46.4 In-situ California bearing ratio (CBR) test

46.4.1 General

The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of road subgrades and base courses. The test should be performed in accordance with BS 1377-9:1990, 4.3.

NOTE 1 The test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The CBR test is fully described in BS 1377-4.

NOTE 2 The CBR method of flexible pavement design is essentially an empirical method in which design curves are used to estimate pavement thickness appropriate to the CBR of the soil. There is no unique CBR of a soil and the value obtained in any test depends very much on the manner in which the test is conducted. The design curves are usually based on one carefully specified method of measuring the CBR, which is usually a laboratory method. The parameter required for the design of flexible pavements is the CBR attained by the soil at formation level after all necessary compaction has been carried out, the pavement has been laid, and sufficient time has been allowed to elapse for equilibrium water content to become established.

Before embarking on in-situ CBR tests, careful thought should be given to how relevant these are to the design method to be used and whether the condition of equilibrium water content is likely to pertain.

46.4.2 Limitations and uses of the test

COMMENTARY ON 46.4.2

The CBR test is unsuitable for any soil containing particles of longest dimension greater than 20 mm, because the seating of the plunger on a large stone can lead to an unrepresentative result. The test with sands tends to give results much lower than the laboratory tests on which the design charts are based. The test is most suited to clay soils, subject always to the soil under test being at equilibrium water content. The water content at a depth of 1 m to 2 m below ground surface, where the soil is normally unaffected by seasonal changes in water content, often gives a good indication of the equilibrium water content, provided that there is no change of strata. An alternative is to carry out the test directly beneath an existing pavement that has identical soil conditions to those of the proposed construction; this method has been used with some success for the design of airfield pavements. In-situ CBR tests have sometimes been carried out in conjunction with in-situ density and water content tests and then linked with laboratory compaction tests. A judicious study of all the resulting data leads to a reasonable design parameter on suitable soils. Attempts have sometimes been made to use the test as a means of controlling the compaction of fill or natural formations, but they have not usually been successful and the procedure is not recommended.

The CBR test should be used primarily to empirically determine the required thicknesses of flexible pavements. It should normally be performed on remoulded (compacted) specimens, although they might be conducted on undisturbed soils or on soils in the field.

46.4.3 CBR from plate load tests

COMMENTARY ON 46.4.3

Surface plate load tests are often specified for assessing CBR values for fills and sub-bases. The test is a specific application of the surface plate load test. The specification for this test seems to vary around the world but consists of incremental maintained loading of a 750 mm diameter plate. Increments are maintained until all movement has stopped or until a specified creep rate is attained in fine grained soils. A series of increments (between 4 and 7) are applied till a specified settlement is exceeded or a specified load reached (in the UK 1,25 mm to be exceeded by one increment, in the US 0,7 inch or 2,54 mm). From the test results a modulus of subgrade reaction is calculated as the slope of the load settlement curve from zero to 1,25 mm displacement. If smaller plates are used correction factors are applied. The CBR value is calculated from various correlations that are available.

The correlations and test methods used should be compatible. The actual test method used and the correlations used should be reported in the Ground Investigation report.

46.5 Deflectometer test

COMMENTARY ON 46.5

The deflectometer test or falling weight deflectometer (FWD) is a testing device used to evaluate the physical properties of pavement. FWD data is primarily used to estimate pavement structural capacity for use in (but is not limited to) highways, local roads, airport pavements, and railway tracks. The machine is usually contained within a trailer that can be either towed to a location by another vehicle or, when used on railway tracks, placed on a hand trolley and pushed to the location.

The FWD is designed to impart a load pulse to the pavement surface which simulates the load produced by a rolling vehicle wheel. The load is produced by dropping a large weight onto a circular load plate — typically 300 mm diameter. A load cell mounted on top of the load plate measures the load imparted to the pavement surface. The load plate can be solid or segmented. The advantage of a segmented load plate is that it adopts to the shape of the pavement, giving an even distribution of the load on uneven surfaces.

Deflection sensors (geophones; force-balance seismometers) mounted radially from the centre of the load plate measure the deformation of the pavement in response to the load. Some typical offsets are 0 mm, 200 mm, 300 mm, 450 mm, 600 mm, 900 mm, 1 200 mm, 1 500 mm. The deflections measured at these sensors are termed D0, D200, D300, etc.

FWD data is most often used to calculate stiffness-related parameters of a pavement structure. This process is computationally intensive although quick on modern computers. It can give quite misleading results and requires an experienced analyst. Instead, many analysts use simplified methods to calculate related parameters that are empirical in nature.

A light weight deflectometer (LWD) is a portable falling weight deflectometer. It is used primarily to test inset base and subgrade moduli during construction. Light weight deflectometers (LWD) are quicker than the isotope measuring method and requires no reference measurements. The equipment can be operated by one operator, allowing for the analysis of collected data and printing out of data files on site.

A heavy weight deflectometer (HWD) is a falling weight deflectometer that uses higher loads, used primarily for testing airport pavements.

A rolling weight deflectometer (HWD) is a deflectometer that can gather data at a much higher speed (as high as 55 mph) than the FWD. It is a specially designed tractor-trailer with laser measuring devices mounted on a beam under the trailer. Another advantage of the RWD over the FWD is that it can gather continuous deflection data as opposed to discrete deflection data collected by the FWD. RWD development has been carried out independently by Applied Research Associates (ARA) since 2005 and KUAB Sweden since 1991.

The deflectometer test should be carried out and the data processed following the guidance given by the Highways Agency or in the manufacturer's literature. The correlations adopted should be appropriate to the method used for the test but even so the results should be treated with caution.