Filling large hole in a road

JimMorrissey · Sep 10, 2005

So I eventually removed a massive boulder (size of a car) from the middle of my soon to be driveway. I pulled the broken rock out of the hole and filled it in with the original soil and some extra. The problem is the hole now behaves like a giant jelly donut. If I hit it with my backhoe the dirt in the hole moves like jelly. It's about 6' deep and 10' around. I filled it in lifts and attempted to compact it. Not much compaction took place. The soil is damp (partial clay, say 50%) and maybe on the wetter side than damp. I know it's hard to compact damp soils.....there is an optimum water content for correct compaction.

I'm covering the entire sub-base surface with fabric. I'm just not sure if I should remove the soil and replace it with stone and crushed gravel so I don't have a depression in the road in a year or two. I'm thinking I should, but since I'm using fabric, not sure if I need to. The fabric should bridge the soft spots, that's primarily what it's for. What do you think......

inveresk · Sep 10, 2005

If the fill hasn't been properly compacted, you're likely in time to have problems with deformation of the base. How much will depend on loading. If the driveway is only to be lightly loaded you may get away with it for long enough. If you've heavily loaded vehicles tracking the driveway, problems will arise sooner. However, if you've upfilled to the depth of a car,I can see you'll want to avoid excavating this again (shame you didn't break the rock up and just put it back in the hole). If you still have building works to do, can you leave the road base as is until you finish these, allow construction vehicles to do the compaction for you then top the road off once they're finished and gone? Another option, belt and braces, would be to pour a concrete raft locally over the area of the fill (plus an overlap of, say, 2 feet all round) and include some fairly heavy steel mesh, A193 for example, a couple of inches from the bottom of the slab before topping off. If you've concrete works going on elsewhere, this might be the easiest solution - just order an extra couple of cube.

JimMorrissey · Sep 10, 2005

Inver,

Definite shame but I didn't want a bunch of heaving going on when the ground freezes and thaws pushing massive chunks of rock up into the road surface. It's definitely wet down there at the turn. I know exactly where the hole edges are so I think I'll dig the darn hole out again and fill about 30% of it with broken concrete and stones (up to roughly the frost line). Then I'll fill in lifts for about 25%, then fill the rest with the trucked in 1 1/2" crushed gravel when the road base is being brought in. The fabric will flow into the depression and the base fill will be flush with the rest of the road.

I knew in the back of my mind when I was filling it with dirt I was making a mistake. Oh well, live and learn. This will not likely be my last automobile sized boulder. /forums/images/graemlins/mad.gif

beenthere · Sep 10, 2005

I think you are right to dig it out. At least now is better time than later. If its a pothole that will later be able to collect water, I would consider getting a drain tile in there to drain the area, if that is at all possible.
If clay gets wet and/or stays wet - it's trouble like you say - large jelly roll.

have_blue · Sep 10, 2005

Jim,

I hate jelly holes. I've had several, and they're a real pain until they compact. The problem is caused by covering up mud with light, dry soil that just floats on top of the heavier mud.

Don't know the solution in your parts, but the solution here is rain. When it rains, the jelly holes firm up just fine. I suspect the dirt on top of the mud becomes heavy and settles into the underlying mud, and eventually squeezes out the water. My jelly holes eventually become firmer than the surrounding soil just from a hard rain or 2.

beenthere · Sep 10, 2005

Clay won't firm up that way, unless you get the water out.
But some soils work as you describe, and most often I find when it works that way, it is really the rain driving the underlying frost out of the ground. When water sits on top of frost layer, the area is pretty unstable. But get that frost out, and the area firms up pretty good.
Clay is a different sort of animal, and doesn't always act this way as it holds water pretty well and won't let it 'pass through'. It's why clay is used to line a pond or to line a garbage dump and the like.

Egon · Sep 10, 2005

To check clay for optimum compaction moisture grab a handfull and roll it between your hands. If you can roll it into nice pencil sized thread with no problem its probably about right.

If you dig out the hole transition the edges at a minimum of 45 degrees.

Egon

JimMorrissey · Sep 10, 2005

Bob,

That's some good news. Maybe I'll wait a bit before I dig it up. How deep have you jelly holes been in the past.

JimMorrissey · Sep 10, 2005

This material is not pure clay. It's a mix of clay and "dirt". Actaully if you come from a real "clay area", say Texas, then you'd likely not call this clay. Although it certainly has a bit of clay in it.

I'm hoping with a good hard rain and a nice dry period that it'll settle a bit. Not sure I have the time for to wait for those conditions though.

Soundguy · Sep 11, 2005

Uh.. filling it with a 50% (wet) clay mix.. and you wonder why it is a jelly donut?

Either let it dry thouroughly.. or dig it out and put some clean fill in.

I'm not a fan of using a large wad of clay in my road base.. some clays swell and hold water.. that makes for a huge problem under a road...

Soundguy