Site investigation for driveway

[mikec1972]

Good afternoon all, I'm building a new gravel driveway approx 40m long by 3.5m wide, would there be a requirement for any Site investigation to determine CBR's etc or would you use a rule of thumb depth for the sub-base (type 1?) of say 200mm topped with 50-75mm of 20mm limestone chippings. Or would you recommend that CBR's be undertaken by the Contractor to determine the sub-base depth? Am slightly worried that we could specify 150mm of type 1 and then the thing starts to rut once it gets trafficked. I know for standard local authority roads they normally specify minimum overall depth of 450mm for frost susceptibility, and typically 150mm sub-base on 450mm capping then black on top. For a private driveway (6 cars using it) 150mm type 1 sub-base with 50mm chippings just seems a little thin to go straight onto the excavated soil. Thanks.

[DNgroundworks]

For a start you wouldnt build straight onto excavated soil, you want to be building of good stable ground ie: clay, rock, chalk etc

If you dig down 200mm and hit hard clay, terram with 150mm MOT Type 1 put down in two layers and rolled, with 50mm of chippings will be solid i reckon.

The reason the public highways are 450mm deep in construction is because they have to handle the rigor of having 40t artic's, farmer giles in his fastrac, and a constant drone of traffic every day, your driveway isn't getting anywhere near this sort of treatment.

[DNgroundworks]

Ultimately depends on ground conditions. A small trial hole will be enough to ascertain this.

[Carberry]

As DNgroundworks says, depends on ground conditions. I've only done a few driveways where I had to go deeper than 150 and they were all because of builders burying crap under the driveway.

[DNgroundworks]

We have just put a haul road in to take 5 "timber carrying" artics a day, for 6 months, that was 250/300 thick crush with terram, on hard (ish) clay it hasnt moved at all, ill probs be somewhere near saying that these lorrys will weigh 40t

Your looking at half that depth of stone...150mm will be enough to take the movements of 6 1 tonne cars......

but depends on ground conditions

[Pablo]

Anymore pictures of that job DN.

[DNgroundworks]

We are back up there soon putting another road in on other side of fell, so ill get some more pics, no doubt it will be full of mud now with the timber forwarders running on it, here are some pics of the original after we had just put it in - (for the OP)

[Dave_L]

50-75mm of limestone chippings is waaaaayy too thick, 40-50max otherwise it will just rut at the first trafficking.

Suggest leave a bag of chipping with the customer for top-up purposes, never had much luck with gravel driveways, people expect too much from them.

[mikec1972]

Thanks for all your responses, reckon 150mm type 1 plus 50mm (20mm) limestone chippings is the way to go then, would there be any benefit in putting a geotextile membrane beneath the chippings and above the sub-base to prevent one from mixing in the other, this has been suggested but I'm not sure about it, i'd be more inclined to put some terram under the sub-base to guard against any loss of fines in the base or sub-grade getting into the base. How about putting a sand blinding layer atop the sub-base?

[mikec1972]

Thanks for all your responses, reckon 150mm type 1 plus 50mm (20mm) limestone chippings is the way to go then, would there be any benefit in putting a geotextile membrane beneath the chippings and above the sub-base to prevent one from mixing in the other, this has been suggested but I'm not sure about it, i'd be more inclined to put some terram under the sub-base to guard against any loss of fines in the base or sub-grade getting into the base. How about putting a sand blinding layer atop the sub-base?

[Carberry]

Put it below sub base. Geotextile membrane faq

[DNgroundworks]

Do not put terram between the sub base and surface gravel, your wheels will spin for ever more!