5.1 The data from the consolidation test are used to estimate the magnitude and rate of both differential and total settlement of soils or earthfill, or both, induced by loading of a structure or additional earthfill. Estimates of this type are of key importance in the design of engineered structures and the evaluation of their performance.

5.2 The test results can be greatly affected by sample disturbance. Careful selection of field sampling techniques and preparation methods of test specimens is required to reduce the potential of disturbance effects.

5.3 Consolidation test results are affected by the magnitude of the load increments. Traditionally, the axial stress applied to the specimen is doubled for each increment resulting in a load increment ratio of one. For intact samples, this loading procedure has provided data from which estimates of the preconsolidation stress, using established interpretation techniques, compare favorably with field observations. Other loading schedules may be used to model particular field conditions or meet special requirements. For example, it may be desirable to inundate and load the specimen in accordance with the wetting or loading pattern expected in the field in order to best evaluate the response. Load increment ratios of less than one may be desirable for soils that are highly sensitive or whose response is highly dependent on strain rate.

5.4 The interpretation procedure specified by these test methods to estimate the preconsolidation stress provides a technique to verify that one set of time readings are taken after the preconsolidation stress is exceeded and that the specimen is loaded to a sufficiently high stress level. Several other evaluation techniques exist and may yield different estimates of the preconsolidation stress. Alternative techniques to estimate the preconsolidation stress may be used when agreed to by the requesting agency and still be in conformance with these test methods.

5.5 Consolidation test results are dependent upon the duration of each load increment. Traditionally, the load duration is the same for each increment and equal to 24 h. For some soils, the rate of consolidation is such that complete consolidation (dissipation of excess pore pressure) will require more or less than 24 h. The apparatus in general use does not have provisions for verification of pore pressure dissipation. It is necessary to use an interpretation technique which indirectly determines that consolidation is essentially complete. These test methods specify procedures for two techniques (Interpretation Procedure 1 and Interpretation Procedure 2), however alternative techniques may be used when agreed to by the requesting agency and still be in conformance with these test methods.

5.6 The apparatus in general use for these test methods do not have provisions for verification of the degree of saturation. Most intact samples taken from below the water table will be saturated to a degree sufficient to assume fully saturated conditions. However, the time rate of deformation is very sensitive to degree of saturation and caution must be exercised regarding estimates for duration of settlements when partially saturated conditions prevail. For intact specimens obtained below the water table, inundation of the test specimen may not significantly change the degree of saturation of the test specimen but rather provides boundary water to eliminate negative pore pressure associated with sampling and prevents evaporation during the test. The extent to which partial saturation influences the test results may be a part of the test evaluation and may include application of theoretical models other than conventional consolidation theory. Alternatively, the test may be performed using an apparatus equipped to saturate the specimen.

5.7 These test methods use conventional consolidation theory based on Terzaghi's consolidation equation to compute the coefficient of consolidation, c_v. The analysis is based upon the following assumptions:

5.7.1 The soil is saturated and has homogeneous properties;

5.7.2 The flow of pore water is in the vertical direction;

5.7.3 The compressibility of soil particles and pore water is negligible compared to the compressibility of the soil skeleton;

5.7.4 The stress-strain relationship is linear over the load increment;

5.7.5 The ratio of soil permeability to soil compressibility is constant over the load increment;

5.7.6 Darcy’s law for flow through porous media applies; and

5.7.7 The deformation (strain) of the soil is one-dimensional in the axial direction.

Note 3: The quality of the result produced by this standard is dependent on the competence of the personnel performing it and the suitability of the equipment and facility used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some, but not all, of those factors.

1.1 These test methods cover procedures for determining the magnitude and rate of consolidation of soil when it is restrained laterally and drained axially while subjected to incrementally applied controlled-stress loading. Two alternative methods are provided as follows:

1.1.1 Test Method A—This test method is performed with constant load increment duration of 24 h, or multiples thereof. Time-deformation readings are required on a minimum of two load increments. This test method provides only the compression curve of the specimen and the results combine both primary consolidation and secondary compression deformations.

1.1.2 Test Method B—Time-deformation readings are required on all load increments. Successive load increments are applied after 100 % primary consolidation is reached, or at constant time increments as described in Test Method A. This test method provides the compression curve with explicit data to account for secondary compression, the coefficient of consolidation for saturated soils, and the rate of secondary compression.

Note 1: The determination of the rate and magnitude of consolidation of soil when it is subjected to controlled-strain loading is covered by Test Method D4186/D4186M.

1.2 These test methods are most commonly performed on saturated intact samples of fine grained soils naturally sedimented in water, however, the basic test procedure is applicable, as well, to specimens of compacted soils and intact samples of soils formed by other processes such as weathering or chemical alteration. Evaluation techniques specified in these test methods assume the pore space is fully saturated and are generally applicable to soils naturally sedimented in water. Tests performed on other unsaturated soils such as compacted and residual (weathered or chemically altered) soils may require special evaluation techniques. In particular, the rate of consolidation (interpretation of the time curves) is only applicable to fully saturated specimens.

1.3 It shall be the responsibility of the agency requesting this test to specify the magnitude and sequence of each load increment, including the location of a rebound cycle, if required, and, for Test Method A, the load increments for which time-deformation readings are desired. The required maximum stress level depends on the purpose of the test and must be agreed on with the requesting agency. In the absence of specific instructions, Section 11 provides the default load increment and load duration schedule for a standard test.

Note 2: Time-deformation readings are required to determine the time for completion of primary consolidation and for evaluating the coefficient of consolidation, c_v. Since c_v varies with stress level and loading type (loading or unloading), the load increments with timed readings must be selected with specific reference to the individual project. Alternatively, the requesting agency may specify Test Method B wherein the time-deformation readings are taken on all load increments.

1.4 These test methods do not address the use of a back pressure to saturate the specimen. Equipment is available to perform consolidation tests using back pressure saturation. The addition of back pressure saturation does not constitute non-conformance to these test methods.

1.5 It is important to clarify that although the procedures outlined in this standard do not directly measure pore water pressures, and therefore effective stress, the results of this test may be applied to effective stress analyses through engineering interpretation. Pore pressure is assumed to be dissipated based on interpretation of the time deformation under constant axial stress. This interpretation is founded on the assumption that the soil is 100 % saturated.

1.6 Units—The values stated in either SI units or inch-pound units [given in brackets] are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.

1.6.1 In the engineering profession it is customary practice to use, interchangeably, units representing both mass and force, unless dynamic calculations (F = Ma) are involved. This implicitly combines two separate systems of units, that is, the absolute system and the gravimetric system. It is scientifically undesirable to combine two separate systems within a single standard. This test method has been written using SI units; however, inch-pound conversions are given in the gravimetric system, where the pound (lbf) represents a unit of force (weight). The use of balances or scales recording pounds of mass (lbm), or the recording of density in lb/ft³ should not be regarded as nonconformance with this test method.

1.7 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026, unless superseded by this test method.

1.7.1 The procedures used to specify how data are collected/recorded and calculated in the standard are regarded as the industry standard. In addition, they are representative of the significant digits that generally should be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations or the user’s objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these conditions. It is beyond the scope of these test methods to consider significant digits used in analysis methods for engineering data.

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.