5.5.7 Consistency limits determination Objective and requirements

(1) The consistency limits (Atterberg limits) comprise the liquid limit, plastic limit and shrinkage limit. Only the determination of the liquid limit and the plastic limit are covered.

(2) The consistency limits are used to characterise the behaviour of clays and silty soil when the water content is changing. Classification of clays and silty soil is mainly based on the consistency limits.

(3)P The testing method to be used for determining the liquid limit (fall cone or Casagrande apparatus) shall be specified.

(4) In general for the liquid limit, the fall cone method should be preferred to the Casagrande method. The fall cone method gives more reliable results particularly for low plasticity soil.

(5) The specimens should at least be of Quality Class 4, according to 3.4, if the test results are supposed to characterise the soil in situ.

NOTE Further information on a procedure, presentation and evaluation of the determine lion of consistency limits can be found in CEN ISO/TS 17892-12, (see X.4.1.6). Evaluation and use of results

(1) Different geotechnical properties, for example compressibility or optimum water content, can be derived from correlations with the liquid or plastic limits.

(2) The value of the plasticity index IP can be computed from liquid and plastic limits. IP can be used in soil classification and in correlations with some geotechnical soil properties, for example with soil strength.

(3) The value of the consistency index IC (or the liquidity index IL) can be computed from liquid and plasticity limits and from the current water content of the soil. It can be used to represent soil consistency and in correlations with some geotechnical properties.

(4) The activity index IA can be computed from IP and the percentage of clay particles, (IA) can be used in soil classification and in correlations with different geotechnical soil properties, for example with soil strength.

5.5.8 Determination of the density index of granular soil Objective and requirements

(1) The density index relates the void ratio of a soil sample to reference values determined by standard laboratory procedures. It gives an indication of the state of compaction of a free draining granular soil.

(2)P The following shall be specified or checked:

  • the quantity and quality of samples;
  • the type of testing procedure to be applied;
  • the method of preparation of each test specimen.

(3) The tested soil should contain less than 10 % of fines (particles passing through the 0,063 mm sieve) and less than 10 % of gravel (particles retained on the 2 mm sieve).

(4)P Density index test results shall be reported together with the available particle size analysis results, natural water content, particle density and percentage of oversize fraction (the hitter if applicable). Any deviation with respect to (3) shall be reported.

NOTE Further information on a procedure, presentation and evaluation the determination of the density index can be round in X.4.1.7. Evaluation and use of test results

(1) When evaluating density indexes, it should be taken into account that the maximum and minimum densities obtained in the laboratory do not necessarily represent limiting densities. It is also generally recognised that these tests give densities with a high degree of variability.

(2) The density index can be used to characterise the shear strength and the compressibility of coarse soil.

5.5.9 Soil dispersibility determination Objective

(1) The objective of the test is to identify the dispersive characteristics of clayey soil. Standard tests for classifying soil for engineering purposes do not identify the dispersive characteristics of a soil. Tests for dispersibility are carried out on clayey soil, primarily in connection with earth embankments, mineral sealings and other geotechnical structures in contact with water.

(2) Four test types are considered, (see M.7) :

  • the pinhole test, which models the action of water flowing along a crack;
  • the double hydrometer test, which compares the dispersion of clay particles in plain water without mechanical stirring with that obtained using a dispersant solution and mechanical stirring;
  • the crumb test, which shows the behaviour of crumbs of soil placed in a dilute solution of sodium hydroxide;
  • the determination of soluble salts in the pore water, which allows the correlation of the percentage of sodium to the total dissolved salts in a saturation extract. Requirements

(1)P The following shall be specified:

  • the storage of samples such that the samples are not allowed to dry before testing;
  • the testing procedures to be applied;
  • the specimen preparation method.

(2)P The results from the dispersibility tests shall be linked to the grain size distribution and consistency limits of the sample.

(3) For the pinhole test, the compaction conditions of the soil specimens, for example wet or dry of optimum, and the mixing water (e.g. distilled versus reservoir water) should be specified.

(4) For the double hydrometer test, a third hydrometer test may be specified if it appears necessary to study the effect of reservoir water on the soil in suspension.

(5) For the crumb test, the use of distilled water may be requested in addition to the sodium hydroxide solution.

5.5.10 Frost susceptibility Objective

(1) The frost susceptibility of soil materials plays an essential role in the design of foundations placed above the freezing front in frost susceptible soil.

(2) Roads, airport runways, railways, buildings on spread foundations, buried pipelines, dams and other structures can be subject to frost heave due to freezing of a frost-susceptible soil having access to water. Frost-susceptible soil can be used in its natural state or as a constructed base for structures.

(3) The risk of frost heaving may be estimated from correlation with soil classification properties (particle size distribution, height of capillary rise and/or fines content) or from laboratory tests on natural, re-compacted and re-consolidated, or reconstituted samples.

NOTE An example can be found in M.8 and X.5. Requirements

(1) If the estimation of frost susceptibility based on classification properties of the soil does not clearly indicate the absence of risk of frost heaving, frost heaving tests in the laboratory should be run. Examples of soil types indicating the need of laboratory tests in addition to correlations to classification properties include organic soil, peat, saline soil, artificial soil and coarse soil with a wide range of grain size.

(2) To determine the frost susceptibility of a soil in its natural state, natural samples should be tested. To estimate the frost susceptibility of a constructed fill, frost heave tests should be run on re-compacted and then re-consolidated specimens or on reconstituted specimens.

(3) The frost susceptibility test in the laboratory is a frost heave test. If the risk of thaw weakening is to be tested, a California Bearing Ratio test should be carried out after thawing of the specimen. The re-compacted or reconstituted specimen should be subjected to one or more freeze-thaw cycles before testing. Evaluation of test results

(1) The results should be interpreted as a function of the type of construction work, the rules used in design and the available comparable experience, considering the consequence of the frost effects.

Eurocode 7: Geotechnical design — Part 2: Ground investigation and testing