Where poor ground conditions make traditional forms of construction expensive,
it may be economically viable to attempt to improve the engineering properties
of the ground before building on it. This can be done by reducing the pore
water pressure, by reducing the volume of voids in the soil, or by adding
The properties of soil which most affect the cost of construction are strengthand
compressibility. Both can be improved by reducing the volume of the voids
in the soil mass. Water must be displaced from saturated soils in order
to reduce the volume of the voids. This may take months if the permeability
of the soil is low.
Soil which is highly compressible is prone to volume change when a load
is applied. This leads to settlement. Fine-grained soils which have been
compressed and then allowed to swell, experience a smaller volume change
when re-compressed. Loosely-compacted coarse-grained soils may exhibit
little change in volume under static loads, but become unstable and exhibit
large volume changes when either vibrated or flooded and then drained.
The sudden application
of a load to a saturated soil produces an immediate increase in porewater
pressure. Over time, the excess porewater pressure will dissipate, the
effective stress in the soil will increase and settlement will increase.
Since shear strength is related to effective stress, it may be necessary
to control the rate of construction to avoid a shear failure. This was
the case, for example, when approach embankments were constructed on soft
alluvium, for the bridge which carried the M180 motorway over the River
Trent near Scunthorpe. The rate at which the excess water pressure dissipates,
and settlement occurs, depends on the permeability of the soil, the amount
of water to be expelled and the distance the water must travel.
Collapse will occur if the shear stress along a potential failure surface
exceeds the shear strength of the soil. Shear strength depends on the effective
normal stress, which depends on the porewater pressure. Undrained loading
causes an increase in porewater pressure equal to the change in the total
normal stress so that there is no increase in strength to match the change
in the shear stress.
The shear strength can be increased either by decreasing the water pressure
or reducing the void ratio of the soil to produce a peak strength which
exceeds the critical shear stress.
Fine-grained soils have a lower permeability than coarse-grained soils,
thus excess porewater pressures take longer to dissipate. Consolidation
reduces the void ratio of the soil and further decreases the permeability.
Real soils are not hydraulically isotropic: the natural orientation of
particles in soils which have been consolidated vertically tends to produce
a horizontal permeability which is greater than the vertical permeability.
Thin horizontal layers of coarse-grained soil in a mass of fine-grained
soil may dramatically increase the horizontal permeability while having
little effect on the vertical permeability. It is possible to increase
the drainage rate without changing the permeability of the bulk of the
soil by introducing layer drains (sandwicks) or fracturing the soil. The
most effective way to reduce seepage into an excavation, through or under
a dam, or away from contaminated ground is to create a low permeability
zone perpendicular to the direction of flow.
|Pumping water out of the ground will cause a local lowering of the
ground water level and a decrease in water pressure. Both will return to
their natural state when pumping stops. The rate of drawdown and the radius
of influence depend on the permeability of the soil: Low permeability implies
slow drawdown and large radius. Decreasing the water pressure increases
the effective stress, which increases the shear strength and causes settlement.
|The introduction of a grid of vertical drains, connected by layer of
highly permeable soil, reduces the distance water has to travel through
the natural soil and facilitates horizontal flow. This limits the excess
water pressure generated during and after construction and increases the
rate of settlement.
to an applied pressure occurs over a period of time. A proportion of the
final settlement can be achieved prior to construction by pre-loading the
soil. The larger the pre-load, the less time it will take to achieve the
final settlement. Pre-consolidating the ground in this way tends to be
an expensive solution compared with the use of piles to support localised
loads such as columns. Pre-consolidation may be a cost-effective way of
reducing the settlement due to lightly distributed loads from roads or
warehouse or supermarket floors provided that material is readily available
to provide the pre-loading. Pre-consolidation is normally designed to take
6 - 9 months.
Compaction is a dynamic process, reducing the volume of soil by expelling
air. The moisture content is not altered significantly under normal circumstances.
(Water may migrate a short distance from the point of application but is
forced to return when compaction is applied to the adjacent soil). Compaction
is most effective when applied to a thin layer because the energy dissipates
with distance. Vibration is the most effective method of compacting loose
Compaction of fill
Fills are normally compacted in layers between 300mm and 600mm thick. For
granular soils, a motor on the back of the roller is used to rotate an
eccentric mass causing the roller to vibrate. For fine-grained soils, the
roller may be fitted with blunt spikes known as sheep's feet. Sheep's foot
rollers produce a kneeding action which changes the shape of clods of soil
and displaces air from the spaces between the clods.
involves lifting and dropping a heavy weight several times in one place.
The process is repeated on a grid pattern across the site. Trials in the
UK indicate that the masses in the range 5 to 10 tonnes and drops in the
range 5 to 10m are effective for compacting loose sand but not clay. Masses
up to 190 tonnes and drops of 25m are used by TLM (Technique Louis Ménard)
in France. Such heavy compaction causes fractures through which water can
flow. This, according to the proponents of the system, enables fine-grained
soils to be compacted. Heavy compaction tends to annoy the neighbours,
which limits its use in built-up areas.
compactive energy per blow = m.g.h
where m = mass, g = gravitational constant, h = drop.
estimated depth of compaction = n.Ö(m.h)
where n is an empirical constant between 0.3 and 1 depending on
the grain size distribution and degree of saturation (0.5-1 for sands,
0.3-0.5 for silts and clayey soils).
and vibro-replacement use a vibrating poker to make a hole in the ground.
Soil is displaced sideways, not removed from the ground.
in coarse-grained soils the poker may be removed slowly while still
vibrating. This causes the sides of the hole to collapse and results in
a depression in the ground surface.
in fine-grained soils it is usual to fill the hole with coarse aggregate
(up to 50mm). The poker may be used to compact the stone column in layers.
A typical column might be 5m deep and 500mm diameter. A line of columns
at say 3m centres can be used to support a reinforced concrete ground beam
effectively producing a piled foundation.
Injecting cementitious material into a soil mass tends to reduce permeability,
cause swelling and may increase strength.
Grout injection into fractured rock which forms the foundation of a
dam is possibly the oldest and best known application. Grout injection
has been used successfully to strengthen and reduce permeability of soil
around a basement excavation below the water table. It has also been used
to control the settlement of structures adjacent to tunnel excavations
in London: predicted settlements of 60mm, which would have caused extensive
damage to old buildings, were limited to 10mm.
Silty soils with high water contents are unsuitable for embankment construction
in their natural state because they are difficult to compact. They can
be improved by mixing hydrated lime with the soil.
Geo-textiles can be used for:
segregation of layers
Rock-fill laid on soft ground to form a road or embankment base can be
prevented from punching into the soil below using a geotextile underlay.
Horizontal membranes can be used to provide tensile re-inforcement and
reduce settlement. There are two primary difficulties:
(i) aligning the mebrane in the direction of the principal tensile stress,
which is probably not horizontal, and
(ii) the fact that geotextiles have a low modulus of elasticity and are
plastic and therefore tend to creep.
a drainage layer
Either as a water-conductor or as a filter to reduce the migration of fine
particles into a granular soil drain.
an impermeable barrier
To prevent or control the flow of contaminated groundwater from or in land-fill