11.3 Drilled Shafts

11.3 Drilled Shafts

The geotechnical report should address axial and lateral pile capacity, axial pile load test for drilled shafts with reaction piles or bidirectional load cells, integrity testing using cross-hole sonic logging, cross-hole Gamma-Gamma logging, thermal testing, or a combination of these methods, as appropriate.

End bearing for shafts is normally ignored unless pile capacity can be verified by top-down or by using bidirectional load tests. For end bearing in dense sand or bedrock, the bottom of a shaft should be cleaned out thoroughly and tested using Mini SID (Shaft Identification Device) or similar tools for evaluating proper clean out.

12. SHORING, DEWATERING, EXCAVATION AND UNDERPINNING

The geotechnical report should address shoring, dewatering, and underpinning. Design of the shoring, dewatering, and underpinning system is usually provided by specialty contractors, with design parameters (soil and groundwater pressure) provided by the GEOR. If shoring is used to support an adjacent building, the design soil pressure should correspond to the at-rest pressure and account for building surcharge. The GEOR and the EDRT should review the contractor’s analysis and design to evaluate that the design has used appropriate soil and groundwater pressures. The GEOR and the EDRT should also review the contractor’s Plan of Action for trigger levels (e.g., Warning Level or Design Limit) of lateral and vertical movement of the shoring and underpinning system before the start of construction.

Because of the potential presence of confined aquifers within or below the BM and OBC, nested piezometers should be installed outside of the excavation for monitoring of drop in groundwater table and water head within various sand layers, as appropriate.

Bottom of excavation should be evaluated for expected conditions for stability. If cohesionless soil is exposed at the bottom of the excavation, the factor of safety against bottom instability should be calculated to check that piping (quick sand condition) is prevented. If cohesive soil is exposed at bottom of excavation, the factor of safety against basal heave should be calculated. Finally, if a cohesionless soil layer at depth is overlain by a layer of cohesive soil at the bottom of excavation, the blowout condition should be carefully analyzed and, if necessary, the cohesionless soil layer should be depressurized to prevent a bottom blowout condition.

13. INSTRUMENTATION AND CONSTRUCTION MONITORING

The GEOR should provide recommendation for geotechnical instrumentation and construction monitoring at locations where ground conditions, type of loading, or proximity of existing structures could be adversely affected by planned construction.