We were on a 10-story foundation job off South Tryon Street last year when the geotechnical report came back with conservative modulus values based on SPT alone. The structural engineer needed a better estimate of lateral stiffness for the mat foundation, so we ran a Flat Dilatometer Test (DMT) at three borehole locations. The DMT gave us a horizontal stress index and constrained modulus that allowed the design team to reduce the mat thickness by almost 20%. That kind of savings comes from understanding the in-situ stress state, which is exactly what the dilatometer provides in Charlotte’s deep residual soils. Before you lock in a foundation design, we recommend pairing DMT data with a MASW Vs30 survey to cross-check stiffness profiles across the site.
One DMT profile in Charlotte’s saprolite can replace three SPT borings for modulus estimation, saving time and reducing uncertainty in foundation design.
Scope of work
Area-specific notes
Designing in Charlotte without in-situ stress data carries real risk. We have seen projects where SPT N-values indicated medium-dense sand but the actual lateral earth pressure was significantly higher, leading to under-designed shoring walls that deflected beyond tolerance. A DMT profile captures K0 and OCR directly, which is critical for retaining wall design and braced excavations. In the I-277 corridor fill areas, the dilatometer can identify overconsolidated zones that behave differently under load, preventing differential settlement between adjacent footings. Skipping this test means relying on empirical correlations that may not hold for local Piedmont soils.
Standards used
ASTM D6635-15 – Standard Test Method for Performing the Flat Plate Dilatometer, IBC 2021 – Chapter 18, Soil Investigations and Foundations, FHWA-NHI-16-072 – Geotechnical Circular No. 6, Shallow Foundations
Linked services
DMT for Foundation Design
Constrained modulus and OCR profiles for mat, spread, and pile foundations. We correlate with local saprolite data to recommend bearing pressures and settlement estimates.
DMT for Retaining Walls & Excavations
K0 and lateral stiffness parameters for tieback walls, soldier piles, and braced cuts. Used directly in limit equilibrium and finite element models.
DMT for Liquefaction Screening
In sands, KD correlates with cyclic resistance ratio (CRR). We apply the Robertson 2012 method to flag liquefaction potential in Charlotte’s alluvial deposits near the Catawba River.
DMT for Pavement & Embankment Design
Modulus profiles for flexible pavement layer coefficients and embankment settlement analysis. Combined with CBR and resilient modulus testing for full characterization.
Typical parameters
Top questions
What does the Flat Dilatometer Test measure in Charlotte soils?
The DMT measures the horizontal stress and stiffness of the soil at discrete depths. From the test we derive the material index (ID), horizontal stress index (KD), and dilatometer modulus (ED). These are used to estimate K0, OCR, constrained modulus, and undrained shear strength. In Charlotte’s residual soils, KD values typically range from 2.5 to 6.0, indicating moderate to high overconsolidation from desiccation and weathering.
How does DMT compare to SPT or CPT for foundation design?
The DMT provides a direct measurement of in-situ lateral stress and stiffness, which SPT cannot do. While CPT gives continuous tip resistance and friction, the DMT adds the horizontal stress component that is critical for modulus and K0 estimates. For Charlotte projects, combining DMT with CPT gives the most complete picture: CPT for stratigraphy and DMT for stress state. The DMT is also faster than SPT since no soil sampling is needed.
How much does a Flat Dilatometer Test cost in Charlotte?
A typical DMT sounding in Charlotte ranges from US$870 to US$1,200 per test point, depending on depth (up to 40 m) and site access. This includes mobilization, field execution, data reduction, and a report with interpreted parameters. Volume discounts apply for multiple test points on the same site. The investment often pays for itself by reducing foundation concrete volumes or avoiding over-designed shoring systems.