Page 95 - RoadCem Manual - PCT BV

Once the soil/sub base bearing strength capacity has been determined it is necessary to determine the soil grain composition as this is the main characteristic that governs the soil/sub base bearing strength.
A number of samples needs to be taken for this purpose and and this is a function of the location and the variability of soil types along the route. It is essential that a soil/sub base bearing strength is determined for all locations with different characteristics.
Determination of soil/sub base bearing strength
There are 4 different methods to determine the soil/sub base bearing strength:
• The A.A.S.H.T.O method which gives an indicative value for the bearing strength. In this method the CBR value can be determined on the basis of the grain composition of the soil/sub base in conformance with the A.A.S.H.T.O method.
• Carrying out a CBR test in the laboratory.
• Carrying out a Dynamic Cone Penetrometer (DCP) test in-situ.
• Carrying out a ground drilling in situ and taking cores for further analysis
It is noted that a combination of these tests is always recommended as it adds additional insight into the expected behavior of the soil/subbase under load.
The A.A.S.H.T.O. method is an easy and inexpensive way to get an indication of the grain composition and estimate the soil/sub base bearing strength. The indicative value obtained can be used in preliminary design but additional analysis and measurements which are more accurate should be used for final design and once the contract has been awarded. It should be noted that this must be agreed with the client up front as additional measurements may not be acceptable to all the clients. In such a situation a more accurate method of measurement should be done at the early stage.
A DCP test is much quicker to do than a ground drilling and coring. It is recommended that that ground drilling takes place every 100 m, along the path of the road and a DCP test every 50 m. These advised distances should be changed up or down if the soils are unusually uniform or unusually variable.
Laboratory tests are conducted on the samples that are used for the Proctor test and they can also be used for a CBR test. With the results of the Proctor test the optimal water content is determined and thus the maximum compaction of the soil/sub base that can be achieved in the field is established. Good compaction of the sub-base is very important to get a bearing strength value for the sub-base and the pavement structure.
A.A.S.H.T.O Method
The A.S.S.H.T.O method gives insight into what types of material are present. An estimation can be made of what the bearing strength of the sub-base might be.
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