Hi, On site yesterday we cut a building platform down to subgrade RL. 14.445. We then proof rolled it with a 6 tonne smooth drum roller. The site has very poor soil with a mixture of wet clay and peat. When I checked the subgrade height it had settled between 80mm and 100mm. We are the stage 1 contractors and we must hand over the site with a 25mm tolerance on subgrade heights. Any suggestions on the best and cheapest way to bring the subgrade back up to RL. 14.445 Cheers. PS. I realise we probably cut the subgrade too low prior to proof rolling.
If that is a building platform, and my name was on it, and going to be responsible for warranteeing it if it selltes and the foundation cracks, I would be digging it out to competent material, and filling it back in with properly compacted soil. All under the watchful eye of a soils engineer.
Jerry's right. If it settled up to 10 cm overnight, how much is it going to settle when they pour the foundation ?
Wet clay and peat for a foundation??? Do yourself and the customer a favor and drive some piling...at his expense. Or tell him to find another site, and if he doesn't like that, get another contractor. You have to watch out for ***-hats who scheme to throw you in default so you can pick up their costs.
Let's think about this for a moment and ask a few questions. Was their a soil boring report in the contract documents? Does the owner/contractor have a materials engineer on the job that has approved the use of the clay and peat for sub grade? Did the sub grade settle from compaction or did it settle on it's own? How big is the site and what is it going to be? I agree that clay and peat is not the best material to build anything on but site and foundations designs vary significantly around the globe. I have seen wet pumping clay pass compaction test that I wouldn't be comfortable building a shed on. If anything, I don't think the OP should be picking up the costs for any undercutting and importing of fill unless it was in the contract and they included it in their bid or they just missed it. If you missed it then you'll have to suck it up. On the other hand, if you undercut the sub grade then that's in your court as well. Depending on the volume and cost, you may want to consider bringing in crushed stone to bring it to grade if the soils/materials engineer gives the approval. Another idea would be to scarify your existing sub grade, add and mix material back in, compact it and cut it to grade. BTW - What does RL 14.445 mean?
Nailed it.
Yair . . . There are different standards, understandings, and contracts. I've always run with what I've brung and worked with what I've got. No way would I accept responsibility for subgrade if I'd been employed to cut it . . . and if I over cut it I would 'make good' with equivalent or better material. And yes I always like to whack down a tad high and cut it back to grade. CM1995. Reduced Level is at 14.445 meters in relation to some fixed bench mark. Cheers
Thanks Scrub, learned something new today. We use Cut (C), Fill (F) and sub grade (SG) terminology.
We have already over excavated peat pockets at subgrade level and backfilled with suitable material. most of the fill area on site was lime stabilised as well. The building will be founded on timber piles (I prefer concrete to be honest) and the concrete slabs will be the only part of the structure resting on the soil. I did some scala penetrometer tests last week and the cbr was unacceptable to depths of 150mm. We also installed subsoil drains. The site has been a nightmare because in the cut we have excavated down 4m and we are still coming across bits of wire, peat and vegetation. It once was a japanese garden and probably a tip site. I think the geotechnical engineers failed to pay enough attention to the site and did a poor geotechnical report. We have already to date removed approx 2,000 m3 of peat from site.
The subgrade was only 20mm too low before proof rolling. it is now 100mm too low. Its is going to be a warehouse and production facility founded on timber piles and ground beams. Yes there was a borehole report and the soil in places has a layer of topsoil followed by subsoil then topsoil then peat and the subsoil. It is a nightmare of a site and it has really frustrated the earthworks contractor who has to segregate the cut material and pick out the best of it to be used as fill. When you are being paid on a m3 rate it is not that profitable.
If it was me I would have a little pow-wow with the jobsite forman and the soils enginer. Get a solid game plan going. This way everyone will be on the same page and you can move foward with a plan. This will also CYA for some future problems.
I agree
Have you come to a solution Piltownmax?
yes. i talked to our surveyour and we lowered the site datum by 30mm. We can do this because the datum was installed with gps and it is generally agreed that gps has a 30mm error. this means that areas that were 50mm too low are now only 20mm. we also have a 25mm tolerance on subgrade heights. i think we are out of the woods thanks to an experienced surveyour.
Good solution, glad it worked out for ya'll.:thumbsup
sounds like you were never in the woods, you just thought you were... Thats ok, been there myself