm{L}\mathrm{L}-\mathrm{P}\mathrm{L}$$

(6)

$$\mathrm{L}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{I}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x} \left(\mathrm{L}\mathrm{I}\right)={\displaystyle \frac{\mathrm{W}-\mathrm{P}\mathrm{L}}{\mathrm{P}\mathrm{I}}}$$

(7)

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y} \mathrm{I}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x} \left(\mathrm{C}\mathrm{I}\right)={\displaystyle \frac{\mathrm{L}\mathrm{L}-\mathrm{W}}{\mathrm{P}\mathrm{I}}}$$

(8)

LL stands for Liquid limit, PL refers to Plastic limit, and W is soil’s Natural moisture content or water content.

The activity of 16 soil samples was calculated using the plasticity index and grain size analysis values, as shown in Equation (9). Subsequently, it was grouped by [33] standard, which describes soils with activity less than 0.75 as inactive, between 0.75 and 1.25 normal, and those with greater than 1.25 as active.

$$\mathrm{A}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}={\displaystyle \frac{\mathrm{P}\mathrm{I}}{\mathrm{P}\mathrm{e}\mathrm{r}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{l}\mathrm{a}\mathrm{y} \mathrm{f}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{f}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r} \mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n} 2 \mathrm{\mu }\mathrm{m}}}$$

(9)

Linear Shrinkage Limit of Soils

Linear shrinkage limit test of soils was performed for a soil that passes the 425-µm (No. 40) sieve and determined for only fine-grained (cohesive) soils that exhibit a dry strength when air-dried. The linear shrinkage limit of soil samples from 15 test pits was determined following ASTM standards. First, the soil was mixed, soaked with water, and placed in the liquid limit apparatus for liquid limit determination. Adjusting the instrument, counting the number of drops, and stopping when the number of blows was 25. Then, the specimen was placed into a brass mold with an original length of 140 mm (14 cm). It was inserted into the oven for one day, and then the change in length due to the shrinkage of the soil during drying was recorded. Finally, it was calculated using Equation (10) and described in Reference [34] as the classification of soil expansiveness based on shrinkage index values and [35] as the classification system of soils by their degree of shrinkage.

$$\mathrm{S}\mathrm{r}={\displaystyle \frac{\mathrm{L}\mathrm{i}-\mathrm{L}\mathrm{f}}{\mathrm{L}\mathrm{i}}}\times 100\mathrm{\%}$$

(10)

Li is the initial length of the soil sample, and Lf is the final length of the sample.

Free Swell

To perform the free swell test, the soil was dried in an oven for 24 h. Then, 10 mL of oven-dry soil, which passed through a No. 40 (0.425 mm) sieve, was added to a 100 mL graduated cylinder. The cylinder was filled with distilled water up to the 100 mL mark. After 24 h, the soil completely settled, and the swelled volume or suspension was read. Finally, it was calculated using Equation (11) and described by the following [36] standards.

$$\mathrm{F}\mathrm{r}\mathrm{e}\mathrm{e} \mathrm{S}\mathrm{w}\mathrm{e}\mathrm{l}\mathrm{l}={\displaystyle \frac{\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l} \mathrm{v}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}-\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{v}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{v}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}}}\times 100\mathrm{\%}$$

(11)

Unconfined Compressive Strength

The unconfined compressive strength was determined using the ASTM standard laboratory procedure. The test was conducted by remolding a failed undisturbed soil specimen. The remolded soil specimen was placed in a loading device at the center of the bottom plate. The upper platen and the soil specimen should be in contact with each other by adjusting the loading device and setting the deformation indicator labels to zero. Finally, the load was applied to produce axial strain and to record deformation, load and time values. The loading was continuous until it decreased with increasing axial strain. The maximum axial stress results in the soil specimen failing and was taken as an unconfined compressive strength value. Then, undrained shear strength (cu) of cohesive soils was taken as one-half of the unconfined compressive strength (qu) when the soil is under ∅ = 0 condition (∅ = the angle of internal friction), according to [37], the relation between consistency and unconfined compression strength, soils of the study area were grouped and discussed.

2.3.4. Data Processing and Interpretation

All the data collected in the fieldwork and recorded in the laboratory analysis were processed and interpreted. Soils were classified based on unified and British soil classification systems. Rock mass classification based on the degree of weathering and UCS of rocks was carried out using the [27] method. Classification based on RQD and RMR values of the rocks was conducted by [29] method. A geological map, an engineering geological map, a groundwater contour map, and a physiographic map of the study area were produced using ArcGIS (version: 10.8), Global Mapper (version: 25.1), and Surfer software (version: 29.1.267) at a scale of 1:50,000. A physiographic map of the area was created from a digital elevation model of 30 m resolution. Data collected in the field and obtained through laboratory analysis were compiled and interpreted from both geological and engineering perspectives to conclude. Finally, the writing and organization of the complete research were performed.

3. Results

A field description of the soils and rocks in the study area was conducted. The rocks were described based on their degree of weathering, color, texture, strength, and type of exposure. The description of rocks was included in the characterization and classification part.

3.1. Engineering Geological Characterization and Classification of Soils

3.1.1. Engineering Geological Characterization of Soils

From an engineering perspective, the soils of Wolaita Sodo Town were characterized. The characterization was performed based on determining soil’s index and engineering properties. Some of the index and engineering properties were chosen and described as follows.

Natural Moisture Content

The natural moisture or water content of soil samples from 15 test pits was determined. It ranges from 23.47% to 44.21%. A water content ranging from 23.47% to 44.21% suggests that some soils could be prone to volume changes, particularly if they are clayey. Each soil sample’s water content is described in the table below (

Table 1. Water content and specific gravity of soil samples of the study area.

Test Pits	Location (UTM, N and E with Elev)	Name of Local Place	Depth (m)	Water Content %	Specific Gravity (Gs)
TP1	N0362460, 0759693 E 2080 m	Kidane Mihret	1.50	44.21	2.75
TP2	N0361690, 0759106 E 2053 m	Geneme/Radio station	1.30	34.26	2.68
TP3	N0362261, 0759000 E 2049 m	Aroge Arada Condominium	3	38.27	2.68
TP4	N0362464, 0756179 E 2003 m	Above university-Adebabay	1.20	37.68	2.74
TP5	N0362867, 0759874 E 2112 m	Teklehaimanot Gedam	1.25	31.57	2.68
TP6	N0362432, 0758188 E 2024 m	Rufael sefer-mar sefer	1	23.47	2.69
	N0362432, 0758188 E 2024 m	Rufael sefer-mar sefer	1.50	29.56	2.75
TP7	N0362027, 0757081 E 2043 m	Primary academy	1.20	35.24	2.69
TP8	N0363410, 0759163 E 2110 m	Gola sefer-Mezegaja	1.20	38.08	2.75
TP9	N0364049, 0758777 E 2035 m	Merkato Menafesha	2	35.47	2.68
TP10	N0363397, 0756740 E 2042 m	Buna kimsha area	2	40.73	2.81
TP11	N0361664, 0756123 E 1961 m	Exit of Sawla road	1	33.46	2.69
TP12	N0363843, 0757530 E 1977 m	Kera-Koka sefer	3	33.66	2.81
TP13	N0363570, 0757903 E 2016 m	Mariam church school	1.20	30.09	2.73
TP14	N0363978, 0758218 E 2015 m	Bunabort near Gebeya sefer	2	38.18	2.69
TP15	N0362145, 0755119	Wolaita Sodo University	2	29.81	2.81

).

