Construction Methods Of Mat Footing On Wet Ground Level 2

[aldamal]

Our foundation is designed to be Mat Footing. Per Soil Test Report, water table is found at elev -1.8m with sandy material. The Geo-Tech Engineer recommends to backfill rock material to a depth of 1.0m. As backfilling is progressing up to a depth of 60cm, compaction was started per direction of consultant, but the rock material sink down even if we add more fill material.

My question is, what is the appropriate method of construction on this kind foundation? Any expert on this, please extend your help. I am new on this kind of site condition.

 

[Ron]

You might consider placing a "mud slab", which is a concrete placement using lean concrete (lower strength than you6 mat foundation) that you can allow to harden and settle a bit by self weight, that will give you a good stable base for your mat. This is also called a "work slab", "blinding" or a variety of other terms depending on your locale.

 

[aldamal]

Thanks a lot Ron,

So no need of rock fill as recommended by Geo-Tech Engr?He is requiring us to place rock fill with sizes range from 2" - 6", up to the level of 1.0m from the soft ground level and then place a blind concrete of 150mm thick.Please clarify further on this point.

 

[SlideRuleEra]

aldamal - As Ron mentioned, "mud slab" has several names and is only to provide a suitable working surface for placement of the structural concrete. The mud slab does not furnish significant support to the mat footing. A believe that the term "blind concrete" is just another term for "mud slab".

Since the mat foundation needs proper soil to bear on, suggest that you follow to Geotech Engineer's recommendation. You also asked about the appropriate method of construction when ground water is a problem. The best way is to dewater the soil. For a relatively shallow excavation, such as yours, this MAY be performed by using diaphragm (mud hog) pumps to keep the excavation free of standing water while work is underway. Under severe groundwater conditions, pumping may be needed continuously - day & night. There are other ways to handle high water levels such as wellpoints and cofferdams, but I doubt if you will need to them.

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[aldamal]

Thanks sliderule,

Water is not a problem. The problem is, when compacting the fill material in layer of 60cm thick, the material sink down even if we add more rock material. In this case, we cannot achieve the required compaction density of 95%. In other words, the ground beneath the fill material is not stable. Please kindly give your opinion.

 

[Ron]

The ground can ultimately be stabilized by adding gravel and mixing in, but that will be to a limited depth. The mud slab or blinding will likely solve your issue or you can over-excavate and backfill with controlled low strength material (CLSM). If your foundation soils are unstable and loose, expect settlement of your mat.

 

[aldamal]

Thanks ron