Download SOME ASPECTS OF CLAY MINERALS IN CIVIL ENGINEERING*

S O M E A S P E C T S OF C L A Y M I N E R A L S
CIVIL E N G I N E E R I N G *
IN
By E. H. STEGER
Central Laboratory, George Wimpey & Co. Ltd., Hayes, Middlesex
[Read 15th April, 1959.]
ABSTRACT
The utilization of clay minerals in grouting is discussed.
Very generally, the design of an economic foundation for a given
structure resolves itself into considerations of bearing capacity,
settlements and permeability. When dealing with cohesive soils-clays--methods have been developed in the last thirty to forty years
which enable the determination of mechanical properties like shear
strength, compressibility, plasticity, permeability, etc., by laboratory
tests. There is no doubt that all these properties depend basically
on the nature of the clay mineral present in any particular soil, but,
for practical purposes, the vast majority of problems can be solved
mathematically or experimentally without any detailed knowledge
of these clay minerals. It may well be that the understanding of
mechanical properties of soils can be made easier by knowing the
minerals which are present, but methods of identification like differential thermal analysis or X-ray diffraction are currently far too
costly. A promising development in this connection seems to have
been made by work on dye-staining methods.
In all problems involving work in clay soils, the foundation engineer has to accept their properties as they are and make the best of
them. Compared to the structural engineer, who can determine
the strength of his steel or his concrete, this is rather an unenviable
position.
This passive dependence on the natural subsoil can be changed in
the case of some non-cohesive soils, i.e., sands and gravels. Here
it is often possible to influence their properties--mainly permeability
and strength--by grouting, a process which might be defined as the
forcing of one or more chemicals, suspensions, or emulsions into the
voids of the soil; only suspensions will be considered here.t
*This paper was read at a discussion on "Clay Minerals and Industry" and the
main emphasis is on engineering aspects. Nevertheless, the particular industrial
use of clay minerals described may interest a wider circle of scientists.
?Similar suspensions are also used for surface work, such as waterproofing
reservoirs and canals.
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CLAY
MINERALS IN CIVIL ENGINEERING
107
The suspensions which are used to reduce permeability are almost
always clay slurries, with or without the addition of a chemical or
cement, depending on the specific problem. This type of grout will
penetrate gravel and coarse and medium sands, rendering the soil
impermeable without significantly increasing its strength. It has
been used frequently in alluvial deposits to prevent the flow of water
through and underneath the foundations of large dams, to enable
cut-off trenches, and other excavations, to be carried out without
sheet piling or lowering of the ground-water table, and for the driving
of tunnels by means of compressed air through sands and gravels.
The most recent applications in this country are the extensive grouting
carried out during the construction of tunnels under the Thames at
Dartford, Kent, and under the Clyde at Whiteinch, Glasgow.
The purpose of injecting a clay grout is the formation of a gel in
the voids. This gel formation is made possible by the thixotropy
exhibited by certain clay slurries. Thixotropy is shown most strongly
by some bentonites--Na- and K-montmorillonites. When agitated,
the suspension is fluid and can be forced into voids, but as soon as
agitation ceases, a gel begins to form. Owing to its ability to absorb
water up to 700 times its dry weight, the clay filling the voids expands
and thus effectively prevents the flow of water.
With a suitable concentration - - usually between 10 per cent.
and 15 per cent.--no d'.cantation takes place except if the stability of
the grout is reduced by the electrolyte content of the soil. In some
instances the stability is purposely reduced or neutralized by means
of the addition of chemicaAs in order to delay the gelling time. This
may be done to ensure a greater i:adius of penetration when it is desired
to force the grout a given distance from the injection pipes.
