Building a road is an intricate business. The topography of the site, the soil conditions, the kind of natural occurrences that may take place there- landslide, earthquake, excessive rainfall- are all factors that go into the determination of the kind of road that needs to be built. Unless engineers know what the load- bearing capacity of the soil is, they cannot go ahead with finalizing the construction plans.
There are various types of soil tests, including cone tests and sieve analysis. But the more common method is CBR Testing.
What is CBR Testing? CBR Testing or California bearing ratio testing has been in existence since its development in 1939. It gets its name from where it originated- the California Department of Transportation.
The test works on a pressure principle, where observations are made to assess the amount of pressure required to penetrate turf- soil, wet soil, clay, etc. This unit is divided by the pressure used to penetrate a crushed rock surface, normally limestone. The CBR rating is higher for harder surfaces.
While limestone is the most commonly used, the onsite strength test can also check penetration against a wide variety of material, from tough substances like concrete, asphalt and stone to different types of soil.
In highways, it’s important to check the quality of the subbase- this is the layer atop the soil and often bears the maximum weight of traffic-vehicular or foot. In the case of footfalls, you may not even see a subbase, but it is an essential component in the case of a road with vehicular traffic.
Sometimes it may happen that field conditions are not conducive to testing for water. If that be the case, the test is then conducted in conditions that mimic it. For instance, in soil with a high content of sand and gravel, there is better bearing of change in moisture content. Engineers will also try to find soil which is non-clay and therefore does not cohere and hold water. For example, they will look for higher saturation in the soil.
It is also important to ensure that the soil has not undergone too many changes because of construction work; this doesn’t mean that there is no water or a steady level of water, but the fluctuation is less. Information from these field tests can also be used to determine the soil’s load bearing abilities.
Once the test is done, care must be taken that no activity that affects the nature of the soil is performed- grading of soil or compacting it will change the composition of the soil and then there will be not much point of the test. In fact, soil samples from the same site can show different load bearing capabilities and so all the more reason why the site not be changed in any way.
Once the test is done and the results analyzed, the next course of action can be determined. There might be a requirement for chemical analysis of the soil or plate load testing, another investigation of the site might be warranted too.
Seeing how a road needs to stand up to heavy vehicles and heavy traffic, CBR testing is crucial and must be done with care.