Many construction teams in San Bernardino assume a standard SPT blow count is enough for foundation design on the deep alluvial fans that dominate the valley floor. That assumption can lead to overly conservative footings or, worse, unexpected differential settlement when the sand layers vary laterally. The Flat Dilatometer Test (DMT) captures the in-situ horizontal stress index and dilatometer modulus that SPT alone cannot resolve. We deploy this test to map lateral variability across the site before integrating results with a consolidation analysis for clayey interbeds common near the Santa Ana River. The DMT provides a continuous profile every 20 cm, revealing thin layers that would be missed by discrete sampling.
The DMT delivers a continuous log of soil stiffness at 20 cm intervals, something no discrete sampling method can match in these alluvial deposits.
Approach and scope
San Bernardino sits on Quaternary alluvial deposits with sand, gravel, and occasional silty clay lenses from the San Bernardino Mountains. Groundwater depth fluctuates between 10 and 30 meters depending on the season and proximity to the river. During a DMT, we advance the blade at 20 mm/s while recording the lift-off pressure (A-reading) and the 1.1 mm expansion pressure (B-reading). The equipment is internally calibrated and traceable to ASTM D6635-15. From these readings we derive the horizontal stress index (Kd), material index (Id), and dilatometer modulus (Ed). These parameters feed directly into settlement estimates for shallow foundations and help differentiate loose sand zones that require Improvement. For sites where collapsible soils are suspected, we pair the DMT with a study of collapsible soils to confirm the collapse potential under wetting. The test is fast — a 20-meter profile takes under 45 minutes — and produces a digital record that can be reviewed remotely.
Technical reference image — San Bernardino
Site-specific factors
ASCE 7-22 requires site-specific ground motion parameters for Seismic Design Categories D through F, which apply to most of San Bernardino. The DMT measures the small-strain shear modulus (G0) via the dilatometer modulus, which directly correlates to the shear wave velocity used in site class determination. Ignoring lateral stress variations in these alluvial fans can lead to incorrect site classification and underestimated seismic demand. We have seen cases where loose sand pockets detected only by DMT triggered the need for deep foundations or ground densification, saving the owner from a retrofit after construction.
Kd, Id, Ed, horizontal stress, modulus of subgrade reaction
Depth Range
Up to 40 meters (sandy soils), 25 meters (clayey soils)
Data Format
Digital .DMT file with raw A/B readings and interpreted curves
Related technical services
01
Single-Borehole DMT Profiling
A single dilatometer test to a depth of 30 meters, typically used for small to mid-size commercial projects. Includes A and B readings every 20 cm, calculation of Kd, Id, Ed, and a summary table of estimated modulus of subgrade reaction. Ideal for pavement design or shallow foundation sizing on uniform alluvial fans.
02
Multi-Borehole DMT Investigation
Three or more DMT soundings across the site to map lateral variability. We overlay results on a geotechnical cross-section to identify loose zones, stiff layers, and potential settlement differentials. Commonly used for multi-story buildings, warehouses, and bridge abutments where soil heterogeneity is a concern.
03
DMT with Seismic Module
Adds a seismic receiver at the blade to measure shear wave velocity (Vs) at each test depth. The Seismic DMT provides both stiffness and Vs in one sounding, eliminating the need for a separate MASW survey. Directly applicable to site class determination per ASCE 7-22 and liquefaction triggering analysis.
Relevant standards
ASTM D6635-15 – Standard Test Method for Performing the Flat Plate Dilatometer, ASCE 7-22 – Minimum Design Loads and Associated Criteria for Buildings, Chapter 20 (Site Classification), IBC 2021 – Section 1803 (Geotechnical Investigations)
Quick answers
What is the difference between DMT and SPT?
The SPT measures resistance to dynamic penetration and provides a blow count (N-value) that correlates to relative density and shear strength. The DMT measures horizontal stress and stiffness under quasi-static conditions, yielding a continuous profile of modulus, lateral stress index, and material index. The DMT is more sensitive to thin layers and provides data that directly feeds into settlement and modulus calculations, whereas SPT is better suited for liquefaction correlations and coarse gravels.
How much does a Flat Dilatometer Test cost in San Bernardino?
A single-borehole DMT to 20 meters typically ranges between US$920 and US$1,050, including mobilization within San Bernardino County, field execution, and a basic report. Multi-borehole investigations or projects requiring the seismic module may cost more depending on depth and number of soundings. Contact us for a firm quote based on your specific site conditions.
Can DMT be performed in gravelly soils common in San Bernardino?
Yes, but with limitations. The dilatometer blade can penetrate sand and fine gravel up to about 10 mm particle size. In cobble-rich zones near the mountain front, the blade may refuse or suffer damage. We typically review soil logs from nearby borings before scheduling DMT in those areas. If the site has significant cobbles, we recommend a combined approach with SPT and geophysics.
What parameters are obtained from a DMT?
From the A and B readings we compute the material index (Id), horizontal stress index (Kd), dilatometer modulus (Ed), and lateral stress ratio (sigma_h/sigma_v). These are used to estimate constrained modulus (M), undrained shear strength in clays, coefficient of lateral earth pressure at rest (K0), and the modulus of subgrade reaction (k). The Seismic DMT also provides shear wave velocity (Vs).
How deep can a DMT profile go in San Bernardino soils?
In sandy alluvium typical of the valley floor, 30 to 40 meters is achievable with a standard hydraulic rig. In clayey or silty interbeds, we typically reach 25 meters before penetration resistance exceeds the blade capacity. The maximum depth is ultimately limited by the rig's thrust capacity and the presence of dense gravel layers.
