Wednesday, February 25, 2009


Introduction and General Notes
The sub-base layer is often the main load-bearing layer of a pavement. It is designed to evenly spread the load of the paving, and any traffic thereon, to the sub-grade below. A well-constructed sub-base will prevent settlement and channelisation - the phenomenon common on cheap installations of block paving, where two 'ruts' develop in the paving. These 'ruts' are caused when a car travels over the same line of paving to the garage, every morning and every night. Channelisation is also apparent on carriageways, particularly at the approach to traffic lights and on upward gradients. The sub-base is intended to prevent channelisation and settlement. 

All too often, cut-price residential block-paving installations omit the sub-base as a major cost-saver. Householders are unlikely to be aware of the need for such a layer, and, if out all day when the work is being done, may never notice its absence. A common tale I hear is....."such-and-such-abody said we didn't need any stone, because the dirt under the old flagstones will be firm enough!" - I usually get this tale once the paving has sunk, and the original contractor is long gone. 

When discussing the construction of driveways with clients, I liken the sub-base to a carpet can spend a fortune on the finest Axminster carpet, but if you don't use an underlay, it's never going to look or feel right. If your contractor tells you that a sub-base isn't required beneath your new drive, then ask how the sub-grade (soil beneath drive) is going to spread the load of the paving and what guarantee is offered with regard to settlement. Alternatively, get a different contractor to give his opinion. 
Once a good sub-base has been installed, it can be re-used without any further work, if you decide to change the surfacing/paving of your driveway. Many newer houses have tarmac drives with a good sub-base beneath. In such cases, the tarmac can be stripped off, the sub-base checked and re-levelled if required, and the new paving can be laid over the old sub-base. 

A finished sub-base should not deviate from the correct level by more than 10mm, and should reflect the final profile of the paving. The bedding layer above the sub-base ought to be a constant thickness to avoid differential settlement.

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