Soft spots and sub-grades with less than 1.5% CBR shall be laid with both an approved geotextile and geogrid or an approved geo-composite of both and the sub-base may be reduced to 300mm over an additional 410mm of 6F5 capping layer granular fill.
Low trafficked residential new roads with either a single access or accesses at both ends to the same road with no bus stops proposed shall be built to the following build depths in millimetres (mm).
Table 1 – Low trafficked asphalt residential new roads
Subgrade CBR value
Sub-base without geotextile
Sub-base with geotextile
Binder course
Surface course
Total depth without geotextile
Below 1.5% & soft spots
N/A
620
130
40
N/A
1.5%
N/A
440
130
40
N/A
2%
400
340
130
40
570
3%
310
300
130
40
480
4% or greater
300
N/A
130
40
470
Table 2 – Low trafficked block paved residential new roads
Subgrade CBR value
Sub-base without geotextile
Sub-base with geotextile
Binder course
Laying course
Block paver
Below 1.5% & soft spots
N/A
620
90
30
60
1.5%
N/A
440
90
30
60
2%
400
340
90
30
60
3%
310
300
90
30
60
4% or greater
270
N/A
90
30
60
60mm pavers may be replaced with 80mm pavers.
In specific exceptional circumstances, we might allow reducing the build depths in Tables 1 and 2 to 200mm binder course over 150mm sub-base with an approved subsoil strengthening technique on a subgrade below 2% CBR.
Table 3 – Low trafficked permeable paved residential new roads
Subgrade CBR value
Sub-base Type 3
Binder course
Laying course
Block paver
Total depth
2.5% or greater
300
130
50
80
560
Permeable paving is not suitable on a subgrade with a CBR value less than 2.5%. Additional sub-base may be proposed to meet hydraulic requirements. Binder course shall be perforated by 75mm cores at 750mm centres.
High trafficked residential new roads with access at both ends to different roads or to the same road with bus stops proposed and all industrial new roads, shall be designed with the following build depths in millimetres (mm).
Table 4 – High trafficked asphalt residential and industrial new roads
Subgrade CBR value
Sub-base without geotextile
Sub-base with geotextile
Binder course
Surface course
Total depth without geotextile
Below 1.5% & soft spots
N/A
620
180
40
N/A
1.5%
N/A
440
180
40
N/A
2%
400
340
180
40
620
3%
310
300
180
40
530
4%
260
260
180
40
480
5% or greater
230
N/A
180
40
450
In specific exceptional circumstances, we might allow reducing the build depths in Tables 4 and 5 to 260mm binder course over 150mm sub-base with an approved subsoil strengthening technique on a subgrade below 2% CBR.
Public road improvements
Public road improvements shall be designed to the relevant road type build as confirmed by us during the review. The surface course for road type 0 and type 1 shall be Hot Rolled Asphalt.
The sub-base depth shall be no less than 620 mm for all road types where laboratory CBR test results are not provided.
Where the subgrade has not been confirmed as non–frost-susceptible, the sub-base must also be of sufficient thickness to provide a minimum of 450 mm of frost protection.
