Study on the Influence of Pile Installation Disturbance on the Settlement of Composite Foundation

Featured Application

This study offers insights for civil engineers and geotechnical practitioners involved in the design and construction of composite foundations, particularly in soft soil environments. The methods and findings can be applied to optimize the selection of pile installation techniques—specifically, choosing between cement mixing piles (CMP) and pre-stressed high-strength concrete pipe (PHC) piles—to effectively manage soil disturbance and predict settlement. The research can inform decision-making processes in highway and infrastructure projects, leading to enhanced stability and performance of foundations subjected to varying embankment loads.

Abstract

The disturbance generated during the pile installation process increased the compressibility of the soil between the piles, thereby causing additional settlement of the composite foundation. This study was conducted against the background of a highway soft foundation treatment project in Zhongshan. Before and after the installation of cement mixing piles (CMP) and pre-stressed high-strength concrete pipe pile (PHC), a piezocone penetration test (CPTU) was applied to assess the soil between the piles and quantify the disturbance. Based on this, the settlement of the composite foundation was predicted using the stress modified method and finite element method (FEM). The results indicate that under low embankment load (<2.5 m), the installation of PHC piles reduces the compression modulus of the soil between the piles by approximately 30%, while CMP results in a reduction of about 10%. Disturbance settlement from CMP and PHC piles accounts for approximately 3–10% and 17–28% of the total settlement, respectively. This impact diminishes with the increase of embankment load.

1. Introduction

Composite foundation is a practical and effective method for treating soft foundations. However, the pile installation process can cause structural damage to the soil surrounding the pile, resulting in a significant increase in pore pressures, a degradation in the strength and modulus of the soil [1,2,3]. Especially in low subgrade, the settlement caused by the load is even less than the settlement caused by the reconsolidation of the soil around the piles. For instance, it was reported that, under a vertical load of 500 kN, the settlement caused by the reconsolidation of the soft clay around jacked pile accounts for 5/7 of the total settlement [4]. Moreover, the method of pile installation has a significant impact on the settlement [5,6,7,8]. Sales et al. reported that in sandy soil foundations, the settlement response of non-displacement piles under the same load is only 1/10 to 1/2 that of driven piles [9]. Therefore, determining the impact of the pile installation process on settlement is of great significance for subgrade quality control and cost savings.

Settlement changes caused by pile installation disturbances are commonly discussed in research on the variations in stress and displacement of the pile and the surrounding soil. Common research methods include analytical solutions, numerical simulations, model tests, and full-scale tests.

The analytical solution is to solve the settlement by establishing a coordinated equation of pile-soil interaction. Pile installation causes high pore water pressure around the pile, which, when dissipated, leads to further consolidation of the soil around pile. Under the action of negative skin friction, this results in additional settlement of the pile. Based on the analytical solution method, scholars have studied the characteristics of pile-soil interaction for various types of piles, including jacked piles [4], circular piles [10], and XCC piles [11].

In terms of model testing, strain gauges are attached to the model pile, or stress meters are embedded in the soil layers to record changes in the stress field. Digital image correlation (DIC) was employed to document variations in the displacement field [12]. To enhance visibility of these changes, transparent soil was utilized as an alternative to conventional soil [13].

Through selecting appropriate models and parameters, numerical simulation methods can efficiently implement comparisons of displacement fields and stress fields for different types of piles. For instance, Lin et al. simulated the installation of three pile types, finding that the peak soil horizontal deformation occurs at the surface with normalized values of 15.2 for static pressure piles, 7.5 for bag grouting piles, and 1.1 for bored piles [14]. Additionally, the numerical simulation method is also capable of accounting for the effects of other factors, such as soil layers [15].

In-situ test results excel in authenticity and reliability, and test designs based on standard testing methods are commonly recognized. For example, Zhan et al. discovered that the increase in pore-water pressure caused by high-frequency vibratory pile driving extends to a range of approximately 3.6 times the pile diameter [16]. Research by Shao et al. indicates that both the soil pressure and the maximum lateral displacement caused by PGP pile construction are approximately half of those caused by driven piles [17]. Duffy et al. measured the base residual stress and found that the installation process of driven precast piles causes greater disturbance compared to driven cast-in-situ piles and screw injection piles [18].

