Settlement on Structural FIll

[reshmislal]

Hi,

We have a site where the existing material is organic, silty and which is replaced with compacted structural fill. There is an additional 10 ft of fill as well. Do we need to wait 2 months to build on it. Or can we monitor settlement daily and begin construction after 30 days.

 

[GeoEnvGuy]

You are waiting for lower permeability material to drain. If the geotechnical report tells you to wait 2 months that is the estimate to achieve 90% or more of the settlement. If you want to accelerate the process you would need to preload and remove. These are all great things to talk to the geotechnical engineer about.

 

[BigH]

Is all the organic and "silty" being removed? What will the compacted structural fill gain purchase on? What is the nature of the structural fill? What will the additional 10 ft of fill consist of? What kind of loads are you putting on the fills? As GeoEnvGuy says - ask your geotech guy!

 

[GC_Hopi]

Best bet to accelerate settlement is to place fill, install settlement plates, place preload, measure settlement plates weekly and have a geotech analyze info to let you know when construction can proceed. Also this assumes you have done a boring to get a soil profile and soil parameters.

 

[reshmislal]

Thank you everyone, yes there are borings available.

 

[BigH]

GC Hopi - why do you need to do preload if "all" the organic and silty has been removed and replaced?

If the underlying material is competent (and we do not know what it is), the issues will be what was the nature of the structural fill and the 3.5 m additional fill. I agree if the competent material was a silty clay to clayey silt, there might be some consolidation settlement due to the 3.5 m of increased fill load and the difference of the structural fill unit weight and the unit weight of the material which was replaced - but unless we know the materials we do not know if there is a need for preload or not. The OP did not supply, nor in his second comment about boreholes are available any information that someone in this forum could judge as to what he needs to do as far as monitoring or supporting a structure (if there is one)

 

[GC_Hopi]

BigH - You are correct there is not enough information to make a well informed judgement. The OP mentioned removal of material and over 10 ft of fill (this depth of fill is likely to settle), settlement monitoring and accelerated construction schedule so I am making an inference that preload is the way to go.

 

[reshmislal]

Thank you all. The existing soil is silty sand in few locations and silty clay in couple locations. Much of the existing soil is being replaced. However the 60 day period was recommended without any basis. There was no calc to support that. So that's why the contractor would like to reduce the waiting period. I thought if we monitor settlement and see no changes then the 60 day can be reduced.

 

[GC_Hopi]

Unless you are willing to take on the geotechnical risk (I would not since that is not my area of practice), you should explain the request to the geotech who did the original report and see if they can respond. Otherwise the contractor can hire their own geotech or cover the cost of getting the original geotech to investigate this if the accelerated schedule is that important then they should not bat an eye , might take longer the 60 days though.