Benefits of the Dilatometer Test
- Determine accurate values for strength or compressibility of the soil strata
- Provides accurate parameters for P-y analyses for lateral loads on deep foundations
- Accurately measures the low strain shear wave velocity using the true interval method
- Accurately evaluates ground improvement by performing before and after DMT
To assure high quality control, all dilatometer tests are performed by a registered professional engineer. We have performed the deepest dilatometer test in the world at Calvert Cliffs Nuclear Power Plant at a depth of 398 feet. We were the organizers of the Second International Conference on the Flat Dilatometer Test (2006).
Dilatometer Test (DMT), ASTM D 6635: In 1975, Dr. Silvano Marchetti invented the Flat Dilatometer, consisting of sharpened blade with a circular membrane located on one side, to investigate H-pile behavior for lateral loads. He performed tests at ten well-documented research sites and developed empirical correlations with classical soil properties. In 1980, he published a classic paper presenting those correlations; most of which are routinely used today. (Marchetti, 1980) In 1981, Marchetti traveled to the United States on sabbatical and worked with Drs. John Schmertmann and David Crapps. While they were initially skeptical of Dr. Marchetti’s invention, they were convinced by the impressive speed and accuracy of the results.
Figure 1 shows a photograph of the stainless steel Dilatometer blade under a direct push rig. The blade, 15 mm thick and 96 mm wide in cross-section, is pushed into the soil at a constant rate of 2 cm/sec, preferably using a load cell to measure the penetration thrust as shown in Figure 2.
Generally the operator stops penetration at 20 cm depth intervals, records the thrust at the test depth using a load cell, and then inflates the membrane.
The surrounding soil usually collapses the 60-mm-diameter stainless steel membrane flush against the blade during the penetration. (In very weak soils, a vacuum must be applied prior to pushing.)
Electrical conductivity between the center of the membrane and the underlying body of the blade completes a circuit that activates a buzzer and a light on the dilatometer control unit. To run the test, the operator slowly inflates the membrane with nitrogen gas supplied from the control unit. When the membrane center moves away from the blade, the electrical continuity is lost and the light and buzzer go off. At that instant the operator reads the gas pressure at the control unit and records the membrane lift-off pressure as the “A‑pressure” on the data sheet. The operator then continues to inflate the membrane. When the membrane has inflated an additional 1.1 mm at its center, an electrical switch inside the blade reestablishes the electrical circuit and reactivates the buzzer and light, prompting the operator to record the corresponding gas pressure as the “B‑pressure”. When below the water table, the operator can slowly deflate the membrane, and record the water pressure that pushes the membrane back in contact with the blade as the “C‑pressure”. Nearly all of the correlations are based on the thrust, “A‑pressure” and “B-pressure”. The “C‑pressure” can be used to determine the groundwater table in clean sands and to determine the undrained shear strengths of soft clay (Lutenegger, 2006).
The dilatometer blade has a cross-sectional area of about 14 cm2 and can be pushed with a direct push rig into soil with an N60-value of about 45 blows per foot or with a heavy drill rig into soil with an N60-value of about 35 blows per foot. Tests can be successfully performed in all penetrable soils, including clay, silt, and sand. If the soil contains a significant amount of gravel, there may be point contacts against the membrane instead of a continuous medium, causing inaccurate results. Furthermore, the gravel will often tear a hole in the membrane.
DMT results have been correlated with the parameters that geotechnical engineers need the most — soil shear strength and deformation properties. The computer program for the dilatometer data reduction evaluates and outputs the following soil properties and parameters:
- Tangent vertical constrained modulus [M],
- Undrained shear strength for clays [cu],
- Drained friction angle for sands [Φ],
- Total unit weight of soil [γt],
- Coefficient of lateral earth pressure at rest [ko],
- Preconsolidation pressure [pc], and
- Over consolidation ratio [OCR].
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