ASTM D1586 isn't just a reference number — it's the standard that governs every SPT we run in Bakersfield. The city sits on deep alluvial fan deposits from the Kern River, a setting where loose sands and silty lenses can look stable but perform poorly under seismic load. Our team operates truck-mounted CME rigs with automatic trip hammers calibrated to the energy ratio correction required by ASCE 7-22 for Seismic Design Category D sites. We deliver split-spoon samples bagged and logged within 24 hours of rig demobilization. For projects near the Kern River floodplain, where channel migration has left alternating layers of fine sand and organic silt, the liquefaction assessment we provide alongside SPT data becomes essential for complying with CBC Section 1803 requirements.
A corrected N60 value from a calibrated hammer is the difference between a footing that performs for 50 years and one that settles unevenly after the first wet winter.
How we work
Local considerations
A mistake we see repeatedly in Bakersfield is design firms using uncorrected N-values from old Caltrans logs without accounting for hammer efficiency. An SPT run with an old donut hammer delivering 45 percent energy versus a modern automatic hammer at 62 percent can undercount blow counts by 30 percent or more. That discrepancy translates directly into underestimated bearing capacity and overdesigned footings — or worse, under-designed ones. Another common shortcut is skipping the saturation check on clay seams. During the rainy season of 2023, we documented N-values in the northwest industrial corridor that dropped from 18 to 7 when a thin clay layer was tested in its natural moisture state versus after 48 hours of soaking. If the geotechnical report doesn't flag that sensitivity, the contractor pours concrete over a time bomb.
Relevant standards
ASTM D1586-18 provides the standard method for the Standard Penetration Test; ASCE 7-22 governs minimum design loads for site class determination; CBC Section 1803 outlines requirements for geotechnical investigations; and ASTM D2487-17e1 establishes the classification of soils for engineering purposes.
Associated technical services
Residential Foundation SPT Package
Two to four borings to 25-foot depth, N-value logging, lab classification, and a signed report with allowable bearing pressure and settlement estimates per CBC 1806. Typical turnaround is five business days.
Commercial & Industrial Deep Borings
SPT borings to 60-80 feet with continuous sampling through soft zones. Includes liquefaction triggering analysis using the NCEER simplified procedure and residual settlement calculations for post-earthquake performance.
Forensic & Supplemental Investigation
Targeted SPT borings adjacent to existing structures where settlement has occurred. We correlate new blow counts with historic logs, identify layer changes, and recommend underpinning depths if bearing strata have shifted.
Typical parameters
Common questions
What does an SPT test cost for a single-family home lot in Bakersfield?
How deep do SPT borings need to go for a two-story commercial building?
The CBC requires borings to extend to a depth where the stress increase from the foundation is less than 10 percent of the existing overburden pressure. For a typical two-story commercial slab in Bakersfield's alluvial soils, that usually means 40 to 50 feet, but we confirm during drilling based on real-time N-value trends.
Do you report raw N-values or corrected N60 values?
We report both. Every log shows the raw field N-value for each 6-inch increment, the N60 corrected for hammer energy ratio, and the N1(60) normalized to 1 ton per square foot overburden pressure. This gives your structural engineer the full dataset without having to request corrections later.