It may be of interest to describe the technique of grouting. One
method which is widely used, and which was developed in France,*
is briefly as follows: a 3 in. diameter casing is sunk into the ground
to the full depth which has to be grouted. Inside this casing the
injection pipe is lowered. This consists of a 1~ in. diameter hollow
tube with groups of four 5 mm holes at 12 in. centres. Each group
of holes is covered by a tightly fitting rubber sleeve. The grout is
applied through an inner 1 in. pipe, choosing the depth of application
by using packers above and below the appropriate group of four
holes. When the 1~ in. pipe is in position a weak cement-clay grout
is pumped through the bottom of this pipe so that it completely fills
*Most of the clay grouting techniques originated in France where some injections were done ill the early 19th century,
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E . H . STEGER
the space between it and the 3 in. casing. This is then withdrawn.
Now grout is forced through the 189 in. pipe, through the cementclay lining and into the ground. The rubber sheath ensures that
there can be no back flow and it also makes it possible to keep the
11- in. tube permanently clear by flushing it through with water under
pressure. Generally, grout is injected until refusal, which means a
gradual building-up of pressures up to between 200 and 301) lb/in. 2
This type of installation permits various types of grouts, like pure
clay slurries, clay-cement mixes or chemicals, to be injected at
various times and depths, according to the nature of the ground and
the purpose of the treatment.
It will be obvious that, in order to achieve success, it is essential
to design a suitable grout. This can only be done when all the
relevant properties of the grout can be defined and controlled. The
ideal is to make this process similar to the design of a concrete mix.
After a study of the grain-size and permeability characteristics of
the material to be grouted, and of the properties of the groundwater, it should be possible to design an injection liquid of known
viscosity. Depending on the local conditions, it is then possible to
calculate the radius of penetration from each pipe and so determine
the distances at which injections will have to be made in order to
ensure a continuous grout curtain. This presupposes a reliable
knowledge of the thixotropic behaviour of the grout and the ability
to delay its natural gelling time to a calculated amount. It should
further be possible to reduce site testing and control to a minimum
--something like the Marsh funnel used to determine a suitable
viscosity for drilling fluids. These are, of course, the ideal conditions which are still very much a matter of the future.
A knowledge of the lattice-structure and colloid-chemical properties of clay minerals makes it possible, in the first place, to choose
a suitable raw material. Na-montmorillonite has already been
mentioned as the ideal substance. It has been shown, for example,
that a Na-montmorillonite has a liquid limit of 700 per cent. compared to 160 per cent. for a Ca-montmoriUonite. The free swell of
the latter, in water, is one tenth that of the Na-saturated mineral.
Consequently, there is considerable difference even within montmorillonites. Neither Na- or K-montmorillonite may be readily
available at a particular site, but, from a knowledge of the cationexchange properties of clay minerals it is often possible to use a
local clay and, by chemical treatment, effect cation-exchange by
replacing the natural ions with Na + and K +. Usually this is done
CLAY MINERALS IN CIVIL ENGINEERING
109
in suspension and the product is then dried and powdered. A local
clay which has a large proportion of colloidal particles and which
has been activated in this way will have the required properties of
thixotropy and will, of course, be much more economical to use
than :an imported material.
Assuming that a thixotropic grout is available, its viscosity is of
great importance for the actual injection process. It must be possible
to pump it easily so that no pressure is wasted above ground or in the
pipes. Gelling should generally not take place the moment the rate
of flow is reduced as this might prevent the grout from penetrating
the smallest voids. Once in place, however, swelling and thixotropic gelling must take place at the required distance and time.
It might be noted here that thixotropic suspensions behave as nonNewtonian liquids and the rheological laws governing their flow
are not yet fully explained.
Although many clay (and clay-cement or clay-chemical) permeability reducing operations have been carried out successfully all over
the world and on very large scales, the methods and processes used
still depend to a large extent on practical experience, if not on
trial-and-error methods. Engineers are stilt some way from being
able to design mixes with the degree of confidence one would wish.
This is largely due to the fact that the knowledge of certain aspects
of the theory of clay suspensions is still in its infancy. It seems also
that new laboratory methods are needed to explain the thixotropic
behaviour of clay suspensions and to establish a natural relationship
between this and the viscosity of such fluids.