In summary, settlement caused by pile installation should not be overlooked, and the piling method is one of the key factors influencing settlement. This article conducts in-situ tests on cement mixing pile (CMP) and hammer-driven pre-stressed high-strength concrete pipe pile (PHC pile), compares the disturbance degree ($SD$) of soil around the piles, and calculates the settlement caused by pile installation using the stress modified method. It also explains the reasons for the abnormal settlement observed in the case of soft foundation treatment under a low-fill embankment.

2. Materials

2.1. Test Site

The Zhongshan Highway Project is located in the river network area of the Pearl River Delta plain, China (see Figure 1). The stretch from K1 + 111.5 to K3 + 645.3 comprises an embankment section. This section is covered by an average 3 m layer of plain fill soil. The soft soil beneath is mainly composed of mucky soil, and its thickness varies from 17.6 m to 52.30 m.

The embankment fill height ranges from 0.5 m to 2.3 m, characterized as a low fill embankment, with a road width of 33.5 m and a slope ratio of 1:1.5 for the embankment slope. Besides, the average depth of the groundwater table is −2.0 m.

The original site investigation from K1 + 100 to K2 + 417 indicated that the stratigraphic distribution in this section consists of plain soil (average thickness of 2 m), muddy soil (average thickness of 18 m), and moderately weathered rock layer. Considering the relatively low fill height in this section, CMP was adopted for road foundation treatment. The design length of CMP was 8 m, with a spacing of 1.2 m between piles.

When the CMP construction reached K2 + 350, subsequent supplementary investigations revealed that the actual thickness of the muddy soil from K1 + 800 to K3 + 635 exceeded 35 m. Therefore, the untreated section K2 + 350~K3 + 635 was changed to be improved with 20-m-long PHC pile. The spacing between piles is 1.8 m, arranged in a square pattern.

The physical and mechanical parameters of mucky soil, the main soft soil layer, are shown in

Table 1. Physical and mechanical properties of mucky soil from K1 + 100 to K3 + 635.

Chainage	${\mathit{\rho }}_0$	$\mathit{\omega }$	${\mathit{e}}_0$	${\mathit{S}}_{\mathit{r}}$	${\mathit{\omega }}_{\mathit{L}}$	${\mathit{\omega }}_{\mathit{P}}$	${\mathit{I}}_{\mathit{P}}$	${\mathit{I}}_{\mathit{L}}$	${\mathit{a}}_{\mathit{v}1-2}$	${\mathit{E}}_{\mathit{s}1-2}$	$\mathit{c}$	$\mathit{\phi }$
	g/cm3	%	/	%	%	%	/	/	MPa−1	MPa	kPa	°
K1 + 100	1.73	55.1	1.45	99.8	47.8	29.5	18.4	1.52	1.26	3.19	9.2	7.5
K1 + 490	1.75	52.2	1.38	99.5	42.4	26.5	15.9	1.64	1.08	3.39	7.4	9.4
K1 + 950	1.70	49.4	1.32	98.5	42.4	26.5	15.9	1.44	0.95	2.47	10.8	6.2
K2 + 600	1.72	55.7	1.48	99.8	43.0	26.8	16.2	1.86	1.27	2.94	8.2	4.9
K2 + 945	1.68	54.5	1.45	99.3	43.3	27.0	16.4	1.72	1.24	2.57	9.1	5.8
K3 + 445	1.77	50.7	1.34	99.7	46.1	28.8	17.3	1.35	1.09	3.41	8.9	5.3

. Besides, the technical parameters of CMP and PHC pile are shown in

Table 2. Technical parameters of CMP and PHC pile.

