What is an "Expansive Soil"?
Expansive soils contain minerals such as
smectite clays that are capable of absorbing water. When they absorb water they
increase in volume. The more water they absorb the more their volume increases.
Expansions of ten percent or more are not uncommon. This change in volume can
exert enough force on a building or other structure to cause damage.
Cracked foundations, floors and basement walls
are typical types of damage done by swelling soils. Damage to the upper floors
of the building can occur when motion in the structure is significant.
Expansive soils will also shrink when they dry
out. This shrinkage can remove support from buildings or other structures and
result in damaging subsidence. Fissures in the soil can also develop. These
fissures can facilitate the deep penetration of water when moist conditions or
runoff occurs. This produces a cycle of shrinkage and swelling that places
repetitive stress on structures.
Why Do These Soils Expand?
Soils are composed of a variety of materials,
most of which do not expand in the presence of moisture. However, a number of
clay minerals are expansive. These include: smectite, bentonite,
montmorillonite, beidellite, vermiculite, attapulgite, nontronite, illite and
chlorite. There are also some sulfate salts that will expand with changes in
temperature. When a soil contains a large amount of expansive minerals it has
the potential of significant expansion. When the soil contains very little
expansive minerals it has little expansive potential.
Changes in Moisture Content Trigger Damage
When expansive soils are present they will
generally not cause a problem if their water content remains constant. The
situation where greatest damage occurs is when there are significant or
repeated moisture content changes.
Problems often associated with expansive soils include:
1.
Foundation cracks;
2.
Heaving and cracking of floor slabs and walls;
3.
Jammed doors and windows;
4.
Ruptured pipeliness; and
5.
Heaving and cracking of sidewalks and roads
Geologists work with geotechnical engineers to evaluate soil and
rock prone to shrinking and swelling. These areas are mapped and denoted for
their expansion potential. Expansive soil and rock be removed and replaced with
non-expansive materials to provide a suitable foundation for new structures.
Expansive materials can also be chemically treated, preloaded, or prewetted to
decrease swell potential
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Foundation Practices & Rehabilitation of
Structures on Expansive Soils
Rama Subbarao G.V., Associate Professor,
Department of Civil Engineering, S.R.K. Institute of Technology, Enikepadu,
Vijayawada.
Expansive soils popularly known as Black cotton soils in India are
highly problematic, as they swell on absorption of water and shrink on
evaporation thereof. Because of this alternate swell and shrinkage, distress is
caused to the foundations of structures laid on such soils. Extensive research
is going on to find the solutions to black cotton soils. The present paper
reviews innovative solutions along with conventional foundation practices to
counteract the dual problem of swelling and shrinkage posed by expansive soils.
Besides, the present paper throws a light on causes of distress in lightly
loaded structures founded on expansive soils and also various measures to
rehabilitate the distressed structure founded on them.
Introduction
Expansive
soil is commonly known as black cotton soils, because of their color and their
suitability for growing cotton. Black cotton soil is one of the major regional
soil deposits in India,
covering an area of about 3.0 lakh sq.km. Expansive soils are problematic soils
because of their inherent potential to undergo volume changes corresponding to
changes in the moisture regime. When they imbibe water during monsoon, they
expand and on evaporation there of in summer, they shrink. Because of this
alternate swelling and shrinkage, structures founded on them are severally
damaged. The annul cost of damage to the civil engineering structures is
estimated at £150 million in the UK, $1000 million in the USA and many billion
of pounds worldwide (Gourley et al. 1993).
In India,
black cotton soils have liquid limit values ranging from 50 to 100%, plasticity
index ranging from 20 to 65% and shrinkage limit from 9 to 14%. The amount of
swell generally increases with increase in the plasticity index. The swelling
potential depends on the type of clay mineral, crystal lattice structure,
cation exchange capacity, ability of water absorption, density and water
content. Swell in the vertical direction is called heave. Among the illite,
kaolinite and montmorillinite clay minerals, the montmorillinite possesses the
greatest ability to swell by illite. The Kaolinite does not swell. Black cotton
soils are very hard in dry state and possess high bearing capacity. In summer,
it is very common to see shrinkage cracks with hexagonal columnar structure,
with vertical cracks as wide as 10mm extending up to a depth of 3m or more.
