Support of Deep Excavation Using Contiguous Pile—A Case Study

The design of deep excavations requires careful consideration of the strength and stability of the various structural elements and soil properties at all stages during the construction process. In addition, the ground movements induced by the excavation need to be carefully studied and controlled, to ensure no damage to nearby properties and services and to keep these within serviceable limits. The performance of a deep excavation depends on the method of construction as well as precise assessment of the local ground conditions. Making reliable predictions of performance often presents a considerable challenge looking at various factors and uncertainties involved. This paper presents a case history on the use of temporary support of excavation using contiguous bored piled wall technique for deep basement excavation in Chennai. Case history presents an excavation of up to 9.3 m deep for the construction of a basement for a commercial building. The site stratification is sedimentary formation (Sandy Clay underlain by Stiff Clay) and the rock is at very deep depths beyond even shoring pile socketing extents. The geotechnical challenge is to design and construct a deep basement up to 9.3 m depth where SPT ‘N’ values are less than 10 up to 15 m below Existing Ground Level (EGL), high ground water table, close proximity of high-rise structures and adjacency of underground metro line to the shoring system. Finite element analysis using PLAXIS was used to analyse this staged excavation. Based on the finite element analyses, temporary strut supports were used to provide lateral support for contiguous piles. Monitoring works all along the shoring line was carried out using total station on regular intervals, and these readings were compared with the analysis results.