Example 6: Braced Cofferdam TERS in Soft and Stiff Cohesive Soil
- May 19
- 5 min read
1. INTRODUCTION
This case study examines a 15-foot-deep excavation through a stratified soil profile consisting of a 5-footthick layer of loose fine sand above a 10-foot-thick deposit of soft clay, which is underlain by firm clay. The properties of the soil and groundwater conditions are illustrated in Figure 1, and a 360 psf surcharge pressure is applied. A braced cofferdam is proposed as the support system for the excavation, with the strut placed 3 feet below the top of the excavation. The analysis utilizes three approaches: the Limit Equilibrium Method (LEM), Non-Linear Analysis (NL), and the Finite Element Method (FEM), all of which are implemented using the DeepEX software. The following sections detail the analytical framework and present the results obtained.
2. ABOUT DEEPEX
DeepEX is a superior software solution for the design, analysis, and optimization of deep excavation projects and tunnels. Its superior interactive interface allows users to generate, analyze, review, and evaluate any model quickly and efficiently.
Implemented analysis methods: Limit Equilibrium, Non-Linear analysis (soil springs), and 2D & 3D Finite Element analysis.
Implemented soil pressure methods (LEM): Active/Passive, At-rest, FHWA Apparent, German EAB, Custom Trapezoidal, 2-step Rectangular, WMATA, NYC and more.
Implemented water pressure methods: Hydrostatic, Simplified flow, Full 2D flownet, Balanced pressures, unconfined flow.
Beam analysis methods (LEM): Blum’s method (continuous beam), FHWA simple span, CALTRANS, WMATA, and more.
3. SOIL PROPERTIES
The soil profile is stratified as follows: from 0 to 5 ft, a loose fine sand layer (ϕ′ = 32°, γ = 109.20 pcf); from 5 to 15 ft, a saturated soft clay layer (cu = 500 psf, γsat = 118.37 pcf); and below 15 ft, a firmer clay layer with undrained shear strength cu = 1000 psf and γsat = 118.37 pcf. Figure 2 presents the soil properties as displayed in the input window.
4. WALL SECTION PROPERTIES
The retaining wall is composed of SZ-10 section sheet piles, with the configurations displayed in Figure 3, which presents the geometry, and material properties are described in Figure 4.
Figure 3 – Geometry of the sheet piles section.
5. SUPPORT PROPERTIES
The proposed support is composed of a single level bracing system (PP12x0.375), with the properties presented in Figure 5.
5. CONSTRUCTION STAGES
The excavation process is modelled by the following stages:
1-Initial installation (Figure 6 a): The sheet piles are driven through the loose sand and the soft clay into the firm clay layer.
2- Partial excavation (Figure 6 b): Excavation proceeds to -5 ft, entirely through the loose sand.
3- Activation of strut support (Figure 6 c): The support system is activated at a depth of -3 ft.
4-Dewatering and final excavation (Figure 6 d): The excavation is dewatered to the dredge line, and excavation continues to the final level at -15 ft.
Figure 6 – Model geometry and loading conditions. Initial phase (a), excavation to 4 ft (b), activation of the support and prestress-for NL and FEM (c), final stage of excavation (d).
7. ANALYSIS ASSUMPTIONS
In this example, the following assumptions were made:
Groundwater conditions: The analysis implements simplified flow to represent groundwater behavior effectively.
Surcharge pressure: applied as a field load.
LEM:
Shear forces, moments and support reaction was performed with Blum's method.
Soil pressures: Active and passive pressure methods; active and passive pressures.
Optimization of wall embedment for FS = 1.5.
N-L and FEM: Prestress equivalent to 80% of the maximum axial force in the support for LEM analyses.
N-L and FEM: The friction between wall and soil equivalent to 33% of the friction angle.
FEM: medium refinement of finite element mesh.
7. DEEPEX ANALYSIS RESULTS
This section presents the results of the analysis, including plots of horizontal earth pressures, bending moment diagrams, horizontal displacements, and shear strength distributions for each method, as detailed below.
A. Limit Equilibrium Analysis - (show moments and horizontal pressures (Figure 7), shear stress (Figure 8), for apparent FHWA pressure as driving pressure (the more unfavorable for the support solicitation).
B. Non-Linear Analysis - (soil springs) (show moments and horizontal pressures (Figure 9), shear stress and displacements (Figure 10).
C. Finite Element Analysis - (same as NL analysis (Figure 11, 12) + FEM Mesh, horizontal (Figure 13) and vertical displacement shadings (Figure 14).
8. ANALYSIS SUMMARY & CONCLUSION
This document presents an analysis of a 15-foot-deep braced excavation through a stratified soil profile consisting of a 5-foot-thick layer of loose fine sand, underlain by a 10-foot-thick layer of saturated soft clay, and subsequently firm clay. The support system comprises a braced cofferdam with a single level of struts installed 3 feet below the top of the excavation. The analysis is performed using DeepEX software and incorporates three design approaches: the Limit Equilibrium Method (LEM), Non-Linear Analysis (NL), and the Finite Element Method (FEM). These methods are applied to evaluate the performance of the excavation support system under the given soil and loading conditions. The results demonstrate DeepEX’s capability to model complex geotechnical problems involving retaining structures in stratified soil conditions. The software effectively captures the behavior of the system through each method, offering a comprehensive understanding of wall performance and soilstructure interaction.
Table 1 summarizes the key results from all three analysis methods, including maximum horizontal displacements, bending moments, shear forces, moment and shear check ratios, and the factor of safety for wall embedment. Table 2 presents verification results for the support system, detailing strut reactions and capacity ratios. These outcomes highlight the strengths and applicability of each analysis approach for practical excavation design in layered soil conditions.
Table 1: Critical wall results for each method.
Table notes:
STR Moment: Moment stress check, assuming constant axial load on wall (demand/capacity).
STR Shear: Shear stress check (shear force demand/wall shear capacity)
Table: Critical support results for each method.
Table notes:
STR Support ratio: Critical structural stress check for support (force demand/structural capacity). Critical support check: Critical demand/design capacity ratio (structural or geotechnical).
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