Kent Design - Technical Appendix - Kent County Council

A separate chemical contamination risk assessment should also be
undertaken for the whole development.
3.2.1.3 SUB-SOIL DRAINAGE
Every endeavour should be made to prevent the water table from
rising to within 600mm of formation level. Methods of preventing this
should be discussed with the Engineer at time of designing the works
3.2.1.4 EARTHWORKS
All turf and topsoil shall be removed from the formation of the new road.
It shall be stored carefully on site up to a maximum of 2m in height to
prevent deterioration and contamination with sub-soil etc and shall
wherever possible be reused within the site. Only with the agreement of
the Engineer may surplus material be removed from the site.
If in-situ stabilisation of the existing soil is to be used instead of
imported granular capping layer there will be considerably less
material to be removed
Other organic materials within the site of the new road shall be
identified, removed and their disposal agreed with the Engineer.
Wherever possible they shall be reused within the site by the
construction of appropriate mounds/landscaping features.
The area of the carriageway shall be excavated or filled to produce
the formation level.
Where local areas are uncovered which are softer than the general
sub-grade they shall be excavated out to a depth appropriate for the
soft area and backfilled with material similar to the surrounding
formation. Where soft areas exceed 25% of the total area of
carriageway in which they occur, the whole area shall be reduced in
level and suitable material substituted. All other unsuitable material,
other than organic material, shall be considered for treatment to
make it suitable for use in the carriageway works, footways, verges
or other parts of the site. Only with the agreement of the Engineer
may surplus material be removed from the site.
Fill material shall comply with ‘suitable material’ as defined in the
Specification.
3.2.1.5 SERVICE DUCTS AND PIPEWORK
Ducts and pipework shall wherever possible not be located in the
carriageway. Where service crossings make this unavoidable, they
shall be installed prior to carriageway construction and protected
from damage by subsequent construction.
3.3 CARRIAGEWAY- SUB-BASE AND CAPPING LAYER
3.3.1.1 SUB -BASE AND CAPPING LAYER DESIGN THICKNESS
For District Distributor roads and Industrial Access Roads the thickness
of sub-base shall be determined in accordance with the Design
Manual for Road and Bridge Works Volume 7 Section 2 Chapter
3 HD25 for the estimated volume of traffic as agreed with the Engineer.
Only ‘Flexible pavements’ are permitted. Where the capping layer
option is preferred, the reasons for not improving the CBR of the subgrade
by stabilisation shall be discussed with the Engineer
For Local Distributor, Major Access Roads, Minor Access Roads, Minor
Access Ways and Homezones the design process shall be as follows:
Carry out four thickness designs using the equilibrium CBR of the
formation for the long term condition and the anticipated CBR at the
time of construction for the short term condition. The latter will
need to be rechecked at time of construction.
Select the thickness for sub-base alone and for Capping layer options
which are the more conservative.
Determine on economic practicality or sustainability grounds
whether to use the sub-base option or capping layer option.
It is not intended that the sub-base be used as a access for the construction
of buildings. Access should be provided separately or by using the surface
of the basecourse prior to laying the blacktop or block surfacing.
The thickness design below is based on the works being carried out by a
company quality assured for this activity. Records of the CBR at the time
of construction, the thickness of the laid materials and the compaction
plant used shall be made available to the Engineer. Where the works are
not carried out by a company quality assured for this activity, the thickness
of sub-base and capping shall each be increased by 20%.
For the purposes of this design guide the following traffic figures in
table 10 have been assumed. For the purpose of structural pavement
design the roads should be designated as shown in the table
irrespective of descriptions used for other purposes.
TABLE 10: ASSUMED TRAFFIC FOR DESIGN PURPOSES
Road Type Length of Construction Commercial Standard axles
Road Traffic Vehicles over the
constructed standard axles per day design life
(lin. meters)
(millions)
Local Distributor 250 1000 125 1.2
Major Access Road 250 1000 50 0.5
Minor Access Road 150 500 10 0.3
Minor Access Way 50 200 1 0.05
Homezone 0 50 0.3 0.01
SUB-BASE THICKNESS - SUB-BASE ONLY OPTION -SHORT TERM CONDITION
(The design is based on that necessary for construction of the road
and the construction of dwellings using the top of bituminous
basecourse as a site access)
TABLE 11: SUB BASE THICKNESS - SHORT TERM
CBR (%) at time < 2 2 to3 >3to