		Parameters	Value
CMP	Machine	Drilling speed	0.5~0.8 m/min
		Rotation speed	40~50 r/min
		Drill diameter	500 mm
	Pile	Pile length	8 m
		Pile spacing	1.2 m
		Installation process	four stirring and four spraying
		Curing agent	PO42.5 Portland cement
		Water cement ratio	0.6
		Cement dosage per meter	65 kg
PHC pile	Machine	Pile hammer	Tubular diesel pile hammer
		Hammer Weight	4.0 × 103 kg
		Hammer drop distance	1.0~1.5 m
	Pile	Pile length	20 m
		Pile spacing	1.8 m
		Outside diameter	400 mm
		Wall thickness	70 mm
		Water cement ratio	0.31
		Aggregate content	Cement = 436 kg/m3
			Fly ash = 35 kg/m3
			slag powder = 75 kg/m3
			fine aggregate = 705 kg/m3
			coarse aggregate = 1032 kg/m3

.

2.2. In-Situ CPTU

Compression modulus is an essential parameter for settlement calculation and prediction. In order to eliminate disturbances beyond pile installation, in-situ testing is a suitable method for obtaining compression modulus.

The commonly used in-situ methods for testing and estimating soil mechanical properties include standard penetration test (SPT), pressuremeter test, flat dilatometer test (DMT), vane shear test (VBT), and piezocone penetration test (CPTU).

The SPT, DMT, and VBT methods are not suitable for the compression modulus determination. The SPT assesses foundation bearing capacity, the DMT measures lateral pressure and consolidation coefficients, and the VBT determines soil sensitivity. Although the pressuremeter test can directly obtain deformation parameters such as the pressuremeter modulus, the method for deducing the compression modulus is still not mature. While the CPTU cannot directly obtain deformation parameters, numerous studies have shown the correlation between cone resistance and compression modulus [19,20]. Due to its low disturbance and ease of implementation, CPTU is often combined with laboratory tests for compression modulus determination.

Therefore, this paper used CPTU to estimate the compression modulus change of soil between piles before and after pile installation. See

Table 3. Technical parameters of CPTU equipment.

${\mathit{D}}_{\mathit{m}}$	${\mathit{F}}_{\mathit{r}\mathit{p}}$	${\mathit{F}}_{\mathit{r}\mathit{u}}$	${\mathit{V}}_{\mathit{p}}$	${\mathit{A}}_{\mathit{c}\mathit{p}\mathit{p}}$
20~30 m	30 kN	I: 15 kN II: 25 kN	2 cm/s	10 cm2

where $D_m$ = measurement depth, $F_{rp}$ = rated penetration force, $F_{ru}$ = rated uplift force, $V_p$ = penetration rate, $A_{cpp}$ = cross-section area of cone penetration probe.

for the equipment parameters used in CPTU.

In the section before K2 + 350, CMP was used, and after that, PHC pile was adopted. Four tests were conducted at K2 + 335 and K2 + 365, and the specific location was in the middle of the roadbed cross-section. The first test (A1 & B1) was conducted to determine the parameters of undisturbed soil layer. Then, after the pile installation, a second test (A2 & B2) was carried out. As shown in Figure 2, the distance between first and second test points are less than 5 m. It is aimed at minimizing the impact of actual geological differences. The experimental results will be discussed in the conclusion section.

3. Methodology

This section introduced the methodology for determining the compression modulus based on in-situ CPTU test results and calculating the composite foundation settlement considering pile installation disturbance.

3.1. Determination of Compression Modulus

Quantifying the disturbance in the soil layer solely through changes in CPTU test indicators lacks clear physical significance. Numerous studies have shown that in cohesive soil layers, there is a high linear correlation between the compressibility modulus of the stratum soil and $q_t$. Therefore, it is necessary to first clarify the relationship.

Sampling was conducted with fixed piston thin-walled sampler in the deep silty soil layer, with a total of three sampling points set up. After vacuum saturation in water for 24 h, the prepared soil samples were installed in a consolidation apparatus. Different levels of axial loads are applied to the sample, specifically 50, 100, 200, 300, 400, and 500 kPa, with each load applied for 24 h before proceeding to the next level. Finally, we got the $e_p-p$ curve shown in Figure 3.

