Pavement Design as per UK Standards

FOUNDATION DESIGN

The main function of a pavement foundation is to distribute the applied vehicle loads to the underlying subgrade, without causing distress in the foundation layers or in the overlying layers. This is required both during construction and during the service life of the pavement.

Pavement foundations are designed on the basis of practical minimum layer thicknesses for construction, protection of the subgrade during construction and long-term provision of support to the overlying pavement layer. Other considerations include drainage and durability

The subgrade surface modulus shall be used in the pavement foundation design.

For widening schemes, where the subgrade is consistent across the width of the existing carriageway and the proposed widening, use of the in situ long-term subgrade surface modulus of the subgrade below the existing carriageway may be used for design purposes.

CBR is traditionally used as indirect test for sub grade strength.

The following equation shall be used where California bearing ratio (CBR) is used in the estimation of short-term and/or long-term subgrade surface modulus:

E = 17.6 (CBR)0.64 MPa

The minimum permitted sub grade stiffness modulus is 30 MPa (approx. 2.5% CBR). Where a sub grade has lower natural stiffness modulus. It is considered unsuitable support for pavement foundation and it should be improved by appropriate methods.

There are three design approaches for foundation

1. Restricted Design Approach – It allows a limited number of designs to be applied for foundation classes 2 and 3 and is particularly intended for use on schemes of limited extent. The designs are conservative, making allowances for uncertainty in material performance and construction level tolerance.
2. Performance Based Design Approach – This method is used to enable the efficient use of materials. It covers all four foundation classes and provides more flexibility to the designer. The criteria for construction of an acceptable foundation must be measure of its constructed performance. This method is used for wider sites.
3. Widening Design Approach – It utilises a restricted or performance design approach to assure the performance of the foundation whilst considering the additional requirements to provide sub-surface drainage continuity between the existing pavement and the widening.

The foundation classes are given below:

• Foundation Class 1 ≥ 50MPa – Capping Layer only – not used for traffic > 20msa – for non-trunk roads only.
• Foundation Class 2 ≥ 100MPa – Capping Layer plus/or granular subbase material
• Foundation Class 3 ≥ 200MPa – Hydraulically bound material (using cement, furnace slag, fly ash etc.) are used
• Foundation Class 4 ≥ 400MPa – Hydraulically bound material (using cement furnace slag, fly ash etc.) are used

(Note: Better foundation class needs less thickness of required overlying thickness of bituminous mix.)

For restricted design approach, thickness charts are available in the pavement section of Design Manual for Roads and Bridges (DMRB). They provide thicknesse for capping layer only (applicable for Class 1 only) or capping layer plus subbase layer (applicable for Class 2 & 3)

Foundation class 4 is not available for the restricted foundation design approach due to a lack of performance data sets generated since the inclusion of this foundation class in 2006. Therefore, foundation class 4 is only permitted as a performance foundation

Performance based foundation designs shall be subject to performance testing in accordance with Series 800 of the MCHW. For this, a demonstration area shall confirm the performance of the foundation design.

For widening of pavement foundations, the pavement and the foundation of the existing adjacent carriageway shall be assessed to establish the material type, condition and the thickness of each layer. The type, condition, construction thicknesses and levels of the layers in the adjacent pavement and foundation are key to the design of the widened pavement and foundation. For widening, the design for each foundation area shall follow either a restricted or performance foundation design approach. The foundation design for widening shall provide continuity of drainage. The requirement for drainage continuity can result in a thicker subbase layer than required by the foundation design.

TRAFFIC ASSESSMENT

Design Traffic = Commercial vehicle loading over the design period (expressed as the number of equivalent standard axle load (80KN or 8.16T)

For new scheme AADF from Traffic Appraisal Manual 12.1.1

For existing road scheme, a classified count shall be carried out over a 12, 16 or 24-hour period. This must be converted to an AADF using the principles given in the COBA manual 13.1.0

AADF is flow in one direction. AADT is traffic on both directions.

Commercial vehicles at opening (F):

Min 3.5 tonnes gross weight

PSV – Public Service Vehicles - Buses and Coaches

OGV1 – 2 & 3 Axle rigid

OGV2 – 4 or more axle articulated

Percentage of commercial vehicles within the AADF (in one direction)

For new road schemes, the commercial vehicle flows by class / category shall be determined from traffic transport analysis using the principles described in the Department for Transport's WebTAG Unit

The flow of other goods vehicle 2 (OGV2) vehicles as a percentage of all commercial vehicles shall be obtained by calculation or modelling.

The resulting percentage of OGV2 vehicles shall not be less than 70 per cent.

For existing road schemes (maintenance design or re-alignment) where there are existing AADF data for the scheme, they shall be used to determine the commercial vehicle flow. Where there are no existing AADF data, a classified count shall be carried out and converted to AADF using the principles given in the COBA Manual

Design Period (Y):

A design period of 40 years is used.

Growth Factor (G):

Following table is used for growth factors:

Wear Factor (W):

Structural wear is proportional to 4th power of axle load. Thus a 50% increase in Axle load results in a five-fold increase in calculated structural wear. The wear factors to be used for the Maintenance (Wm) and New Design (Wn) cases are shown in the following table.

Percentage of Commercial Vehicles in Heaviest Loaded Lane (P):

As all lanes are designed as for the heaviest loaded lane. For new and existing carriageways with 2 or more lanes in one direction, the proportion of vehicles in the most heavily loaded lane shall be estimated using following table.

DESIGN TRAFFIC = 365*F*Y*G*W*P*10-6

Nomographs for determining the design thickness of flexible pavement are shown below:

Notes:

• Beyond design traffic >80msa the thickness becomes constant
• Thickness to be rounded up to next 10mm
• Min. asphalt thickness is 200mm
• Min. HBM base is 150mm
• Binder course must be provided beneath TSCS (optional with other surfacing)
• Binder course is assumed to be the same as base but other can be used.
• In excess of 80msa binder course and base must contain crushed rock or slag course aggregate
• Permitted base and binder course materials are AC 40/60 and EME2
• Permitted asphalt base and binder course materials for flexible pavements with an HBM base are AC 40/60, EME2, HRA, SMA