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Drilled Shafts Vs. Driven Piles

Drilled Shafts

Drilled shafts are nondisplacement reinforced concrete deep foundation elements constructed in dry, cased, or slurry-filled boreholes. A properly constructed drilled shaft will not cause any heave or loss of ground near the shaft and will minimize vibration and soil disturbance. Dry holes may often be bored within 30 minutes leading to a rapidly constructed, economical foundation. Single drilled shafts may be built with large diameters and can extend to deep depths to support large loads.

Lateral expansion and rebound of adjacent soil into the bored hole may decrease pore pressures. Heavily overconsolidated clays and shales may weaken and transfer some load to the shaft base where pore pressures may be positive.

Rebound of soil at the bottom of the excavation and water collecting at the bottom of an open bore hole may reduce end bearing capacity and may require construction using slurry.

Drilled shafts tend to be preferred to driven piles as the soil becomes harder, pile driving becomes difficult, and driving vibrations affect nearby structures. Good information concerning rock is required when drilled shafts are carried to rock. Rock that is more weathered or of lesser quality than expected may require shaft bases to be placed deeper than expected. Cost overruns can be significant unless good information is available.

Driven Piles

Driven piles are displacement deep foundation elements driven into the ground causing the soil to be displaced and disturbed or remolded. Driving often temporarily increases pore pressures and reduces short term bearing capacity, but may increase long term bearing capacity. Driven piles are often constructed in groups to provide adequate bearing capacity.

Driven piles are frequently used to support hydraulic structures such as locks and retaining walls and to support bridges and highway overpasses. Piles are also useful in flood areas with unreliable soils.

Pile driving causes vibration with considerable noise and may interfere with the performance of nearby structures and operations. A preconstruction survey of nearby structures may be required.

The cross-section and length of individual piles are restricted by the capacity of equipment to drive piles into the ground.

Driven piles tend to densify cohesionless soils and may cause settlement of the surface, particularly if the soil is loose.

Heave may occur at the surface when piles are driven into clay, but a net settlement may occur over the longterm. Soil heave will be greater in the direction toward which piles are placed and driven. The lateral extent of ground heave is approximately equal to the depth of the bottom of the clay layer.

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