Aggregate piers: stress transfer mechanism and construction effect

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Montana State University - Bozeman, College of Engineering


This thesis is a compilation of two different papers based on the behavior of aggregate piers which is a soil improvement method used to increase the bearing capacity and reduce the expected settlements of soils in which different types of structures are supported. The first paper describes the results of two modulus load tests and a dimensional finite difference analysis (FDA) conducted to evaluate the load-displacement response of isolated aggregate piers. Load test aggregate piers were constructed with two different materials: the first one with 38 mm base coarse and the second one with 75 mm subbase coarse materials. The numerical analyses provided reasonable predictions of the load-displacement response of the isolated aggregate piers. Parametric analyses using the validated numerical model illustrate that the lateral stress increment on the soil around the pier during the installation process of the pier should be considered in the numerical analysis, otherwise the settlement can be overestimated. The second paper is based on two full-scale load tests that were conducted to examine the load transfer mechanisms of end-bearing single and group aggregate piers. The first included a load test on a 0.76 m diameter isolated aggregate pier. The length of the aggregate pier was 4.3 m. The second test included a 2.1 m square reinforced concrete footing supported by four 0.76 m diameter aggregate piers of 4.3 m long. The soil consists of soft to medium stiff layers of sandy and clayey silt overlain by a 2-m-thick, softer silty clay layer. At the bottom, weathered rock was found. The load transfer mechanism within the length of the piers was examined using a series of load cells and tell-tale reference plates installed at different depths of the aggregate piers. Additionally, the installation effect was investigated using Cone Penetration Tests (CPT) conducted prior and after construction of the aggregate piers and inclinometers installed at multiple locations around the aggregate piers. The results of the experiments were compared with those in the literature to provide insights on the performance of aggregate piers with different configurations (single vs. group) and depths (floating vs end-bearing) in different soil profiles.




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