Specific Gravity

The specific gravity of the soils in the study area ranges from 2.68 to 2.81, which falls within the typical range for common soil types, such as sand, silt, and clay. Soils with a specific gravity of 2.68 to 2.81 are likely to perform well in terms of load distribution, as they exhibit moderate density, which is generally beneficial for foundation stability and reduces the risk of settlement or structural failure. The test results of the specific gravity of 16 soil samples are shown in the table below (Table 1).

Grain Size Analysis

The grain size analysis of the soils in the study area revealed that gravel soils range from 0% to 11.67%, sand soils range from 6.01% to 26.80%, silt soils range from 19.91% to 47.11%, and clay soils range from 34.61% to 60.1% (

Table 2. Grain size analysis of soils of the study area.

Test Pits	Depth (m)	%Gravel	%Sand	%Silt	%Clay
TP1	1.50	0.00	19.98	19.91	60.11
TP2	1.30	0.00	19.14	26.94	53.92
TP3	3	0.10	15.59	34.28	50.03
TP4	1.20	0.00	6.41	35.82	57.77
TP5	1.25	0.00	11.07	43.82	45.11
TP6	1	1.80	21.04	38.1	39.05
	1.50	11.67	26.80	26.91	34.61
TP7	1.20	1.45	16.69	40.42	41.43
TP8	1.20	0.40	6.01	43.88	49.71
TP9	2	0.00	13.03	41.74	45.23
TP10	2	0.30	12.83	43.17	43.70
TP11	1	0.10	11.77	46.33	41.80
TP12	3	0.20	6.97	44.12	48.71
TP13	1.20	0.50	23.10	33.24	43.16
TP14	2	0.00	10.38	47.11	42.51
TP15	2	0.15	25.13	34.82	39.90

and Figure 4). The gravel content ranges from 0 to 11.67%, indicating that the soils are predominantly fine-grained, with only a small fraction of coarse particles. The sand content ranges from 6.01% to 26.80%, indicating that the soils may have a moderate proportion of sand, which contributes to improved drainage properties and higher shear strength compared to more clayey soils. The silt content ranges from 19.91% to 47.11%, implying the presence of moderately cohesive material that can impact the soil’s compaction and plasticity. The clay content ranges from 34.61% to 60.1%, indicating a significant proportion of fine, cohesive particles.

Atterberg (Consistency) Limit

The Atterberg limits determination of 16 soil specimens are shown in the table below (

Table 3. Atterberg limits with their index values and activities of 16 soil samples.

Test Pits	Depth (m)	W%	LL%	PL%	PI	LI	CI	% Clay Fraction	Activity
TP1	1.50	44.21	41.48	20.03	21.45	1.13	−0.13	60.11	0.36
TP2	1.30	34.26	48.28	19.64	28.64	0.51	0.49	53.92	0.53
TP3	3	38.27	51.42	22.06	29.36	0.55	0.45	50.03	0.59
TP4	1.20	37.68	53.48	21.62	31.85	0.5	0.5	57.77	0.55
TP5	1.25	31.57	43.66	19.48	24.18	0.5	0.5	45.11	0.54
TP6	1	23.47	41.96	24.51	17.45	−0.06	1.06	39.05	0.45
	1.50	29.56	45.58	18.55	27.03	0.41	0.59	34.61	0.78
TP7	1.20	35.24	45.28	24.51	20.77	0.52	0.48	41.43	0.5
TP8	1.20	38.08	46.29	16.25	30.04	0.73	0.27	49.71	0.6
TP9	2	35.47	48.50	19.48	29.03	0.55	0.45	45.23	0.64
TP10	2	40.73	54.72	25.54	29.18	0.52	0.48	43.70	0.67
TP11	1	33.46	41.11	27.08	14.02	0.46	0.55	41.80	0.34
TP12	3	33.66	39.97	15.47	24.50	0.74	0.26	48.71	0.5
TP13	1.20	30.09	43.53	21.62	21.91	0.39	0.61	43.16	0.51
TP14	2	38.18	43.93	27.08	16.84	0.66	0.34	42.51	0.4
TP15	2	29.81	40.86	16.99	23.87	0.54	0.46	39.90	0.6

). The liquid limit of soils in the study area ranges from 39.97 to 54.72%. The plastic limit of 16 soil samples was determined, ranging from 15.47% to 27.08%. The plasticity index, liquidity index, and consistency index values of soils of the study area range from 14.02 to 31.85%, −0.06 to 1.13%, and −0.13 to 1.06%, respectively. The plasticity index value indicates that the soils have low to marginal swelling potential. The consistency limits, as indicated by index values and activity, of Wolaita Sodo town soil samples are described in the table below (Table 3).

The plasticity chart (Figure 5 and Figure 6) shows that most study area soils, except for two test pit samples, all fall above the A-line. Soils under the A-line are silt. Soils that fall under CL have low to intermediate plasticity (11 soil samples), ML has low plasticity (two soil samples), and CH has high plasticity (three soil samples). No soil sample falls under the MH or elastic silt category. The plasticity chart (Figure 6) based on BSCS shows that most of the soils fall above the A-line in the CI (clay of intermediate plasticity) category, two soil samples under the MI (silt of intermediate plasticity), and three soil samples are under the CH (clay of high plasticity) category.

Activity of Soils

The activity of soils in the study area (Table 3) ranges from 0.34 to 0.78. Test pit 6B has an activity value between 0.75 and 1.25, indicating it is a typical soil. The remaining 15 soil samples have an activity value below 0.75, referred to as inactive soils (Figure 7). Consequently, they provide a relatively stable foundation for engineering structures. The normal soil in Test Pit 6B requires more careful attention to moisture fluctuations, but it is still a manageable condition with proper design.

Linear Shrinkage Limit of Soils

According to the test results, the percentage of linear shrinkage limit of the soils under investigation ranges from 5% to 11.43%. The shrinkage index ranges from 14.29% to 26.9%. Based on [35] classification system, the degree of shrinkage of soils of the study area shows poor quality (TP5, TP7, TP10) to medium quality (the remaining 13 test pits). The shrinkage limit and shrinkage index values of 16 test pits are described in

Table 4. Linear shrinkage limit values of soils of the study area.

Test Pits	Depth (m)	Li (cm)	Lf (cm)	Sr %	SI	Quality Based on Shrinkage Limit Value	Potential of Swelling Based on the Shrinkage Index Value
TP1	1.50	14	12.8	8.57	19.21	Medium	Low
TP2	1.30	14	12.9	7.86	22.7	Medium	Medium
TP3	3	14	13.1	6.43	26.9	Medium	Medium
TP4	1.20	14	13	7.14	26.19	Medium	Medium
TP5	1.25	14	12.5	10.71	17.07	Poor soil	Low
TP6	1	14	13.3	5	15.37	Medium	Low
	1.50	14	13.2	5.71	22.07	Medium	Medium
TP7	1.20	14	12.5	10.71	14.29	Poor soil	Low
TP8	1.20	14	13	7.14	20.64	Medium	Medium
TP9	2	14	13.1	6.43	21.35	Medium	Medium
TP10	2	14	12.4	11.43	14.11	Poor	Low
TP11	1	14	13	7.14	17.86	Medium	Low
TP12	3	14	12.9	7.86	22.7	Medium	Medium
TP13	1.20	14	12.8	8.57	19.21	Medium	Low
TP14	2	14	12.7	9.29	15.71	Medium	Low
TP15	2	14	13	7.14	17.86	Medium	Low

.

