Dilatometer test

Chapter 3

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].

In-Situ Soil Testing, L.C. specializes in dilatometer tests to support geotechnical engineering and ground improvement firms. As engineers, we know that one of the most important challenges the geotechnical engineer faces is determining accurate values for the strength or compressibility of the soil strata at the project site. The weaker the soils are, the more critical they are to the geotechnical engineering design. We generally perform dilatometer tests at 20 centimeter (8 inch) depth intervals. From the high quality test data at close depth intervals, the soil profile and the soils’ strength and deformation properties can be accurately defined. By having sufficiently high quality test data, your engineers can confidently perform design computations and safely optimize the design. Additionally, the data can be analyzed using probablistic methods.

The accuracy of settlement computations (Schmertmann, 1986 and Monaco, 2006) based on dilatometer test data has been demonstrated by many researchers, as shown in Figure 3. From a data base of various projects in a wide variety of soil types,

the average ratio of predicted settlement from dilatometer analysis versus actual measured long-term settlement is 1.07 with a standard deviation of 0.22. We provide our clients with an Excel spreadsheet template that is used to compute settlement beneath a spread footing from dilatometer test data.

Dr. Silvano Marchetti originally developed the dilatometer to predict the lateral load capacity of piles. Because the DMT tests the soil horizontally, it is an excellent method to evaluate lateral capacity (Marchetti, et al., 1991 and Robertson, et al., 1989). The engineer can determine accurate P-y curves and continuous P-y profiles from those methods and use them with numerical computer programs such as LPILE and COM624. (Figure 4)

For ground improvement projects, we recommend performing dilatometer tests before, during and at completion of the improvement. By performing tests during the improvement phase, the amount of improvement achieved can be evaluated.

In cohesionless soils, ground improvement techniques often both increase lateral stresses and compact the soil. These changes lead to both a greater friction angle and increased stiffness as any excess pore pressures rapidly dissipate.

They also may encourage an “ageing” process that further increases the shear strength and stiffness. The amount of improvement that occurs depends on the dynamic effort and the distance away from the dynamic source. The improved soils will be fairly heterogeneous in both the vertical and horizontal directions. A large number of tests are needed to confirm that the soils have been adequately improved at all desired locations.

In-situ tests with high shear strain and disturbance effects measure ground improvement poorly because they destroy the improvement during the test. Because the DMT accurately measures both the soil’s deformation modulus and the at rest lateral pressure with minimal ground disturbance, they provide an excellent choice to determine whether sufficient ground improvement has been performed (see Figure 5). As documented at the St. Johns River Power Plant near Jacksonville, Florida, the dilatometer M values were found to be more accurate in evaluating soil improvement than relative density correlations based on electronic cone qc values. (Schmertmann, et al., 1986).

At the Second International Conference on the Flat Dilatometer in April 2006, the true interval seismic test was unveiled. The geophones are spaced exactly 0.50 meters apart in a module located directly above the blade. When a plate is struck horizontally at the ground surface, a shear wave is sent through the soil and received at the upper geophone first and later at the lower geophone. Both waves are recorded, digitally processed, and transmitted serially through the single wire DMT cable to the computer at the surface. The second wave is shifted to the left by a delta time until it is superimposed on the first wave. The shear wave velocity is easily computed as the difference in the shear wave travel distances between the upper and lower geophones by this computed delta time. Photo 6 shows the seismic module, control unit and computer and Photo 7 shows the shear waves before and after superimposition.