Methodology
Our methodology for standard penetration test in Spokane follows ASTM D1586, using a 63.5 kg hammer dropped 760 mm to drive a split-barrel sampler. Samples are recovered at 1.5 m intervals to 30 m depth, with N60 values corrected for hammer energy. Data is correlated with local soil types—till, loess, and basalt—to estimate relative density, strength, and liquefaction potential. For continuous profiling, we recommend cone penetration test (cpt).
Reference Technical Parameters
| Parameter | Reference Value |
|---|---|
| Predominant soil type | Glacial till, loess, basalt |
| Maximum seismic acceleration (PGA) | 0.3g – 0.4g (ASCE 7-16) |
| Typical groundwater level | 10 – 25 m below surface |
| Bedrock depth | 5 – 30 m (basalt or granite) |
| Typical N60 range | 10 – 50 blows/0.3 m |
Local Considerations — Spokane
Spokane’s geology is dominated by glacial till from the Missoula Floods, overlying Columbia River Basalt. In the South Hill, thick loess deposits require careful SPT sampling to assess collapse potential. Our team has executed diverse projects across the city, from the downtown core to the Spokane Valley, adapting methods to local conditions. For deeper investigation, we also offer dpsh dynamic probing as a complementary tool. The region's moderate seismicity (PGA 0.3g–0.4g) necessitates site-specific liquefaction analysis per ASCE 7.
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Services in Spokane
Applicable Standards
- ASTM D1586
- ASCE 7-16
Frequently Asked Questions
What soil types are common in Spokane?
Spokane soils include glacial till (sand, gravel, cobbles), loess (silt) on the South Hill, and alluvial deposits along the Spokane River. Basalt bedrock is typically encountered at depths of 5 to 30 meters.
Is seismic site classification required in Spokane?
Yes, ASCE 7-16 requires site classification (A–F) based on shear wave velocity or SPT N-values. Spokane’s seismicity (PGA 0.3g–0.4g) makes this mandatory for most commercial and multi-family structures.
How many SPT borings are needed for a typical residential project?
For a single-family home, one or two borings to 10–15 m depth are usually sufficient. Larger projects require more borings per ASCE 7 or local building codes. Our firm tailors the scope to site variability.