Free Swell

The free swell value of soils of the study area ranges from 5% (TP-13) to 23% (TP-9) (

Table 5. Free swell values of soil specimens of the study area.

Test Pits	Depth (m)	Initial Volume	Final Volume		Average Final Volume	Free Swell Index (%)
			Sample No. 1	Sample No. 2
TP1	1.50	10	10	14	12	20
TP2	1.30	10	10	14	12	20
TP3	3	10	10	12.5	11.3	13
TP4	1.20	10	10	12	11	10
TP5	1.25	10	10	13	11.5	15
TP6	1	10	10	13	11.5	15
	1.50	10	10	13	11.5	15
TP7	1.20	10	10	13	11.5	15
TP8	1.20	10	10	12	11	10
TP9	2	10	10	14.5	12.3	23
TP10	2	10	10	13	11.5	15
TP11	1	10	10	12	11	10
TP12	3	10	10	13.5	11.8	18
TP13	1.20	10	10	11	10.5	5
TP14	2	10	10	12	11	10
TP15	2	10	10	13	11.5	15

), which is relatively low. Therefore, soils with a swelling potential below 50% have low expansion potential, making them less likely to undergo significant volume changes when subjected to moisture fluctuations.

Unconfined Compressive Strength

The unconfined compressive strength of 10 soil samples was determined, with values ranging from 215.8 to 333.5 kPa (

Table 6. Unconfined compressive strength values of 10 soil samples.

Test Pits	Sampling Depth (m)	Unconfined Compressive Strength (qu), kPa	Undrained Shear Strength (cu), kPa
TP1	1.50	238.9	119.5
TP3	3	316.1	158
TP4	1.20	333.5	166.7
TP6	1	267.6	133.8
	1.50	228.8	114.4
TP7	1.20	221.0	110.52
TP9	2	215.8	107.9
TP10	2	311.6	155.8
TP12	3	241.9	121
TP13	1.20	296.1	148
TP15	2	257.2	128.6

and Figure 8). Soils with UCS values closer to 333.5 kPa will likely provide a better load-bearing capacity, making them suitable for heavy civil engineering structures, such as high-rise buildings or bridges, without requiring extensive soil modification. On the other hand, soils with UCS values around 215.8 kPa may be weaker and more prone to deformation under load, necessitating additional measures like soil stabilization, deep foundations, or ground improvement techniques to ensure the stability of structures built on these soils. The relation between consistency and unconfined compressive strength of these ten soil samples was described according to [37] standards.

3.1.2. Engineering Geological Classification of Soils

According to the Unified Soil Classification System (USCS), soils of the study area are classified into lean clay, lean clay with sand, fat clay, fat clay with sand, and clayey silt with slight plasticity (

Table 7. Classification of soils based on Unified soil classification system (USCS).

Test Pits	Depth (m)	Sieve Analysis (%)				Consistency Limits (%)		USCS
		Gravel	Sand	Silt	Clay	LL	PI	Symbol	Soil Name Description
TP1	1.50	0.00	19.98	19.91	60.11	41.48	21.45	CL	Lean clay with sand
TP2	1.30	0.00	19.14	26.94	53.92	48.28	28.64	CL	Lean clay with sand
TP3	3	0.10	15.59	34.28	50.03	51.42	29.36	CH	Fat clay with sand
TP4	1.20	0.00	6.41	35.82	57.77	53.48	31.85	CH	Fat clay
TP5	1.25	0.00	11.07	43.82	45.11	43.66	24.18	CL	Lean clay
TP6	1	1.80	21.04	38.1	39.05	41.96	17.45	CL	Lean clay with sand
	1.50	11.67	26.80	26.91	34.61	45.58	27.03	CL	Lean clay with sand
TP7	1.20	1.45	16.69	40.42	41.43	45.28	20.77	CL	Lean clay with sand
TP8	1.20	0.40	6.01	43.88	49.71	46.29	30.04	CL	Lean clay
TP9	2	0.00	13.03	41.74	45.23	48.50	29.03	CL	Lean clay
TP10	2	0.30	12.83	43.17	43.70	54.72	29.18	CH	Fat clay
TP11	1	0.10	11.77	46.33	41.80	41.11	14.02	ML	Clayey silt with slight plasticity
TP12	3	0.20	6.97	44.12	48.71	39.97	24.50	CL	Lean clay
TP13	1.20	0.50	23.10	33.24	43.16	43.53	21.91	CL	Lean clay with sand
TP14	2	0.00	10.38	47.11	42.51	43.93	16.84	ML	Clayey silt with slight plasticity
TP15	2	0.15	25.13	34.82	39.90	40.86	23.87	CL	Lean clay with sand

). Plots of the soils in the study area on the Casagrande plasticity chart, based on the Unified Soil Classification System (USCS), are shown in Figure 5. According to the British Soil Classification System (BSCS), soils of the study area are classified as clay of intermediate plasticity, clay of high plasticity, and silt of intermediate plasticity (

Table 8. Classification of soils based on the British Soil Classification System (BSCS).

Test Pits	Depth (m)	Sieve Analysis (%)				Consistency Limits (%)		BSCS
		Gravel	Sand	Silt	Clay	LL	PI	Symbol	Soil Name Description
TP1	1.50	0.00	19.98	19.91	60.11	41.48	21.45	CI	Clay of intermediate plasticity
TP2	1.30	0.00	19.14	26.94	53.92	48.28	28.64	CI	Clay of intermediate plasticity
TP3	3	0.10	15.59	34.28	50.03	51.42	29.36	CH	Clay of high plasticity
TP4	1.20	0.00	6.41	35.82	57.77	53.48	31.85	CH	Clay of high plasticity
TP5	1.25	0.00	11.07	43.82	45.11	43.66	24.18	CI	Clay of intermediate plasticity
TP6	1	1.80	21.04	38.1	39.05	41.96	17.45	CI	Clay of intermediate plasticity
	1.50	11.67	26.80	26.91	34.61	45.58	27.03	CI	Clay of intermediate plasticity
TP7	1.20	1.45	16.69	40.42	41.43	45.28	20.77	CI	Clay of intermediate plasticity
TP8	1.20	0.40	6.01	43.88	49.71	46.29	30.04	CI	Clay of intermediate plasticity
TP9	2	0.00	13.03	41.74	45.23	48.50	29.03	CI	Clay of intermediate plasticity
TP10	2	0.30	12.83	43.17	43.70	54.72	29.18	CH	Clay of high plasticity
TP11	1	0.10	11.77	46.33	41.80	41.11	14.02	MI	Silt of intermediate plasticity
TP12	3	0.20	6.97	44.12	48.71	39.97	24.50	CI	Clay of intermediate plasticity
TP13	1.20	0.50	23.10	33.24	43.16	43.53	21.91	CI	Clay of intermediate plasticity
TP14	2	0.00	10.38	47.11	42.51	43.93	16.84	MI	Silt of intermediate plasticity
TP15	2	0.15	25.13	34.82	39.90	40.86	23.87	CI	Clay of intermediate plasticity

).

