SPT Testing in Billings, Montana: Geotechnical Data You Can Build On

For a typical residential or light commercial project in the Billings area, a single SPT borehole to 30-40 feet, including mobilization, drilling, sampling, and a brief report, generally runs between US$610 and US$850 per hole. Larger programs with multiple deep borings, energy calibration, and full engineering reports will scale accordingly. The biggest cost variable is access—tight urban lots on the South Side or steep slopes near the Rims can add setup time.

The IBC requires borings to extend deep enough to stress the soil at a depth where the net pressure increase from the foundation is less than 10% of the existing effective stress. In the Yellowstone Valley, that often means 30 to 50 feet for a two-story structure, but on the Rims where bedrock is shallow, you may hit refusal at 10 feet. We size the boring plan to your specific structural loads and footprint.

Yes. We measure transferred energy with an instrumented rod per ASTM D4633 when requested, and we always apply standard overburden and rod-length corrections. An uncorrected N-value is nearly meaningless in design—especially in the silty sands of the Billings alluvium, where overburden effects can mask loose layers.

It's the most widely used field test for that exact purpose. Using the Seed & Idriss (1971) framework and subsequent updates, we compare your corrected N-values against the cyclic stress ratio for the design earthquake. Billings sits in a region where the IBC maps a moderate seismic hazard, and liquefaction of loose saturated sands beneath the water table is a legitimate concern for certain sites near the river.

The city generally follows IBC Table 1803.1, which scales the minimum number of borings with building footprint. For a typical 5,000-square-foot commercial slab, expect at least two borings. Geotechnical variability here is high enough that we often recommend one additional boring beyond the code minimum to catch the lensing common in the fluvial deposits.