New Build Snagging Inspection

Concrete Block Driveway

I recently completed a new build snagging inspection in Kent and thought it would be useful to cover a very common issue, which is where the concrete block driveway starts to sink under the weight of vehicular traffic. What we commonly see is tramlines in the drive which align with the car tyres. So why exactly does this happen and what is the technical specification for the concrete blocks, the laying course and the sub base? For this particular drive,, which under the standard (BS7533-13-2009 Pavements constructed with clay, natural stone or concrete pavers Part 13: Guide for the design of permeable pavements constructed with concrete paving blocks and flags, natural stone slabs and setts and clay pavers) would be regarded as a Type A-Domestic drive way with total infiltration construction; meaning that rainwater will essentially permeate and drain through the blocks and sub base. We therefore have an unbound surface course (Concrete Blocks) and an unbound base (Type 1 MOT).

On this particular drive we measured a trough of 15mm where the car tyres commonly cover the drive.

Driveway Design

In choosing how deep the sub base should be (Type 1 MOT) or the laying course (Finer graded material), we have to understand the soil type, which in this case is heavy clay.

BS7533, states that, ‘A permeable pavement needs to be able to capture the specified amount of water to be retained and then discharge it in a controlled manner through the subgrade or drainage system. At the same time, the pavement has to be able to withstand the loadings imposed by vehicles. The design of the pavement is approached by considering the sub-base thickness required to meet both the hydraulic factors
and the loading factors. The greater sub-base thickness identified for either of these factors is adopted.’
Clearly this driveway is not able to withstand the load imposed by vehicles.

The depth of the sub base is determined by taking a soil sample and determining its CBR (California Bearing Ratio) and whilst I don’t have that information, and I very much doubt that the developer did, we know that for heavy clay (Which is what we found), it has an estimated CBR of 2-5% and a plasticity index of 40-70. From this information, we can assess the guidelines in BS7533 and determine that the minimum depth of sub base required is at least 250mm, and up to 425mm, determined upon the actual CBR test result (A CBR of 5% needs no additional depth). From table 7, taken from BS 7533, we see that a minimum 250mm of course graded material is needed, but due to the low CBR, we may also need an additional depth of material, which is seen in Table 9. However, it should be noted that a small amount of cement can be added to the sub base in order to stabilise the coarse graded material and to reduce the depth of the sub base. This will of course reduce the permeability of the driveway.

Depth of MOT Type 1
Depth of MOT Type 1

How was This Driveway Constructed?

We removed a number of concrete blocks and determined the following information:

Depth of laying course = 0mm

Depth of sub base = 160mm (Loose material with no cement added)

Joint filling material = Sand

Sand 'laying course' found at 100mm depth
Sand ‘laying course’ found at 100mm depth

Unusually what we also found after excavating circa 100mm into the Type 1 MOT, was a layer of sand. This of course should not be there and suggests that the installer placed an incorrect laying course (Sand), before determining that the drive needed to be higher. The sand then has another 100mm of Type 1 MOT placed on top.

Total depth of sub base is 160mm
Total depth of sub base is 160mm

We do need a laying course of finer graded material (Circa 40mm thick), but for total infiltration drives, such as this, neither the laying course or the jointing material should be sand, since conventional jointing sand is not suitable as a medium for surface water to pass down through the pavement.

Why did this drive Sink?

Hopefully, from our measurements, you will already have determined that there was simply insufficient depth of sub base to support the weight of vehicular traffic, and this is what we commonly find on concrete block driveways. This drive required an additional 150mm of sub base material in addition to a 40mm deep laying course. Of course, we have an additional issue, in that the layer of sand installed will significantly reduce the infiltration rate, and may cause ponding on the driveway due to that reduced permeability.

Repairing the Driveway

This driveway is shared between the neighbouring property, so it won’t be possible to take up and correct one half of the drive, without also taking up the neighbouring drive. The blocks need taking up for reuse, the sub base needs excavating, and the sub grade (Clay) needs excavating a further 190mm, in order to achieve the required depth of sub base material. The drive is measured at 92 square metres, and will likely cost in the region of £8k-£10k to correct.

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