Testing/ Characteristics Flashcards
What is CBR?
California Bearing Ratio
It is the bearing capacity of a material compared to a well graded crushed stone
High quality material should have CBR= 100%
Sandy soils= 5-40%
Silty soils= 3-15%
Clay soils= 3-10%
More useful for new construction as old roads won’t change.
How do you test for CBR?
DCP/ LWD/ FWD site testing allows you to establish CBR for unbound materials (subbase, subgrade). It is a test recognized in DMRB CS229
Trial pits will be located in areas of widening, where full depth pavement construction is required. Test pits will allow to collect samples of subgrade soil material to establish long-term equilibrium subgrade modulus (and/or CBR) as required by DMRB CD225. and for laboratory testing (lab CBR and Atterberg limits).
MCHW cl 882- states The subgrade surface modulus shall be determined via:
(i) Dynamic Cone Penetrometer (DCP);
(ii) calibrated Falling Weight Deflectometer (FWD); or
(iii) calibrated Lightweight Deflectometer (LWD).
In-situ CBR tests are not practical as they require a large plan- test pit and take time.
What is DCP? How do you undertake a DCP?
What is a low CBR?
What is it a useful reading for?
Dynamic cone penetrator- 20mm diameter cone is put on ground, 8kg weight is dropped from a height of 575mm and it measures penetration which can be converted to CBR. the more it penetrates/ falls the softer the subgrade is.
You place it inside empty corehole
Graph measures it in separate layers it shows no of blows vs depth of penetration. steep line= softer material
CBR <2.5% is BAD
It is useful for showing high flood water level/ slippage info on emankment
How do you measure stiffness?
R value/ Resistance test which is a modified triaxial compression test