Soils containing expansive clays become very sticky when wet and usually are
characterized by surface cracks or a “popcorn” texture (Fig.1) when dry.
Therefore, the presence of surface cracks (Fig.2) is usually an indication of
an expansive soil.
Problems With Expansive Soils
The
problem is more in case of light structures; those cannot counteract the upward
thrust posed by expansive soils. The damage will be apparent, usually, several
years after construction. The soil below will exert swelling pressure both
upwards and laterally. As a result, the floor slab is lifted up, leading to
cracking of floor. Cracking is normally evident at the corners of window and
door openings. These usually assume in the form of diagonal cracks-a
consequence of differential settlement in the wall (Fig. 3 & 4). Often,
utilities buried in soil as the water pipelines and sewage lines, get damaged
due to displacement in the soil in which they are buried. The ensuing leakage
further aggravate the situation. Roads that pass through expansive soil
sub-grade are subjected to heaving and shrinkage settlement of these
treacherous soils. Both the lined and unlined canals are subjected to the
vagaries of expansive soils. The unlined canal slopes erode and become soft.
Canal beds heave up obstructing the functioning of the canal. The concrete
linings splinter like glass pieces on account of deleterious cyclic movement of
background swelling clay. This heavy results in seepage losses.
Foundation Practices on Expansive Soils
The following
conventional foundation practices and innovative techniques can provide
solutions to problematic soils.
Sub excavating or replacing the Expansive Soil
by Cushions
In this technique, the
expansive soil is replaced either in part or full (Fig. 5) with a material that
doesn’t undergo swell. The load of the cushion provides the load necessary to
counter heave.
Sand Cushion Method
Satyanarayana (1969)
has suggested that the entire depth of the expansive soil stratum or a part
there of may be removed and replaced with a sand cushion, compacted to the
desired density and thickness. Swelling pressure varies inversely as the thickness
of the sand layer and directly as its density. Therefore, generally sand
cushions are formed in their loosest possible state without, however, violating
the bearing capacity criterion. The basic advantage of the sand cushion method
is its ability to adapt itself to volume changes in the soil. However, the sand
cushion method has several limitations particularly when it is adopted in deep
strata. Most of the foundation engineers often suggest some arbitrary thickness
for the sand cushion without consideration to the depth of the zone of
potential volume change which itself is difficult to determine. The high
permeability of sand creates conditions conducive to easy ingress and
accumulation of water from surface runoff.
CNS Layer Method
Replacement by soils
with relatively impervious material may, to a great extent offset the
disadvantages of sand cushion method. Katti (1978) has developed a technique
where by removal of about 1m of expansive soil and replacement by cohesive
non-swelling soils (CNS) layer beneath foundations has yielded satisfactory
results. Katti has successfully adopted it for prevention of heave and
resultant cracking of canal beds and linings and recommends it for use in
foundations of residential buildings also. According to Katti cohesive forces
of significant magnitude are developed with depth in an expansive soil system
during saturation which is responsible for reducing heave and counteracting
swelling pressure. The behaviour is mainly attributed to the influence of
electrical charges present on the surface of clay particles on the dipolar
nature of water molecules, producing absorbed water bonds that give rise to
cohesion.
Moorum is a typical example of CNS material. The cohesive bonds develop around
the particles at a faster rate than the ingress of water molecules into the
interlayer of the expanding lattices of montmorillinite, thereby reducing
heave. The heave of expansive soil underlying a CNS layer reduces exponentially
with increase in thickness of the CNS layer and attains a value of no heave
around a depth of 1.0m.The shear strength of the underlying expansive soil at
the interface and below increases with the thickness of CNS layer. The ultimate
bearing capacity after saturation at the interface and 1m below interface have
been found to increase compared with the value of expansive soil in winter.