The fitted result of the $e_p-p$ curve is:

$$e_p=1.412·\exp (-0.406p^{0.334}).$$

(1)

According to Equation (1), Equation (2) is obtained for the tangent modulus of soil layer along depth.

$${E_s}^{\prime }=-{\displaystyle \frac{(1+e_p)dp}{de}}={\displaystyle \frac{-(1+e_p)}{e_p^{’}}}={\displaystyle \frac{\exp (0.406p^{0.334})+1.412}{0.191·p^{-0.666}}}$$

(2)

Based on the CPTU test results and Equation (2), the scatter plot of ${E_s}^{\prime }$ and $q_t$, as well as the results of the linear fit were shown in Figure 4.

Thus, the change in ${E_s}^{\prime }$ can be quantified based on the CPTU test results.

3.2. Composite Foundation Settlement Calculation

Composite modulus method and stress modified method are commonly used in settlement calculation. When considering the former, ${E_{sp}}^{\prime }$ can be represented as:

$${E_{sp}}^{\prime }=m{E}_P+(1-m)E_s$$

(3)

where $m$ = pile replacement ratio; $E_P$ = deformation modulus of pile.

$E_P$ of CMP and PHC pile is typically tens of times and hundreds of times, respectively, higher than $E_s$ of the surrounding soil between the piles. According to Equation (3), when calculating ${E_{sp}}^{\prime }$, the contribution of $E_s$ is minimal. Therefore, the composite modulus method cannot effectively reflect the impact of pile driving on settlement.

When considering the stress modified method, it determines the load shared by the soil between the piles based on the pile-soil stress ratio. The presence of the reinforced body is ignored, and only the compression of the soil between the piles is calculated.

The load shared by soil between piles, $p_s$, is:

$$p_s={\displaystyle \frac{1}{1+m(n-1)}}p={\mu }_{s}p$$

(4)

where $p$ = embankment load; $n$ = pile-soil stress ratio, which can be obtained through static load test; ${\mu }_s$ = disturbance coefficient.

According to layer-wise summation method, the settlement of the reinforced area, $S_e$, is:

$$S_e={\mu }_s{\displaystyle \sum _{i=1}^n\frac{\Delta p_i}{E_{si}^{\prime }}}{H}_i$$

(5)

where $\Delta p_i$ = additional stress increment of the $i$-th reinforced layer; $H_i$, ${E_{si}}^{\prime }$ = thickness and compression modulus of the $i$-th soil layer between piles after pile driving.

3.3. Pile-Soil Stress Ratio

To determine the pile-soil stress ratio, an in-situ static load test was conducted after pile installation, as shown in Figure 5. The load was applied in 8 stages, each with 35 kPa, resulting in a final load of 280 kPa, over twice the design load of 120 kPa. The $p_{sl}-n$ curve of CMP and PHC pile is shown in Figure 6.

The test result showed that pile-soil stress ratio is positively correlated with the magnitude of the embankment load. The pile-soil stress ratio varies widely, ranging from 8 to 42 for PHC pile and 2 to 10 for CMP. Based on the working conditions in this case, within $H_1$ from 0.5 m to 2.5 m, the stress ratio of pile-soil for pile-composite foundations can be taken as 8 to 20, while for CMP, the stress ratio of pile-soil can be taken as 2 to 7.

According to Equation (4), the ${\mu }_s$ can be calculated for different fill heights. Then, based on Equation (5), the total settlement results considering disturbances can be calculated.

3.4. Finite Element Method

To compare with the results of the stress modified method, FEM is also conducted in settlement calculations using PLAXIS 2D 2024 software. An axisymmetric model was built, and 15-node triangular elements were adopted for higher computational accuracy. In order to reduce the influence of model boundaries on the main deformation area, the considered range extends to a depth of 60 m underground and a horizontal width of 85 m. The established model is shown in Figure 7.

The model restricts horizontal displacement of left and right boundaries, and limits both horizontal and vertical displacements of the bottom. The pore water pressure in the soil above the water table is set to zero. All soil material is modeled using the Mohr-Coulomb model, while the pile is simulated using the embedded pile element built into the model.

When reducing the compression modulus, layer-by-layer weakening was conducted every 2 m within the pile range.

