Effect of Surcharge Analysis on Cantilever Excavation Design

A. Introduction

Selecting the appropriate surcharge method can have a significant impact in cantilever sheet pile design. In this article we examine the impact of several external surcharge distribution approaches on the design of a cantilever excavation system. Different scenarios for an external distributed surcharge and a 3-Dimensional load are considered. The wall embedment and sheet pile section is optimized for each case with our shoring design software – DeepEX.

Tables 1 and 2 summarize assumed soil layer and initial wall section properties respectively. Figure 1 presents the generated model in DeepEX.

Table 1: Soil Layers (Stratigraphy) – Soil Properties

Cantilever Excavation Example - Soil Properties and Stratigraphy

Table 2: Default Wall Properties

Cantilever Excavation Example - Sheet Pile wall section properties

Cantilever Excavation Example - Model in DeepEX Software

Figure 1: Cantilever Sheet Pile Wall – DeepEX Model

B. Examined External Surcharge Cases

The following cases are evaluated:

Case 1: Distributed Surface Load – Elasticity Equations (Boussinesq etc.)

Case 2: Distributed Surface Load – 2-Way Distribution Method

Case 3: Distributed Surface Load – 1-Way Distribution Method

Case 4: Distributed Surface Load – Elasticity Equations x2 (AREMA Specifications)

Case 5: 3D Load (Footing) – Distributed Load – Elasticity Equations

Case 6: 3D Load (Footing) – Distributed Load – 2-Way Distribution from Soil Friction

The following figures illustrate the related settings both for the external strip load and for the 3-D Footing load in DeepEX Software.

External Surcharge Analysis Methods in DeepEX Software

Figure 2: Distributed Load Settings & External Load Analysis Options – DeepEX Software

3D Footing Load Options and Analysis Methods in DeepEX software

Figure 3: 3D Footing Load Settings & Analysis Options – DeepEX Software

C. Analysis Settings

All the examined case models will be analyzed with the Free Earth Method for cantilever excavations (LEM approach) assuming a wall-soil interface friction angle at 33% of the soil friction angle.

Analysis Settings in DeepEX Software

Figure 4: Analysis Settings in DeepEX Software

Wall embedment will be optimized by requiring a minimum wall embedment FS of 1.5 on rotation.

Automatic Wall Depth Optimization Options in DeepEX Software

Figure 5: Automatic Depth Optimization Options in DeepEX

D. Analysis Results & Wall Optimization

Elasticity Equations for Surcharges - Cantilever Excavation Analysis Results

Figure 6: Case 1 – Elasticity Equations: Wall Embedment FS, Wall Moment & Surcharge Pressure Diagrams

2-Way Distribution Method - Cantilever Excavation Analysis Results and Section Optimization

Figure 7: Case 2 – 2-Way Distribution: Wall Embedment FS, Wall Moment & Surcharge Pressure Diagrams, Pile Section Optimization Options

1-Way Distribution Method - Cantilever Excavation Analysis Results

Figure 8: Case 3 – 1-Way Distribution: Wall Embedment FS, Wall Moment & Surcharge Pressure Diagrams

Elasticity Equations x 2 for Surcharges - Cantilever Excavation Analysis Results

Figure 9: Case 4 – Elasticity x2 (AREMA): Wall Embedment FS, Wall Moment & Surcharge Pressure Diagrams

3D Footing Load Elasticity Equations - Analysis Results in DeepEX

Figure 10: Case 5 – 3D Footing – Elasticity Equations: Wall Embedment FS, Wall Moment & Surcharge Pressure Diagrams

3D Footing Load 1-Way Distribution from Soil Friction Method - Analysis Results

Figure 11: Case 6 – 3D Footing – 2-Way Distribution from Soil Friction: Wall Embedment FS, Wall Moment & Surcharge Pressure Diagrams

E. Analysis Summary & Conclusions

Table 3 summarizes the calculated maximum moments, the minimum required wall embedment depths and the optimum structural section for each load case.

Table 3: Analysis Summary

 

Optimum Wall Embedment

Min Wall Embedment FS

Optimum Structural Section

Maximum Moment

Moment Check Ratio

Case 1

18.5 ft

1.569

AZ12-770

45.6 ksf

0.788

Case 2

18 ft

1.6

AZ12-770

37.7 ksf

0.651

Case 3

18.5 ft

1.569

AZ12-770

41.9 ksf

0.723

Case 4

22 ft

1.57

AZ20-800

74.5 ksf

0.803

Case 5

15.5 ft

1.572

AZ12-770

27.8 ksf

0.48

Case 6

16 ft

1.581

AZ12-770

27.8 ksf

0.48

From all the above examined scenarios it is clear that the selection of the surcharge distribution approach can have a big impact on the design project’s cost. Our clients and our workshop participants already know the best method! Sign up for a free trial and learn what method you should use in your shoring design projects!

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