Award Date

2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering

Department

Civil and Environmental Engineering

Advisor 1

Moses Karakouzian, Committee Chair

First Committee Member

Samaan G. Ladkany

Second Committee Member

Mohamed Trabia

Third Committee Member

Vernon F. Hodge

Fourth Committee Member

Douglas B. Rigby

Number of Pages

321

Abstract

Cemented carbonate deposits (known locally as "caliche") in Las Vegas have been used to support shallow and deep foundations with relatively high bearing pressures compared to soil. Most high rise structures in Las Vegas are founded on either a large mat or a long pile foundation. Recently, a new foundation type consisting of a short pile system bonded to shallow cemented layers was utilized for a large high rise building in Las Vegas and settlements during construction were recorded. The bonding of caliche layers together with short piles forms a caliche stiffened pile (CSP) foundation. The CSP foundation is unique since it derives stiffness from both piles and near surface caliche layers. This type of foundation is a new concept for building support, so this research formulates a method of analysis for the CSP foundation, and compares predicted to measured foundation settlement. The performance of the CSP foundation is also compared to a conventional pile foundation. The settlement behavior of a spread footing, single pile and 4 pile group in a layered soil-caliche profile has been studied using both 2D and 3D finite element models. The results indicate that the presence of a thin layer of high elastic modulus in a soil profile has a significant settlement reducing effect. Regarding a pile group in this profile, the settlement reducing effect due to the presence of caliche layers in a soil profile is greatest when stiff layers are present at both the top and below the pile tip. For a single pile in a caliche stiffened profile, the presence of the upper caliche layer causes an increase in the vertical stress adjacent to the pile due to the plate or beam effect. The analysis of a caliche stiffened pile (CSP) foundation system affects the load distribution and results in a more uniform stress distribution at the base of the lower caliche layer compared to a pile foundation in soil. The load distribution of a full scale pile load test in a soil/caliche profile was accurately predicted using both 2D and 3D finite element models. A case study building foundation was modeled using 2D and 3D models, and predicted settlements are compared to measured data. An analysis of the case study foundation indicates that increasing the pile length by 100 percent reduces the settlement by only 10 percent. Predictions of excess pore pressures and tensile stress in the caliche layer below pile tips were similar for both the 2D and 3D models. The settlement distribution along the building length including the building ends was reasonably predicted by the 3D model, but the model over predicted settlements where the upper caliche layer was thickest. The research indicates that the simpler 2D plane strain model can provide a reasonable initial prediction of settlement but limited information regarding anticipated differential settlements. Based on this research effort, guidelines for design of a CSP foundation are presented.

Keywords

Analysis; Bearing pressures; Building foundations; Caliche; Caliche stiffened pile (CSP); Piles; Rocks; Settlement

Disciplines

Civil Engineering

Language

English


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