Driveways & Depths of Foundation

Driveways & Depths of Foundation

With any pavement construction scheme, great care must be taken to consider the intended use of the project, especially so in the case of a driveway or car parking area.

The first step must be to examine the sub-grade – in several areas – across the intended plot, to determine the design of the driveway with regard to the base and possible sub-base layers that will be required.

British Standard specification BS7533:101 and BS7533:102 recommends using a California Bearing Ratio (CBR) which tests the strength of the sub-grade. Its values are recorded, and the results used to design the base and possible sub-base in accordance with the information provided by BS7533:101. The CBR guidance within BS7533:102 will direct the contractor regarding steps they may carry out on site themselves.

The California Bearing Ratio plays a pivotal role in the design of pavements (which includes driveways and car parks). Pavement design considers two main factors: the strength of the sub-grade, as measured by the CBR after construction, and the projected volume and type of traffic expected on the pavement throughout its lifespan. This ensures that the stresses experienced by the paving and sub-grade do not exceed permissible limits.

CBR values are influenced by various factors, including soil type, moisture content and compaction. Each soil type has its own load-bearing capacity, which directly effects the CBR values obtained through testing. Moreover, the moisture content of a soil specimen is a critical factor in determining the CBR value.

The Dynamic Cone Penetrometer is the most common apparatus used to record CBR readings on- site by professional companies. In BS7533:102 there is a guide on soil type and sub-grade evaluation to provide guidance with designing your project. (Note – Pavements designed in accordance with BS7533:101 have an expected lifespan of forty years, making them very sustainable.)

A site survey may also include a Hydrological Survey (see Hydrological Surveys) to establish the likely ground water movement across the area, as well as neighbouring ground in case of any springs, ditches, culverts or swales that may cause a high-water table at any time, resulting in areas of soft ground. A high-water table of 300mm below the surface would require suitable sub-surface drainage.

Chalk soil – especially shallow chalk, may cause issues with frost heave, where water trapped within the porous stone may expand dramatically in freezing conditions. Sandy soils may be too light, and problems occur whereby the edges of the driveway start to migrate laterally.

As with all projects requiring excavation, any possible services should be investigated by hand digging, and using a CAT scan to detect any cables or pipes.

The majority of driveways will require an excavated depth of around 300mm, the site taken down to ambient ground, which is often the same level as the depth of topsoil. Assuming firm ground, compact the sub-grade with a ride-on vibrating roller – preferably with a forward and reverse gearbox – with a minimum weight of 265 kgs.

Once fully compacted, a layer of Terram non-woven geotextile membrane should be laid out over the area, overlapping by 300mm or so to ensure complete coverage. MOT Type 1 should be spread over the site, levelled and compacted in layers not exceeding 100mm, making at least four passes with the roller, in two opposing directions. The fall on the base level should reflect the finished surface course.

Ensure the outer edges of the driveway are restrained to prevent migration, (see Edge Restraints) Once the supporting concrete has cured sufficiently to prevent any damage, the perimeter of the driveway will require further compaction to ensure a uniform strength. Due consideration must be given to the weight of vehicles – including visitors such as dustcarts and oil delivery lorries when assessing the required strength of the project.

If you are intending to use a block paving style finish to the driveway, 45 or 90 degree patterns are the strongest design-wise. Be sure to take into consideration the likely movement of traffic across the site, including tight corners, slopes, or where the effects of braking or sideways pressure can cause the surface to ‘swim’ and become disrupted. In all such cases, extra care and thought should be given to lateral as well as downward forces that may come into play when specifying edge restraints.

If there are any ‘soft-spots’ that appear during the excavation and preparation operations, consider changing the specification from MOT to lean mix concrete. This may require additional fees under a Variation Order (see Variation Orders). A minimum longitudinal fall of 1:80 is required, and a cross-fall should also be installed if possible.

Assuming block paving or similar small element units, a layer of 30mm of crushed rock or sharp sand should be spread uniformly as a lightly compacted bedding layer, to receive either 50mm or 75mm pavers, depending on the weight of intended traffic. Always err on the side of caution, making changes to suit the site conditions as discovered, and not according to a general ‘standard’ specification.