Specifing what type of Geogrid to use in a gravel raft foundation

[EireChch]

Mo Chara,

I am working in Chirstchurch in New Zealand. As some of you might we have had some big EQs over the last 3 years which has made everyone realize the phenomenon on liquefaction.
Long storey short, the residential project i am working on has been given low risk liquefaction, however it would be prudent not to consider the risk of liquefaction.

A number of possible foundation options have been given by the local authority (Ministry of Business, Innovation and Employment).

The option that we have gone with is an enhanced concrete slab (rib raft slab) over an engineered hardfill raft incorperating a layer of geogrid, placed at the base of the raft. The hardfill raft has to be a minimum thickness of 0.8m. The idea of this foundation is that if the ground liquefies beneath the raft should bridge over the liquefiable soils and reduce the potential for differential settlmeent and any differential settlement can (hopefully) be absorbed in the enhanced slab. The ground conditions are loose sand to at least 2m depth, groundwater is at 1.2m depth.

My question is how do i specify what type of geogrid to use. I have seen some geogrids, bi-axial for example that offer a ultimate tensile strength of 55KN,. There has been no liquefaction analysis undertaken as it is not a requirment for the area where my site is (i.e. Technical Categorey 2 (TC2), for TC3 sites a liquefaction analysis is required).

Any feedback or direction is appreciated.

Regards
Matt

 

[GeoPaveTraffic]

I would use a Bi-Axial or Tri-Axial grid for what you are doing.

Mike Lambert

 

[EireChch]

Hi GPT, thanks for the reply.

Yes I agree it should be a bi axial or tri axial grid.

I am wondering what technical input is required to determine what strength of grid?

My line of thought is that the soils beneath the raft will liquefy and not provide adequate support to the raft resulting in say 100mm of settlement. howeverthat's where the Geogrid will come in. The geogrid will start to sag with the raft but the tensile strength should resist the sag.

Is it a case of working out how much deflection the raft will have and specify a grid that will only allow 50mm or less ?

 

[GeoPaveTraffic]

Now you are getting into whether the rock, with or without geogrid, will reduce damage to the structure. That is a much more complicated question.

Generally speaking, I don't believe that the rock without geogrid is likely to improve performance during an earthquake. I also do not believe that rock with grid, any grid, will significantly improve peformance during an earthquake. The grid will increase the stiffness of the rock some, but not much and compared to the mat foundation under the house not at all.

The mat foundation will likely prevent most houses form failing, i.e. falling down, due to liquefaction. However, the foundation may shift or settle unevenly and I really don't see the rock pad preveneting or changing that.

To directly answer your question you would need to have a detailed study indicating the amount of movement and how that movement is expected to change across the footprint of the house.

Mike Lambert