4.4 Pressuremeter tests (PMT)
4.4.1 Objectives
(1) The objective of the pressuremeter test is to measure in-situ the deformation of soil and soft rock caused by the expansion of a cylindrical flexible membrane under pressure.
(2)P The test consists of inserting a probe containing a cylindrical flexible membrane into the ground either into a pre-formed borehole, or by self-boring or by full displacement pushing. Once at a predetermined depth the membrane is expanded under pressure and readings of pressure and expansion are recorded until a maximum expansion for the particular device is reached.
NOTE Expansion is measured from radial displacement, or calculated From volume change of the cylindrical membrane.
(3) The test should be used to derive strength and/or deformation parameters of the ground or specific pressuremeter parameters.
(4) The results may be used to derive stress-strain curves in fine soil and soft rock.
4.4.2 Specific requirements
(1)P When planning a test programme for a project the type of pressuremeter to be used shall be specified.
(2)P There are four different types of apparatus generally available, for which the corresponding standards shall be used:
- pre-bored pressuremeters (PBP), e.g. the flexible dilatometer test (FDT), according to EN ISO 22476-5;
- the Ménard pressuremeter (MPM), a specific form of the PBP, according to EN ISO 22476-4;
- the self-boring pressuremeter (SBP), according to EN ISO 22476-6;
- the full-displacement pressuremeter (FDP), according to EN ISO 22476-8.
NOTE The PBP and the MPM are lowered into a test hole created specifically for the pressuremeter test. The SBP is drilled into the ground using an integral cutting head at its lower end such that the probe replaces the material it removes thereby creating its own test hole. The FDP is usually pushed into the ground with an integral cone at its lower end, thereby creating its own test hole. The MPM may in some instances be pushed or driven into the ground. PBP, SBP and FDP probes may take a number of forms, in accordance with the type of installation and measuring systems.
(3) Two different basic test procedures may be used:
– a procedure to obtain a pressuremeter modulus, (Em),and limit pressure, (plm), that may be used in design procedures formulated for the Ménard pressuremeter;
and
– a procedure to obtain other stiffness and strength parameters.
(4)P The tests shall be carried out and reported in accordance with a test method that conforms to the requirements for the particular instrument type to be used, (see 4.4.2 (2)P).
(5)P Any deviations from the requirements given in the corresponding standard shall be justified and in particular their influence on the results shall be commented upon.
4.4.3 Evaluation of test results
(1)P If necessary, the applied pressure shall be corrected for membrane stiffness to obtain the true pressure applied to the cylindrical ground contact surface around the probe.
(2)P If a radial displacement type pressuremeter is used, the displacement readings shall be converted to cavity strain and, if testing weak rock, corrected for membrane compression and thinning.
(3)P If a volume displacement type pressuremeter is used (e.g. MPM), the volume reading shall be corrected for system expansion.
(4)P In addition to the requirements given in 4.2, the field and test reports according to EN ISO 22476-4, EN ISO 22476-5, EN ISO 22476-6 and EN ISO 22476-8 for the specific test type shall be used for the basis of any further evaluation.
(5) In addition to the plots required by the individual equipment test standard, the list of additional plots in Table 4.1 should be considered.
| Probe | Ground type | Abscissa | Ordinate |
| Radial displacement type | |||
| Self-bored, pushed in | All | Cavity strain for each arm | Applied pressure |
| Pre-bored | All | Cavity strain for each pair of arms | Applied pressure |
| Self bored | All | initial cavity strain for each arm |
Applied pressure |
| All | All | Cavity strain for unload-reload cycle for each arm | Applied pressure |
| All | Clay | Logarithm of cavity strain for each arm | Applied pressure |
| All | Sands | Natural logarithm of current cavity strain for each arm | Natural logarithm of effective applied pressure |
| Volume displacement type (except MPM) a | |||
| Pre-bored | All | Volume change | Applied pressure |
| Pre-bored | All | Rate of change of volume | Applied pressure |
| a For MPM tests, the pressure is plotted as abscissa and the volume change as ordinate. | |||
4.4.4 Use of test results and derived values
4.4.4.1 General criteria
(1)P If an indirect or analytical design method is used, the geotechnical parameters of shear strength and shear modulus shall be derived from the pressuremeter curve using methods relevant for (he particular test and equipment type.
(2)P If a direct or semi-empirical design method is used, all the features of the method shall be taken into account.
NOTE Direct foundation design procedures use directly the measurements from field tests, instead of conventional soil properties.
(3)P If, for instance, the semi-empirical method to determine the settlement of spread foundations from MPM results is used, only the modulus (EM) determined from Ménard pressuremeter results shall be applied in this particular method.
NOTE Examples of calculations of settlements can be found in E.2.
4.4.4.2 Bearing resistance of spread foundations
(1)P If a semi-empirical method is used, then all aspects relating to the method need to be followed, in particular the specification for the pressuremeter type used in establishing the method. EN ISO 22476-4 shall be followed.
NOTE 1 The semi-empirical method according to Ménard is given in EN 1997-1:2004, Annex E.
NOTE 2 An example of the calculation of the bearing resistance is given in E.1.
(2) If an analytical method is used, the strength of the soil may be determined using empirical and theoretical methods but only on the basis of local experience.
NOTE Examples of analytical methods arc given in EN 1997-1:2004, Annex D.
(3) The angle of shearing resistance (φ') may be determined from an SBP test in coarse soil by theoretical methods and from FDP and PBP tests using empirical correlations but only on the basis of local experience.
4.4.4.3 Settlement of spread foundations
(1) The settlement of spread foundations may be determined from MPM tests using a semi-empirical method,
NOTE An example of the calculation is given in E.2.
(2) If an analytical method is used, the stiffness of the soil may be determined using theoretical models to interpret the pressuremeter test but only on the basis of local experience.
NOTE Examples of analytical methods are given in EN 1997-1:2004, Annex F.
4.4.4.4 Pile bearing resistance
(1) The ultimate compressive resistance of piles may be derived directly from stress controlled tests.
NOTE An example of the calculation of the ultimate compressive resistance is given in E.3.
(2) When the ultimate compressive or tensile resistance of a pile is derived indirectly from pressuremeter test results, an analytical method may be applied to derive values of base and shaft resistance but only on the basis on local experience.