The physical and mechanical properties of the embankment soil and foundation soil layers in sections K1 + 111.5 to K3 + 645.3 are listed in

Table 4. Physical and mechanical parameters of soil layers.

No.	Soil Layer	${\mathit{G}}_{\mathit{s}}$	$\mathit{e}$	$\mathit{c}$	$\mathit{\phi }$	${\mathit{E}}_{\mathit{s}}$	$\mathit{\lambda }$
		kN·m3	-	kPa	°	MPa	-
1	Embankment soil	18.6	0.78	10.5	21.4	8.1	0.30
2	Plain soil	17.6	0.80	7.0	15.0	4.1	0.30
3	Muddy soil	16.3	1.20	4.8	3.8	2.1	0.32
4	Moderately weathered rock	18.0	0.50	21.0	36.0	20.5	0.30

, while the calculation parameters for CMP and PHC pile are provided in

Table 5. Physical and mechanical parameters of CMP and PHC.

Composite Foundation	${\mathit{E}}_{\mathit{p}}$	${\mathit{G}}_{\mathit{s}}$	$\mathit{\lambda }$
	Mpa	kN·m3
CMP	150	21	0.3
PHC	1000	29	0.2

.

4. Results

4.1. CPTU Parameters Before and After Pile Installation

The CPTU results before and after pile installation are shown in Figure 8, where $q_c$ is cone tip resistance, $u_2$ is pore water pressure, and $q_t$ is corrected cone tip resistance. The three parameters have the following relationship:

$$q_t=q_c+u_2(1-a)$$

(6)

where $a$ is the net area ratio, taken as 0.8, and is only related to the shape of the cone tip.

To quantify the degree of disturbance in the soil layer before and after pile installation, a disturbance index was established based on corrected cone tip resistance from first and second test, which is:

$$SD={\displaystyle \frac{q_t-{q_t}^{\prime }}{q_t}}\times 100\%$$

(7)

The test results of $SD$ are shown in Figure 9. $SD(h)=a{h}^{-b}$ is applied to fit the test results. For CMP and PHC pile, respectively, $a_1=0.497$, $b_1=-0.757$ and $a_2=0.645$, $b_2=-0.374$.

From the test data, it can be observed that in shallow strata (0 to 3 m), the disturbance generated during the piling process of both types of piles is relatively large, exceeding 50%. However, as the treatment depth increases, the disturbance within the CMP length remains at around 10% to 15%, and there is essentially no disturbance below a depth of 2 m from the pile bottom. The reasons for this pattern in the impact behavior of the surrounding soil during the piling process can be attributed to three factors:

Firstly, in addition to being influenced by cutting in shallow soil layers, the lifting force of the mixing shaft also affects them. Due to the small self-weight stress of shallow soil layers, the lifting force causes significant disturbance, resulting in a disturbance exceeding 60%.

Secondly, as the drilling depth increases, the surrounding soil is increasingly affected by the cutting action of the mixing shaft, but this impact on the disturbance of the surrounding soil is approximately 5% to 10%.

Thirdly, the disturbance experienced by the soil below the pile bottom is mainly caused by the requirement for the drilling shaft to hover for a period and inject slurry after reaching the design depth. According to the test results, the depth of this disturbance is approximately 2 m, with an average disturbance of 5%.

Compared to CMP, the hammer-driven piling process of PHC pile results in greater disturbance to the surrounding soil, with an average disturbance exceeding 30%. This is due to several reasons:

Firstly, during on-site hammer-driven piling, noticeable undulations in the surface soil within a range of 1 m to 2 m around the pile are observed, indicating significant disturbance of the soil. The instantaneous mechanical energy diffusion caused by hammer impacts disrupts the skeleton structure between soil particles, resulting in a disturbance exceeding 60% in shallow soil layers.

Secondly, disturbance in deeper soil layers mainly stems from soil squeezing effects. The disturbance within the pile body gradually attenuates with depth, consistent with the pattern of lateral displacement along the depth. By the time it reaches the pile bottom, the disturbance around the pile is approximately 10% to 20%.

