On a recent mid-rise development in Calgary's Beltline district, the geotechnical team encountered glacial till with interbedded cobbles and variable silt layers. Standard penetration tests alone could not reliably capture the lateral stress history or the small-strain stiffness of these overconsolidated deposits. That is exactly where the Flat Dilatometer Test (DMT) becomes indispensable. By inserting a thin steel blade into the ground and measuring the pressure required to inflate a steel membrane against the soil, the DMT delivers direct readings of horizontal stress, modulus, and soil type index. For Calgary's complex glacial geology, this in-situ method provides a level of detail that complements data from ensayo CPT and conventional borings, especially when assessing settlement and excavation support in dense tills.
The DMT provides direct measurements of lateral stress and stiffness in overconsolidated glacial tills, reducing settlement uncertainty in Calgary's mid-rise projects.
Methodology and scope
Calgary experiences a semi-arid climate with deep seasonal frost penetration, which means the moisture regime in the upper soil profile fluctuates significantly from spring thaw to late summer. These changes affect the borehole stability and the repeatability of dilatometer readings if not accounted for in the testing sequence. The DMT procedure follows ASTM D6635 (also CFEM Ch 4) (also CFEM Ch 4) (also CFEM Ch 4) (also CFEM Ch 4), with readings taken every 20 cm to build a continuous profile of lateral stress index (KD), dilatometer modulus (ED), and material index (ID). In projects where the design relies on small-strain parameters, such as high-rise foundations or bridge abutments, the DMT data can be correlated to constrained modulus (M) and shear modulus (G0) using established empirical relationships. This test pairs well with asentamientos analysis to refine settlement predictions and with suelos-expansivos evaluations when clay-rich layers are present. The team also cross-references the DMT results with laboratory index tests to validate the soil behavior classification under local conditions.
Technical reference image — Calgary
Local considerations
The difference in soil response between Calgary's northwest residential areas underlain by lacustrine clays and the southeast industrial zones atop fluvial sands is dramatic. In the northwest, the DMT reveals high KD values and elevated lateral stresses from post-glacial desiccation and overconsolidation, meaning excavation support designs must account for locked-in horizontal pressures. Conversely, in the southeast valley where the Bow River deposited loose sands, low KD readings indicate potentially contractive behavior under shear, raising the risk of liquefaction or bearing capacity failure. Ignoring these lateral stress contrasts leads to either over-designed retaining walls or unsafe excavation shoring. A targeted DMT program across the site captures this variability and informs both the ensayo-spt correlation and the final foundation solution.
Continuous profiles of ID, KD, and ED at 20 cm intervals for bearing capacity and settlement analysis in tills and clays.
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Lateral Stress Assessment for Excavation Support
Focused DMT soundings near shoring walls and deep excavations to measure in-situ horizontal stresses and optimize tieback or soldier pile designs.
Applicable standards
ASTM D6635 (also CFEM Ch 4) (also CFEM Ch 4) (also CFEM Ch 4) (also CFEM Ch 4) Standard Test Method for Performing the Flat Plate Dilatometer, NBCC 2020 Division B, Part 4 – Geotechnical Resistance, CSA A23.3-19 Design of Concrete Structures (annex on lateral earth pressures)
Frequently asked questions
What is the difference between DMT and CPT for Calgary soils?
The CPT measures cone resistance and sleeve friction continuously but does not directly quantify lateral stress or small-strain stiffness. The DMT provides a direct measurement of horizontal stress (through KD) and the dilatometer modulus (ED), which correlates better to settlement behavior in overconsolidated tills typical of Calgary. Many geotechnical engineers use both methods together: CPT for stratigraphy and DMT for stress-strain parameters.
How deep can the Flat Dilatometer Test reach in Calgary's glacial till?
In most Calgary tills, the DMT can be advanced to depths of 30–40 m using a standard drill rig with a push system. However, the presence of cobbles or boulders at depth may limit penetration or cause blade damage. Pre-drilling through major obstructions is sometimes necessary. The practical maximum depth depends on the rig capacity and the site's specific cobble content.
What is the typical cost range for a DMT sounding in Calgary?
For a single DMT sounding to 20 m depth in Calgary, the cost typically ranges from CA$1.190 to CA$1.490, including mobilization within the city and a basic report. The final price may vary depending on the number of soundings, total depth, access conditions, and whether pre-drilling is required. A multi-sounding program with bulk discounting can reduce the per-sounding rate.
Which Calgary soil types are best suited for DMT interpretation?
The DMT performs best in fine-grained soils such as clays, silts, and sandy silts, where the membrane inflation pressure gives reliable readings. In Calgary's lacustrine clays and glacial tills, the correlations to constrained modulus and undrained shear strength are well established. The test is less reliable in coarse gravels or cobble-dominated layers, where the blade may not seal properly or the membrane can be damaged.
How long does a DMT sounding take on a typical Calgary site?
A single DMT sounding to 20 m depth, including setup, readings every 20 cm, and retrieval, usually takes 2 to 3 hours on site. If the soil contains frequent cobbles or requires pre-drilling, the time can increase to 4–5 hours. The data reduction and reporting take an additional 1–2 days, depending on the project's complexity and the number of soundings.
