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Methodology of the winning prediction

Tim Chapman and Isobel Coman, Arup Geotechnics, explain the methodology behind their prediction

Basic assumptions

In producing a prediction for the pile competition a simple analysis procedure was followed to obtain the ultimate load capacities for both the control and grouted pile. It was assumed that the sand inside the piles would have formed a plug during installation, and that the shaft capacity for both piles was the same.

Pile load capacity

Shaft capacity

The value of sand density was taken as = 20kN/m3. The top surface of the water table was assumed to be 4m below ground level, with a hydrostatic profile.

As piles were driven the shaft capacity was calculated assuming k =2 and = p =36degrees. The friction angle was deduced from BS8002, table 3.

The total shaft capacity of the pile was then calculated using the relationship:

Qs = K . vave . tan . As

= 1775kN

Base capacity of control pile (C1)

The base capacity of the control pile was calculated assuming an Nq value of 85 (after Berezantsev) and from the relationship

Qb =Nq . vbase . Ab (assuming no limit on the base stress)

= 1950kN

Base capacity of grouted pile (JP1)

In calculating the base capacity of the jet grouted pile it was assumed from the data provided that UCS of the grouted sand was 10MPa and the maximum base stress of the pile would equal approximately twice the UCS value of the sand, ie 200MPa. The base capacity of the pile was calculated using the relationship:

Qb = maximum base stress . Ab

= 3280kN

Pile settlement

The pile settlement used in the production of the load settlement curves was estimated from previous experience of pile load tests in dense sand.

Pile settlement at a maximum shaft capacity, 1775kN, was assumed to be about 1% of the pile diameter (approximately 5mm).

The pile settlement at 60% of ultimate load was assumed to be 2.5% of the pile diameter (approximately 12mm).

The pile settlement at ultimate loading was assumed to be 10 to 12% of the pile diameter (approximately 50mm).

The performance of the pile was assumed to be elastic until loading equalled full shaft capacity. The pile was then assumed to behave in an increasingly plastic way towards failure load.

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