Dispersion of Surface Load

[dik]

I have to design a tunnel that has a 50,000 lb load located over about 5' of compacted granular fill. Is there a reference that can be used for distributing the load laterally to reduce the effect of this on the roof? Also looking at using a geogrid material to help disburse the load. Is this normally cost effective?

any help appreciated, Dik

 

[civilperson]

Boussinesq solved the problem of stresses produced at any point in a theoretical soil as a result of a point load. A stiff reinforced concrete beam can spread the load out to the length of the beam.

 

[GeoPaveTraffic]

Depending on the geometry, a tunnel may be stiffer than the soil on either side and could attract the entire 50kip load. It becomes a matter of relative stiffness and requires a careful study.

 

[cap4000]

Boussinesq's formula for truly elastic materials and Westergaard's formula with geogrids are typically used. You can also use a 1H:2V formula for a quick and dirty method to determine your pressure at 5 foot down. There are also typical "pressure bulb" or elongated tear drop shape curves that may also help you at various levels down into the gravel layers. The vertical stess diminishes and eventually vanishes to zero due to the friction angle of the gravel. As a last resort you may want to consider a concrete slab subbase to spread the 50k load even quicker.

 

[dik]

Thanks gentlemen... I thought it would be a difficult analysis... I think I'll just treat it as a point load distributed over about a 2'x2' area...

Dik