The expansion index, EI, is used to measure a basic index property of soil and therefore, the EI is comparable to other indices such as the liquid limit, plastic limit, and plasticity index of soils, as in Test Method D 4318
The EI is not used to duplicate any particular field conditions such as soil density, water content, loading, in-place soil structure or soil water chemistry. However, the test procedure keeps all conditions constant allowing direct correlation of data between organizations. All organizations can benefit from these collective experiences.
The classification of a potentially expansive soil is based on the following table:
| Expansion Index, EI | Potential Expansion |
| 0–20 | Very Low |
| 21–50 | Low |
| 51–90 | Medium |
| 91–130 | High |
| >130 | Very High |
Note 18212;Notwithstanding the statements on precision and bias contained in this test method: the precision of this test method is dependent on the competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies which meet the criteria of Practice D 3740 1.1 This test method provides an index to the expansion potential of compacted soils when inundated with distilled water. 1.2 This test method controls variables that influence the expansive characteristics of soils and still retains a relatively simple test for practical engineering applications. 1.3 The values stated in SI units are to be regarded as the standard. The values stated in inch-pound units are approximate. 1.4 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D 6026 1.4.1 The method used to specify how data are collected, calculated, or recorded in this standard is not directly related to the accuracy to which the data can be applied in design or other uses, or both. How one applies the results obtained using this standard is beyond its scope. 1.5 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 and health practices and determine the applicability of regulatory limitations prior to use.
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