Thus the expansive soil should be excavated up to of 1m below the footing level
and replaced with CNS layer, compacted to modified AASHO specifications,
projecting up to 1m beyond the foundations.
However, studies conducted later (Subba Rao et al., 1995) indicated that CNS
Cushion was effective in arresting heave only during the first cycle of
seasonal moisture fluctuations and, during the subsequent cycles, the heave may
be more than that recorded by a black cotton soil without cushion. Besides, a
soil conforming to the specifications suggested by Katti (1978) for suitability
as CNS material is difficult to find.
Fly Ash Cushion
Each one of the above
methods has one limitation or the other, in terms of its efficacy or economy.
The studies have been carried out using fly ash as a cushioning material (Sree
Ramarao et al., 2005). Developments of cohesive bonds in a lime-stabilized fly
ash cushion, when stabilized with lime, is expected to produce an environment
similar to the one obtained in CNS material following saturation and
consequently arrest heave. The results of the study showed a new solution to
the problem heave of expansive soil in the form of “Fly ash cushion method.” It
also solves the problem of fly ash utilization and disposal to some extent. If
at a site containing black cotton soil, the depth of the active zone is 3m, it
would be sufficient if 1.5m of expansive clay is removed and replaced with fly
ash cushion to get the heave reduced significantly. With the superstructure
load causing further reduction of heave, the amount of sub-excavation and
replacement with lime stabilized fly ash cushion can be further reduced.
Deep Foundation Techniques
In this case, the
foundation is made to rest at some depth by passing the soil in the active
zone, i.e. the zone within which volume changes in the soil occur due to
seasonal moisture changes.
Under-Reamed Piles
Under-reamed
bored piles were introduced in India
by Central Building Research Institute (C.B.R.I), Roorkee. In India, at about
3.5m below the ground, movements are negligible and if foundations are anchored
at that depth, they will remain stable. Based on this principle, under-reamed
piles (Sharma et al, 1978) were adopted for foundations in expansive soils in India. The
bulbs are provided generally in the inactive zone where sufficient anchorage is
available. The diameter of the stem of the under-reamed pile ranges from
20-50cms and the diameter of the bulb is normally 2 ½ times the diameter of the
stem. The spacing of the bulbs, in the case of multi under-reamed pile, should
not exceed 1 ½ times the bulb diameter. The Bureau of Indian Standards has also
brought a code IS 2911: Part III-1980 on under-reammed piles (Fig. 6).
Granular Pile-Anchor Foundation (GPAF) System
It has been observed
that under-reamed pile foundations suffer from the difficulty of both formation
upon which the whole mechanism of anchoring depends. Phanikumar et al (1996)
felt that the cost of under-reamed pile foundation is more for light structures
where the cost of structure itself is very low. In this technique, the
foundation is anchored at the bottom a granular pile to mild steel anchor plate
with the help of a mild steel anchor rod. This is called a granular pile-anchor
(Fig. 7) also counteracts the problem of shrinkage acting as a storage medium.
As the granular pile is a particulate medium, it cannot resist the tensile
uplift force on the foundation, and as such needs to be modified into a
pile-anchor by the above mechanism. As the expansive soil absorbs water, it
swells and uplifts the foundation. But, an enormous resistance to uplift is
mobilized along the cylindrical pile–soil interface because of the shear
parameters of the Pile-soil Interface, and the shear resistance augmented by
the lateral swelling pressure. Model tests conducted in the laboratory revealed
that heave and swell potential are enormously reduced by the installation of
granular pile anchors. The % reduction was about 90 to 95. It has also been
observed that the strength characteristics of the ambient soil surrounding the
granular pile-anchor showed a large improvement and that the composite ground
showed improved bearing capacity.
Chemical Stabilization
Chemical stabilization
of expansive soils can be adopted to alleviate the problems posed by these
soils to civil engineering structures. Chemical stabilization of expansive
clays consists of changing the physico-chemical around and inside of clay
particles where by the clay requires less water to satisfy the static imbalance
and making it difficult for water that moves into and out of the system. The
most common chemical admixtures used in soil stabilization are lime and cement.
