ABSTRACT:

A project was undertaken in Colombo for construction of a housing scheme comprising 340 units of twostorey apartment blocks. Subsurface investigations revealed the existence of substantial amounts of compressible material, which would not only have a large primary compression, but also a significant secondary compression. To eliminate these undesirable qualities in the foundation material ground improvement was considered necessary. This paper describes the use of heavy tamping in this project. Particular to this project are treatment of peat and peaty clay under saturated conditions, use of granular and cohesive material, and free fall of the tamper for increased efficiency.

INTRODUCTION

In the outskirts of Colombo, Sri Lanka, Urban Development Authority of Sri Lanka decided to inaugurate a housing scheme within a 5-km radius of the New Parliament Building. The chosen project site is situated on the Pitakotte - Talawatugoda Road. The site consists of a low-lying area between two rolling hills. During the rainy season the site goes partially under water while during the dry periods the ground water table recedes to about lm to 1.5m below the existing ground surface.

SITE INVESTIGATION

Site investigation conducted prior to commencement of work revealed that the subsoil is composed of 3m to 3.5m of peat, dark grey to black in colour and partly to fully decomposed, underlain by 1m to 1.5 m of grey plastic clay followed by fine to medium sandy clay. At a depth of about 6m partially weathered to hard rock is encountered. The typical SPT N value in peat and clay is 0; N value in sand as it approached the base rock would rise to about 5 to 9. A comparative Mackintosh probe blow count in Peat and Clay is about 5 to 10; in sand the blow count ranges typically between 20 and 70.

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