# The deformation of retaining piles and ground surface under various support systems during deep excavation

- Yiqun Tang
^{1, 2}Email author, - Wenqiang Zhao
^{2}and - Jie Zhou
^{1, 2}

**Received: **19 May 2017

**Accepted: **3 October 2017

**Published: **18 October 2017

## Abstract

### Background

Since the high-rise buildings and deep underground structures dramatically increases in urban area, the decent design and construction of deep excavations is essential before these structures are actually built up. The building area is very susceptible to the changes of their geo-environment, and even minor insecurity of a deep excavation may lead to a catastrophic failure of structures and deformation of the ground. Therefore, the adverse influence of deep excavation on the surrounding environment should be monitored and controlled stringently.

### Results

Based on the real case of deep excavation in Xuzhou, China, a FDM numerical model, which was accomplished by FLAC-3D, was developed to evaluate the deformation of retaining piles and ground surface under various excavation support systems. The original pile-anchor support system was simulated as well as the proposed quincunx double-row piles support system. For the original support system, the deformations were discussed by comparing the numerical results with monitoring data. For the proposed support system, orthogonal tests were designed to evaluate the influence of multiple factors on the effectiveness of excavation support. The optimum solution for the proposed support system was obtained through orthogonal tests.

### Conclusion

Results show that the pile space is the primary factor for the excavation-induced deformations, which can be reflected by the lateral displacement of retaining piles and the settlement around the excavation, while the row space has insignificant influence on the deformations. The conclusions of this paper can contribute to the design of similar projects and avoid excessive deformation of the ground during excavation in the future.

## Keywords

## Background

Since the demand for space dramatically increases in modern cities, numerous high-rise buildings and deep underground structures (like parking structure, underground mall, metro system, and basement) appears in urban area. Before these structures are actually built up, the decent design and construction of deep excavations is essential. Unfortunately, the building area, which requires deep excavations, is commonly the area full of buildings, roads, and municipal pipeline. They are very susceptible to the changes of their geo-environment. Therefore, the adverse influence of deep excavation on the surrounding environment should be monitored and controlled stringently.

Though the deep excavation is a temporary project, its importance should not be underestimated. Even minor insecurity of a deep excavation may lead to a catastrophic failure of structures and deformation of the ground. The study of the adverse effect induced by deep excavation is urgently required. Peck (1969) proposed a dimensionless curve method to estimate the ground settlement around the excavation. This method is widely accepted and applied in practices. Ou et al. (1993) selected ten excavation cases in Taipei with good-quality construction and field observation data to study the characteristics of ground surface settlement during excavation, and proposed an empirical formula to predict the ground surface settlement profile at the center section of an excavation. A clear understanding of the pile behavior during nearby excavations can be obtained from case studies with proper instrumentation. Finno et al. (1991) demonstrated that lateral soil movements from excavation activities could be detrimental to nearby existing piles by field observations, including lateral deformations of the sheet pile and lateral and vertical deformations of the main columns. Goh et al. (2003) carried out an actual full-scale instrumented study to examine the behavior of an existing pile due to nearby excavation activities resulting from the construction of a cut-and-cover tunnel. However, no comparisons could be made between the pile and wall behavior because of an inclinometer installed in the diaphragm wall was damaged.

Meanwhile, the modern computer technology is capable of processing staggering amount of data and simulating practical geotechnical engineering problems. Professional FEM and FDM simulation software products can consider the pile-soil interaction, group effect, complex boundary conditions and soil non-homogeneity (Zhang et al. 2011). Poulos and Chen (1997) established design charts for estimating pile bending moments and deflections based on results from a two-stage analysis procedure. The finite-element method was used first to simulate the excavation procedure and to generate free-field soil movements, that is, the soil movements that would occur without the presence of the pile. These generated lateral soil movements were then used as input into a boundary-element program to analyze pile response. Gao et al. (2010) developed a 3D model in FLAC-3D, which simulated the whole construction process of a deep excavation adjacent to a metro tunnel in Shanghai. The simulated results showed the reinforced foundations and underground structures around the deep excavation could effectively decrease nearby displacements. Zhou et al. (2011) developed a three dimensional numerical model through finite difference method based on a real dewatering case during the construction of Xujiahui metro station in Shanghai. The model was applied to calculate the dewatering-induced settlement around the deep excavation. The results were proven effective according to the monitoring data afterward. In the papers mentioned above, the numerical models were all applied to evaluate one specific support system for each case. However, the comparison of various support systems for the same case can also be realized through the numerical models.

