The subgrade strength needs to be assessed, together with the traffic loading (ESA), to determine the pavement thickness. Subgrade strength is influenced by the physical and mineralogical characteristics of the material, and its density and moisture content, both when compacted and in service. The best outcome is usually determined through practical experience backed up with field testing. If there are still concerns on the subgrade properties then laboratory testing may be required.
Subgrade strength can vary greatly along any stretch of newly formed road. Often, the key thing is to identify where the subgrade strength is very low. This might be evident from the wheel rut depth from construction vehicles or become visible during compaction of the subgrade. Areas of high moisture content or high clay content are of particular concern. Getting the subgrade reasonably consistent will lead to a more evenly strengthened road. Weak areas need to be fixed because they will cause subgrade failure once the road is in use. If the subgrade fails, then typically the section of road must be dug up and laid down again.
6.2.1 Using the California bearing ratio (CBR)
CBR test apparatusThe most commonly used measure of subgrade strength is the California bearing ratio (CBR). It was developed back in the 1930s by the Californian Department of Transportation, to measure the load-bearing capacity of soils used for building roads. The basic test is performed by measuring the pressure required to penetrate soil or aggregate with a plunger of standard area. The rate of this penetration is then compared to a standardised California crushed limestone aggregate, and the result expressed as a percentage.
In terms of CBR values, the Californian crushed limestone is very strong (by definition 100%), and most highway pavement charts only require material with strength of CBR = 80%. Extensive testing of pavement material used in New Zealand forest roads show a great range from 20% through to 80%. Most subgrades at the time of construction will typically have values from 3 to 15%. A subgrade with 8% is considered just strong enough to build on. However, a CBR of less than 4% has marginal capacity to carry traffic loads. Some form of stabilisation treatment or other techniques will be required, or an alternative road alignment may be necessary to avoid the weak soils. Stabilisation techniques are discussed later in this chapter. For example, daylighting will reduce the moisture content of the subgrade and result in a significant increase of CBR.
The pragmatic problem with the laboratory-based CBR test is that it is both very time consuming and costly to complete a single measurement, and the test itself relies on simplifications and assumptions. For example, all particles > 40 mm are sieved out or removed as they can affect the result, but most pavement aggregates have a relatively high percentage of this larger material. Also, because the tests are for highway design, it very much focuses on strength at very low levels of deformation, up to 20 mm. For lower class forest roads, ruts of 50-100 mm are accepted before a pavement is considered failed.
A number of in-situ alternatives have been developed over time and most still report the outcome as a CBR value. The laboratory CBR tests are usually used as a control for field penetrometer determinations.
6.2.2 In-situ measurements of CBR
Dynamic cone penetrometer resultsThe aim of subgrade evaluation is to determine a subgrade CBR value for the density and moisture conditions that are expected to prevail in service. The procedure used is influenced by factors such as the importance of the road, and the amount of experience the designer has had with similar subgrade material in similar locations and environments.
In variable soil conditions more measurements are warranted. A design CBR must take into account the changes in topography, drainage and soil type along the road. Correct documentation of the changes in soil type and CBR sample positions will provide accurate information for the subsequent pavement design.
For lightly trafficked roads, or where subgrade and climatic conditions are similar over large areas, extensive investigations might not be warranted if the road designer has considerable expertise and experience. However, it is still important to formalise the process and record the outcome. For example, the test might be primarily through visual inspection of soil samples to determine its uniformity with depth, and call upon supplementary tests with the dynamic cone penetrometer or impact hammer for weak areas.
Dynamic cone penetrometer
The dynamic cone penetrometer is an instrument that drops a standard weight a fixed height to drive a small cone into the ground. The incremental changes in depth are correlated to CBR measures. The dynamic cone penetrometer permits the measurement of CBR through to a depth of 800 mm, or deeper if extension rods are used. As such, it is an excellent tool to use prior to road construction to ascertain the subgrade strength below the surface where the road will be built.
From the CBR profile, variability of the subgrade material properties can be determined. The dynamic cone penetrometer test is most reliable in fine-grained soils. High subgrade CBRs, obtained from this penetrometer test in sands and gravels, should be further checked using laboratory testing or in-situ CBR equipment. The use of a cone penetrometer for the measurement of CBR in soils with a CBR < 3% will give unreliable results. This is due to the tendency of the penetrometer to sink into the soil under its own weight.
The graph above can be used to determine the CBR value of the subgrade material from the blows per mm ratio obtained from the penetrometer tests.
Impact soil test (Clegg Hammer & Loadmaster)
The impact soil test is a dynamic test procedure using a falling weight dropped directly onto the pavement. An internal accelerometer measures the rate of deceleration. The peak deceleration of the compaction hammer is directly related to the resistance offered at contact. This resistance results from the stiffness and shearing resistance of the material. Several dynamic falling weight apparatuses are available, including the Loadmaster and the Clegg Hammer Impact Soil Tester.
From various comparison studies, it has been shown that there is a correlation between the impact test results and the CBR of the material, although this relationship varies with soil types. For example, for the Clegg Hammer, a useful starting point is the form CBR=O.07 (IV)2, where IV=Clegg Impact Value. Although this relationship must be used with some caution, the impact test procedure has wider applications including checking variations during construction, and monitoring changes over time.
For further information on the equipment and test procedures consult the manufacturers or suppliers of the soil impact test equipment.