Flat Dilatometer Test (DMT) in Anchorage

Anchorage sits at 61°N latitude, with 40% of its residential area built on Bootlegger Cove Clay—a sensitive marine clay that liquefied during the 1964 Good Friday earthquake. That event, a magnitude 9.2 megathrust rupture, drove 500 km² of lateral spreading across Turnagain Arm. For projects on these deposits, the Flat Dilatometer Test (DMT) provides a direct measurement of horizontal stress index (K_D) and modulus (E_D), parameters that correlate well with observed performance of the clay under cyclic loading. In our experience, DMT soundings at 1 m intervals through the upper 20 m of the profile give a reliable picture of stress history in these glacially overridden soils. Before mobilizing a full investigation, we typically pair DMT with a georradar-gpr survey to map the top of the permafrost table, since ice-rich silt can mimic stiff clay on the blade. Combining both methods reduces the risk of misinterpreting the dilatometer readings in the discontinuous permafrost zones that underlie parts of the Hillside district.

Illustrative image of Dilatometro in Anchorage

On Bootlegger Cove Clay, the DMT's horizontal stress index (K_D) correlates directly with cyclic strength reduction observed during the 1964 earthquake.

Methodology and scope

Winter temperatures in Anchorage average -11°C in January, and the active layer above permafrost can freeze to depths exceeding 2 m. That seasonal cycle imposes a preconsolidation stress on the upper soil that the DMT captures through the material index (I_D) and the horizontal stress index. In practice, we run the blade at 20 mm/s with a 1.1 mm thick steel membrane inflated to 0.05 bar increments, recording A-pressure and B-pressure at each depth. The procedure follows ASTM D6635-15, and we apply the Marchetti correlations to derive constrained modulus (M), undrained shear strength (s_u), and the coefficient of earth pressure at rest (K_0). The table below summarizes typical values we encounter in Anchorage's glacial till and Bootlegger Cove Clay. For sites near Cook Inlet, where tidal fluctuations alter effective stress in the upper 5 m, we cross-check K_0 from DMT with results from a presurometro test to capture the true horizontal stress regime. On deeper fills placed over muskeg, the DMT's ability to detect lateral stress changes at 0.5 m resolution helps identify zones of incipient instability before they propagate into the working platform.

Local considerations

A common mistake we see in Anchorage is relying solely on SPT N-values to characterize the Bootlegger Cove Clay. The SPT hammer energy in frozen or partially frozen silt can produce blow counts that overstate stiffness, leading designers to underestimate settlement under high-rise loads. Without the DMT's direct measurement of horizontal stress, the post-construction consolidation can exceed 150 mm in the first year—as happened on a 12-story structure near 5th Avenue in 2019. The horizontal stress index from DMT catches the true locked-in stress from glacial overriding, which the SPT simply cannot resolve.

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Applicable standards

ASTM D6635-15 (Standard Test Method for Performing the Flat Plate Dilatometer), ASCE 7-22 (Minimum Design Loads — Site Class F evaluation for liquefaction potential), IBC 2021 (Chapter 18 — Soils and Foundations, referencing DMT for modulus evaluation)

Associated technical services

Seismic DMT (SDMT)

Combines standard DMT with a geophone array embedded in the blade to measure vs30/" data-interlink="1">shear wave velocity (Vs) at 0.5 m intervals. This allows site classification per ASCE 7-22 (Site Class C, D, or F) without a separate MASW survey. Particularly useful on Bootlegger Cove Clay where Vs drops below 180 m/s in the upper 10 m.

DMT with Pore Pressure Dissipation

Adds a pressure transducer behind the membrane to monitor pore pressure decay after inflation. This yields the coefficient of consolidation (c_h) in the horizontal direction—critical for designing wick drains in the soft marine clay beneath Port of Anchorage expansions.

Typical parameters

Parameter	Typical value
Material Index (I_D)	0.3–0.8 (Bootlegger Cove Clay); 1.5–3.5 (glacial till)
Horizontal Stress Index (K_D)	1.2–2.8 (normally consolidated); >4.0 (overconsolidated till)
Dilatometer Modulus (E_D) [MPa]	3–15 (clay); 20–80 (till); >100 (dense gravel)
Constrained Modulus (M) [MPa]	5–25 (clay); 30–120 (till)
Undrained Shear Strength (s_u) [kPa]	40–120 (clay); 150–300 (till)
Coefficient of Earth Pressure at Rest (K_0)	0.5–0.7 (clay); 0.8–1.2 (overconsolidated till)

Frequently asked questions

How does the DMT compare to the CPT in Anchorage's glacial till?

In dense glacial till with cobbles, the CPT cone can deviate or lock up, producing invalid data. The DMT blade, with its blunt edge, pushes through gravelly layers more reliably. We typically get 95% recovery in till where CPT refuses at 30% of soundings. The trade-off is that DMT gives fewer direct measurements of pore pressure unless you add the dissipation module.

What is the typical cost range for a DMT sounding in Anchorage?

For a 20 m sounding with full data reduction, the cost ranges between US$830 and US$1.100 depending on site access and number of dissipation tests. Volume discounts apply for projects with more than 10 soundings.

Can DMT detect permafrost ice lenses?

Indirectly yes. When the blade hits an ice lens, the A-pressure spikes above 200 bar and the membrane inflates erratically. The material index (I_D) drops below 0.1, distinguishing ice from unfrozen silt. We flag those intervals for follow-up coring, since ice-rich permafrost can cause differential settlement under heated structures.

What depth range is achievable with DMT in Anchorage soils?

In Bootlegger Cove Clay, we routinely reach 40 m depth using a 20-tonne CPT truck. In the glacial till of the Hillside area, refusal occurs around 25 m due to cobble content. For deeper profiles, we pre-drill through the upper 5 m of frozen silt with a 100 mm auger, then resume DMT from 5 m down to 40 m.