5.9 Dissipation test

If the drainage and/or consolidation characteristics of the soil are to be evaluated, dissipation tests may be carried out at pre-selected depths in the deposit. In a dissipation test, the pore pressure decay is obtained by recording the values of pore pressure with time. In fine, low permeability soils, the pore pressure record is used to evaluate the coefficient of consolidation.

NOTE 1 A dissipation test may be used to estimate the in situ pore pressure.

Pore pressure and cone resistance shall be measured with time. It is particularly important to take frequent readings at the beginning of the dissipation test. The dissipation shall be performed with the rods unclamped.

If the weight of the equipment is greater than the total resistance of the soil, the dissipation test may be performed with clamped rods. If the rods are clamped during the dissipation test, this shall be recorded as a deviation from this part of ISO 22476 (7.2.1.d).

NOTE 2 The required depth and minimum duration of a dissipation test depends on the soil conditions and the purpose of the measurement. A maximum duration is also a common reference condition for avoiding inappropriately long interruptions.

The logging frequency should be at least 1 Hz for the first minute of the dissipation test, but may be halved for every log (time) cycle thereafter.

The duration of the dissipation test should normally correspond to at least t₅₀, the time needed for 50 % pore pressure dissipation (u_t ≤ u₀ + 0,5 × Δu_i), since t₅₀ is the time used in most interpretation methods.

If u₀ is not known, a cautious estimate of the value should be made.

NOTE 3 Variation in cone resistance is unavoidable in practice and will depend on factors such as type of equipment and soil conditions.

5.10 Test completion

The penetration of the cone penetrometer and the push rods shall be terminated when

• the required penetration length has been reached, or
• the agreed maximum thrust or maximum capacity of the measuring system has been reached. Possible damage to the equipment can also be a valid reason to end the test.

The reference readings of the measured parameters shall be recorded after extraction of the cone penetrometer from the soil and, if necessary, after cleaning of the cone penetrometer. If the zero drift of the measured parameters is larger than the allowable minimum accuracy according to the required application class of Table 2, then the results should be neglected, or the test can be assigned to a lower class. After completion of the test, inspect the cone penetrometer and note any excessive wear or damage.

NOTE The zero drift determined from zero load output before test and after cleaning is a measure of the correct functioning of the equipment and is used to evaluate if the requirements of Table 2 can be fulfilled. The zero load outputs from the uncleaned cone are important for the interpretation of test results.

Measured parameters can be corrected for zero drift, if appropriate.

5.11 Equipment checks and calibrations

The overall equipment shall undergo regular maintenance, checking and calibration procedures as specified in Annex A. The calibration interval shall be such that the accuracy required for the application class can be verified.

NOTE Verification of the required accuracy can be based on the track record.

5.12 Safety requirements

National safety regulations shall be followed; for instance for:

• personal health and safety equipment;
• clean air if working in confined spaces;
• ensuring the safety of the equipment.

6 Test results

6.1 Measured parameters

The following parameters shall be determined:

f_s measured sleeve friction;

l penetration length;

q_c measured cone resistance;

u pore pressure, one or more of the following (for ТЕ2):

 u₁ pore pressure in the face of the cone,

 u₂ pore pressure at the cylindrical extension of the cone,

 u₃ pore pressure measured above the friction sleeve.

α measured total angle between the vertical axis and the axis of the cone penetrometer.

6.2 Correction of parameters

Recorded values that are not representative due to penetration interruption shall be ignored.

NOTE 1 The surrounding water pressure will influence the measured cone resistance and sleeve friction. This is explained by the effect of the water pressure in the gaps between the cone and the friction sleeve, and in the gap above the friction sleeve.

When water pressure is measured using a filter element at the cylindrical extension of the cone (u₂), measured cone resistance shall be corrected by using Formula (6):

where

q_t is the corrected cone resistance, in MPa;

q_c is the measured cone resistance, in MPa;

pressure in the gap between the cone and the sleeve}, in MPa:

a is the net area ratio:

a = A_n/A_c where

A_c is the cross-sectional projected area of the cone (see Figure 6), in mm²;

A_n is the cross-sectional area of load cell or shaft (see Figure 6), in mm².

Key

• 1 cross sectional area (top) A_st
• 2 friction sleeve surface area A_s
• 3 cross sectional area (bottom) A_sb
• 4 cross sectional area A_c

Measurements should only be corrected if u₂ is measured.

NOTE 2 u₂ can be estimated from u₁ or u₃ by empirical relations, to obtain an approximately corrected cone resistance. In soft clays and silts, the correction can get substantial, and u₂ should preferably be measured and corrections carried out if the cone resistance recordings are used for interpretation of mechanical parameters.

The area ratio cannot be determined from geometrical considerations alone, but should be determined by tests in a pressure chamber or similar.

NOTE 3 The measured sleeve friction is influenced by surrounding water pressure. Since it is not standard practice to measure the pore pressure u₃ above the friction sleeve, the uncorrected sleeve friction f_s is commonly used. A possible correction method for the recorded sleeve friction is given in Annex C.

Various other corrections can be necessary to meet the requirements of the application class, like temperature effects, cross-sectional area of cone, compression of the push rods and rebound of the thrust machine.

6.3 Calculated parameters

The following parameters shall be calculated, based on the measured parameters:

R_f friction ratio

z penetration depth

Correction for inclination, like calculation of penetration depth from penetration length, shall be carried out for application classes 1 and 2 according to the procedure given in Annex B.

The following parameter shall be calculated, based on the corrected parameters:

R_ft corrected friction ratio.