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.
