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GRANULAR PILES-CONSTRUCTION, DESIGN AND BEHAVIOUR Madhira R. Madhav J.N.T.Universit y, Hyderabad, India [email protected] ABSTRACT: Granular piles have been developed initially to resist compressive loads through predominantly pile action. However, the capacity of single or small groups of GPs is restricted by the bulging capacity. The carrying capacity of GP can be enhanced by reinforcing it with geosynthetics (geogrids or geotextiles) in the form of sheets or by jacketing, to inhibit bulging near the t op. The failure modes of small groups of granular piles range from simple punching of short GP, axial compression of long ones, to shear buckling of peripheral GP. However, the y offer cost effective alternative to expensive pile groups. By a simple modification of placing a steel plate, a concrete pedestal or a geogrid at the bas and by attaching a steel rod or a cable to the footing, a granular pile can be made to resist uplift or pullout forces. The response of Granular Pile Anchors (GPA) is much superior to that of solid piles as the load is directly transferred to the tip where the bulging capacity is the highest. This improvement is orders of magnitude more in case of normally consolidated soils whose undrained strength increases with depth. The paper summarizes and presents an overview of the recent development pertaining to granular piles. INTRODUCTION In general, the term soft ground includes soft clay soils, soils with large fractions of fine particles such as silts, clays soils which have high moisture content, peat foundations, and loose sand deposits near or under the water table. For clayey soils, the softness of the ground can be assessed by its undrained strength, c u , or by its unconfined compression strength, q u . on the other hand, the SPT N values are utilized to ascertain the consistency of the ground and its relative density. Table 1 outlines the identification of soft ground according to the types of the structures using the aforementioned assessment methods. Considering such factors as the significance of the structure, applied loading, site conditions, period of construction, etc., it becomes important to select appropriate method suitable for specific soil types as tabulated in Table 2 For soft and cohesive soils in subsiding environments, ground improvement by reinforcement (i.e. sand compaction piles), by admixtures (i.e. deep mixing method), and by dewatering (i.e. vertical drains) are applicable. For loose sand deposits, various in-situ compaction methods are applicable such as dynamic compaction, resonance compaction, and vibroflotation. Above the ground, such techniques as earth reinforcement or mechanically stabilized earth (MSE) and the utilization of lightweight synthetic materials as applicable. Amongst various techniques for improving in-situ ground conditions, granular piles or stone columns are probably the most versatile. They provide primary functions of reinforcement and drainage, and in addition, improve the strength and deformation properties of soft soil in post installation and reconsolidation phase. Granular piles increase the unit weight by replacement, drain rapidly the excess pore pressures generated, act as strong and stiff elements and carry higher shear stresses. They are installed in wide variety of soils, ranging from loose sands to soft clays and organic soils. Granular piles are cost effective, more so, in end bearing conditions. They are installed using vibro-replacement, composer (sand compaction piles), rammed stone columns and even by heavy tamping. The rammed stone columns incorporate the additional benefits of heavy tamping as they in effect are preloaded.
