SPT (Standard Penetration Test) for Reliable Site Characterization in Regina

We still see projects around Regina where the foundation design moves forward with nothing more than a desktop study and a few hand auger samples. The problem is, Regina sits on a deep sequence of glaciolacustrine clays and silts deposited by glacial Lake Regina — stiff at the surface, yes, but with a plasticity that can shift dramatically once you get past the desiccated crust. The SPT (Standard Penetration Test) cuts through the guesswork. It delivers N-values at regular depth intervals, and when the split spoon comes up, you get a disturbed sample you can actually look at. For the kind of lightly overconsolidated tills we encounter east of the city toward the Condie Moraine, we often pair the SPT with a CPT test to get a continuous tip resistance profile without gaps between spoon intervals. Understanding how the blow counts change with depth is what separates a footing that settles evenly from one that tilts toward the utility trench after a wet spring.

In Regina's glaciolacustrine clay, the difference between an N-value of 8 and an N-value of 15 can change your allowable bearing pressure by over 50 kPa — that is a foundation cost driver you want to nail down before the excavator arrives.

Methodology and scope

Regina winters put the ground through a freeze-thaw cycle that reaches over two metres deep in an average year, and the summer brings rapid drying that shrinks the surface clays into the classic polygonal cracks you see in back alleys across the Cathedral neighbourhood. Those swings matter when you are reading SPT results. A blow count taken in late August, when the crust is baked hard, can look ten blows higher than the same material tested in May. Our field crew works year-round, so we account for seasonal moisture conditions in the borehole log notes, not just the N-value on the chart. In the deeper Regina clay — the stuff that stayed wet since the lake drained 10,000 years ago — we often see undrained shear strengths that correlate directly with N-values through the relationships Stroud and others developed. For projects where the clay transitions into the Condie silt or the till underneath, we also run a grain size analysis on the split spoon samples to confirm the stratigraphy we are interpreting from the blow count shift. When the client needs bearing capacity estimates that go beyond presumptive table values, the SPT data feeds directly into the footing design calculations using the Modified Meyerhof method referenced in the Canadian Foundation Engineering Manual.

Local geotechnical context

Section 4.2 of the National Building Code of Canada (NBCC 2020) and CSA A23.3 put the responsibility on the geotechnical engineer to characterize the site with sufficient subsurface data — and in Regina, 'sufficient' means you have to sample through the entire clay sequence until you hit competent till or bedrock. The biggest risk we see is stopping the borehole too soon. A contractor drills to eight metres, gets N-values in the high teens, and calls it good — but at twelve metres there is a soft, normally consolidated layer that nobody found. Under the sustained load of a slab-on-grade warehouse in the Ross Industrial Park, that hidden layer compresses, and three years later the floor is out of level by forty millimetres. Another risk is misinterpreting gravel refusal. The Condie Moraine and the buried valleys around Regina can have lenses of coarse outwash sitting right on top of soft silt — the spoon hits gravel, the driller calls refusal, and the soft layer underneath gets missed entirely. When we see that scenario, we switch to rotary drilling to core through the obstruction and keep sampling. The cost of one extra hour of drilling is nothing compared to a differential settlement claim.

Typical values

Parameter	Typical value
Standard followed	ASTM D1586-18 / CSA A119.2
Hammer type	Automatic trip hammer (safety hammer) with energy calibration
Borehole diameter	100 mm to 150 mm, hollow-stem auger or mud rotary
Sampling interval	Every 1.5 m depth increment, or at stratum change
Sampler	Standard 50 mm O.D. split spoon, 450 mm drive
Energy correction (ER)	Measured rod energy ratio applied per seed-whitman correlations
N60 reporting	Raw N-value and normalized N60 reported on all logs
Typical depth range in Regina	10 m to 25 m, depending on till depth

Related services

Foundation Design SPT Package

Boreholes laid out to match your building footprint, with sampling every 1.5 m through the full clay depth. We provide N60-corrected logs, undrained shear strength profiles, and allowable bearing pressure recommendations for spread footings or mat foundations per NBCC 2020 and CFEM. Includes settlement estimates based on the Schmertmann method.

Road and Utility Corridor Investigation

For linear infrastructure crossing Regina's variable glacial deposits, we run SPT soundings at 50 m to 100 m spacing along the alignment. The report includes subgrade modulus estimates, frost susceptibility classification based on grain size and N-value, and recommendations for pavement structure design using the AASHTO 1993 method adapted for local conditions.

Liquefaction Screening and Seismic Assessment

SPT data fed into the simplified procedure (Seed and Idriss, modified by Youd et al.) to assess liquefaction potential in saturated silts and sands lenses within the Regina clay sequence. We apply magnitude scaling for the Prairie seismic setting and report factor of safety against liquefaction at each sampled depth, along with post-liquefaction settlement estimates.

Applicable standards

ASTM D1586-18: Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils, CSA A23.3: Design of Concrete Structures — Foundation geotechnical input requirements, NBCC 2020, Division B, Part 4: Structural Design — Subsurface investigation provisions, Canadian Foundation Engineering Manual (CFEM), 4th Edition — SPT correlations for bearing capacity and settlement

Common questions

How much does an SPT investigation cost for a typical residential lot in Regina?

For a single-family residential lot inside city limits, with two boreholes to a depth of around 10 metres each, you are generally looking at CA$640 to CA$930 per borehole depending on access, drilling method, and whether we need to core through gravel lenses. That includes the field work, laboratory classification of the split spoon samples, and the geotechnical report with bearing capacity and settlement recommendations.

How deep do SPT boreholes need to go in Regina to satisfy the building code?

The NBCC does not prescribe a fixed depth — it requires that you investigate the soil that will actually be stressed by the foundation. In Regina, that means going through the entire glaciolacustrine clay sequence until you hit the glacial till or at least a depth where the stress increase from the footing drops below 10% of the in-situ effective stress. For a typical two-storey building on spread footings, that is usually between 12 and 18 metres, but we adjust based on the footing width and the loads.

What is the difference between the raw N-value and the N60 value reported on the log?

The raw N-value is the number of blows the hammer actually delivered in the field. The N60 corrects that number to a standard 60% hammer energy efficiency, because the actual energy delivered depends on the hammer type, the rod length, and whether the crew is using an automatic trip hammer or a rope-and-cathead setup. On our Regina jobs we use an automatic hammer with a known energy ratio, so the correction factor is measured, not assumed. You design with N60, not the raw number.

Can you do SPT testing in winter when the ground is frozen solid?

Yes — we work year-round in Regina. When the surface is frozen, we start the borehole with a core barrel or a solid-stem auger to get through the frozen crust, which can be over two metres thick in February. Once we are into unfrozen soil, we switch to the split spoon and run the SPT normally. The log notes will clearly mark the depth of the frozen zone so nobody misinterprets those refusal-like blow counts as a dense layer deeper down.