3.2. Engineering Geological Characterization and Classification of Rock Masses

3.2.1. Engineering Geological Characterization of Rock Masses

Some rocks exposed in the study area were affected by weathering. Discoloration of the surface and the presence of rock fragments were observed during field investigation. Rocks exposed on natural slopes around Wolaita Sodo town radio station in the northwest direction were slight to moderately weathered. Rocks exposed by a road cut at the center of the town (southwest of the bus station) and north of the town near Damota mountain were high to completely weathered. Fresh rocks exposed by excavation were observed in the southeast direction of the town and in the north direction of the city below the road of Wolaita to Addis Ababa. Strength was determined using Schmidt hammer and geological hammer tests. The strength of rock masses was correlated to uniaxial compressive strength. The number of 18 sampling points were taken in different locations following the ISRM Schmidt hammer test procedure (

Table 9. Classification of rocks of the study area based on their UCS value.

Location (UTM, N and E with Elev.)	Lithology	Average SHV	Average UCS (MPa)	Qualitative Strength Based on Reference [27]
0361887, 0759145 E 2031 m	Slightly weathered ignimbrite	33.6	40.25	Medium strength
0361493, 0758930 E 2045 m	Fresh ignimbrite	42.5	68.64	High strength
0361406, 0758919 E 2046 m	Slightly weathered ignimbrite	31.7	35.26	Medium strength
0361403, 0758909 E 2039 m	Fresh ignimbrite	42.7	69.38	High strength
0361602, 0758976 E 2052 m	Completely weathered ignimbrite	10.9	3.12	Very low strength
0362727, 0757181 E 2017 m	Highly weathered ignimbrite	11.7	3.66	Very low strength
0362728, 0757176 E 2016 m	Completely weathered ignimbrite	10.5	2.86	Very low strength
0362725, 0757183 E 2019 m	Highly weathered ignimbrite	11.5	3.52	Very low strength
0362663, 0757120 E 2007 m	Highly weathered ignimbrite	13.3	4.9	Very low strength
0362717, 0757127 E 2011 m	Moderately weathered ignimbrite	16.6	8.11	Low strength
0363814, 0755668 E 1883 m	Slightly weathered ignimbrite	33.0	38.63	Medium strength
0363814, 0755656 E 1879 m	Fresh ignimbrite	39.6	58.46	High strength
0363801, 0755683 E 1891 m	slightly weathered ignimbrite	33.0	38.63	Medium strength
0363800, 0759465 E 2137 m	Highly weathered ignimbrite	11.1	3.25	Very low strength
0363882, 0755875 E 1901	Highly weathered ignimbrite	13	4.65	Very low strength
0364291, 0759575 E 2114	Slightly weathered ignimbrite	29.8	30.64	Medium strength
0364304, 0759510 E 2112	Slightly weathered ignimbrite	34.3	42.18	Medium strength
0364429, 0759415 E 2097	Fresh ignimbrite	41.7	65.74	High strength

). For each sampling point, 20 rebound values were recorded, averaged, and used. The scanline sampling technique was used to collect discontinuity data and determine the RQD of rock masses in the study area.

3.2.2. Engineering Geological Classification of Rock Masses

Rock Mass Classification Based on Degree of Weathering and Strength Tests

Rock masses in the study area were classified based on their degree of weathering and strength. The degree of weathering categorized rocks as fresh, slightly weathered, moderately weathered, highly weathered, and completely weathered. Their strength also classified them. Consequently, the rock masses exhibit very low, low, medium, and high mass strengths.

• Highly to completely weathered, very weak ignimbrite (Rocks with very low mass strength)

The study found highly to completely weathered ignimbrite rocks exposed at three locations in the study area near the bus station in the southwest of the town, near Damota mountain to the north, and along a river cut between the radio station and Woin kebele (Figure 9). These rocks had a fine-grained texture and dark to light brown weathered color, showing a high degree of weathering. They were friable, with a dull surface luster, and over half of them had disintegrated into smaller fragments and soils. The joints in these rocks had very close spacing and were filled with clay, which reduced the rock’s strength. The strike and dip of the master joint were S10°W, 81°NW, and the slope strike was S60E, 34NW direction. The average Schmidt hammer value of these rocks ranged from 10.5 to 13.3, with the derived strength values ranging from 2.86 to 4.9 MPa. The rocks could easily be broken under firm blows with a geological hammer and peeled with a pocketknife.

• Moderately weathered, weak ignimbrite (Rocks with low mass strength)

Moderately weathered, weak ignimbrite rocks were observed at the excavation or quarry site. Less than half of the rock was disintegrated into the soil, exhibiting a grey color in the fresh part and a dark brown color in the weathered part (Figure 10). The rock had an aphanitic texture, with surface discoloration observed. The groundwater condition of the joints was completely dry. The joints had less than 1 m of persistence, 3 mm aperture, and no filling material. The average spacing between the joints was 0.2 m, with the joints being closely spaced. This rock’s average Schmidt hammer value was 16.6, and the uniaxial compressive strength was 8.11 MPa. These rocks could be easily peeled by a pocketknife and broken with a firm blow using a geological hammer.

• Slightly weathered, medium-strong ignimbrite (Rocks with medium mass strength)

Slightly weathered, medium-strength rocks were observed in the study area, exposed by river/stream cuts near the radio station, road cuts around the center of the town, and through excavations for quarry sites to the southwest and north of the town. They were also observed on the natural slope around Ganamie Mountain (Figure 11). These rocks have a fine-grained texture with fresh grey and weathered light brown color. Vertically oriented columnar joints were found in the excavation face or quarry site, with visible ends at the top and extending beyond the exposure limits. Blocks of rock and soil obscured the extended ends. The joints between the columns were closed by rock fragments at the top and clay at the bottom. On the natural slope around Ganamie Mountain, joints were not visible at either end and were open, with a persistence of 3 m, a 3 cm aperture, a strike of N70° E, and a dip of 25° NW. The average joint spacing was 1.2 cm. The average Schmidt hammer values for these rock units ranged from 29.8 to 34.3, and the uniaxial compressive strength was determined to range from 30.64 to 42.18 MPa. These rocks could not be scraped or peeled with a pocketknife but could be fractured with a single firm blow using a geological hammer.

• Fresh, strong ignimbrite (Rocks with high mass strength)

Fresh, strong ignimbrites are exposed on the hillside, and excavation faces are in two areas. These rocks show minimal weathering effects compared to others in the study area. The rocks have an aphanitic texture with a grey color (Figure 12). Joints with high spacing values were observed in these rocks. The master joint in these rocks has no visible ends (obscured by blocks of rock), is open, and has an aperture ranging from a minimum of 1 cm to a maximum of 3 cm. The joint is straight with a persistency of 6 m. The strike and dip of the master joint are N32° E and 65° SE. The average Schmidt hammer values for these rocks range from 39.6 to 42.7. The uniaxial compressive strength, derived from Schmidt hammer values, ranges from 58.46 to 69.36 MPa. These rocks require more than one blow of a geological hammer to be fractured, with lumps breaking only under heavy hammer blows. These rocks are commonly excavated in the area to construct buildings, other structures, and fences.

