Laying block paving over clay

[Mikecb1]

I am planning to replace my 150 sq metre shingle drive with block paving. I will be using Marshalls Tegula blocks. The underlying soil in my location is clay, but I have not excavated the existing shingle drive to see exactly what is underneath. It appears to be some kind of compacted hardcore. I have obtained quotes for the new drive from 3 Marshalls registered installers, but because of the clay I've had 3 different opinions on the required sub base. One guy says geotextile membrane, 2 x 75mm layers of compacted mot1 aggregate and 40mm sand. Second chap says first put down a 40mm layer of weak mix concrete, then the other layers. Third guy says no membrane, 75 mm premix concrete, 75mm mot1, 40mm sand. I contacted Marshalls for advice, but they say it's best to rely on the experience of local installers, and wouldn't make any recommendation.

I would be very grateful for some advice.

TIA

Mike

[dig dug dan]

The first option is the best. The geotextile is designed to stop the sub base mixing in with the subgrade. We have similar soil conditions here, and I have never had problems.

[Mikecb1]

Dan

Thanks for the advice. The fact that two out of three of the prospective contractors were adamant that a layer of concrete was needed on top of the clay bothered me. When you're laying out 20 grand you want to be sure it's going to be done right!

Mike

[lutonlagerlout]

in fairness all are right in different ways
I have seen lads use lean mix where the sub grade is bad and it lasts the test of time
technically geo membrane and 2 layers of 75mm type 1 wacked is correct , but the others will probably work too
cheers LLL

[Mikecb1]

Thanks LLL. I can sort of understand how lean mix could stabilise the underlying clay, but would a 75mm slab of premix also be satisfactory? I wonder if it might crack if the underlying clays moves, and if it would lead to drainage problems?

[lemoncurd1702]

Geo-text and possibly a geo-grid which is a plasticy reinforcing mesh sitting between the layers of sub-base giving it tensile strength similar to how reinforced concrete works but naturally retaining flexibility and drainage properties.
Haven't tried it myself yet but it is increasingly common on civil projects.

Maybe Sean could enlighten further.

I have however used cement dusted over top of first layer prior to topping with second layer to stabilise the base material in wet conditions. Helps dry it out and stiffen up a bit without compromising the permeability too much.

[Mikecb1]

Thanks lemoncurd

The geogrid sounds like a good idea, as does adding some cement to the first layer of aggregate, though none of the people who have quoted suggested either of these. Closest was the chap who wanted to put down a layer of lean mix first. As I said a couple of posts back, the proposal that bothered me most was putting down a 75mm concrete slab first, followed by a single 75mm layer of aggregate. Seemed to me to be likely to crack and/or create drainage problems.

[lutonlagerlout]

its not a slab per se ,its just very dry leanmix
I know of 2 blokes who use this technique when the underlying ground is very poor ,and it works for them
I also know for a fact that it is specified on some civil projects,right or wrong
LLL

[Mikecb1]

LLL

Thanks. Yes, the lean mix approach seems favourite from what everyone has said, unless, after excavation, the sub-soil turns out to be reasonably firm. In this case the lean mix may not be needed.

What no-one has commented on explicitly was the proposal to put down a slab of pre-mix instead of the first layer of aggregate.

[mickg]

We compact the mot on top of the geotextile membrane laid onto the clay then place the lean mix concrete then lay the paving on the laying course

[seanandruby]

First off you need to check the sub base crushed concrete and any textile if any to see if it's sound, could be you could lay straight on top of that. A geo grid ground reinforcement mesh would normally go straight on top of inferior sub grade if needed to stabilize.

[Mikecb1]

Oh dear

I started off with 3 different proposals from 3 contractors, but instead of getting strong support for any of them I now seem to have at least 3 more alternative recommendations!

Since the 3 quotations I have are all around the same price, how do I choose???

Or should I get more quotes?

Now very confused.

[lemoncurd1702]

First off you need to check the sub base crushed concrete and any textile if any to see if it's sound, could be you could lay straight on top of that. A geo grid ground reinforcement mesh would normally go straight on top of inferior sub grade if needed to stabilize.

Is a geo-textile also incorporated somewhere in the sub-base make-up or just the grid.

[lemoncurd1702]

Oh dear

I started off with 3 different proposals from 3 contractors, but instead of getting strong support for any of them I now seem to have at least 3 more alternative recommendations!

Since the 3 quotations I have are all around the same price, how do I choose???

Or should I get more quotes?

Now very confused.

I think anymore quotes would confuse the issue more. If they are all Marshalls register and are providing the Marshalls guarantee then you should have no worries.

[mickg]

are they offering you a Marshalls 10 year guarantee ?