Example: Cofferdam - Combined Sheet Pile Walls - LEM, NL, FEM Analysis
This example presents the design of a cofferdam (combined sheet piles braced with pipe struts), supporting a 30 ft excavation.
The 2D model is generated and analyzed in DeepEX software, using all the available analysis methods (Limit Equilibrium, Non-Linear and Finite Element Analysis). The rectangular 3D model of the cofferdam is created with the DeepEX model wizard, and analyzed as well.
A. Introduction - Project Description
In this example we will design a 40’ x 80’ cofferdam, supported by combined sheet pile walls (HZ 680 M LT Piles with AZ 14 steel sheets) with internal bracing (PP14x0.5 and PP18x0.5 pipes). The Figure below presents the cofferdam model. Tables 1 and 2 present the soil properties and the stratigraphy respectively. Table 3 presents the support properties. The general ground surface is at El. -3 ft and the general water table is at El. 2 ft.
Table 1: Soil properties.
Table 2: Stratigraphy.
Soil Layer | Elevation (m) | OCR | Ko |
F | 5 | 1 | 0.500 |
CL | -15 | 3 | 0.905 |
S1 | -22 | 1 | 0.441 |
GT | -35 | 1 | 0.413 |
Table 3: Support parameters.
| Strut Level 1 | Strut Level 2 |
Strut elevation on wall | 0 | -10 ft |
Strut Section | PP16x0.75 | PP24x1 |
Strut Spacing | 20 ft | 20 ft |
Steel Material | A50 | A50 |
B. Modeling with DeepEX
B.1. Edit Soil Properties and Stratigraphy
In DeepEX we can create an unlimited number of soils and define the soil type and soil properties for each soil. The soil properties can be defined manually for each soil, or be estimated from SPT values and other test results. The following figure presents some soil properties and the project stratigraphy as defined in Tables 1 and 2.
B.2. Examined Wall Section
In DeepEX, we need to manually define the wall type and wall section to be examined in a model. The software will calculate all wall stresses and capacities according to the selected design standard and provide extensive results and check ratios for each construction stage. Based on the results, we can easily optimize the wall section, either manually, or with the use of DeepEX automatic optimization tools.
In this example, we assume the use of a combined sheet pile wall section (HZ 680 M LT steel beams, combined with AZ 14-770 sheet piles). Table 4 presents the wall section parameters.
Table 4: Wall parameters.
| Left Wall | Right Wall |
X wall Coordinate (top left point) | 0 | 40ft |
Wall Type | Combined sheet piles | Combined sheet piles |
Steel Beam Section | HZ 680 M LT | HZ 680 M LT |
Sheet Pile Section | AZ 14-770 | AZ 14-770 |
Beam Spacing | 6.782 ft | 6.782 ft |
Steel Material | A50 | A50 |
Wall depth | 45ft | 45ft |
B.3. Construction Stages
In DeepEX, we can create and analyze all construction stages of a model. The construction stages both allow the more advanced analysis methods (Non-Linear – Finite Element Analysis) to converge, and can assist us with the final model optimization.
B.4. Analysis Methods and Analysis Settings
The generated model will be analyzed with all analysis methods available in DeepEX Software – Limit Equilibrium Analysis, Non-Linear Analysis (Elastoplastic springs) and Finite Element Analysis.
For the Limit Equilibrium Analysis method, we will use Active and Passive soil pressures for the driving and resisting side, in all construction stages with cantilever excavation and 1 support level (Stages 0 to 3). For all Stages with multiple support levels, we will use the FHWA Apparent Pressures for the driving side (Stages 3 to 5).
For the calculation of wall moment and shear diagrams (Beam Analysis) we will use Blum’s method, which considers the support locations are hinges and uses a virtual support below the excavation, at the point of zero net pressures.
We will calculate the water pressures, considering the Simplified Flow method.
In all Stages, we will use as wall friction 33% of the soil friction for both the driving and the resisting sides.
C. LEM, NL and FEM Analysis Results – 2D Model
The table below includes the most critical results and checks (displacements, wall moment, wall shear, support reactions, wall embedment FS and more), for each analysis method, among all construction stages. All calculated result graphs can be also presented on the model area for each constructions stage.
Table 5: DeepEX critical results/analysis method
D. 3D Model Generation and Analysis
With DeepEX 3D version, we can use the created and analyzed 2D sections in order to generate the full 3D Frame model, with the use of the 3D Model Wizard. In the Wizard, we can define the shaft dimensions, the first support spacing from each corner and the 2D design section we wish to use, in order to model all surrounding walls. As soon as the rectangular model is created, we can access it on the model area, edit the surrounding node locations in order to simulate the actual plan view shape, access and apply different load combinations on the surrounding walls and walers, edit the location and structural section of each support and more.
DeepEX can analyze the 3D models and design all created walers and supports. All 3D analysis results can be reviewed in comprehensive summary tables and graphically on the model area, for each construction stage.
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