A laboratory investigation into the stabilization of natural soils using two waste materials
Date
2014-12-04
Authors
Barrett, Andrew John
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Abstract
Soil stabilization is the generic term for any process which
has as its aim the upgrading or improvement of one or more
soil properties. In South Africa, stabilization is usually
effected using a combination of mechanical densification and
the addition of a binder to the soil. Two waste materials, PFA
(a power station waste) and phosphogypsum (a waste from the
fertilizer industry) have been shown to be useful binders
overseas. The purpose of this project was to investigate
local samples of these waste materials to establish their
potential as soil stabilizers.
A limited amount of research into the use of these two
materials has been carried out in South Africa. The results of
these programmes are discussed. Generally, however, none
of these programmes were comprehensive enough to reach
categoric conclusions regarding either binder.
The soils used in the investigation were taken from various
locations in the Johannesburg Municipal area in which it is
intended to construct surfaced roads for light traffic in the
near future. The soils chosen cover a range of plasticity
indices from 0 to 30, but unfortunately, were very similar
mineralogically.
The phosphogypsum used was supplied by Triomf Fertilizer (Pty)
Limited. Two samples of PFA were used.
One of them was supplied by Darling and Hodgson, Limited, who
are extracting a "selected" ash from Grootvlei power station,
and intend to make it commercially available at some stage.
The other ash was obtained from Kelvin power station.
Tests were done to establish both the short and long term
effects of the binders on the soils. The short term effects
were investigated by looking at changes in the plasticity and
moisture density relationships for the soils immediately after
the addition of the binders. The long term effects were
investigated by looking at the changes in unconfined compressive
strength of moulded soil samples with the addition of the
binders. (Samples were moulded at 100% Mod AASHO density and
optimum moisture content in a tapered cylindrical mould by
static compaction). These effects were looked at from 3
different angles.
i) the effect of binder composition (i.e.
different ratios of PFA/lime and PFA/cement)
ii) the effect of binder content
iii) the effect of age on strength gain
The results of the testing programme were not very encouraging.
For the binder contents used, there were minimum short term
effects for both binder types. The addition of phosphogypsum
to the soils resulted in only very small strength increases.
For the PFA binders, it appears that the lime content of many
of the PFA/lime binders fell in a range where only short term
reactions could be expected. However, even where sufficient
lime was present to satisfy the initial lime demand of the
soil, the PFA appeared to act mainly as a diluent to the lime.
Similar conclusions apply to the PFA/cement binders.
The general nature of PFA/lime stabilization was uncovered in
this testing programme, but further tests will have conducted
before conclusions regarding the quality of our local materials
can be drawn.
Description
A Dissertation Submitted to the Faculty of Engineering,
University of the Witwatersrand, Johannesburg, in partial
fulfillment for the Degree of Master of Science in Engineering.
Johannesburg, 1979.