Carriageway sub-base and capping layer

Table 34 must be used to find the thickness of capping and sub-base required.

Table 34: Carriageway sub-base and capping thickness(a)
  Min 450mm Frost Susceptibility
  Major Residential Access Road / Residential Access Road (250mm bituminous layer) Shared Surface Residential Access Industrial Road (300mm bituminous layer)
CBR Value

Capping

Sub Base

Capping

Sub Base

Capping

Sub Base

Less than 2%

550

200

500

250

600

150

2%

400

200

350

250

450

150

3%

300

200

250

250

350

150

4%

250

200

200

250

300

150

5 to 15%

200

200

200

250

250

150

More than 15%

 

200

 

250

 

150

The foundation design should not typically vary along the road. An appropriate value must be selected for each significant change in the subgrade properties.

Where the equilibrium California Bearing Ratio (CBR) falls between values in the above table, the value should be rounded down to the lower value.

Where results are below 2% a suitable design needs to be submitted to Road Adoptions and ground improvement will be required. The use of geo-textile will only be agreed in certain situations. Further guidance can be found in Design Manual for Roads and Bridges CD225 Design for new pavement foundations.

Frost susceptibility

Material within 450mm of the finished road surface must not be frost susceptible.

Capping materials

Approval of the capping layer must be given by the council for each site and must comply with National Highways Specification Table 6/1, Class 6F2, 6F3 or 6F5. The capping layer must be tested as necessary to demonstrate that it has an in-situ CBR of 15% (or equivalent test result). Alternative materials may be approved by the council subject to demonstrating that they will achieve an in-situ CBR of 15% (or equivalent test results).

Sub-base

Sub-base must be MCHW Clause 803, Type 1 Unbound Mixtures (National Highways Specification

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