Calgary’s rapid expansion since the 1950s oil boom pushed development onto glacio-lacustrine plains and river terraces with highly variable subsurface conditions. Before any foundation design begins, engineers need direct visual access to the soil profile. An exploratory test pit provides that access — a machine-excavated trench that exposes stratigraphy, groundwater seepage, and buried obstructions in real time. For shallow foundations on Calgary’s clayey tills and silty sands, this method often reveals critical details that boreholes miss, such as gravel lenses or perched water tables. When combined with a plate load test on the excavated base, the team obtains immediate deformation modulus values for footing design.
A single test pit can expose more stratigraphic detail than three boreholes in Calgary’s interbedded till and sand deposits.
Methodology and scope
Calgary’s near-surface geology consists of dense glacial till overlying sandstone and shale of the Paskapoo Formation. Exploratory test pits in this setting typically reach depths of 3 to 5 meters using a rubber-tracked excavator. The operator digs a trench about 1.2 meters wide, and the geotechnical engineer logs each layer according to CFEM — noting color, moisture, plasticity, and consistency. A key advantage of the test pit is the ability to perform in-situ density tests using the sand cone method on each distinct stratum. The team also collects bulk disturbed samples for laboratory classification and undisturbed block samples when cohesive tills are encountered. Depth to groundwater is recorded immediately after excavation, as Calgary’s water table can rise rapidly in spring snowmelt periods.
Technical reference image — Calgary
Local considerations
In Calgary, many sites underlain by glacial till contain isolated cobbles or boulders that can stall a test pit excavation. The operator must stop immediately if a large obstruction is struck — continuing risks damaging the excavator or creating an unstable cavity. Another common issue is sudden groundwater inflow from a confined sand lens, which can collapse the pit walls. The supervising engineer conducts a daily hazard assessment before entry, per Alberta OHS regulations. For deep pits exceeding 1.5 meters, a trench box or sloping benching is mandatory to prevent wall failure.
Field work + lab classification in 5–7 business days
Associated technical services
01
Residential Foundation Test Pits
Excavation and logging for single-family homes and duplexes. Focus on bearing capacity of till or sand layers at typical footing depth of 1.2–1.5 m. Includes in-situ density and recovery of samples for Atterberg limits and grain size.
02
Commercial & Industrial Test Pits
Deeper pits (up to 6 m) for strip footings, mat foundations, or retaining wall bases. Performed in compliance with ASTM D420 and Alberta OHS requirements. Groundwater monitoring over 24 hours is standard for dewatering design.
03
Utility & Pipeline Trench Investigations
Linear test pits along proposed utility routes to identify shallow bedrock, boulders, or high water table zones. Data supports trenchless technology feasibility and shoring system selection.
Applicable standards
CFEM (Standard Practice for Description and Identification of Soils), ASTM D420 (Standard Guide for Site Characterization for Engineering Design), Alberta OHS Code Part 24 (Excavations and Trenching), NBCC 2020 Division B, Part 4 (foundation design referencing subsurface conditions)
Frequently asked questions
How deep can an exploratory test pit go in Calgary?
In typical glacial till and sand deposits, the maximum safe depth is around 5 meters using a standard rubber-tracked excavator. Deeper pits require benching or trench boxes to maintain wall stability. The practical limit is usually set by groundwater inflow or the presence of large boulders that cannot be removed safely.
What does a test pit investigation cost for a Calgary residential lot?
For a standard single-family lot, the cost ranges between CA$640 and CA$1,050 depending on pit depth, number of pits, and whether laboratory testing is included. This covers mobilization, excavation, field logging, in-situ density tests, and basic classification. Additional lab work or groundwater monitoring adds to the total.
How do test pits compare to boreholes for Calgary sites?
Test pits provide direct visual observation of soil layers, which is a major advantage in Calgary’s variable tills where borehole samples can lose fine-grained lenses. However, boreholes can reach greater depths (10–30 m) and are required when deep foundations or bedrock investigations are needed. The choice depends on the project’s depth requirements and the level of stratigraphic detail needed.