Rock Mass Classification Based on Uniaxial Compressive Strength (UCS)

UCS was estimated indirectly using a portable Schmidt hammer, as other equipment was unavailable for this purpose. Subsequently, the uniaxial compressive strength of the rocks was estimated from the recorded SHV of 18 sampling points. Uniaxial compressive strength values for rocks of the study area ranged from 2.86 MPa to 69.38 MPa, with Schmidt hammer values (SHV) ranging from 10.5 to 42.7 (Table 9). The classification of rocks based on UCS values was done according to the method suggested by [27]. The rocks were classified into four categories based on their mass strength: rocks with very low mass strength (seven sampling points), rocks with low mass strength (one sampling point), rocks with medium mass strength (six sampling points), and rocks with high mass strength (four sampling points).

Figure 13 presents the variation of SHV and UCS across different lithological units, primarily ignimbrite of varying weathering degrees. The plot shows that fresh ignimbrite and slightly weathered ignimbrite (SW ignimbrite) consistently exhibit higher UCS values (up to ~70 MPa) and higher SHV readings, indicating strong and competent rock suitable for heavy construction. In contrast, completely weathered and highly weathered ignimbrite show a significant drop in both UCS and SHV (often below 10 MPa), reflecting poor mechanical properties and higher susceptibility to failure. This trend highlights the clear impact of weathering on rock strength and reinforces the need to avoid heavily weathered zones for critical foundations without proper treatment.

Rock Mass Classification Based on Rock Quality Designation (RQD)

In the absence of borehole core data, the RQD for the rock masses of the study area was estimated from fracture frequency using the formula proposed by [28]. Scanned line sampling was used to collect discontinuity data on fractured rock surfaces. 16 scanline data were collected from different areas, ranging from a minimum of 4 m long (containing two to 19 discontinuities) to a maximum of 30 m long (containing 36 discontinuities). The scanline data were measured, counted, and described, summarized in the table below (

Table 10. Scanline data collected in the field.

Scanline Data
Scanline Number	Lithology	Location: UTM (Northing and Easting)	Length (m)	No of Discontinuity
1	Slightly weathered ignimbrite	0361406, 0758919	5	4
2	Fresh ignimbrite	0361403, 0758909	4	2
3	Highly weathered ignimbrite	0362727, 0757181	18	30
4	Completely weathered ignimbrite	0362728, 0757176	21	37
5	Highly weathered ignimbrite	0362725, 0757183	7	9
6	Moderately weathered ignimbrite	0362717, 0757127	6	7
7	Slightly weathered ignimbrite	0363814, 0755668	11	9
8	Slightly weathered ignimbrite	0363801, 0755683	13	11
9	Highly weathered ignimbrite	0363800, 0759465	30	36
10	Slightly weathered ignimbrite	0361887, 0759145	5	5
11	Fresh ignimbrite	0363814, 0755656	12	5
12	Highly weathered ignimbrite	0363882, 0755875	4	19
13	Slightly weathered ignimbrite	0364291, 0759575	15	12
14	Slightly weathered ignimbrite	0364304, 0759510	10	9
15	Fresh ignimbrite	0364429, 0759415	10	2
16	Highly weathered ignimbrite	0362663, 0757120	5	27

).

Table 11. RQD, fracture frequency, and mean spacing of rocks in the study area.

Scanline Number	Lithology	Fracture Frequency, λ (m−1)	Mean Spacing (m)	RQD (%)	Quality
1	Slightly weathered ignimbrite	0.8	1.25	80.88	Good
2	Fresh ignimbrite	0.5	2	90.98	Excellent
3	Highly weathered ignimbrite	1.67	0.59	50.26	Fair
4	Completely weathered ignimbrite	1.76	0.57	47.48	Poor
5	Highly weathered ignimbrite	1.29	0.78	63.04	Fair
6	Moderately weathered ignimbrite	1.17	0.85	67.35	Fair
7	Slightly weathered ignimbrite	0.82	1.22	80.16	Good
8	Slightly weathered ignimbrite	0.85	1.18	79.07	Good
9	Highly weathered ignimbrite	1.2	0.83	66.26	Fair
10	Slightly weathered ignimbrite	1	1	73.58	Fair
11	Fresh ignimbrite	0.42	2.38	93.3	Excellent
12	Highly weathered ignimbrite	4.75	0.21	58.15	Fair
13	Slightly weathered ignimbrite	0.8	1.25	80.88	Good
14	Slightly weathered ignimbrite	0.9	1.11	77.25	Good
15	Fresh ignimbrite	0.2	5	98.25	Excellent
16	Highly weathered ignimbrite	5.4	0.19	51.85	Fair

, and Figure 14 and Figure 15, provides a comprehensive assessment of rock mass quality across the study area, based on lithology, weathering degree, fracture characteristics, and RQD values. The data clearly show that rock mass quality is strongly influenced by the degree of weathering. Fresh ignimbrite consistently exhibits excellent rock quality, with RQD values above 90% (Table 11; Figure 14), low fracture frequencies (0.2–0.5 m⁻¹), and wide mean fracture spacing (2–5 m) (Table 11; Figure 15), as observed in scanlines 2, 11, and 15. These properties indicate massive, competent rock ideal for supporting heavy engineering structures. In contrast, highly weathered and completely weathered ignimbrite demonstrate significantly reduced rock quality. RQD values drop below 60% in several scanlines (e.g., scanlines 3, 4, 12, and 16), placing these rocks in the “Fair” to “Poor” quality range (Table 11; Figure 14). These zones also show high fracture frequency values (up to 5.4 m⁻¹) and very close mean spacing (as low as 0.19 m) (Figure 15), reflecting extensive rock mass fragmentation and a higher likelihood of structural instability. Slightly and moderately weathered ignimbrite generally fall into the “Good” quality category, with RQD values between 70% and 80%, moderate fracture frequency (~0.8–1.2 m⁻¹), and spacing between 1.0 and 1.3 m (Table 11). These zones are considered moderately favorable for construction, with localized stabilization or reinforcement as needed.

Rock Mass Classification Based on Rock Mass Rating (RMR)

The Rock Mass Rating (RMR) system was applied to classify the rock masses of the study area. The classification was based on the five primary parameters determined from field surveys. These parameters include the uniaxial compressive strength of intact rock material, Rock Quality Designation (RQD), spacing of discontinuities, condition of discontinuities (persistence, separation, smoothness, infilling, and weathering), and groundwater conditions. To determine the RMR, values corresponding to these parameters were calculated and summed to obtain the basic RMR value. As for the sixth parameter, the orientation of discontinuities, it was not considered in this classification in the current research due to its dependence on specific engineering applications, such as slope, tunnel, and foundation design. The detailed RMR_basic values for the rock masses in the study area were calculated and are provided in the table below (

Table 12. The rating values of the five parameters and their summation.

