DESIGN OF EXCAVATION SUPPORT SYSTEM FOR A COMMERCIAL PLAZA IN RAWALPINDI

Abstract

In Pakistan, commercial construction is leaning towards high rise buildings with multi-story basements which require substantial excavations. These type of excavations can lead to settlement and caving in of soil into the excavation. Which can lead to collapse of the surrounding buildings. Therefore, an efficient excavation support system is needed to prevent this from happening. The practices used in design of excavation supports are based on empirical methods and are highly conservative thus resulting in heavy costs, rendering the projects less feasible. Therefore, there is a need to use such methodologies which can meet the design requirements and reduce the cost at the same time. The aim of this thesis is to highlight the design of three different excavation support systems for a commercial plaza in Rawalpindi, Pakistan and to compare its costs to provide the most cost effective solution. The assigned site is located behind Rania Mall, Saddar Rawalpindi and was surrounded by mega buildings on two sides and service roads on remaining other two. The excavation support systems undertaken for design included drilled shafts, piles with tie-backs and diaphragm wall with internal bracings. Field analysis was carried out to find the composition and shear strength parameters of the soil. The borehole data obtained as a result of extensive tests classified the soil as stiff silty clay. A 20ft excavation was to be carried out on site for the construction of two basements, each of 10ft height. The buildings located just over the edge of the excavation trench developed very heavy lateral stresses which was to be catered for to place the foundation of the plaza. The design guide lines for each of the support system was followed differently, FHWA Circular No.4 for the design of secant piles with tiebacks, AASHTO manual on bridge design (2012) for drilled shafts and Deep Excavations by Chang Yu Ou for diaphragm wall with internal bracing. This thesis includes a short comparison of all the three support systems based on design and cost. Extensive Cost analysis is carried out for each support system. Government issued rates of materials in Rawalpindi were used.