Elsevier

Computers and Geotechnics

Volume 86, June 2017, Pages 141-149
Computers and Geotechnics

Research Paper
Assessment of strut forces for braced excavation in clays from numerical analysis and field measurements

https://doi.org/10.1016/j.compgeo.2017.01.012Get rights and content

Abstract

One important consideration in the design of a braced excavation system is to ensure that the structural bracing system is designed both safely and economically. The forces acting on the struts are often determined using empirical methods such as the Apparent Pressure Diagram (APD) method developed by Peck (1969). Most of these empirical methods that were developed from either numerical analysis or field studies have been for excavations with flexible wall types such as sheetpile walls. There have been only limited studies on the excavation performance for stiffer wall systems such as diaphragm walls and bored piles. In this paper, both 2D and 3D finite element analyses were carried out to study the forces acting on the struts for braced excavations in clays, with focus on the performance for the stiffer wall systems. Subsequently, based on this numerical study as well as field measurements from a number of reported case histories, empirical charts have been proposed for determining strut loads for excavations in stiff wall systems.

Introduction

Construction of a basement structure using a braced retaining wall system will inevitably result in wall deflections and ground settlement. Excessive ground settlement will frequently cause damage to adjacent properties in urban areas. The total amount of ground settlement associated with deep excavations is closely related to the type of support system, the properties of the in situ soils, and the excavation procedure. For excavations in clays, basal heave stability also needs to be considered.

Another important design issue is to ensure the structural safety of the bracing system. The forces acting on the struts are often determined using empirical methods such as the Apparent Pressure Diagram (APD) method. Terzaghi and Peck [22] and Peck [20] recommended the widely used APD, to estimate the magnitude and distribution of prop loads. They proposed different APDs for braced excavation in sands, stiff fissured clays, and soft to medium clays. This method was developed based on field measured data for braced excavations with flexible wall systems.

Ou [18] summarized Peck [20]’s work on APD to estimate the magnitude and distribution of strut loads in different clays as shown in Fig. 1, where the Rankine’s coefficient of lateral active earth pressure Ka is expressed as:Ka=1-m4cuγHewhere cu is the soil undrained shear strength (in kPa), γ is the soil unit weight (in kN/m3), He is the depth of the excavation (in meter) and m is an empirical coefficient. Most of the commonly used empirical methods that were developed from either numerical results or field studies have been for excavations with flexible wall systems such as sheetpile walls. To date, there have been limited studies on the excavation performance for stiffer wall systems such as diaphragm walls and bored piles.

Chang and Wong [4] proposed a modified APD for diaphragm walls in deep clay deposits. The research was based on a case study and a parametric finite element study. Their research showed that strut loads computed using the Peck’s APD underestimated the strut loads significantly. By introducing a strut force exceedance ratio α, as functions of the soil stiffness ratio and undrained shear strength, a modified APD was proposed. They commented that the amended APD was derived from the cases with T/B ratio greater than 1 (where T is the clay thickness below the final excavation level, and B is the excavation width, as illustrated in Fig. 2). If T/B is less than 1, they inferred that there would be strong restraining effect from the hard stratum reducing the strut force.

Hashash and Whittle [8] compared Peck’s conventional APD with their FE results which considered undrained strength anisotropy and strength non-homogeneity. Their research indicated that the conventional APD was smaller than the finite-element results for diaphragm wall, especially for deep excavations. Also, the wall stiffness plays an important role in the apparent earth pressures. As the wall stiffness decreased, the apparent earth pressure decreased.

Hsiung et al. [10] reported the well-instrumented strut behavior of a 16-m deep excavation with seven level struts restrained by a diaphragm wall in Taipei. They found that the Peck’s APD underestimated the measured apparent pressure for this case. Sze [21] carried out a series of centrifuge tests to investigate the apparent earth pressure for an undrained excavation. According to Sze’s test results, Peck’s APD underestimated the measured apparent pressure by 30 % for the case of excavations supported by diaphragm walls. Wong et al. [25] observed that for the construction of a major Singapore underground expressway project, most of the data were within the vertical boundary of the apparent earth pressure diagram proposed by Terzaghi and Peck [22]. However, they recommended that the vertical pressure diagram should extend to the ground surface instead of decreasing to zero to fit all the measured data.

Twine and Roscoe [23] enhanced Terzaghi and Peck's work and introduced the Distributed Prop Load (DPL) method based on 81 case histories and field measurements of prop loads. However, of the 81 case histories, 28 cases are for flexible walls in soft to medium clays (denoted as class AF) while only 2 cases are for stiff walls (class AS). In addition, although there are 10 reported cases for stiff walls in stiff clays (class BS), 5 of them are singly propped while 2 cases have two strut levels and only the remaining 3 cases have three levels of struts. In view of these limited published data, it is therefore relevant to reassess the DPL method for the class AS and class BS excavation types.

All these studies outlined earlier generally indicated that Peck’s APD under-predicted the apparent earth pressure of the braced excavations, especially for those involving diaphragm walls and large excavation depths. As various factors are likely to influence the APD such as the clay thickness, soil strength and stiffness, wall stiffness, excavation width, and strut stiffness, this paper explores the performance of the strutting system, including the apparent earth pressure, through a series of plane strain and three-dimensional finite element analysis. Some differences were observed between the numerical results and Peck’s APD.

As discussed previously, for the case of excavations in stiff wall systems, the proposed distributed prop loads (DPL) by Twine and Roscoe [23] were based on only very limited measured data. The main contribution from this paper is to propose updated APD for stiff wall systems based on extensive numerical analyses supplemented by additional measured data (8 cases in soft clays and 8 cases in stiff clays, with up to five levels of struts).

Section snippets

Details of numerical models

For this study, the finite element analyses were carried out using the geotechnical software PLAXIS 2D (V9.0) and PLAXIS 3D Foundation [1]. Fig. 2 shows a typical cross-section and plan view for the cases considered. The parameters shown in the figure include: L = excavation length, B = excavation width, D = wall penetration depth, T = clay thickness below the final excavation level (FEL), SH = horizontal strut spacing, SV = vertical strut spacing and He = depth of final excavation, all in meters. In this

Numerical results

In this section, the general trends of the strut forces with different wall stiffness and for the three clay types are presented. Fig. 4 depicts the plan view arrangement of the struts and walers (depicted schematically in red1) for the 3D analyses. The symbol x denotes the horizontal distance from the center of the excavation. There is no strut located at the middle section of the

Apparent earth pressure envelope for stiff walls

For design of braced deep excavation in soils, apparent earth pressure diagrams are usually used to calculate the minimum horizontal strut loads for each supporting level during vertical excavation. Herein the results from the 2D and 3D maximum strut forces for He = 14 m and He = 16 m were used to derive the apparent earth pressures for excavations in stiff walls. The tributary area load distribution procedure proposed by Peck [20] was used. The assumed loading area for each strut is plotted in Fig. 6

Summary and conclusions

A detailed study of strut forces were carried out through a series of 2D and 3D finite element analyses for excavations in clays. The general trend was for the forces in struts to decrease with increasing soil strength and to increase with increasing wall system stiffness. The results indicated that the differences in the strut forces between different L/B (and 2D vs 3D) depend on the clay types, L/B ratios, and the strut levels. For various L/B ratios in stiff clay, the differences are less

Acknowledgements

Part of this research is supported by the LTIF project titled “Braced Excavation-induced Ground Movements”, funded by the Land Transport Authority (LTA), Singapore. The authors would like to acknowledge the financial support from LTA. The corresponding author is also grateful to the support by the National Natural Science Foundation of China (Nos. 51608071 and 51420105013).

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