4.10 Flat dilatometer test (DMT)
4.10.1 Objectives
(1) The objectives of the flat dilatometer test are the determination of the in-situ strength and deformation properties of soil by expanding a thin circular steel membrane mounted flush on one face of a blade-shaped steel probe inserted vertically into the ground.
(2)P The test consists of measuring the pressures when the membrane is flush with the blade and just begins to move and when the displacement in the centre of the membrane reaches 1,10 mm into the soil. The test shall be performed at selected depths or in a semi-continuous manner.
(3) The results of DMT tests may be used to obtain information on soil stratigraphy, in-situ state of stress, deformation properties and shear strength.
(4) The DMT test should primarily be used in clays, silts and sands where particles are small compared to (he size of the membrane,
NOTE Further information on a procedure, presentation and evaluation of the DMT can be found in CEN ISO/TS 22476-11, (see X.3.7).
4.10.2 Specific requirements
(1) The tests should be carried out and reported in accordance with a recognised method.
(2)P Any deviations from the requirements given in the method referred to in (1) shall be justified and in particular their influence on the results shall be commented upon.
NOTE Further information on a procedure, presentation and evaluation of the DMT can be found in CEN ISO/TS 22476-11
4.10.3 Evaluation of test results
(1)P The requirements given in 4.2, shall be followed for the evaluation of the test results.
(2) In addition the field and test reports, according to the method referred to in 4.10.2 (1), should be used for evaluation purposes.
NOTE Further information on a procedure, presentation and evaluation of the DMT can be found in CEN ISO/TS 22476-11
4.10.4 Use of test results and derived values
4.10.4.1 Bearing resistance and settlement of spread foundations
(1)P If the bearing resistance of spread foundations is derived from DMT results, an analytical design method shall be used.
(2) If an analytical method is used, the derived value of the undrained shear strength (cu) of non-cemented clays, for which the DMT test results show material index IDMT < 0,8, may be determined using the following relationship:
where
KDMT is the horizontal stress index or any other well documented relationship based on local experience.
NOTE Examples of analytical methods are given in EN 1997-1:2004, Annex D.
(3) If applying an adjusted elasticity method, the one-dimensional settlement of spread foundations may be calculated using values of the one-dimensional tangent modulus (Eeod) determined from results of DMT tests. In fine soil, such procedures should be applied only when the sum of the effective overburden pressure and the stress increase induced by the foundation load is less than the pre-consolidation pressure.
NOTE 1 Such an adjusted elasticity method is given in EN 1997-1:2004, Annex F.
NOTE 2 An example of such a determination of settlements is shown in Annex J.
4.10.4.2 Pile bearing resistance
(1)P If the ultimate compressive or tensile resistance of piles is derived from DMT results, an analytical calculation method shall be applied to derive the values of base and shaft resistance,
4.11 Plate loading test (PLT)
4.11.1 Objectives
(1) The objective of the plate loading test is the determination of the vertical deformation and strength properties of soil and rock masses in-situ by recording the load and the corresponding settlement when a rigid plate modelling a foundation is loading the ground.
(2)P The plate loading test shall be carried out on a thoroughly levelled and undisturbed surface either at ground level or on the bottom of an excavation at a certain depth or the bottom of a large diameter borehole, an exploration shaft or gallery.
(3) The test is applied in all soils, fills and rock but normally should not be used for very soft fine soil.
4.11.2 Specific requirements
(1)P The test shall be carried out and reported in accordance with EN ISO 22476-13.
(2)P Any deviation from the requirements given in EN ISO 22476-13 shall be justified and in particular its influence on the results shall be commented upon.
NOTE Deviations exist, for example with respect to plate size and test procedure (incremental loading, constant rate of deformation).
4.11.3 Evaluation of test results
(1)P In addition to the requirements given in 4.2, the field and the test reports according to EN ISO 22476-13 shall be used for evaluation purposes.
4.11.4 Use of test results and derived values
(1) The results of a PLT may be used to predict the behaviour of spread foundations.
(2) For deriving geotechnical parameters of a homogeneous layer (for use in indirect design methods), the layer should have a thickness beneath the plate of at least two times the width or diameter of the plate.
(3) Results of a PLT may only be used for direct design methods if:
– the size of the plate has been chosen considering the width of the planned spread foundation (in which case the observations are transformed directly);
– a homogeneous layer up to two times the width of the planned spread foundation exists (in which case the results of smaller sized plates - not considering the planned foundation width - are used to transform the results on an empirical basis to the actual foundation size).
(4) If an analytical design method for bearing resistance is used, the undrained shear strength (cu) may be derived from a PLT conducted at a constant rate of penetration, sufficiently fast to practically preclude any drainage.
NOTE 1 Examples of analytical design methods for bearing resistance are given in EN 1997-1:2004, Annex D.
NOTE 2 An example of a relationship used for deriving the value of cu is given in K.1.
(5) If an adjusted elasticity method for settlement evaluation is used, the Young's modulus of elasticity (E) may be derived from the plate settlement modulus (EPLT),based on established experience.
NOTE 1 Such an adjusted elasticity method fur settlement evaluation is given in EN 1997-1:2004, Annex F.
NOTE 2 The determination of EPLT is shown in K.2.
(6) The coefficient of sub-grade reaction (ks) for evaluating deformations may be derived from results of an incremental loading test.
NOTE An example of the calculation of ks, is given in K.3.
(7) For direct design, the results of PLT may be transferred directly to the foundation problem without using any geotechnical parameters.
(8) Settlements of footings in sand may be derived from PLT results.
NOTE An example is given in K.4.
