Keywords

1 Introduction

SCP has been applied earlier and more widely in treating soft soil foundations along the coast of Japan. Since 1996, China has begun to explore the application technology of SCP in port engineering. The HKZM Bridge adopts the bridge-island-tunnel combination scheme, in which the foundations of the island-tunnel are mostly covered with deep soft clay, and the bearing capacity is very low. SCP foundation is adopted in the island wall and transition section, with relatively high replacement rate. Because there is very little research on the high-replacement rate SCP foundation in China, the in-site load plate test is used to study the deformation and bearing capacity of the high-replacement SCP.

Through field tests, Yea and Kim [1] pointed out that the replacement rate of SCP was the main factor determining the undrained shear strength and consolidation characteristics of Marine sedimentary clay in Busan New port. Zhang and Wang [2] and Wang [3] pointed out that consolidation settlement of SCP with large diameter and high replacement rate mainly occurred in the early loading stage, while the average settlement and residual settlement of SCP were smaller than those areas of low replacement rate. Heo et al. [4] specially studied the combined influence of replacement rate and penetration ratio of SCP on the foundation reinforcement effect through centrifuge test. According to previous studies, most of the researches on SCP mainly focus on the low replacement rate (30–50%), and few researches focus on the bearing capacity of high replacement rate (50–80%).

Most of the existing methods for calculating the bearing capacity of SCP are based on the failure form of single pile swelling, and there is a large error when applied to the high displacement rate. At present, there are few researches on failure mode of high-displacement.

In this paper, the settlement and deformation of high displacement rate SCP are analyzed by in-situ test data and post-construction settlement monitoring data, and the deep-water and in-situ load test methods and modified settlement calculation formula are obtained, which can be used to guide the design and construction of SCP foundation.

2 Project Overview

In the main structure island tunnel project of the HKZM Bridge, SCPs are used as the foundation treatment technology on a large-scale [5, 6], and different compacted sand pile replacement rates are used in different areas. Among them, the SCP is adopted at the rescue wharf side which diameter is 1.6 m, arrangement is square layout, spacing is 1.8 m, and with 62% replacement rate. Based on the previous engineering experience, it is necessary to carry out large-scale underwater load test, to study the consolidation settlement, deformation modulus, stress sharing ratio and soil index change of SCP foundation comprehensively.

3 Experiment

3.1 Experiment Design

The anchor pile method is used to carry out the load test of the underwater SCP foundation. The bearing plate size is 5.4 m × 5.4 m. Anchor piles are made of steel piles with diameter of 1.2 m, wall thickness of 16 mm; there are four piles in total. The layout is a square with a spacing of 9.4 m. The benchmark piles are also steel piles and the layout is the square with a spacing of 14 m.

The underwater SCP foundation load test platform is mainly composed of a reaction force system, a loading system, a measurement system, and a limit device. The offshore test platform is always in a low-frequency vibration state under the influence of strong wind and ocean currents. Therefore, on the underwater load board set up a set of high-precision static leveling system for settlement observation, as shown in Fig. 1. After improvement, the system has good sealing performance and can be applied to deep underwater measurement [7, 8]. The measurement accuracy of this system is ± 0.4 mm, and the range is 400 mm. The loading mode adopted two-cycle loading. When the settlement increases sharply, or the cumulative settlement is greater than 6% of the width of the load plate; or the settlement is greater than twice the previous level of load, and has not stabilized after 24 h, the test will be terminated.

Fig. 1
A photograph of a leveling system includes the dial indicator system and the level gauge system on a screen at the bottom.

Test settlement measurement system

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3.2 Analysis of Test Results

The p-s curve of settlement under various loads is shown in Fig. 2. In the first loading cycle of the test, the settlement amount was 53.69 mm, the settlement after unloading was 48.18 mm, and the rebound value was 5.51 mm. In the second cycle, the settlement is 124.29 mm, and the settlement after unloading to 274.35 kPa and maintaining the load for 50 days is 148.51 mm. The settlement after unloading is 136.82 mm, and the rebound value is 11.69 mm. This test did not reach failure, so the ultimate bearing capacity should be greater than 340 kPa.

