4.1. CPTU Parameters Before and After Pile Installation
The CPTU results before and after pile installation are shown in
Figure 8, where
is cone tip resistance,
is pore water pressure, and
is corrected cone tip resistance. The three parameters have the following relationship:
where
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:
The test results of
are shown in
Figure 9.
is applied to fit the test results. For CMP and PHC pile, respectively,
,
and
,
.
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, 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
and
: 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: “
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
and disturbance-induced settlement
were conducted using the stress modified method (SMM) and finite element method (FEM) proposed in
Section 3.
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.
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,
is approximately 6 to 7 mm for CMP composite foundations. With the gradual increase in fill height, the settlement
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
. For PHC pile, it is
. When the fill height is 2.5 m, the proportion for CMP is
, and for PHC pile it is
.
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.