Influence of Cement SAND in Pile Capacity

[vinothsekar]

Hi,

I'm working on pile capacity calculations over on carbonate soil for Container berth terminal construction. We are going to construct 580m length and 56m width berth. Berth have 6 grids with an equal width spacing of 11.5m expect the first grid. Pile diameter is 1.4m.

Each grids line hols 64 pile with centre to centre 9m approx spacing. The dredging level at A grid is - 14.5 which is followed by the slope 1:3 till the 6 grid.

Embedded length of piles in all grids by Calculating pile capacity is 30m. Reference Euro code 7 part-1 and Geotechnical characters have taken reference from Tomlinson 5th edition.

Considering the parameters Phi 38 deg by taking avg N value of 40. Laboratory test show 36 to 38 deg from direct shear. The design safe load is 1200 T for each pile

General Soil strata is 6 m Sand from top level followed by 9m cemented sand again 2.5m Sand followed by 2m cemented sand intermixed with Fragments of Falun which is followed 2m Sand underneath by 2m of cemented sand followed by 25m Sand.

The problem which I'm facing here is 5th & 6th grid pile foundation is placing over the Cemented Sand [Caco3 % varying from 30 to 60 %]. Increasing pile embedded depth will resolve this issue but it will affect over excess cost.

Will carbonate sand influences in reduction of end bearing capacity.

What will be the percentage of reduction will be there in Pile capacity?

Because as of now there is no exact design method for pile foundation in carbonate sand.

Kindly share your valuable feedback on this scenario if possible.

 

[EireChch]

We found piles in Calcarenite (carbonate sandstone) all the time. I don’t see wha the big issue is? What are you afraid of cemented sand loosing its cohesion? If so, this is not an input in your pile design and if there is some loss of cohesion will it reduce your phi angle? Probably not. the shear box test you have done would have had no cohesion due to remoulding of the sand and as such your phi from your shear box should be somewhat reliable

 

[vinothsekar]

Carbonate soils differ in many ways from the siliceous sands. An important distinction is that the major constituent of carbonate soils is calcium carbonate which has a low hardness value compared to quartz, the predominant constituent of the silica rich sediments. Susceptibility of carbonate soils to disintegration (crushing) into smaller fractions at relatively low stress levels is partly attributed to this condition. Typically, carbonate soils have large inter-particle and intra-particle porosity resulting high void ratio and low density and hence are more compressible than soils from a silica deposit. A soil matrix which is predominately carbonate is more likely to undergo degradation due to crushing and compressibility of the material than soil which has low carbonate.
I would like to know how these crushability of calcareous sand will impact in reduction of pile capacity.

Vinothkumar S
Geotechnical Engineer

 

[maxim22]

"Carbonate soils differ in many ways from the siliceous sands. An important distinction is that the major constituent of carbonate soils is calcium carbonate which has a low hardness value compared to quartz, the predominant constituent of the silica rich sediments. Susceptibility of carbonate soils to disintegration (crushing) into smaller fractions at relatively low stress levels is partly attributed to this condition. Typically, carbonate soils have large inter-particle and intra-particle porosity resulting high void ratio and low density and hence are more compressible than soils from a silica deposit. A soil matrix which is predominately carbonate is more likely to undergo degradation due to crushing and compressibility of the material than soil which has low carbonate."

The above seems to be a general comment from published text maybe not applicable to piles.

Get good core samples to check the material parameters as average N=40 is a weak rock/strong soil boundary. Do void ratio lab tests as well. How much stress/acid do you need to disintegrate the sample?

If you are too late and in design stage and if end bearing is a concern, I guess there is reason enough to increase the FoS to say 10 like we do in carbonate rocks if there are dissolution features from geological history. I doubt skin friction will be affected for design life.

If the ground is unstable maybe you need deeper piles/inject grout around piles.

Allow for test piles and loading pre-contract.