11.5 Interpretation of the results

(1) The results of a PLT are presented as applied contact pressure (p_u) versus settlement curves and an indication of the ultimate contact pressure (see figure 11.2).

(2) The ultimate contact pressure (p_u) from the PLT results may be taken as:

— the largest possible pressure p_u1, in sensitive clays or dense sands, (see figure 11.2);

— the pressure p_u2 at which the creep s₂ = s(t + Δt) − s(t) increases considerably (see figure 11.3);

— equal to the pressure p_u3 at a defined settlement e.g. equal to 15 % of the diameter or width of the plate.

11.6 Reporting of results

(1)P in addition to the requirements given in 2.6 the report shall include the following information:

• number of the plate;
• size of the plate;
• location;
• method of gaining access to plate test position, e.g. by borehole, test pit etc.;
• size/area of test pit, diameter of borehole etc.;
• elevation of the test plate;
• time (start and end of test);
• description of tested soil or rock;
• description of the test arrangements (reaction and loading systems and settlement measurement system);
• calibration data and checks of measuring equipment;
• rate of settlement during each load increment (loading programme);
• time and applied contact pressure dependent readings of settlements
• applied contact pressure versus settlement graphs including loading and unloading cycles;
• creep during each increment or loading cycle;
• time versus settlement graphs from load increments of interest;
• time versus temperature (at the test plate) readings;
• any relevant observations of the operator which may affect the interpretation of test results.

11.7 Derived values of geotechnical parameters

(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) , the layer should have a thickness beneath the plate of at least 2 times the width or diameter of the plate.

(3) Results of PLT may only be used for direct design 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 2 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 the sample analytical method for bearing capacity of annex B of ENV 1997-1 is used, the undrained shear strength c_u may be derived from a PLT conducted at a constant rate of penetration.

(5) In annex I.1 an example of the assessment of the derived value of c_u is given.

(6) If the sample adjusted elasticity method for settlement evaluation of annex D of ENV 1997-1 is used, the Young's modulus E_m may be derived from the plate settlement modulus E_PLT, based on established experience.

(7) In annex I.2 the determination of E_PLT is shown.

(8) The coefficient of subgrade reaction k_s for evaluating settlements may be derived from results of an incremental loading test.

(9) In annex I.3 an example is shown for the calculation of k_s.

(10) For direct design, the results of PLT may be transferred directly to the foundation problem without using any geotechnical parameters.

(11) In order to determine the settlement behaviour of strip foundations from PLT results directly, the plate size is recommended to be of a certain ratio to the footing width.

Recommended plate sizes are tabulated below for different footings:

(12) In case plate sizes smaller than defined in 11.7 (11) have to be used, the settlements of the footing in sand may be derived according to annex I.4.