Geotechnical Testing in the 21st Century – The Dilatometer

Introducing our Guest Article Series on Advanced Geotechnical Testing

Inertia can sometimes prevent progress.  In Ontario, there is a massive historical database and a lot of momentum that supports the use of the Standard Penetration Test (SPT) as a benchmark for geotechnical investigations.  The SPT test has changed little since its invention in 1902 and remains at the core of standard industry practice, despite evidence to suggest that it is not suitable for all soil conditions.  Our three-part series on Advanced Geotechnical Testing will provide you with insight into alternative testing methods and how they can deliver meaningful value on your projects.
– Mark Tigchelaar, President, P.Eng.

 

Geotechnical Testing in the 21st Century – The Dilatometer

To assess the characteristics of a site for construction, geotechnical investigations must be carried out to determine that site’s response to any change in external conditions. This change may involve applying load or modifying the surface profile. Regardless of what changes are made, the response on? the ground must be determined prior to effecting that change.

Typically, a geotechnical investigation involves sampling the overburden soils and carrying out appropriate programs of both in situ testing and laboratory testing. Commonly, the fieldwork section of the geotechnical study involves sampling by the standard penetration test (SPT) method, which includes determining the penetration index value (N-value) of the soils. The N-value is commonly used to estimate the condition of the soils subject to test. This test methodology is characteristically unsophisticated and imprecise, however a very large body of data supports its use and interpretation. Shear strength testing of cohesive (clayey) soil is carried out by shear vane testing, which measures the torque required to cause shear failure in a cylindrical profile of soil. The in situ test data are supplemented by a variety of tests performed in the laboratory on samples transported from the site. It should be recognized that laboratory tests are carried out on samples which have experienced disturbance during sampling and have additionally experienced stress changes from their in situ condition.

There are significant advantages to project design in making a more precise determination of soil properties by utilizing more sophisticated in situ testing methods. There are methods that cause substantially less stress change in the tested soil, and measure characteristics such as resistance to penetration or force required to cause deformation providing much-enhanced data for soil property determination. Tools available for this work include the static cone penetrometer (CPT), the pressure meter (PMT) and the Marchetti Flat Plate Dilatometer (DMT). Each has advantages and disadvantages when compared to the others, but all involve carrying out precise measurements which eliminates a large measure of the guesswork involved in cruder test methods.  The following discussion focuses on the DMT and its advantages:

Invented by Dr. Silvano Marchetti in 1975, the Dilatometer (DMT) consists of a thin blade shape probe which incorporates a pressure cell. The shape and aspect ratio of the probe results in a greatly reduced disturbance to the enclosing soil as the probe inserted in the ground. In turn, this results in a more accurate assessment of the material properties. The test probe is advanced into the ground using driving rods and its downward progress is arrested at 200 mm depth increments, at which test points the pressure cell is activated to record:

  • Enclosing pressure;
  • Force required to cause deformation in the enclosing soil; and,
  • Residual pressure after deflation of the pressure cell.

The traditional geotechnical parameters are inferred from the recorded measurements – parameters such as angle of internal friction (granular soils), undrained shear strength (cohesive soils), constrained (deformation) modulus, overconsolidation ratio and unit weight. An interpretation of probable soil behaviour is made based on the recorded measurements. The data developed from the test are particularly valuable with regard to its determination of constrained modulus, which is used in making assessments of settlement of buildings and deformation of the soils under an applied load. The nearly-continuous testing of the materials enables an interpretation of the soil profile with regard to deformation characteristics based on a set of soil units and sub-units. Layer definition can be made on the basis of predicted deformation behaviour.

The DMT enables design engineers to produce consistently high-quality estimates of ground response to site and load changes. Soil layers which are more prone to consolidation can be identified by the test method, as can thin, weak soil layers.  Tools such as the DMT are well able to quantify the beneficial effects of ground improvement work. More sophisticated test methods such as DMT are currently underutilized and underappreciated in Canada, particularly in Ontario. Carrying out such testing often results in significant cost savings being made in future construction projects, and those savings can be attained with minimal additional cost in data acquisition.

Colin Alston is an engineer with more than 50 years of experience including work for both contractors and consultants.  Colin has a particular interest in in situ testing techniques, working with a mechanical CPT in early 1970’s.  He has been working with the DMT since 1980.  Colin can be reached at  colin_agc@bellnet.ca

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