Lithology	UCS (Mpa)	RQD %	Joint Condition	Joint Spacing (m)	Groundwater Situation	RMRbasic	Class	Quality
	Rating Value of Each Parameter
Slightly weathered ignimbrite	4	17	24	15	15	75	II	Good
Fresh ignimbrite	7	20	29	15	10	81	I	Very good
Highly weathered ignimbrite	1	13	12	10	15	51	III	Fair
Completely weathered ignimbrite	1	8	15	10	10	44	III	Fair
Highly weathered ignimbrite	1	13	12	15	7	48	III	Fair
Moderately weathered ignimbrite	2	13	19	15	15	64	II	Good
Slightly weathered ignimbrite	4	17	22	15	15	73	II	Good
Slightly weathered ignimbrite	4	17	26	15	15	77	II	Good
Highly weathered ignimbrite	1	13	18	10	10	52	III	Fair
Slightly weathered ignimbrite	4	13	25	15	15	72	II	Good
fresh ignimbrite	7	20	27	20	15	89	I	Very good
Highly weathered ignimbrite	1	13	16	10	15	55	III	Fair
Slightly weathered ignimbrite	4	17	22	15	7	65	II	Good
Slightly weathered ignimbrite	4	17	20	15	15	71	II	Good
Fresh ignimbrite	7	20	28	20	15	90	I	Very good
Highly weathered ignimbrite	1	13	14	10	15	53	III	Fair

).

Based on the values of RQD and RMR (Table 11 and Table 12), the quality of the rock masses in the study area was assessed. The RQD values ranged from 47.48% to 98.25%, which reflects a range of rock quality from poor to excellent. Meanwhile, the RMR values ranged from 44% to 90%, categorizing the rocks into three major classes such as:

• Class I (Very Good)—Higher rock mass quality.
• Class II (Good)—Moderate rock mass quality.
• Class III (Fair)—Lower rock mass quality.

As shown in Figure 16, fresh and slightly weathered ignimbrites display high RQD values (above 85%) and correspondingly high RMR scores (above 70), classifying them as “Good” to “Very Good” quality rock masses. These rocks are structurally competent, with low fracturing and high bearing capacity—suitable for engineering structures with minimal reinforcement. In contrast, highly weathered (HW) and completely weathered (CW) ignimbrites exhibit much lower RQD values (as low as ~45–55%), which coincide with RMR scores around or below 50. These values fall into the “Fair” to “Poor” quality categories, indicating fractured, weak rock masses prone to deformation and failure under load. This trend aligns well with the field observations and test results. The close tracking of RQD and RMR across all lithologies reinforces that weathering degree is a dominant factor influencing rock mass behavior. Thus, both parameters together provide reliable input for engineering geological classification and site suitability analysis.

3.3. Surface Geodynamic Processes

This research observed and assessed surface geodynamic processes in the study area. A significant geodynamic process was observed in the area, contributing to soil and rock displacement. Additionally, notable erosion features were identified, indicating the movement of soil and sediment due to natural forces. Weathering processes were also observed, indicating the breakdown of rocks due to environmental factors. Furthermore, some karst features were observed in the region, indicating the dissolution of soluble rocks, such as limestone. Areas with potential collapsibility were identified, where rocks or soil could become unstable under certain conditions.

3.3.1. Rill Erosion

Rill erosion was observed in the northwest, northeast, and southwest directions of Wolayita Sodo town. This type of erosion originates from a hillside of a small mountain, with water flowing downhill toward the town (Figure 17). The erosion is more pronounced on steep slopes, and as the water continues to flow, it eventually forms gully erosion. The affected area exhibits landforms typical of rill erosion, characterized by small, concentrated channels that result from the movement of runoff water down the slope.

3.3.2. Gully Erosion

Gully erosion was observed in the southeast and south directions of the study area, particularly near the road from Sodo town to Arba Minch (Figure 18). The erosion has created a steep-sided channel, cutting the land by up to 5 m in some areas. Subsurface water is draining out due to the effects of this erosion. However, the road has not yet been directly affected by the gully erosion, although it is at risk if no remedial measures are taken. The landform created by this process is identified as a gully.

3.3.3. Weathering

Weathering was observed to affect rocks exposed on road cuts, hillsides, and natural slopes in the study area (Figure 19). The rocks showed significant development of discontinuities, and further weathering occurred due to water infiltration through these cracks. This led to observable discoloration and staining on the rock surfaces, along with changes in texture, disintegration, and decomposition. The rocks’ strength was significantly reduced compared to fresh or slightly weathered rocks.

3.3.4. Rockslide

A rockslide was observed in the northeast direction of the town near a river (Figure 20). The rockslide exhibited a planar mode of failure, with the sliding mass being primarily composed of slightly to moderately weathered rock, which is covered by soil at the top. The leading cause of this rockslide is the removal of the toe of the slope due to excavations and quarrying activities. This human-induced process has led to instability, with the potential for the sliding mass to block the surrounding rivers, causing flooding.

3.3.5. Rock Toppling

Toppling was observed in the north and southeast directions of Wolayita Sodo town. The toppling in the north direction is above the town, where widely spaced large joints in the rock unit cause large blocks to topple (Figure 21). This movement poses a threat to infrastructure, as it may disrupt asphalt roads, buildings, and houses, potentially resulting in fatalities. Additionally, the toppling could lead to slope instability or trigger landslides. In the southeast direction, toppling occurred on outcrops of welded ignimbrites, where excavation for construction purposes promoted the process. Nearby settlements and rivers could be impacted if these rock blocks topple, potentially blocking rivers or damaging the settlement. The toppling blocks become especially dangerous during heavy rainfall or when subjected to additional loads, and without proper mitigation measures, such as retaining walls or slope cutting, they pose a significant risk.

3.4. Effects of Groundwater on Engineering Structures

In the study area, no clear surface manifestations of groundwater effects were observed in relation to the engineering problems. However, there was noticeable seepage, with water draining out from the subsurface towards the gully in the south direction. Additionally, a swampy area was observed, suggesting the presence of shallow groundwater. To further investigate the groundwater conditions, a groundwater table contour map was produced for the area, with an interval of 15 m, and the flow direction was plotted (Figure 22).

3.5. Geology of the Study Area

The study area is characterized by an ignimbrite rock unit and residual soil, commonly referred to as laterite soil. It was also challenging to find rock exposure in the town, except on the small riverbanks and excavation areas.

3.5.1. Ignimbrite

Ignimbrite is exposed in the study area on natural slopes, river cuts, road cuts, and excavation faces (Figure 23). It has an aphanitic to eutaxitic texture, and the color ranges from light gray to gray for the fresh part and from brown to dark for the weathered part. Larger crystals, phenocrysts, or clasts have been observed on the fresh part of this rock unit. Silica and feldspar minerals were identified. It exhibits variable strength, ranging from high to very low. The variation in rock strength is due to the degree of weathering, variable exposure, and joint spacing. Most of the rocks exposed by the road cut were weathered entirely. Rocks exposed on the hillside and through the excavation/quarry site were fresh to moderately weathered. The local people highly excavated this rock unit for construction and building stones (commonly used for houses and retaining walls).

