Gravity Wall Analysis
Application Note LS-G-AN3 describes the typical steps followed in setting up and analysing a gravity wall stability problem with LimitState:GEO. It also describes the analysis of a trench excavation in front of the wall. A brief overview of the document is given here.
Within the full pdf document, the key concepts of Soil/structure interface properties and Cohesive soil/structure interface tensile strength are discussed.
All files used in this note are available in a zip file that can be downloaded here.
|The problem involves analysis of an 8m high wall with vertical back retaining 6m of dense gravel overlain by 2m of loose sand (Figure 1). The surface of the sand is level with the top of the wall and supports a surcharge of 5 kN/m2. The wall and the layer of dense gravel are underlain by stiff clay. The problem is to be checked against Eurocode 7 Design Approach 1 Combination 1 and 2 requirements for long and short term conditions. Passive resistance of the soil in front of the wall is to be included. The modelled soils and associated parameters are listed in Tables 1 and 2 below:||
Figure 1: Retaining wall problem geometry
||Loose Sand (Default) - Always drained, Angle of friction =30o, Unit weight=14kN/m3, Saturated unit weight=19kN/m3
|B||Dense Gravel - Always drained, Angle of friction =37o, Unit weight=18kN/m3, Saturated unit weight=21kN/m3|
||Stiff Clay (Default) - Drained / undrained, Cohesion=120kN/m2, Drained cohesion=4kN/m2, Angle of friction =22o, Unit weight=21kN/m3, Saturated unit weight=21kN/m3|
||Cohesion=5000kN/m2, Unit weight=24kN/m3|
Table 1: Soil and wall properties
|Soil / Wall interface||Details|
|Loose sand / wall
||Mohr-Coulomb, Always drained, Angle of friction =20o
|Dense gravel / wall
||Mohr-Coulomb, Always drained, Angle of friction =24.7o|
|Stiff clay / wall
||Mohr-Coulomb, Drained / Undrained, Angle of friction =14.7o, Cohesion=120kN/m2|
Table 2: Soil / wall interface properties
The LimitState:GEO model may be set up manually (steps described in the pdf document). Alternatively the complete model may be loaded by opening the file 'gravity wall.geo' contained within the zip file.
By clicking Solve, the Adequacy Factors listed in Table 3 are obtained.
The critical scenario (depicted in Figure 2) is found to be the Long term analysis with Design Approach 1/2 which has an Adequacy Factor of 1.028. A value above 1.0 means that in Eurocode 7 terms the design is safe. The additional margin of safety (or overdesign factor) of 1.028 is given in terms of the load (or loads) to which the Adequacy Factor was assigned.
The document continues on to describe the analysis of the model when a 4.5m deep trench is excavated at a distance of 3.5m from the toe of the slope. In this problem it is assumed that the short term Design Approach 1/2 analysis is of interest, since this is now a global stability problem for which Design Approach 1/1 is less suited.
Table 3: Scenarios and analysis results
Figure 2: Retaining wall critical failure mechanism