In this paper, numerical models were developed in FLAC-3D to simulate the construction of a deep excavation in Xuzhou. Models of different cases (different support systems) were applied to calculate the deformations of piles and soils around the deep excavation, and the results of these cases were compared. Accordingly, this paper provided some guidelines for the similar projects.

### Study area

^{2}and its circumference was around 634 m. The proposed high-rise double-tower building, which is a reinforced concrete tube frame structure, is 259 m high. The west tower, which is planned as residual area, has 67 stories above the ground and three-level basement. The east tower, which is planned as multi-functional area, has 60 stories and four-level basement. Also, there is a 9-story annex which is a frame-shear wall structure is planned for shopping and hotel area. Through geotechnical investigation, the stratums of construction site are shown in Table 1.

There are two primary types of aquifers in the site. This first one is an unconfined aquifer, which is primarily the pore water in the third stratum. The water table depth is commonly 0.60 to 2.3 m below the ground. The second one is the confined aquifer. There is a weak confined aquifer in the fifth stratum, which is consisted of clay with large amount of lime concretions. Its water head is around 12.0 to 12.5 m below the ground, and it fluctuates between 0.5 m and 1.0 m annually. Another confined aquifer is deposited in the limestone bedrock. Its water head varies from 4.5 m to 7.8 m below the ground, and it fluctuates between 1.0 m and 4.0 m annually.

Characteristic of the stratums in construction site

Stratum | Thickness (m) | Average thickness (m) | Characteristics description |
---|---|---|---|

① Miscellaneous fill | 2.00~4.50 | 2.97 | Variegated in color, and with loose structure. |

② Silty clay | 0.70~3.40 | 1.87 | Brownish yellow in color, and in soft-plastic state. |

③ Fill | 4.60~7.20 | 5.73 | Grey in color, and in soft-plastic state. |

④ Clay | 0.70~3.00 | 1.65 | Yellow in color, and in hard-plastic state. |

⑤ Clay with lime concretion | 1.90~3.20 | 2.74 | Brownish yellow in color, and in-hard plastic state. Particle size of lime concretion is 0.50~5.00 cm. |

⑤-1 Clay | 4.20~7.30 | 5.39 | Brownish yellow in color, and in-hard plastic state. |

⑥ Sandstone | 4.60~7.20 | 6.16 | Purple in color, and strongly weathered. |

⑥-1 | 4.00~7.10 | 5.00 | Grey in color, and slightly weathered. |

Parameters of anchor cables

Layer | Horizontal interval (m) | Number of steel strand | Orifice relative elevation (m) | Drilling angle (°) | Length of free segment (m) | Length of anchoring section (m) | Design value (kN) | Pre-stressing value (kN) |
---|---|---|---|---|---|---|---|---|

1 | 1.20/2.40 | 6 | −1.50 | 25 | 30 | 3 | 500 | 300 |

2 | −4.75 | 25 | 22 | 750 | 450 | |||

3 | −9.50 | 20 | 27 | 850 | 510 | |||

4 | −14.00 | 15 | 19 | 800 | 480 |

Parameters of concrete bracings

Layer | Relative elevation (m) | Section size of bracing (mm × mm) | |
---|---|---|---|

Corner bracing | Connecting rod | ||

1 | −1.75 | 1000 × 900 | 800 × 900 |

2 | −6.45 | 900 × 900 | |

3 | −11.65 | ||

4 | −15.50 |

## Method

^{3}, respectively. The model is divided into five layers corresponding to the natural stratums. The excavation does not reach the fifth layer. According to the geotechnical investigation of the construction site, the shear modulus (G) and bulk modulus (K) are obtained from Poisson’s ratio (ν) and Young’s modulus (E) by conversion formula (1) and (2). The soil parameter applied in model is shown in Table 4.

