Soils under the Memorial area and those near the Houston Ship Channel tell two very different stories. In Memorial, the Beaumont clay formation often presents stiff desiccated crust with high lateral stress, while along the Ship Channel soft alluvial deposits dominate. The Flat Dilatometer Test (DMT) captures these contrasts by measuring the horizontal stress index and material index at every 20 cm, providing a continuous profile that a standard SPT boring would miss. This makes the DMT especially useful for projects transitioning between these geological settings. When combined with other in-situ methods like the ensayo CPT for continuous cone resistance and pore pressure readings, the DMT adds a layer of lateral stress data that refines foundation design parameters for Houston's challenging clay sequences.
The DMT's horizontal stress index in Beaumont clay often exceeds 4, signaling high lateral pressures that directly impact retaining wall and deep foundation design.
Methodology and scope
Houston's geology is dominated by overconsolidated Beaumont clay, a high-plasticity formation that can reach depths of 30 meters. The Flat Dilatometer Test in Houston typically records a material index between 0.1 and 1.2 in the upper clay layers, indicating silty clay to clay behavior, while the horizontal stress index often ranges from 2 to 6, reflecting significant locked-in lateral stresses. These values directly feed into the constrained modulus (M) for settlement analysis and the undrained shear strength (Su) for temporary excavation stability. The DMT blade, with its 95 mm width and 14° apex angle, is pushed at a rate of 20 mm/s per ASTM D6635, and the A, B, and C readings are corrected for membrane stiffness using the calibration data. The test is particularly effective for establishing the coefficient of consolidation (ch) in low-permeability clays, which is critical for evaluating asentamientos over time in Houston's compressible soils.
Technical reference image — Houston
Local considerations
With over 2.3 million residents and a history of hurricanes like Harvey (2017), Houston faces compounded geotechnical risks. The expansive Beaumont clay can undergo volumetric changes exceeding 8%, leading to slab heave and foundation distress if lateral stresses are underestimated. In zones near Addicks and Barker reservoirs, seasonal water table fluctuations further complicate DMT interpretation. The Flat Dilatometer Test helps quantify these risks by providing direct measurements of KD and ID, which correlate with the overconsolidation ratio (OCR) and the coefficient of earth pressure at rest (K0). Ignoring the lateral stress component in Houston clay often results in under-designed deep foundations or excessive slab movements in residential and commercial structures.
Full-profile Flat Dilatometer Test pushed at 20 cm intervals to depths up to 40 m. Includes membrane calibration, real-time data logging, and A/B/C reading corrections per ASTM D6635.
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Lateral Stress and K0 Profiling
Determination of the coefficient of earth pressure at rest (K0) from KD and ID correlations. Essential for retaining wall design, braced excavations, and slope stability in Houston clay.
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Consolidation and Settlement Parameters
Evaluation of constrained modulus (M) and coefficient of consolidation (ch) for primary and secondary settlement analysis in compressible clay layers, calibrated with oedometer tests.
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Seismic DMT (SDMT) Integration
Combines the standard DMT with a seismic module for vs30/" data-interlink="1">shear wave velocity (Vs) measurement. Provides small-strain stiffness (G0) and site class per ASCE 7, critical for seismic design in Houston.
Applicable standards
ASTM D6635-15 (Standard Test Method for Performing the Flat Plate Dilatometer), IBC 2021 (Chapter 18: Soils and Foundations, Site Class Determination), FHWA-NHI-16-072 (Drilled Shafts Manual, lateral load analysis using DMT)
Frequently asked questions
How does the DMT compare to the CPT for characterizing Houston clay?
The DMT provides direct measurements of lateral stress (KD) and material index (ID), which are not obtained from the CPT. In Beaumont clay, the DMT's KD values correlate strongly with the overconsolidation ratio (OCR), while the CPT cone resistance (qc) is more sensitive to soil density and grain size. For projects involving lateral earth pressures or consolidation analysis, the DMT adds unique value that the CPT alone cannot deliver.
At what stage of a project should the DMT be performed in Houston?
Ideally, the DMT is executed during the preliminary geotechnical investigation, after a few borings establish the soil stratigraphy but before detailed foundation design. In Houston, we recommend running DMT soundings alongside SPT borings to capture lateral stress profiles early. This allows the structural engineer to incorporate K0 values into the design of basements, retaining walls, or mat foundations from the start, avoiding costly redesigns later.
What is the typical depth range and cost of a DMT sounding in Houston?
A standard DMT sounding reaches depths between 15 and 30 meters, limited by the thrust capacity of the drill rig and the stiffness of the Beaumont clay. The cost for a single DMT sounding in Houston typically ranges between US$940 and US$960, including mobilization within the metro area, field execution, and a preliminary report with KD, ID, and M profiles. The final price varies with depth, number of soundings, and access conditions.
Location and service area
We serve projects across Houston and its metropolitan area.
