The dry lakebed sediments underlying much of Salt Lake City create a subsurface profile unlike anywhere else along the Wasatch Front. Sitting at 4,265 feet elevation on the ancient floor of Lake Bonneville, the valley contains interbedded silts, clays, and sands that can shift from stiff to soft within a few vertical feet. Running a Standard Penetration Test here is not a routine checkbox: the N-values we record directly feed into liquefaction triggering analyses under the site-specific ground motions required by IBC Chapter 16. We correlate blow counts with undrained shear strength for the soft lacustrine clays found across the valley, using the sampler recovery to flag zones where a CPT test might be warranted for continuous stratigraphic resolution in borderline liquefiable layers. Every SPT boring we complete in Salt Lake County is logged by a field engineer who understands the difference between a false refusal on a gravel stringer and a true bearing stratum in the Lake Bonneville deposits.
An N60 below 10 in saturated silty sand at 15-foot depth triggers mandatory liquefaction review under the Salt Lake Valley IBC amendments — we flag it before the structural engineer sees the log.
Service characteristics in Salt Lake City
Demonstration video
Risks and considerations in Salt Lake City
Salt Lake City sits within 10 miles of the Wasatch Fault, a normal fault system with an estimated recurrence interval of roughly 1,300 years for a magnitude 7.0 event. The last major surface-rupturing earthquake on the central segment occurred approximately 1,400 years ago, placing the region within a recognized seismic gap. For SPT-based site characterization, this translates into a non-negotiable requirement: every borehole penetrating saturated granular soils below the groundwater table must generate N60 values defensible under cross-examination. A single SPT refusal on a boulder at 12 feet can be misread as competent bearing material if the driller does not recognize the refusal mechanism. We record refusal type — whether on a cobble, a caliche cemented layer, or true bedrock — and we do not assign N-values to non-representative blows. In the downtown corridor, where deep excavations for parking structures intersect the sand-and-gravel stringers of the Bonneville shoreline deposits, mischaracterized refusal has led to costly over-excavation and dewatering failures. The risk is not theoretical: it is documented in changed-condition claims filed with the Utah Geological Survey. When SPT data feeds directly into the liquefaction potential index and the foundation bearing capacity report, shortcuts in the field become structural liabilities.
Our services
Our Salt Lake City SPT program is built around the specific subsurface challenges of the Bonneville basin. Each service below connects directly to the data the SPT borehole generates, and we configure the scope based on the project's geotechnical risk category and the site's mapped seismic hazard class.
Liquefaction Triggering Analysis from SPT Data
N60 values corrected for fines content and overburden are run through the Seed-Idriss simplified procedure with site-specific PGA and magnitude from the USGS Wasatch Front model. We deliver cyclic resistance ratio versus cyclic stress ratio plots for each critical layer, with factor of safety contours that the structural engineer can apply directly to foundation design.
Bearing Capacity Determination from N-Values
Using Meyerhof and Bowles correlations tied to N60, we calculate allowable bearing pressure for spread footings and mat foundations on the Lake Bonneville deposits. Settlement estimates at service loads are checked against the IBC 25-mm total settlement limit, with differential settlement projections for multi-column frames.
Seismic Site Classification per ASCE 7-22
We compute the average N60 over the top 100 feet of the profile, classify the site from A to F, and flag Site Class F conditions such as liquefiable soils thicker than 10 feet. The classification feeds directly into the design response spectrum used by the structural engineer.
Correlation Testing Package
Split-spoon samples are rushed to the lab for moisture content, Atterberg limits, and grain-size distribution. We correlate SPT N-values with laboratory undrained shear strength from unconfined compression tests on cohesive samples, building a site-specific calibration for the project's geotechnical model.
Common questions
How much does an SPT borehole cost in Salt Lake City?
For a standard SPT borehole in the Salt Lake Valley, we typically quote between US$510 and US$810 per borehole, depending on depth, access constraints, and whether traffic control is required. Deeper holes beyond 50 feet or those needing drill rig mobilization across steep terrain in the foothills run toward the upper end. The price includes field logging, N60 corrections, groundwater monitoring, and the signed report.
At what depth does SPT refusal occur in Salt Lake City?
Refusal depth varies dramatically across the valley. In the downtown core, we often hit gravel stringers or cobble layers between 25 and 45 feet, yielding N-values over 50 within a single six-inch increment. Along the Wasatch foothills, weathered bedrock can appear as shallow as 10 feet. West of the airport, the Lake Bonneville clays extend past 100 feet without refusal. We log the refusal mechanism — whether on a boulder, a caliche lens, or bedrock — so the geotechnical model reflects what stopped the sampler, not just the blow count.
How are N60 corrections calculated for SPT data in the Salt Lake basin?
We measure hammer energy ratio directly in the field using an instrumented rod section, then apply the standard correction chain: N60 = N_field × (ERi/60) × (rod length factor) × (borehole diameter factor) × (sampler liner correction). Overburden correction uses the Liao and Whitman (1986) equation with effective stress computed from the logged soil unit weights. For the silty sands common along the Jordan River corridor, the fines content correction per NCEER recommendations is applied before running any liquefaction analysis.
Does SPT testing in Salt Lake City require a geologist or engineer on site?
Yes — under Utah state regulations, a licensed professional geologist or geotechnical engineer must be responsible for borehole logging, sample description, and final interpretation. Our field crews are led by a staff engineer who classifies every sample per ASTM D2487 on site, records the drilling conditions and any artesian flow, and signs the boring log. The final report is stamped by a Utah-licensed Professional Engineer with geotechnical scope authority.