Landfill Dewatering SPT Slab Design

[Highstepper]

I am working on a concrete cap design for a landfill that has crushed tile and a slurry mixture of mostly small pebbles and some small amounts of clay in it. The landfill is about 5 acres in surface area and roughly 20 feet deep. The landfill will be partially closed until the open area (which is filled with water) is filled with the tile and then the slab will be connected to another slab. We did the geotech investigation yesterday and the SPTs were 40-50 in the first 18 inches, water was encountered (roughly level with water in the open space), then the SPTs were 4-6 until 9 feet (when borehole collapsed). I told the client that we need to dewater the open area and after that we should use a compactor before placing the slab as there will be several inches of settlement. He does not see the slab as having any problems with water in it. Is my recommendation reasonable? I will wait on the geotech report but this seems prudent or am I off? Client is hot to trot to lay the slab as they need the room for storage and 18-wheelers will be driving on it.

 

[aeoliantexan]

I am guessing that the material was end-dumped in one or several thick lifts and remains loose except for the top 18 inches, which was compacted by the truck traffic. I am assuming that it is pretty clean and coarse and behaves like crushed stone. It probaby does have potential for several inches of settlement, but needs some energy input to cause the settlement. I would try running a big vibratory roller over it to cause some of the settlement to occur and densify the top few feet to better support the trucks. It may not be necessary to dewater it to compact it; the water may help.

Be cautious; if there are too many fines in it, your roller may sink out of site. Keep a big machine with a long cable handy to rescue it.

If you are in a seismic area, it might liquefy during an earthquake. Some one else will need to advise you about that.