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This is a Bentley PLAXIS case study by Strong Force-MGC W.L.L. that examines whether the semi-manual or 2D design methods are always applicable and the most conservative and economical in shoring support for deep excavation.
Strong Force-MGC W.L.L. was approached to provide a shoring support solution for a Bahrain project on a design-and-built basis. The project consisted of a high-rise structure with deep excavations in some areas.
The area of interest in Figure 1 is the roundabout. The shoring system installed at the roundabout area is a tangent piling wall (TPW), consists of piles with 830mm Dia., and ~ 15m deep, each, to support ~ 9m of deep excavation in a silty SAND layer followed by a weak SILTSTONE layer. The GWT is @ 2m begl.
An in-hand 2D design from an independent designer for the shoring support system was provided which covered a straight roadside and the roundabout side.
The proposed supporting system for the TPW at the roundabout side was 39 nos. of uniform single-level Ground anchors with 1.7m spacing, 20 degrees inclination, at 2.7m level & 15m total length. The provided 2D design had failed to address the actual 3D nature of the project (the curve of the roundabout side) because the 2D design assumed that the TPW is straight and continuous in the out-of-plane direction. However, all the proposed ground anchors are clashing & intersecting at one point, affecting their functionality.
Strong Force-MGC W.L.L. decided to utilize PLAXIS 3D for the design and analysis of this case. The conceptual design was brain-stormed on papers, followed by the 3D drawings & sketches. The main idea was to manipulate the ground anchors arrangement in terms of various inclination, spacing, and lengths so the anchors will not intersect or affect each other. Different soil models were used (LE, MC, HS, HSS & GHS) for the sand, rock, and TPW materials covering the expected range of their properties (i.e., sensitivity analysis).
The model's size, quality of the mesh, boundary conditions, water flow regimes, and permeability of the system (fully permeable, impermeable, semi-permeable) were considered in the analysis. Moreover, the TPW was modeled as LE & elastoplastic equivalent plate element (which took less than an hour to analyze), as well as MC volume piles elements using soil clusters to represent the granular details of the actual scenario (which took a couple of days on an overclocked CPU needed to analyze ~ 3 millions of nodes).
The TPW modeled as a plate element might be enhanced by considering the custom connections between the plate panels and the expected water flow through the TPW using the interface permeabilities feature. Nonetheless, one of the major advantages of using plate elements is the straightforwardness of getting the structural forces acting on the TPW.
The final ground anchors arrangement is shown in Figure 4. The maximum deformation of the TPW was
Using Plaxis 3D resulted in reduced anchors of only 16 nos. and not clashing with each other considering all SSI's while the total savings in cost and time was ~ 58% compared to a 2D design approach.
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