3.5.2. Thick Residual Soil Deposit

Thick residual soil deposits highly cover the study area. The thickness of the soil varies from place to place. It is up to 7 m at the foot of river valleys and excavations. These soils are formed through in situ weathering and the decomposition of rock materials. The colour of the soil is almost identical, predominantly red and reddish-brown. The soils have high dry strength. The soil type is termed laterite soil, rich in iron and aluminium.

3.5.3. Geological Map and Structures

The types of geological structures found or observed in the study area are predominantly columnar joints and systematic to non-systematic fractures. Columnar joints were observed in the southeast direction of Wolaita Sodo town on the excavated rock faces/quarry sites. Most of the columns are nearly straight with parallel sides. The joints are partially filled by clay soil and rock fragments. The columnar joints are widely spaced and reach heights of up to 15 m, forming a cliff. The rock exposure locations containing the columnar joints are N0363814, E0755668/N06°50.130′, and E037°46.053′ with an elevation of 1883 m. The ignimbrites are also affected by irregular fractures with varying trends. Some systematic joints were also observed in some locations. Most of the joints are filled with clay, and some are open. Three master joints were observed in different places. The major and minor trends of the joints are the same: NNE-SSW and ENE-WSW. The strike and dip of these joints are N70° E/25° NW, N32° E/65° SE, and S10° W/81° NW (Figure 24).

Figure 25 shows that ignimbrite rock units are distributed as isolated patches primarily in the central, northeastern, and southeastern parts of the study area. These are surrounded by extensive residual soil deposits (in red), which dominate the surface coverage. Two intersecting fault lines are marked, with one crossing the A–B section near the midpoint, influencing both the surface morphology and subsurface structure. The cross-sectional profile along line A–B reveals a variation in elevation from approximately 1890 m to over 2050 m, with ignimbrite rock occurring as topographic highs. The presence of a fault correlates with a visible break and offset in the topographic and lithologic continuity, suggesting it plays a role in lithological juxtaposition and slope instability.

Figure 25. Geological map of the study area and its cross-section from A to B.

Figure 25. Geological map of the study area and its cross-section from A to B.

3.5.4. Geological Log Description of Boreholes in the Study Area

Four borehole points were collected in the study area, providing additional information for this research work (

Table 13. Geological log description of boreholes in the study area.

BH No. 1		BH No. 2		BH No. 3		BH No. 4
Location		LOCATION		LOCATION		LOCATION
N 0757902		N 0757722		N 0755320		N 0754559
E 0362336		E 0364216		E 0361868		E 0361575
E = 2046 m		E = 1964 m		E = 1877 m		E = 1855 m
SWL = 102.20 m		SWL = 93.60 m		SWL = 50.90 m		SWL = 33.11 m
WTE = 1943.8 m		WTE = 1870.4 m		WTE =1826.1 m		WTE = 1821.89 m
Depth (m)	Lithology	Depth (m)	Lithology	Depth (m)	Lithology	Depth (m)	Lithology
0–27	Red clay	0–42	Ignimbrite	0–8	Clay	0–8	Ash
27–50	Ignimbrite	42–48	Weathered ignimbrite	8–17	Highly weathered & fractured ignimbrite	8–20	Slightly Weathered Ignimbrite
50–74	Tuff	48–85	Ignimbrite	17–23	Highly weathered ignimbrite	20–62	Fresh Ignimbrite
74–92	Red clay	85–103	Sand with clay	23–35	Slightly fractured ignimbrite	62–82	Highly Weathered Ignimbrite
92–134	Ash	103–109	Red clay	35–62	Highly welded ignimbrite	82–88	Clay
134–140	Ash With Sand	109–121	Highly weathered ignimbrite	62–65	Slightly weathered & fractured ignimbrite	88–94	Fractured ignimbrite
140–164	Tuff	121–127	Weathered ignimbrite	65–68	Sand	94–97	Pumice
164–167	Silt	127–139	Ignimbrite	68–75	Slightly weathered ignimbrite	97–136	Tuff
167–185	Tuff	139–145	Sandy clay	75–84	Highly welded ignimbrite
185–188	Ignimbrite	145–169	Sand	84–87	Highly fractured ignimbrite
188–194	Sand			87–114	Highly weathered ignimbrite
194–202	Alluvial deposit			114–126	Sand
202–205	Fractured ignimbrite			126–137	Highly weathered ignimbrite

Note: SWL = Static water level, WTE = Water table elevation.

). Their GPS location is also included in the table. The vertical log shows the subsurface soil and rock type that covers the area. The source of the data is the Wolaita Sodo Town Water Supply Office. The geological log for each borehole is provided in the Figure 26. The borehole data provide important vertical information that complements the surface geological observations and test pit profiles. For example, BH1 and BH2 confirm the presence of thick ignimbrite and tuff layers beneath red clay and ash, correlating with mapped units of ignimbrite rock with overlying high-plasticity soils. In contrast, BH3 and BH4 show weathered ignimbrite, welded tuff, and pumice interbedded with clay and sand, which supports the surface classification of these areas as moderately weathered rock with limited foundation capacity.

3.6. Engineering Geological Mapping of Wolaita Sodo Town

An engineering geological map of Wolaita Sodo Town was produced based on a topographic map, field data, in situ test results, descriptions of soils and rocks, and laboratory analysis data. The map provides valuable information, including lithological units, engineering geological units, surface geodynamic processes, geological structure, water table contours, and various sampling points. It serves as a comprehensive, multipurpose, and medium-scale map produced at a scale of 1:50,000 (Figure 27). Areas with low to medium plasticity clay have a more stable base and are suitable for light to medium-weight structures. Nevertheless, this finding may require careful drainage design due to possible variations in moisture content and slope conditions in the locality. These zones can be developed for residential or low-rise commercial structures using traditional shallow foundations.

Designated as high-plasticity clay, they create more engineering challenges due to their higher shrink-swell potential and moderate strength. Site investigation is necessary before building in these zones, and possible reinforcement of foundations or soil stabilization may be required. These areas are more suitable for open spaces, parks, or non-settlement-sensitive structures. Areas with silt of low to moderate plasticity exhibit highly erodible and moderate swelling behavior, particularly where surface water is concentrated. These areas require improved surface water management and the limited use of heavy structures without subgrade treatment. Rock units are also differentiated by mass strength. The high mass strength of fresh ignimbrite is suitable for major infrastructure, such as high-rise buildings, retaining walls, and transportation facilities. Weathered ignimbrite (yellow hatching) is also appropriate and might need to be treated in joints and slopes. In comparison, intermediate weathering and well- to very well-weathered ignimbrite are characteristically mechanically weak and prone to failure. For critical structures, these should be avoided unless extensive engineering measures are implemented.

The borehole log data, combined with surface geological measurements and test pits, show a consistent subsurface pattern for much of Wolaita Sodo Town. Boreholes revealed thick clay layers (up to 5.4 m) with high moisture content and low available shear strength in regions underlain by high-plasticity clays (Gofe and part of Dilla Sefer), with peaks of swelling-related surface deformations of buildings corresponding with these sub-surface conditions. In contrast, the Fana and Soddo Central boreholes intersected shallow bedrock (1.2–2.3 m depth) consistent with outcrops of moderately to slightly weathered ignimbrite at the map surface. Dipping downslope weathering profiles were also consistent with geomorphic processes expected to produce similar patterns based on erosion or slope wash. This correlation also reinforces the reliability of the surface evaluation-based engineering geological map and the subsequent surface and shallow subsurface predicted construction suitability zoning for the town.

