CBR and PI Improvements

Independent test data conducted by The University of Arkansas, Civil Engineering Department, USA, showed typical CBR increases of 250% to 300% with ECOBASE™ Soil Stabilizer.

The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR @ 100%).   It is primarily intended for, but not limited to, evaluating the strength of cohesive materials having maximum particle sizes less than 19 mm (0.75 in.) (AASHTO, 2000).   It was developed by the California Division of Highways around 1930 and was subsequently adopted by numerous states, counties, U.S. federal agencies and internationally.  As a result, most agency and commercial geotechnical laboratories in the U.S. are equipped to perform CBR tests.

The CBR rating was developed for measuring the load-bearing capacity of soils used for building roads.  The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or for soils under paved airstrips.  The harder the surface/density, the higher the CBR rating.

A CBR of 3 equates to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of 10.

High quality crushed rock has a CBR over 80. The standard material for this test is crushed California limestone which has a value of 100.

Typical CBR Ranges
Typical CBR Ranges
General Soil Type
USC Soil Type
CBR Range
Coarse-grained soils
40 - 80
30 - 60
20 - 60
20 - 40
20 - 40
10 - 40
10 - 40
5 - 20
Fine-grained soils
15 or less
CL LL < 50%
15 or less
5 or less
10 or less
CH LL > 50%
15 or less
5 or less

ASTM D6951 / D6951M - Recommended In-situ CBR Test

This test method is used to assess insitu strength of undisturbed soil and compacted materials (or both).  The penetration rate of the 8-kg [17.6-lb] DCP can be used to estimate insitu CBR (California Bearing Ratio), to identify strata thickness, shear strength of strata, and other material characteristics.

A field DCP measurement results in a field or insitu CBR and will not normally correlate with the laboratory or soaked CBR of the same material.  The test is thus intended to evaluate the insitu strength of a material under existing field conditions.

Independent test data conducted by The University of Arkansas, Civil Engineering Department, USA, showed improvements in PI of the treated soil by 50% using the ECOBASE™ Soil Stabilizer.

ASTM D4318 Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils

These test methods cover the determination of the liquid limit, plastic limit, and the plasticity index of soils.

Two methods for preparing test specimens are provided as follows: The Wet preparation method and Dry preparation method.  The method to be used shall be specified by the requesting authority.  If no method is specified, use the wet preparation method.

The liquid and plastic limits of many soils that have been allowed to dry before testing may be considerably different from values obtained on non-dried samples.  If the liquid and plastic limits of soils are used to correlate or estimate the engineering behavior of soils in their natural moist state, samples should not be permitted to dry before testing unless data on dried samples are specifically desired.

Two methods for determining the liquid limit are provided as follows: Method A, Multipoint test.  Method B, One-point test.   The method to be used shall be specified by the requesting authority. If no method is specified, use Method A.

The multipoint liquid limit method is generally more precise than the one-point method.  It is recommended that the multipoint method be used in cases where test results may be subject to dispute, or where greater precision is required.

Because the one-point method requires the operator to judge when the test specimen is approximately at its liquid limit, it is particularly not recommended for use by inexperienced operators.

The correlation on which the calculations of the one-point method are based may not be valid for certain soils, such as organic soils or soils from a marine environment.  It is strongly recommended that the liquid limit of these soils be determined by the multipoint method.

The plastic limit test is performed on material prepared for the liquid limit test.

The liquid limit and plastic limit of soils (along with the shrinkage limit) are often collectively referred to as the Atterberg limits.  These limits distinguished the boundaries of the several consistency states of plastic soils.

The composition and concentration of soluble salts in a soil affect the values of the liquid and plastic limits as well as the water content values of soils (see Method D4542).  Special consideration should therefore be given to soils from a marine environment or other sources where high soluble salt concentrations may be present.  The degree to which the salts present in these soils are diluted or concentrated must be given careful consideration.

The methods described herein are performed only on that portion of a soil that passes the 425-µm (No. 40) sieve.  Therefore, the relative contribution of this portion of the soil to the properties of the sample as a whole must be considered when using these tests to evaluate properties of a soil.

The values stated in SI units are to be regarded as the standard, except as noted below.  The values given in parentheses are for information only.

The standard units for the resilience tester covered in Annex A1 are inch-pound, not SI. The SI values given are for information only.

All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.

For purposes of comparing a measured or calculated value(s) with specified limits, the measured or calculated value(s) shall be rounded to the nearest decimal or significant digits in the specified limits

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


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