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Analysis & Optimization of a Deep Excavation for Eurocode 7 Load Combinations

A. INTRODUCTION

Deep excavations play a vital role in the engineering industry.

From construction of high-rise buildings, underground structures, or transportation systems, the design of deep excavations requires careful consideration to ensure safety, stability, and structural integrity.

In several cases, a deep excavation design needs to comply with internationally recognized standards and guidelines.


In this example, we will create, analyze, and optimize a soldier pile wall deep excavation with two levels of ground anchors, for all available Eurocode 7 load combinations. Eurocode 7, a set of specifications developed by the European Committee for Standardization (CEN), emphasizes the importance of a rigorous and systematic approach to deep excavation modeling, aiming to mitigate risks, enhance stability, and minimize potential hazards.


The following sections present how such a model can be created in minutes with our complete deep excavation design software DeepEX.

We will illustrate important analysis settings software options to quickly and automatic optimize the shoring system, so that it complies with all EC7 requirements. Table 1 presents the soil properties and stratigraphy.

Tables 2 and 3 present the initial wall and support section properties respectively.


Table 1: Soil properties and stratigraphy

​Soil

​Top El.

​Description

​Unit Weight

​Friction Angle

​C’

​Eload

​exp

​qSkin


​(-)

​(m)


​(-)


​(KN/m3)


​(deg)


​(kPa)


​(kPa)


​(-)


​(kPa)


​F

​0

​Fill - Sand

​19.6

​30

​0.6

​15000

​0.5

​50

​S1


​-2

​Medium Dense Sand

​21

​34

​1

​35000

​0.4

​150

​GT

-7


​Glacial Till

​22

​36

​10

​50000

​0.4

​200

Table 2: Initial wall section properties

​Wall Type:

​Soldier Piles (Steel Beams with Timber Lagging)

​Pile Section:

​HE 300A

​Lagging Thickness:

​5 cm

​Steel Material:

A50 Steel

​Wall depth:

​14 m

Table 3: Initial support properties

Anchor:

​Tieback 1

​Tieback 2

Elevation on Wall:

​- 3m

​- 6m

​Structural Section:

​5 x 1.4 cm Strands 270 ksi

​5 x 1.4 cm Strands 270 ksi

​Free Length:

​6.5 m

5.16 m

​Fixed Length

​8 m

​8 m

​Installation Angle:

​20 deg

20 deg

B. MODEL GENERATION – DEEPEX MODEL WIZARD

The model wizard is a powerful tool in DeepEX, that allows users to generate any deep excavation model in seconds, including all construction stages. The wizard utilizes a series of tabs, where we can define all project parameters, from analysis settings, project type, to soil properties, support elevations, design standards and more. Figures 1 to 5 present some of the DeepEX wizard options.

Figure 1: DeepEX Model Wizard – Analysis settings


Figure 2: DeepEX Model Wizard – Project type, dimensions & support data


Figure 3: DeepEX Model Wizard – Soil properties and boring


Figure 4: DeepEX Model Wizard – Wall Section Properties


Figure 5: DeepEX Model Wizard – Stages creation, Support Elevations


The software can automatically create any deep excavation model with all construction stages, as well as linked model copies with each selected standard load combination.

Figure 6 presents the generated construction stages.

Figure 7 shows the load combinations from the selected Eurocode 7 standard.

Figure 6: Generated model in DeepEX (all stages)


Figure 7: Linked design sections with each EC7 load combination


C. MODEL ANALYSIS & OPTIMIZATION


- Initial Model Analysis

Initially, we will perform a Limit Equilibrium analysis for all design sections (base model – service, and the sections with the Eurocode 7 load combination).

Figure 8 presents the DeepEX analysis and checking summary table that becomes visible at the end of all computations.

We come to the following conclusions:


1. The displacements and settlements are significant, but we need to verify that with the Non-Linear analysis engine and the Finite Element method that consider the project staging and soil stiffness.


2. The wall shear check ratios are ok for all Eurocode 7 load combinations, but the check is not satisfied for the combinations DA-2 and DA-3.


3. The support structural sections are fine, but we see that at least one of the supports fails geotechnically (pullout) both in service conditions and with all Eurocode 7 load combinations.


4. The wall embedment FS is above 2 for the service conditions & above 1 for all EC7 load combinations (the wall embedment is sufficient – perhaps we can consider reducing the pile installation depth).


Figure 8: DeepEX – Analysis and checking summary table (initial model)


Figure 9: EC7 – DA-2 Combination Results (Stage 5), top support pullout capacity is not sufficient


- Top tieback fixed length optimization


In the optimize tab of DeepEX we can select to optimize the fixed length of a ground anchor, and then click on our top tieback support.

The software will locate the suitable tieback length so that the geotechnical check will be satisfied for all linked design sections.

In this case, the software returns an optimum fixed length of 12m (compared to the 8m we initially assumed).

Figure 10 shows the fixed length optimization procedure in DeepEX.

Figure 11 presents the analysis and checking summary table after the optimization, where we notice that the geotechnical check issue is resolved.

Figure 10: Tieback fixed length optimization procedure in DeepEX