Soil Parameters applied in numerical model

Layer | Soil group | Thickness (m) | Weight density (kN/m | Cohesion (kPa) | Friction angle (°) | Shear modulus (MPa) | Bulk modulus (MPa) |
---|---|---|---|---|---|---|---|

1 | Miscellaneous fill | 2.30 | 17.50 | 7.00 | 10.00 | 4.48 | 12.50 |

2 | Silty clay | 2.70 | 17.90 | 22.00 | 11.00 | 3.98 | 9.72 |

3 | Fill | 5.00 | 17.80 | 24.00 | 11.60 | 4.55 | 11.11 |

4 | Clay | 10.00 | 19.30 | 75.00 | 11.80 | 13.64 | 33.33 |

5 | Sandstone | 20.00 | 24.00 | 140.00 | 30.00 | 1200.00 | 2000.00 |

There are six primary construction steps for the excavation: (1) lowered the ground water table to −22.6 m (22.6 m below the ground), and constructed retaining piles, stud piles, and waterproof curtain; (2) excavate the first layer of soil to −4.3 m, and constructed the top beam, the first bracing and anchor cables; (3) excavate the second layer of soil to −8.3 m, and constructed the second bracing and anchor cables; (4) excavate the third layer of soil to −12.3 m, and constructed the third bracing and anchor cables; (5) excavate the fourth layer of soil to −16.3 m, and constructed the fourth bracing and anchor cables; (6) excavate to −16.3 m, which is the bottom of the deep excavation.

*d*), pile space (

*S*), and row space (

*L*) on supporting effect were considered in models. And as shown in Table 5, three levels were set for each factor.

Factor level charts

Levels | (A) Pile Diameter ( | (B) Pile Space ( | (C) Row Space ( |
---|---|---|---|

1 | 600 | 1200 | 1200 |

2 | 800 | 2400 | 1800 |

3 | 1000 | 3600 | 2400 |

## Results and discussion

The lateral displacement of retaining piles and the settlement around the excavation can be considered as straightforward and effective indicators, which can reflect the safety of excavation. They can also be used to evaluate the adverse effect brought by excavation on the surrounding environment. Therefore, these two indicators were applied to study the performance of different excavation support systems.

### Pile-anchor support system

Simulated settlement at the first step and the percentage of simulated total settlement

Monitoring point | DB03–1 | DB16–1 | DB20–1 | DB26–2 |
---|---|---|---|---|

Settlement (mm) | 2.74 | 2.74 | 2.74 | 1.90 |

Percentage of Simulated Total Settlement (%) | 15.56 | 15.62 | 17.03 | 10.88 |

It can be seen in Table 6 that the settlement caused by dewatering before excavation are 2.74 mm, 2.74 mm, 2.74 mm and 1.90 mm respectively, and the percentage of total settlement are 15.56%, 15.62%, 17.03% and 10.88% respectively. The settlement at the first step was caused by consolidation, and the influence of dewatering cannot be ignored.

### Quincunx double-row piles support system

*K*

_{ i }represents the summation of the corresponding test results of level

*i*on any column;

*k*

_{ i }represents the arithmetic average of test results of level

*i*on any column;

*R*represents the range, and

*R*= max {

*K*

_{1},

*K*

_{2},

*K*

_{3}} − min {

*K*

_{1},

*K*

_{2},

*K*

_{3}} or

*R*= max {

*k*

_{1},

*k*

_{2},

*k*

_{3}} − min {

*k*

_{1},

*k*

_{2},

*k*

_{3}} (Xu et al. 2002).

