Slab Reconstruction Over Landfill 1

[hayeska]

General Information

Existing PEMB built over landfill (1980)
Previous repairs made to perimeter walls (compaction grouted) show negligible settlement.
Slab replaced 1995 (non-structural) has settled 6"+ in several locations.

Contractor has proposed pressure grouting to raise slab (with no guarantees) or 10'X10' grid of micropiles (80+ @ 30';15'refuse +15' embedment; 20k working load) and structural slab.

We would be concerned with the feasibility of the micropiles through the refuse, eccentricity, unbraced length
etc., along with the potential environmental hazards (methane release, groundwater contamination etc.)

We are considering a series of RC beams (no piles) with a structural slab, (Similar to a waffle slab) vs. using a mat slab.

We know there will be additional settlement and these RC beams will have to span the voids below. We will survey the existing slab and design the spans with respect to anticipated settlement.

We are proposing to install ports that will allow such settlement to be monitored and allow future pressure grouting as warranted.

Looking for a discussion (pros and cons).

Thanks in advance

 

[oldestguy]

A few questions:

Is there a venting layer under the slabs for methane removal, or don't you have concerns? If there is one, will it be compromised by grouting?

Can the grouting contractor come back later and get through his former work for adding more grout?

 

[hayeska]

Oldestguy,

The landfill was closed in the mid 70's. There is no known methane venting system at this or any other developed site.

The owners do not want to pressure grout the existing slab. When it was replaced in 1995 they put no steel in the slab; used fibers. Needless to say the slab has performed poorly.

The intent is to provide a structural slab that will be able to withstand areas of localized settlement.

The slab footprint is 60'X100'.

We are proposing to provide monitoring ports that will also allow the new structural ribbed mat slab to be "mud jacked" in the future as necessary. This is a means to mitigate future issues.

 

[oldestguy]

On the methane situation, it does not smell. Liability for possible future explosion or illness is still there for many more years. I'd ask for a hold harmless agreement for any engineering.

 

[Ron]

OG...good advice!

 

[EireChch]

Apart from the issues raised by OG regarding gas monitoring and control how about trying a re-levellable slab. I am not sure how common they are in the States but we used them quite regularly in Christchurch NZ. The are essentially two slabs a lower 200m waffle slab with a thinner 100mm upper slab. The two slabs have jacking screws placed in a 2m x 2m grid patter which allow the slabs to be jacked apart following differential settlement. They are relatively expensive but may be in the same ball park as the waffle slab and ground beams you have discussed. Search Firth concrete surface Strucutre for more information.

Regards
Eire

 

[hayeska]

OG/Ron,

Yes, that is good advice.

Eire,

I took a look at the system you referenced. Very interesting.

We would be attempting to achieve the same goal by a different means.

Thanks for the information.