Thirdly, due to limitations in testing depth imposed by on-site equipment, test results below the pile bottom are unavailable. However, based on the trend of disturbance degree curves, the disturbance within the pile bottom area is generally less than 10%.

4.2. Settlement Before and After Pile Installation

The field-measured settlement data for this section are shown in

Table 6. Measured settlement.

Section	Number	${\mathit{H}}_{\mathbf{1}}$	${\mathit{H}}_{\mathbf{2}}$	${\mathit{S}}_{\mathit{m}}$
		m	m	mm
①	●K1 + 150	0.8	18.0	84
	●K1 + 400	0.5	17.0	74
	●K1 + 600	1.6	14.0	124
②	●K1 + 900	1.3	35.0	203
	●K2 + 100	0.5	42.0	214
	●K2 + 350	1.0	44.0	248
③	○K2 + 600	2.1	45.0	165
	○K2 + 850	1.3	46.0	172
	○K3 + 050	2.5	42.0	181
	○K3 + 300	1.5	43.0	176
	○K3 + 635	2.1	47.0	181

, where “●” and “○” respectively refer to CMP and PHC pile. And the layout of the observation cross-sections is illustrated in Figure 10.

For ease of analysis, the cross-sections in Table 1 are classified into three categories based on $H_1$ and $H_2$: ① shallow soft soil layer treated with short piles, ② shallow treatment of deep soft soil layer treated with short piles, and ③ deep soft soil layer treated with long piles. The measured average settlements for the three sections are 74 mm, 221 mm, and 175 mm, respectively.

In terms of comprehensive cost investment and settlement control effectiveness, the comprehensive cost per meter for PHC is approximately three times that of CMP, and the treatment depth needs to be 12 m deeper. Considering the foundation replacement rate, the estimated treatment cost of PHC composite foundation is about 2.25 times that of CMP. However, the settlement control is not proportional to the cost investment. Comparing the shallow treatment of the deep soft foundation section with that of the deep treatment section, switching to PHC pile composite foundation treatment and increasing the treatment depth by 12 m only resulted in a 20.8% decrease in settlement.

4.3. Comparison Between Measurement Results and Calculated Results

To facilitate the analysis of the proportion of settlement caused by construction disturbances relative to the total settlement, calculation scenarios were established based on the site cross-section conditions, as shown in

Table 7. Settlement calculated by stress modified method and finite element method.

Case	${\mathit{S}}_{\mathit{e}}$ (mm)		${\mathit{S}}_{\mathit{f}}$ (mm)		$\mathit{k}={\mathit{S}}_{\mathit{f}}/{\mathit{S}}_{\mathit{e}}$ (%)
	SMM	FEM	SMM	FEM	SMM	FEM
$C_1$	56	111	6	7	10.2	6.3
$C_2$	144	260	8	13	5.8	5
$C_3$	223	386	10	19	4.3	4.9
$C_4$	298	489	10	23	3.5	4.7
$C_5$	356	568	12	26	3.3	4.6
$P_1$	29	56	7	16	23.8	28.6
$P_2$	99	146	19	37	19.1	25.3
$P_3$	147	194	27	47	18.2	24.2
$P_4$	178	234	32	55	18.0	23.5
$P_5$	207	265	36	60	17.3	22.6

: “C” represents CMP, “P” represents PHC pile, and “1” to “5” represent the fill heights ranging from 0.5 m to 2.5 m. Total settlement $S_e$ and disturbance-induced settlement $S_f$ were conducted using the stress modified method (SMM) and finite element method (FEM) proposed in Section 3.

• Total settlement after pile installation

Considering the influence of construction disturbances, the total settlement ranges for CMP and PHC pile foundation within fill heights of 0.5~2.5 m are 56~356 mm and 29~207 mm, respectively, using the stress modified method. However, the FEM results are 111~568 mm and 56~265 mm.

As is shown in Figure 11, the FEM results are 30% to 60% larger compared to those obtained using the stress modified method. The results from the stress modified method are closer to the measured settlement, with a difference of 10% to 15%, which falls within an acceptable range.

