The test procedure of the Flat Dilatometer Test (DMT) and the checks on the equipment are covered in detail in the following documents:
TC16 (2001) “The DMT in Soil Investigations”, A Report by the ISSMGE Committee TC16, 41 pp. (see in particular Section 7 of this document).
ASTM (http://www.astm.org/DATABASE.CART/HISTORICAL/D6635-01.htm) D6635-01 (2002) “Standard Test Method for Performing the Flat Plate Dilatometer”. Book of Standards Vol. 04.09, 15 pp.
Eurocode 7 (1997) “Geotechnical Design. Part 3 : Design assisted by field tests, Flat Dilatometer Test (DMT)”, 9 pp.
Another document illustrating in detail, with figures, methods and tools used for the checks is: DMT Course Notes (2001) by Marchetti and Monaco, Bali Insitu 2001, 77 pp.
Scope of this document is to provide a synthetic list of the required checks. It skips the detailed description of the methods and tools used for the checks, covered in the above documents.
CHECKS ON THE EQUIPMENT
- Membrane corrections : initial DA = 5 to 30 kPa, initial DB = 5 to 80 kPa. Frequency: before and after each sounding.
- Sharpness of electrical signal : the signal at off/on inversions must be clear and sharp.
- Airtightness of Blade. The blade submerged and pressurized at 0.5 MPa must not leak. Frequency: when the operator suspects a leak.
- Elevation of Sensing disc above the surrounding plane = 0.04-0.07 mm. Frequency : when the sensing disc is extracted and then reinserted. Tool : Tripod with dial gage.
- Elevation of Feeler above the sensing disc = 0.04-0.07 mm. Frequency : when the sensing disc is extracted and then reinserted. Tool : Tripod with dial gage.
- Quartz cylinder height = 3.90 ±01 mm. Frequency: unnecessary using original cylinders, if cylinder is not damaged.
- Sensing disk extraction force should be higher than the weight of the blade. Thus, if the sensing disk is lifted, the blade is lifted too, without falling. Frequency : when the sensing disk is extracted and then reinserted.
- Blade penetration edge should be sharp and free from severe denting or undulations.
- Coaxiality between blade and axis. Deviation of edge from axis should not exceed 1.5 mm. Frequency: once a year or when the operator suspects bending. Tool : L-square.
- Blade planarity. The “sag” between a 15 cm ruler placed against the face of the blade parallel to its long side and blade should not exceed 0.5 mm. . Frequency: once a year or when the operator suspects bending. Tool : 15 cm ruler.
- Blade wearing. With time, wearing makes the blades thinner. However the “pedestal” underneath the membrane not worn, being protected by the membrane. Hence the recess of the pedestal (initially 0.25 mm) decreases with time. Eventually the pedestal will be higher than the surrounding plane. If such height exceeds 0.10 mm, the blade must be considered worn out and discarded. Frequency: once a year or after intense testing in abrasive soils. Tool : Large tripod with dial gage.
- Control Unit Airtightness. The control unit pressurized at 40 bar, with all its valves closed, should not leak (pressure decrease unnoticeable). Frequency: when the operator suspects a leak.
- Control Unit Pressure Gages : The zero offset of each gage should not exceed ±15 bar. Difference in gage readings : when the low-pressure gage reads 6 bar, the high pressure gage must read in the range 5.80 to 6.20 bar. Since the gages are two, they constitute a check to each other. Therefore other gage calibrations are not required. Frequency: once a year or when the operator suspects a gage damage (e.g. a severe blow).
CHECKS ON TEST EXECUTION
- Time to A : »15 seconds (from stopping advancement of blade).
- Time from A to B : »
- Membrane corrections change: DA or DB before/after a sounding must not exceed 25 kPa.
- Time of deflation from B to C (when taken) : 45-60 seconds.
- There are other checks that it is wise to perform before going to a site. However, strictly, they are not “quality checks”, in that they concern defects making it impossible to take the readings. Example: If the wire inside the cable is interrupted, the operator cannot take the readings.
- The above tolerances for DA, DB and their variations are appropriate for the most critical case, i.e. when the soil is very soft. The more consistent is the soil, the more DA, DB are a small fraction of the readings. In such soils less strict tolerances on the DA, DB variations are acceptable.