Effect of finite layer thickness on the angle of internal friction of sandy soils

Abstract

The present study investigates the effects of sample length and diameter on triaxial friction angle of sand. This was done in an aim to establish the relationship between physical dimensions of sand sample tested and bearing capaci ty factors in particular reference to a footing resting on a sand layer of finite thickness. The end condition of the both triaxial sample and bearing sand layer were maintained identical. Drained triaxial tests were performed on three selected sand samples of Bangladesh. The initial density of each of the soil sample were maintained approximately constant during triaxial test by allowing free fall of the particles to the sample mould. A special arrangement was designed and constructed for the purpose. Triaxial tests were dOlle using three sample diameters varying from 38mm to 71mm and five sample lengths in the range of 38mm to 178mm. As such 14 sample moulds have been designed and fabricated. The maximum and minimum densities of the sand samples were determined using the method of Kolbuszews!ti (1948). Calibration for density against height of fall were carried out in order to obtain uniform density of triaxial sample. Sample end platens of triaxial apparatus were covered with sand paper (grade 3) to achieve rough end conditions. A vacu"m pressure of approximately 1 N/cm2 was used duril}g preparation and placement of the sample. Two specimen were tesLed for each of the triaxial test using cell pressures of 10 N/cm2 and 14 N/cm2 respectively. Resul ts of tr iaxial tes t indicates that the apparent angle of internal friction decreases with the increase of sample length upto a certain value of LID, where Land D are length and diameter of ti,e sample respectively. The angle of friction then becomes constant. The LID ratio at which the friction angle becomes constant are found to be dependent on sample diameter. It is also noticed that there is an optimum diameter at which the angle of internal friction becomes minimum for a particular soil. Beyond this limit (on either side) the apparent angle of friction increases. lising these values of angle of internal friction the bearing capacity factors Ny, Nq given by Terzaghi (1943) and Meyerhof (1963) were estimated. It was observed that these factors also vary with Lin in a similar manner of apparent angle of internal friction. Factors were introduced to consider the effect of LID ratio on these hearing capacity factors. The effects are termed as sample dimensi on factors Fy' Fq for Ny' Nq respectively. They are expressed by tileratio of bearing capacity factor at particular LID to that at which these factors Ny and Nq becomes constant. This factors were compared wi th the thiclmess factors used by Abedin (1986) to COllsider the effect of layer thickness on bearing capacity factors for a strip footing resting on a sand layer of finite thickness. It is observed that the sample dimension factors and thickness factors of Abed in (1986) are of similar nature. As such the present invesl.igation indicates that a modified triaxial test may be introduced to estimate the effect of layer thickness on bearing capacity.