Results of test

Test number | A | B | Blank column | C | Settlement/mm | The maximum lateral displacement of piles/mm |
---|---|---|---|---|---|---|

1 | 1 | 1 | 1 | 1 | 11.53 | 27.08 |

2 | 1 | 2 | 2 | 2 | 13.62 | 29.35 |

3 | 1 | 3 | 3 | 3 | 14.92 | 30.68 |

4 | 2 | 1 | 2 | 3 | 12.32 | 27.69 |

5 | 2 | 2 | 3 | 1 | 14.54 | 30.15 |

6 | 2 | 3 | 1 | 2 | 15.30 | 31.05 |

7 | 3 | 1 | 3 | 2 | 14.04 | 29.32 |

8 | 3 | 2 | 1 | 3 | 15.27 | 30.71 |

9 | 3 | 3 | 2 | 1 | 16.21 | 31.88 |

Analysis of test results

Evaluation index | A | B | C | ||
---|---|---|---|---|---|

Settlement/mm |
| 40.07 | 37.89 | 42.10 | 42.28 |

| 42.16 | 43.43 | 42.15 | 42.96 | |

| 45.52 | 46.43 | 43.50 | 42.51 | |

| 13.36 | 12.63 | 14.03 | 14.09 | |

| 14.05 | 14.48 | 14.05 | 14.32 | |

| 15.17 | 15.48 | 14.50 | 14.17 | |

| 5.45 | 8.54 | 1.40 | 0.68 | |

Influence Degree | B A C | ||||

Optimal Solution | B | ||||

The Maximum Lateral Displacement of Retaining Piles/mm |
| 87.11 | 84.09 | 88.84 | 89.11 |

| 88.89 | 90.21 | 88.92 | 89.72 | |

| 91.91 | 93.61 | 90.15 | 89.08 | |

| 29.04 | 28.03 | 29.61 | 29.70 | |

| 29.63 | 30.07 | 29.64 | 29.91 | |

| 30.64 | 31.20 | 30.05 | 29.69 | |

| 4.80 | 9.52 | 1.31 | 0.64 | |

Influence Degree | B A C | ||||

Optimal Solution | B |

In Table 8, it can be seen that for the both index, it has *R*
_{B}>*R*
_{A}>*R*
_{C}, which indicates that the factor B has greater influence on test results than A, and A is greater than C. The factors can be ranged as follows: pile space, pile diameter, row space. The optimal solutions for two evaluation indexes are determined by the minimum *K*
_{
i
} for each factor, and they are B_{1}A_{1}C_{1} and B_{1}A_{1}C_{3} respectively. When the row space corresponds to 2400 mm, a small lateral displacement of retaining piles is obtained, and it is beneficial to the security of excavation. Hence, B_{1}A_{1}C_{3} is selected as the optimum solution, and the pile diameter, pile space, row space correspond to 600 mm, 1200 mm, 2400 mm respectively.

In Fig.11, it can be seen that with the increase of row space from 1200 mm to 2400 mm, the settlement and the maximum lateral displacement of retaining piles slowly increase at first then decrease slightly, and the variation is very small. It is indicated that the row space of quincunx double-row piles has a little influence on the settlement around the excavation and the lateral displacement of retaining piles than pile diameter and pile space. With the increase of pile space from 1200 mm to 3600 mm, the settlement and the maximum lateral displacement of retaining piles increase, and the change of pile space has a very obvious influence on the lateral displacement of retaining piles.

## Conclusions

The simulated settlement around the excavation caused by dewatering is great, as well as the percentage of simulated total settlement. Hence the influence of dewatering cannot be ignored.

Through the analysis of monitored lateral displacement of retaining piles and numerical simulation results, it is cleared that the lateral displacement of retaining piles is prone to be large at a depth, where the stratum has large thickness and is consisted by soils with poor engineering properties during excavation.

In quincunx double-row piles support system, pile space is the primary factor affecting the settlement around the excavation and the lateral displacement of retaining piles, especially for the latter. Row space has little effect on the settlement around the excavation and lateral displacement of retaining piles.

For the construction of deep excavation in similar geo-environments, the quincunx double-row piles support system with reasonable parameters can effectively restrict the settlement around the excavation and the lateral displacement of retaining piles, therefore, can optimize the supporting effect.

## Declarations

### Authors’ contributions

YT, WZ and JZ participated in the discussion the results of the survey. WZ drafted the manuscript. All authors read and approved the final manuscript.

### Competing interests

The authors declare that they have no competing interests.

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## Authors’ Affiliations

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