soft sub grade

[deefa]

Hi Tony, i,m in the process of laying a new block paved pathway and patio in my back garden, i have excavated down the required amount but i have found that a small section (approx 2sq meters) is as soft as a trifle!, although this soft part of the sub grade falls under the path area i am worried about its effects on differential settlement and am wondering the best way round the problem, i have considered digging out more of the soil in this area until i hopefully hit a more substantial base and then backfilling the hole with m.o.t? also i have read your section concerning geomembrane??..not sure mate so i thought i,d ask the man himself!

[84-1093879891]

Excavate the soft spot. Although there can be many reasons for the existence of soft spots as you describe, it's important to establish that it's not come about because of a leaky water service pipe or a leaky drain. This should become apparent once you've dug down another 2-300mm or so - if it's a water/drain problem, the ground will get wetter and wetter; if it's just a soft spot of no cause, it will start to firm up.

Assuming it is just an isolated dollop of softer material, once it's excavated, you can backfill with Type 1 material, compacting in layers no greater than 150mm until you're back at formation level for the rest of the paving.

It may be worth putting down a genuine separation membrane,such as Terram (not a weed membrane such as Plantex), and then building up your sub-base as per normal. I would probably take this route, given the existence of one identified soft spot and the uncertain nature of the rest of the ground.

You only want to be laying the block paving once, so you might as well get it right. :)

[deefa]

hi tony, well the mysterious trifle patch seems to have sorted itself out!!, after returning to the job on monday the ground is now rock solid and seems ok to lay the sub base onto?? the block paving gods seem to be smiling on us on this one mate

[84-1093879891]

It sounds as though the dry weather has firmed it up, but it's probably still worth using a membrane, for when the damp returns.