ABSTRACT:
This paper presents a case study of caisson foundation system for a high-rise residential building in Kuala Lumpur granitic formation. Due to site congestion and sloping terrain of the project site, hand-dug caisson was adopted as the deep foundation system rather than conventional bored pile or driven pile systems. This paper discusses the design aspects of caisson pile especially on the assessment of rock socket which involves estimation of both shaft and base resistances for rock and their load transfer behaviour. Checking of concrete lining and some interesting findings on the belled-out base caisson is briefly discussed here. The performance of the caisson pile was verified with two numbers of static maintained load tests with test load up to 1,700 tonnes and 3,000 tonnes (instrumented test pile) respectively, and one plate bearing test at the excavated caisson base. Generally, the caisson pile performance was better than prediction due to the inherent rough and irregular rock shaft surface formed by mechanical hacking and the advantage of proper rock socket cleaning prior to concreting compared to conventional bored pile construction. This paper also presents typical construction sequence together with some good practices on 360o rock condition mapping and, safety and health of the workers during construction.
1. INTRODUCTION
The site is located at Kuala Lumpur with development area of 2.34acres. The development comprises 3 blocks of high-rise residential building with about 20 storeys of service apartments, ground floor retailer shops and elevated car parks respectively. The site is underlain by granitic formation with depth of bedrock ranging from 1m to 20m below ground. For area with shallow bedrock, footing was adopted as building foundation system for practical and economy reasons (refer to Fig. 1). Hand-dug caisson was adopted as the deep foundation system due to the existing site condition of partially sloping terrain as the boring rigs or jack-in machine of the conventional foundation piling system require relative flat and large platform for the pile installation. The limited space of the site also restrained the use of the large piling machine. Caisson pile construction involves hand digging of earth materials and rock hacking using portable tools. The staged excavation is temporarily retained by cast in-situ circular concrete linings before subsequent lift excavation until the founding level. Concrete lining is unnecessary in bedrock and stable stiff soil with reasonably dry condition when water table is below excavation. Proper cleaning and checking on the completed digging hole would be carried out before lowering of steel cage and concreting. Figure 1 Foundation system for the development
2. DESIGN OF CAISSON PILE
2.1 Rock Socket Design During the soil excavation in caisson pile, concrete lining would normally be cast in-situ to temporarily support the excavated soil surface. However, the quality of cast concrete is difficult to be controlled due to the local caisson construction norm of only using hand tapping for compaction after concreting to avoid dislodging of the surrounding soil. Hence, the contribution of pile shaft resistance in the soil would be uncertained and is usually ignored in the pile capacity design unless proper site verification test is carried out. The geotechnical capacity of the caisson pile is primarily derived from both the shaft and base resistances of the founding materials. Unlike conventional bored pile usually ignoring the base resistance due to unsatisfactory base cleaning, caisson pile under dry hole construction can easily achieve satisfactory base cleaning and allow inspection of rock socketting condition before concreting. Eq. (1) with design approach established by Rosenberg & Journeaux (1978), Horvath (1978) and Williams & Pells (1981) was adopted to estimate the rock socket resistance with consideration of the respective strengths of intact rock and rock mass effect in association with the inherent discontinuities


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