Friction values for cast-in-place concrete piles in a typical Winnipeg clay

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Yaipukdee, Sopon
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Friction values for cast-in-place piles in a typical Winnipeg clay were studied. Shear conditions in the soil near a pile were simulated in the laboratory, using a direct shear apparatus and undisturbed samples of the clay were placed in contact with wet mortar. Softening of the clay due to soaking under controlled pressures was investigated. In addition, the shear strength of the clay at various distances from the clay-mortar interface was investigated to determine effects of the water/cement ratio and the time of curing of the mortar. It was found that the consolidated undrained shear paramaeter c, decreased with increasing moisture content whereas o remained unchanged. The friction values between the clay and cast-in-place mortar was increased due to the effect of cementing action of the mortar. At the contact surface, the friction was about 2.2 times the shear strength of the clay. The strength of the clay-mortar decreased with increasing distance from the mortar. At a distance of about 0.25 inches from the mortar, its value was approximately equal to the natural shearing strength of the clay for the same moisture content. The weakness plane was found to be a short distance from the clay-mortar interface, it was at the distance where the shear strength of the clay-mortar was about the same as the clay strength... The test results indicated that the friction value of the clay for cast-in-place concrete piles was actually the shearing strength of the clay, reduced for water-softening. The clay used for the test was a clay of medium strength. A safe friction value equal to 200 psf may be used for design practices.