• Disturbance settlement

As illustrated in Figure 12, when the fill height increases, the settlement induced by the pile driving disturbances gradually increases, but the rate of increase slows down. When the embankment fill height is 0.5 m, the settlement, $S_f$ is approximately 6 to 7 mm for CMP composite foundations. With the gradual increase in fill height, the settlement $S_f$ at 2.5 m fill height is approximately 12 to 36 mm. When using PHC pile treatment, with an embankment fill height of 0.5 m, the settlement is approximately 7 to 16 mm, and with an embankment fill height of 2.5 m, the settlement is approximately 16 to 60 mm.

As illustrated in Figure 13, when the fill height increases, the proportion of disturbance settlement to total settlement gradually decreases. When the fill height is 0.5 m, the proportion of disturbance settlement to total settlement for CMP is $k=6\%\sim 10\%$. For PHC pile, it is $k=23\%\sim 28\%$. When the fill height is 2.5 m, the proportion for CMP is $k=3\%\sim 4\%$, and for PHC pile it is $k=17\%\sim 22\%$.

The reasons for the above result are as follows:

Firstly, with the increase in upper load, the influence depth of additional stress increases, leading to greater settlement in the lower subsoil layers and a smaller proportion of disturbance settlement.

Secondly, the magnitude of the upper load affects the pile-soil stress ratio. Under low loads, the soil between piles bears more load. Taking into account both the increase in load and the decrease in the ratio of load borne by the soil between piles, the overall disturbance settlement exhibits a trend of decelerated growth.

5. Discussion

In this manuscript, CPTU test was mainly applied to quantify the influence of CMP and PHC pile installation disturbance. The stress modified method was employed for analyzing settlement of soft soil foundation with low embankment (<2.5 m).

The primary conclusion of this study is that pile installation disturbance weakens the compression modulus of the soil between the piles, and the weakening rapidly decreases with depth. The additional settlement generated accounts for an average of 6% (CMP) and 23% (PHC pile) of the total settlement, respectively.

To mitigate the negative impacts of pile disturbance on settlement, several countermeasures will be discussed. Firstly, if the schedule allows, simple surcharge preloading method can be used to improve the consolidation of the foundation before pile installation.

Secondly, in situations where there are cost and time constraints, improvements in installation techniques become quite important. For CMP, in shallow soil layer, reinforcement can be appropriately enhanced by injecting more slurry to strengthen the soil between piles. For PHC pile, it is advisable to reduce the hammering energy in shallow layers, gradually restoring it to the design value as the depth increases, thus minimizing disturbance to the shallow soil layers.

Besides, the application of the settlement analyze method is almost not limited by soil type, because the disturbance can cause changes in soil compression characteristics. However, it should be noted that when installing displacement pile in sandy soil foundation, the soil compaction behavior is usually obvious. The settlement completed by vibration reduces the settlement experienced during the embankment filling process and the post-construction period, which is beneficial. A case [21] noted that at the moment of pile installation completion, the settlement of the sandy soil foundation was 42 mm, about ten times that of soft soil foundation. Thus, regarding the impact of pile installation disturbance on total settlement, the two conditions of sandy soil foundations and soft soil foundations may yield opposing conclusions.

6. Conclusions

Set against the backdrop of a deep and soft soil low-fill embankment project in the Zhongshan area, this study investigated the influence of CMP and PHC piles on settlement during the pile driving process. The main conclusions are as follows:

1.

Under low embankment load (0.5 to 2.5 m), the settlement of embankment treated with PHC pile (20 m) is only 20% less than those reinforced with CMP (8 m). This occurrence is attributed to the greater additional settlement induced by the former’s pile driving process compared to the latter.

2.

The disturbance caused by pile installation decreases with depth. For embankment fill heights ranging from 0.5 to 2.5 m, the average disturbance caused by the pile installation process of CMP and PHC pile are approximately 10% and 30%, respectively.

3.

Compared to finite element analysis, disturbance settlement calculated through stress modified method aligns more closely with field-measured settlement. As the fill height increases, the proportion of disturbance settlement to total settlement gradually decreases. Hence, considering the impact of construction disturbance is more meaningful for low-fill embankment projects.