9.5 Interpretation of the results

(1) Results of DMT tests can be interpreted using well established correlations to determine the subsoil stratigraphy, the deformation properties of cohesionless and cohesive soils, the in situ state of stress and the undrained shear strength of cohesive soils.

(2) The interpretation of the results of DMT tests requires a knowledge of the in situ pore water pressure u₀ and the effective vertical stress σ'_v0 prior to blade insertion. The value of u₀ at any test depth should be determined from reliable pore water pressure measurements. The value of σ'_v0 at any test depth should be estimated from the unit weight of the soil layers above that depth.

(3) When interpreting the results of DMT tests the values of p₀, p₁, u₀ and σ'_v0 should correspond consistently to the same test location and membrane depth.

(4) The soil pressure p₁ against the DMT membrane when its centre is expanded 1,10 mm should be determined using the following relationship:

p₁ = p_B – ΔP_B;avg – Z_m.

(5) The soil pressure p₀against the DMT membrane when its centre is flush with the blade should be determined with a linear backextrapolation from the soil pressure against the membrane at the two preset deflections, 0,05 mm and 1,10 mm, hence using the following relationship:

p₀ = 1,05 (P_A + ΔP_A;avg − Z_m) − 0,05p₁.

(6) The material index I_DMT, the horizontal stress index K_DMT and the dilatometer modulus E_DMT should be calculated using the following relationships:

— I_DMT = (p₁ – p₀)/(p₀ – u₀),

— K_DMT = (p₀ – u₀)/σ'_v0),

— E_DMT = 34,7(p₁ – p₀).

9.6 Reporting of the results

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

• rig and rod types;
• characteristics of systems used to advance the blade;
• predrilling depth and system to support the borehole if any;
• diameter and location of friction reducer if used;
• thrust applied to the push rods and at the top of the blade if measured;
• elevation of the groundwater table;
• procedures to calculate the pore pressure against the membrane at each test elevation;
• characteristics of the measuring system to obtain the in situ pore pressure when relevant;
• type and size of dilatometer blade and membrane;
• zero readings of pressure measurement devices;
• values of the ΔP_A- and ΔP_B- calibration-pressures measured before and after each dilatometer profiling or single test and corresponding average values;
• tabulated output of values of P_A- and P_B- pressure-readings;
• tabulated output of the calculated values of p₀- and p₁- pressure;
• any relevant observation of the operator such as incidents, equipment damage during testing, repairs and replacements, details not included in the above list which may affect the interpretation of test results.

9.7 Derived values of geotechnical parameters

9.7.1 Bearing capacity of spread foundations

(1)P When the bearing capacity of spread foundations is evaluated based on DMT results, an analytical method shall be used.

(2) When the sample analytical method of annex B in EC 7 Part 1 is used, the derived value of the undrained shear strength c_u, of non cemented clays, for which the DMT test results show I_DMT < 0,8, can be determined using the following relationship:

c_u = 0,22σ'_v0(0,5K_DMT)^1,25

or any other well documented relationship based on local experience.

9.7.2 Settlement of spread foundations

(1) When applying the adjusted elasticity method of annex D in ENV 1997-1 the one dimensional settlement of spread foundations may be calculated using values of the one dimensional tangent modulus E_oed determined from results of DHT tests as shown in annex H. In cohesive soils such procedure should be applied when the stress increase induced by the foundation load is less than the preconsolidation pressure.

9.7.3 Pile foundations

(1)P When the ultimate bearing resistance of piles is evaluated from DMT results an analytical calculation method shall be applied to derive the values of base and shaft resistance.