Fig. 2
A graph of data in millimeters versus data in kilopascals plots 3 concave down decreasing curves on top and 2 dotted line joins the above curves at the ends.

The curve of load and stress

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Pile-soil stress analysis

There is gravel cushion between the bearing plate and the SCP foundation. It is necessary to calculate the stress on the top surface of the sand pile. The stress on the top surface of the sand pile caused by the floating bulk density of the gravel cushion is 11 kPa. The stress diffusion angle of crushed stone is set as 38°. According to the calculation results, the relationship between the calculated average stress and load on the top surface of the SCP is shown in Fig. 3.

Fig. 3
A line graph of stress versus load plots 2 lines. Average stress at the bottom of bearing plate: (754, 25), (9900, 350). Average stress on top of sep: (754, 25), (9900, 210).

The relationship between the calculated average stress and load

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During the load maintenance period, the relationship between the load and the average stress on the top surface of the sand pile, the stress of the sand pile and the stress of the soil between the piles is shown in Fig. 4.

Fig. 4
A graph of stress versus load plots 3 lines. Stress of sep: (2000, 120), (9900, 410). Stress of soil between piles: (2000, 10), (9900, 50). Average stress on top of sep: (2000, 50), (9900, 210).

The relationship between the load and the average stress

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Figure 5 is the pile-soil stress ratio which is measured during the maintenance load. The replacement rate of the sand piles in the test area is measured to be 59.14%. It can be inferred that the load borne by the sand piles is 7700.75 kN, It is estimated that the stress on the top surface of sand piles could be 268 kPa, and the stress of soil between the piles could be 42 kPa. Therefore, the revised pile-soil stress ratio should be 6.3 actually.

Fig. 5
A graph of the value versus date plots a decreasing curve for the measured pile-soil stress ratio.

Pile-soil stress ratio during maintenance load

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It can be concluded that as time goes by, the stress of sand piles and the pile-soil stress ratio decrease. When the stress redistribution is basically completed and the stress redistribution phenomenon no longer occurs, the calculated pile-soil stress ratio reflects the real situation.

Deformation modulus

The deformation modulus E0 of foundation is an important parameter to evaluate the deformation characteristics. This research assumes that the deformation modulus remains unchanged before and after the foundation reinforcement. According to the definition of the deformation modulus [9, 10]:

$$ E_{sp} = I_{0} (1 - \mu^{2} )\frac{{p_{sp} \alpha }}{s} $$
(1)

where, Esp is the deformation modulus of composite foundation respectively (MPa); Ps, Psp is the load borne by the soil between the piles and the foundation (kPa); I0 is the shape coefficient of pressure bearing plate, and the square plate is taken as 0.886; μ is the Poisson’s ratio of soil, 0.30 for sand; s is the settlement corresponding to p (mm).

According to Eq. 1, the deformation modulus of compacted sand piles composite foundation in each stage is calculated as shown in Table 1 [11]. Therefore, the deformation modulus of the composite foundation under the load of 100–200 kPa is 11.07 MPa, and that of the natural foundation before reinforcement is 1.03 Mpa. The effect of foundation reinforcement is obvious.

Table 1 Deformation modulus values of reinforced composite ground
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Bearing capacity of SCP

At present, the calculation of composite foundation bearing capacity is usually obtained by combining the bearing capacity of the pile body and the soil between the piles according to a certain principle, and the calculation formula is:

$$ f_{spk} = mf_{pk} + (1 - m)f_{sk} $$
(2)

where fpk and fak are the ultimate bearing capacity of the pile and the soil between the piles, respectively.

