The subsurface profile of Baltimore is dominated by the Coastal Plain physiographic province, with deep deposits of unconsolidated sands, silts, and clays interbedded with occasional gravel layers. The water table sits within 3-5 m of the surface in many areas, particularly near the harbor and along the Patapsco River. For projects requiring deformation modulus and lateral stress ratio at depth, the Flat Dilatometer Test (DMT) offers a reliable alternative to pressuremeter testing. The DMT blade, advanced at 20 mm/s, measures the lift-off pressure (p0) and the expansion pressure to 1.1 mm (p1) in 20-second intervals. These readings feed directly into the calculation of the horizontal stress index (KD) and the dilatometer modulus (ED), which are then correlated to the constrained modulus (M) and the coefficient of earth pressure at rest (K0). In Baltimore's variable alluvial sequence, a DMT campaign can be complemented with a study of soil mechanics to cross-validate consolidation parameters, and a resistivity survey to map clay versus sand transitions across a site.
In Baltimore's Coastal Plain, the DMT provides continuous K0 and OCR profiles that SPT intervals cannot resolve, directly reducing foundation design uncertainty.
Method and coverage
- Zeroing the membrane at atmospheric pressure before each 20-second cycle.
- Applying gas pressure until membrane lift-off (p0) and then to 1.1 mm expansion (p1).
- Recording the closing pressure when the membrane retracts (p2) for pore pressure correction.
Regional considerations
A common mistake among contractors working in Baltimore's Inner Harbor redevelopment is relying solely on SPT N-values to estimate settlement in soft clays. The DMT reveals that the constrained modulus can vary by a factor of three within a single 5 m stratum — information that SPT cannot capture. Ignoring this variability leads to overestimating bearing capacity and underestimating long-term consolidation settlements. For structures requiring tight differential settlement tolerances, such as hospitals or transit stations, the DMT's continuous profile is essential to avoid costly post-construction repairs.
Process video
Standards that apply
ASTM D6635-15 (Standard Test Method for Performing the Flat Plate Dilatometer), AASHTO T-256-16 (Standard Method of Test for Dilatometer Testing), IBC 2021 Section 1803 (Geotechnical Investigations)
Related services
DMT with Pore Pressure Measurement (DMTu)
A modified blade with a porous element behind the membrane records the pore pressure decay after the expansion cycle. This allows estimation of the coefficient of consolidation (ch) and the horizontal hydraulic conductivity (kh) in saturated clays. Particularly useful for Baltimore's deeper estuarine deposits where drainage conditions control long-term settlement rates.
Seismic DMT (SDMT)
A seismic module attached above the dilatometer blade measures the vs30/" data-interlink="1">shear wave velocity (Vs) at each test depth. The Vs profile is used to classify the site according to IBC 2021 Site Class (A through F) and to calculate the small-strain shear modulus (G0). In Baltimore's coastal plain, the SDMT provides the Vs data needed to evaluate liquefaction susceptibility under the NCEER (Youd-Idriss 2001) method.
Typical parameters
Common questions
What is the main advantage of DMT over CPT in Baltimore's clays?
The DMT measures the horizontal stress index (KD) and the dilatometer modulus (ED), which directly yield the coefficient of earth pressure at rest (K0) and the overconsolidation ratio (OCR). In Baltimore's soft estuarine clays, these parameters are essential for accurate settlement analysis — something the CPT alone cannot provide without empirical correlations.
How deep can the DMT blade reach in Baltimore without predrilling?
Using a 200 kN CPT rig, the DMT blade can be advanced to 30-40 m in Baltimore's unconsolidated sands and clays. In areas with dense gravel layers near the surface, a pilot hole may be required, but in most of the Coastal Plain profile, continuous push is feasible. The maximum depth is typically limited by the thrust capacity of the rig, not by the blade itself.
What are the typical DMT parameters for the Patapsco Formation?
In the Patapsco Formation underlying much of Baltimore, the Material Index (ID) ranges from 0.5 to 1.2, indicating silty sand to clayey silt. The horizontal stress index (KD) varies between 2.0 and 4.5, suggesting moderate overconsolidation from past surcharge. The constrained modulus (M) typically falls between 15 and 60 MPa, which corresponds to a medium-stiff to stiff deposit.
How does the DMT help with liquefaction assessment in Baltimore?
The DMT provides the horizontal stress index (KD), which is used in the method proposed by Monaco and Marchetti (2007) to evaluate cyclic resistance ratio (CRR). Combined with the vs30/" data-interlink="1">shear wave velocity from a Seismic DMT, the site can be classified for liquefaction susceptibility under IBC 2021. This is particularly relevant in Baltimore's younger Holocene alluvium along the harbor and the Patapsco River.
What is the typical cost range for a DMT investigation in Baltimore?
For a standard project in the Baltimore metropolitan area, the cost per DMT sounding ranges between US$810 and US$1,110, depending on total depth, number of soundings, and access conditions. This includes mobilization, field testing, data reduction, and a summary report with interpreted parameters. Large-volume projects (more than 10 soundings) often receive a reduced per-point rate.