4. Discussion

The natural moisture content of the soils in the study area ranges from 23.47% to 44.21%, indicating that soils with higher natural moisture content are weaker due to lower cohesion and friction, while those with lower moisture content are brittle and prone to erosion (Table 1) [38]. Specific gravity values ranging from 2.68 to 2.81 (as measured by a Pycnometer) indicate moderate to high mineralization values, as well as adequate structural strength properties [39,40] (Table 1). Sixteen samples were analyzed by wet sieving, confirming the dominance of fine-grained soils (approximately 73% clay, 24% silt, and 2% sand) (Table 2, Figure 4), as previously noted by [41]. Moderate to low swell potential and a low plasticity index, a favorable range for foundation stability, were also observed after Atterberg limit tests (Table 3). Liquidity index values near zero suggest a low potential for flow under wet conditions, a condition that poses a risk of developing severe flow under such wetting conditions [42]. According to [33,42], most soils are classified as inactive (activity < 0.75) and carry a low expansion risk (Table 3, Figure 7). Linear shrinkage limits classify the soil from poor (TP5, TP7, TP10) to medium quality (others) (Table 4), with correspondent linear shrinkage limits ranging from 5% to 11.43%, classifying low to medium swelling potential again [42]. The free swell values vary from a minimum of 5% (TP-13) to a maximum of 23% (TP-9). However, free swell values are well below the damaging level, which is <30%, thus confirming low swelling risk (Table 5). The results of UCS tests on 10 samples are presented in Table 6, which indicate that the soils are classified as very stiff, with strengths ranging from 215.8 to 333.5 kPa, thereby corroborating [37].

According to the USCS classification, soils are classified as lean clays, fat clays, clayey silts, and their associated sands. The BSCS classification identifies the soils as intermediate to high plasticity clays and silts. Rock mass strength and weathering intensities were assessed using the Schmidt and geological hammer tests, as per the ISRM standards [27], at 18 points. According to, rocks can be broadly classified under four categories: highly to completely weathered, very weak ignimbrite Schmidt, – MPa; moderately weathered, weak ignimbrite Schmidt, MPa; slightly weathered, medium strong ignimbrite—Schmidt,—MPa; fresh, strong results by ignimbrite—Schmidt,—MPa. Weaker rocks were found at the bus station, Damota Mountain, and a river cut near Woin kebele, compounded by a structural weakness that was in part due to the clay-filled joints. The moderately weathered rocks, although less modified structurally, require engineering skills, while strong and fresh ignimbrite rocks can support heavy construction. The RQD results indicate that rock mass variables are poor to excellent. The RMR values were placing the rocks in Class I (very good) to Class III (fair).

Rill erosion has caused damage to slopes, roads, and drainage networks. In steep terrain, it immediately threatens structural stability, making it a crucial consideration in planning. Land degradation by gully erosion adjacent to the Wolaita Sodo–Arba Minch road, with a potential threat to the road (Figure 18). Weathering of rock increases the susceptibility of slopes and road cuts to collapse, with significant structural implications. The quarrying-related rockslide northeast of town (Figure 20) highlights a geohazard in terms of excavation. There are two examples of joint-controlled rock toppling: north (Figure 21) and southeast of town, with the former produced by widely spaced joints above the city and the latter exacerbated by excavation on welded ignimbrite [43]. Although groundwater movement itself will not be directly visible, apart from some seepage into a gully, changes in groundwater conditions may affect the stability of soil-rock structures, either by flooding or through erosional or dissolutive processes [44,45]. Groundwater contours (Figure 22) provide information about flow direction and help aid mitigation efforts. Engineering geological map of Wolaita Sodo Town, combining lithology, structure, geodynamic processes, and groundwater databases for construction assistance planning. It has a scale of 1:50,000 and provides sufficient detail to evaluate hazards and facilitate site selection [46,47]. That map is essential to the success, health, and environmental stewardship of any engineering work that takes place in the region. These findings emphasize the critical role of soil properties, moisture content, and weathered rock conditions in urban infrastructure performance [48,49].

The geotechnical conditions observed in Wolaita Sodo Town show a strong correlation with reported infrastructure problems. In areas such as Gofe and parts of Dilla Sefer, soils exhibit high moisture content and elevated plasticity indices, which are typical of expansive clays. These conditions contribute to seasonal swelling and shrinkage, resulting in road heaving, surface cracking, and foundation displacement, particularly during and after the rainy season. Similarly, in poorly drained zones with thick, low-strength soils, minor settlements and wall cracks in low-rise buildings have been frequently observed. These findings highlight the critical role of soil properties in urban performance and support the need for tailored foundation design and drainage planning in affected areas.

5. Conclusions and Recommendations

5.1. Conclusions

This study provides a comprehensive engineering geological characterization and classification of soils and rocks in Wolaita Sodo Town, Southern Ethiopia, intending to support safe and sustainable urban development. The investigation revealed that the soil profile consists predominantly of lean to fat clays with medium to high plasticity, low free swell potential, and unconfined compressive strength values ranging from 215.8 to 333.5 kPa, indicating stiff to very stiff consistency. Atterberg limits, specific gravity, shrinkage characteristics, and moisture content were analyzed in detail, and statistical parameters such as mean and standard deviation were used to quantify variability across the site. Rock mass conditions were equally diverse, with RQD values ranging from 47.48% to 98.25% and RMR scores from 44 to 90, categorizing rock quality as fair to very good. These variations were spatially mapped to highlight zones of concern for engineering applications. The data also revealed critical zones prone to geotechnical problems such as slope instability, erosion, and foundation settlement—especially in areas underlain by weathered ignimbrites and clay-rich soils. The findings serve as a foundational database for infrastructure planning, design, and construction in the region. The integration of classification systems (USCS, BSCS, RMR) with field mapping and statistical analysis ensures a robust basis for geotechnical decision-making. The accompanying engineering geological map provides a practical tool for land-use management, zoning, and hazard mitigation. It is recommended that future construction projects in the area incorporate these findings during the site selection and design phases to ensure optimal outcomes.

5.2. Recommendations

Areas mapped with shallow bedrock and good rock mass quality (e.g., fresh ignimbrite with high RMR and RQD values) should be prioritized for dense infrastructure development such as public buildings, road networks, and multi-story housing. In zones with thick clay layers of high plasticity or silt of intermediate plasticity, engineers should avoid shallow strip foundations. Alternatives such as deep foundations, soil replacement, or stabilization with lime/cement treatment should be considered to reduce differential settlement. Zones affected by gully and rill erosion, as identified in the surface geodynamic process map, require slope stabilization (e.g., retaining walls, vegetation cover) and improved drainage systems. Urban development should be limited in these areas without remedial measures. The area is part of the central Ethiopian Rift System and lies in Ethiopia’s fourth seismically active zone. It affects multi-story buildings and any other engineering structures. Hence, a detailed seismic investigation is required. Likewise, engineers and concerned bodies should consider the seismicity effect before designing and constructing any engineering structures.