The ultimate bearing capacity of discrete material piles mainly depends on the maximum confining force σru that the soil at the side of the pile can provide. The ultimate bearing capacity of discrete material pile can be calculated according to the passive earth pressure method [12]:

$$ f_{pk} = \sigma_{ru} K_{p} $$
(3)

where, σru is the lateral ultimate stress; Kp is the passive earth pressure coefficient of the pile material.

There are many methods to calculate the maximum lateral limit stress, such as BRauns formula, Wong formula, Hughes & withers formula, and passive soil pressure method [8, 11,12,13,14,15]. In this paper, the ultimate bearing capacity of the discrete material pile is calculated according to above-mentioned theories. The calculation results are compared with the standard penetration test results and load test results in Table 2.

Table 2 Calculated and measured ultimate bearing capacity of composite ground
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The ultimate bearing capacity of the foundation is greater than 340 kPa. The foundation bearing capacity obtained by the passive earth pressure method and the Hughes and Withers calculation formula is close to the test results.

4 Monitoring Data Analysis

With various replacement rates, the settlement of the relevant area of West island transition section is reduced from 314–401 mm to about 28–50 mm, and the settlement is reduced to about 8.9–23.2% than the original foundation.

In West island, three types of replacement rate of SCP are adopted. As shown in Table 4, it can be found that with the increase of replacement rate from 42 to 55%, the settlement of foundation decreases significantly. However, when the replacement rate increases from 55 to 70%, the average settlement of foundation does not decrease significantly.

In East island, the upper part and the lower part of SCP have different replacement rates. The settlement is reduced from 108.4–926.7 mm to 42.6–214.3 mm and it is reduced to 11.2–50.7%.

In fact, the settlement of SCP foundation consists of settlement of reinforcement zone and settlement of underlying layer. A site in Hong Kong, Zhuhai-Macao is selected for specific analysis, whose upper part is clay layer and the lower part is the medium sand layer, this is a typical double-sided drainage foundation. The consolidation characteristics of this layer of soil should be considered when the total settlement of clay layer under preloading is calculated. According to the geological survey report, clay layer is over consolidated soil and the over consolidation ratio average is 1.49. Therefore, the influence of soil stress history should be fully considered when calculating the total settlement. According to the calculation, the stratum stress at the top of the underlying is 226.2 kPa, which is greater than the consolidation pressure in the earlier stage.

Thus, the final settlement of the underlying layer at this point under preloading is 43.9 mm, and the consolidation degree within 4 months of full load during preloading is 47%. The settlement of the underlying layer at this point during preloading is 20.7 mm. As the measured settlement of this point during preloading period is 64.1 mm, so the settlement of the underlying layer accounts for about 32% of the overall settlement.

5 Conclusion

As it is difficult to accurately evaluate the bearing characteristic of underwater SCP foundation, large-scale in-situ load tests and deformation characteristic tests were carried out based on the Hong Kong-Zhuhai-Macao Bridge project. The stress sharing ratio of underwater SCP composite foundation with the same replacement rate is closely related to the load level and time, and the stress sharing ratio decreases with the increase of the load level. The bearing capacity of single pile of SCP is calculated according to the passive earth pressure method, which is close to the load test results. The calculation needs to consider the influence of the underlying clay layer on the overall settlement.

  1. (1)

    The underwater static leveling measurement system proposed in this test project can obtain more realistic and stable settlement data.

  2. (2)

    The pile-soil stress ratio is 6.3, the deformation modulus is about 8 kPa, and the rebound and compression modulus is 32.65 kPa.

  3. (3)

    The ultimate bearing capacity of SCP composite foundation is greater than 340 kPa.

  4. (4)

    The settlement of West island transition section relevant area is reduced from 314–401 mm to about 28–50 mm, and the settlement is reduced to about 8.9–23.2% than the original foundation. The settlement of East island was reduced from 108.4–926.7 mm to 42.6–214.3 mm, and it is reduced to 11.2–50.7%.