Geotechnical recommendations in pile foundation guides in choosing the depth, the type of pile and special precautions to be taken for different soil conditions. Pile Foundations are recommended only when a raft foundation cannot be proposed due to excessive settlement caused due to the underlying loose or compressible soil.
General geotechnical recommendations for all types of pile foundations are explained briefly.
Geotechnical Recommendations for Pile Foundation
The important geotechnical consideration for pile foundation construction are:
- The base level of the piles is determined considering the end resistance of the stratum and settlement behavior of the soil under the pile groups.
- A slip of 5 to 10mm of the soil is enough to develop full skin resistance along the pile whereas a displacement of the order of 10% of the diameter of pile tip is necessary to mobilize full end bearing resistance.
- If piles encounter shrinkable clays near the ground, due allowance may be made for loss of frictional resistance and also for uplift due to swelling.
- In stiff fissured clays, bored cast-in-situ piles or low displacement driven piles are usually recommended. Dense silts cause high penetration resistance for driven piles but the capacity of the pile remains low due to disturbance of the soil during driving.
- Normally consolidated clays cause down-drag on bored cast-in-situ piles due to consolidation on account of drainage occurring as a result of boring.
- Point resistance and skin friction of pile in sand increases as the length of the pile increases. This increase in value is 10 times the pile diameter for loose soil and 20 times for dense sand. Beyond this length, the values remain constant.
- Point resistance of piles longer than 15 to 20 times the diameter, driven through weak strata, into the thick firm sand deposit, increases with the depth of embedment in this stratum. This increase is up to a maximum value corresponding to 8 to 12 times the diameter of the pile.
- Except for bored piles in sand, the capacity of a group of piles equals the sum of the capacities of individual piles in the group. In case of bored piles in sand, the capacity is about two thirds the sum of capacity. Check is necessary for failure of the pile group as a single block.
- Pile capacity may be calculated by several appropriate methods so as to establish upper and lower bound values. Errors are very high when results from one type of soil deposit in one locality or valid for one year of the pile are extrapolated to derive the value for different deposits in another locality or another type of pile involving a different construction technique.
Geotechnical Recommendations for Driven Piles
For driven pile, compact loose and medium dense cohesionless soils are preferred. For such piles, pile driving formulate are more reliable for cohesionless soils than for cohesive soils. Large surface cracks are formed by driven piles in stiff clays. Hence the skin resistance may be neglected up to 1.8m at the top.
Geotechnical Recommendations for Bored Piles
Capacity of bore piles are more dependent on the construction technique than for driven piles. The boring operations make the soil more loosened. Shaft friction values for bored piles in sands may be only half of that for driven piles. This ratio is about one-third of end bearing resistance. If concrete is placed (but not mechanically compacted) while withdrawing the shell tube, the surrounding cohesionless soil may be considered to be in loose condition.
With a view to limit the number of piles in each group, the recommendation should indicate the high possible capacity of the pile considering the soil parameters, the bore log and the appropriate type of pile.The capacity of the desired pile foundation is calculated from the standard codes of the region.
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