Drilled Shafts Guidelines
Drilled Shaft Foundations
Drilled shafts are primary choices for resisting high lateral loads, often proving more economical than spread footings or pile caps when large cofferdams would otherwise be required. In West Virginia, they are frequently selected to satisfy hydraulic or environmental footprint issues and to provide resistance to lateral soil movement, including arresting landslides where practical.
Rock Socket Requirements
All drilled shafts in West Virginia must be socketed into bedrock. Key requirements include:
- Placement: The rock socket typically begins at the rock-line or the scour depth, whichever is deeper.
- Diameter: The rock socket is usually 6 inches smaller in diameter than the shaft to form a ledge for better casing sealing (unless at known dry-socket locations).
- Elevation: The top of the shaft should be at least 1 foot above the normal pool elevation to facilitate latent concrete removal and provide a cap joint above the water surface.
Lateral Analysis and Modeling
Rock sockets should be sized to limit lateral displacement and ensure geotechnical fixity in the service limit state. The WVDOH defines fixity as the point where the deflection curve crosses the zero line for the second time using software such as LPile.
- Strong Rock (UCS ≥ 1,000 psi): Use internal P-Y curves for Strong Rock. Deflection at the top of rock is limited to 0.0004b (where b is the diameter).
- Weak Rock (UCS up to 1,000 psi): Use internal P-Y curves for Weak Rock. The input Krm ranges from 0.0005 for stiffer rock to 0.00005 for softer rock.
Axial Geotechnical Resistance
Design must follow LRFD standards using the RMR system for bearing resistance equations. A resistance factor of 0.45 is used for end bearing and 0.55 for side resistance.
- Inclined Bedding: When bedrock dips between 20° and 80°, resistance factors must be reduced by 50%.
- Non-Redundant Shafts: Factored resistances must be further reduced by 20%.
Intermediate Geomaterials (IGM)
Claystone, mudstone, and IGMs are prone to smearing and rapid deterioration. Side resistance cannot be relied upon unless verified by representative load testing or jar slake testing (material must not "flake" within 96 hours). For preliminary IGM skin friction estimates, refer to FHWA-NHI-10-016.
Construction Quality Control (CSL)
Cross-hole sonic logging (CSL) tubes are installed in all wet shafts and demonstration shafts. For rating CSL results, both the First Arrival Time (FAT) and energy reduction are considered:
| Rating | Criteria |
|---|---|
| Good (G) | FAT increases 0-10% and Energy Reduction < 6 db |
| Questionable (Q) | FAT increases 11-20% and Energy Reduction < 9 db |
| Poor / Flaw (P/F) | FAT increases 21 to 30% or Energy Reduction of 9 to 12 db |
| Poor / Defect (P/D) | FAT increases 31% or more or Energy Reduction > 12 db |
Tolerances and eccentricities
- Horizontal Deviation: A maximum horizontal deviation of 3 inches from the plan location is permitted at the top of the shaft.
- Plumbness: Shaft plumbness must be within 1.5% (0.15 feet per 10 feet).
- Cleanliness: The base of the rock socket is considered clean if no more than 1/2 inch of material exists on 75% of the bottom, and the remaining 25% has no material thicker than 1-1/2 inches.
