A STUDY OF COMBINED INTERMEDIATE DEPTH FOUNDATION SYSTEM

Abstract

The Combined Intermediate Depth Foundation (CIDF) system is a combination of a group of Auger Bored Compaction Under-reamed Piles (ABCUP) of small length with grid beams and top slab supported on soil. The group of ABCUP is bored and compacted with full visibility and control. The combination of elements like ABCUP, grid beams, slab, soil-cement and improved soil mass within this composite system, is more near to a composite block with improved side skin friction and bearing at bottom. Structurally the closely spaced under-reamed bulbs at the same level from the top and the improved confined soil mass gives an action of stiff layer at the level of under reamed bulb.. The construction procedure of the CIDF system has been developed and used in the field by Professor G.C. Nayak. The CIDF system corresponding to isolated footing with five and nine piles and combined footing with five pile system are analysed for pile depths of 10 diameter of pile and spacing of 4,6,8 times the diameter of pile. The analysis is first carried out as a composite block and then an assumed group of pile system for axial load only. An assumed group of pile system gives conservative vlaues of axial load carrying capacity and the share of ABCUP, beams and slab has been worked out. Further the material efficiency for each element in CIDF system has been calculated. The present study attempts to present the cost effectiveness of CIDF system in comparison to the shallow foundations of conventional R.C. isolated and combined footings carrying the same load as the conservative load worked out for group pile system. The cost effectiveness of the system is given in terms of quantity of material / kN load. The conventional foundation e.g. isolated footings and combined footings transmit loads to the soil purly by bearing and by structural action of bending and punching shear and as the CIDF system, transmits the load to the soil by skin friction and full bearing of plan area, directly by ABCUP and structurally by bending action of beams and slab, the CIDF system ig economical and effective. For the same load the CIDF system generally requires less than 1/4th of the material of corresponding conventional foundations for most of the soil strata. In the composite block the amount of reinforced concrete used is only 10 to 16% of the volume of composite block. The speed of construction of CIDF system is found to be much faster than conventional foundation and requires unskilled labour. Finally the summary, conclusions and scope for future work is highlighted along with various areas of applications of this CIDF system.