Lime stabilization has been used successfully on major projects to minimize
swelling of the expansive soil. Generally, 3 to 8% by weight hydrated lime is
added to the top several inches of the soil (John et al). Lime continues to be
widely used additive for modification of expansive clays in view of its
cost-effectiveness although limited success in many instances. Lime is
sparingly soluble in exchange reactions are less. Further, the lime diffusion
into soil either from lime piles or lime slurry pressure injection is hardly 38
to 50mm in 1 to 4 years unless extensive fissure and crack system is present.
The hydration of Portland cement is a complex pozzolanic reaction that produces
a variety of different compounds and gels. The results of mixing cement with
clay soil are similar to that of lime. It reduces liquid limit, the plastic
index and the potential of volume change, it increases the shrinkage limit and
shear strength. For highly plastic clay, it is not effective like lime in
stabilization. Addition of 2 to 6% cement content can produce a soil that acts
as a semi rigid slab (John). Some investigators have tried and succeeded in
minimizing the swelling of expansive soil using chemicals like calcium chloride
(CaCl2), calcium sulfate (CaSo4), potassium chloride
(Kcl), aluminum chloride (AlCl3), etc.
Stabilization by Industrial Wastes
Utilization of
industrial wastes like fly ash, quarry dust, silica fume, copper slag, tannery
sludge, etc (Sabat et al, Stalin et al) in the geotechnical engineering field
will solve the problem of disposal of these wastes. Extensive research is
carried and carrying by the geotechnical investigators to reduce the swelling
of expansive soils by using industrial wastes. Fly ash is a waste material
produced due to burning of coal for thermal power industries. It is a hazardous
material causing environmental pollution degradation. Fly ash is added to soils
treated with lime to increase the pozzolanic reaction and improve the gradation
of granular soils. The pozzolanic activity of silt soils has been improved by
using a lime-fly ash ratio of 1:2. Liquid limit decreases and plastic limit
increases with increase in the percentage of fly ash. Generally, the plasticity
index reduces by about 50% when 20% of fly ash added. The optimum moisture
content decreases and maximum dry unit weight increase with increase in fly ash
content. When the non–plastic fly ash particles are added to the expansive clay
the water content required for the reorientation of the particles will be less
(Pandian et al., 2004).
Stabilization by Reinforcement
Using fibers like jute
fabrics, coir ropes, rubber tire chips, waste plastics, synthetic fibre etc can
successfully stabilize the expansive soils. The work reported by Raid R.
Al-Omari and Faris J. Hamodi (1991) showed the feasibility of using tensile
geogrid for the purpose of controlling the swell of plastic soils. Swelling
tests using an enlarged oedometer revealed promising results. The
reinforcements were cylindrical geogrid of varying stiffness values embedded in
clays of different plasticity indices. The reduction in swell increased with
increasing the geogrid stiffness, apparently due to a strong ‘interference’
bond restricting the relative movement between clay and the grid. A footings
model test confirmed the effectiveness of the proposed technique.
Horizontal Moisture Barriers
Horizontal moisture
barriers can be installed around buildings in the form of membranes or paving,
both flexible and rigid. Horizontal barriers are meant to prevent excessive
intake of moisture. Considerable success has been achieved with asphaltic
membranes-catalytically blown asphalt membranes or prefabricated sheets.
Asphalt membranes can be used to cover the surface of expansive soils so that
non-expansive fill can be placed on top of the membranes. This minimizes
infiltration of surface water into the under slab soils.
Vertical Moisture Barriers
Vertical moisture
barriers using concrete, ferrocement or any other impervious material around
the perimeter of the building, to cut off the source of water, can be very
useful in minimizing seasonal drying and shrinkage of the perimeter foundation
soils and also in maintaining long-term uniform moisture conditions beneath
covered areas. Vertical moisture barriers should be provided to a depth greater
than the depth of seasonal moisture changes.
SERC Roorkee / Ghaziabad
have developed technology for ferrocement waterproofing and water barriers.
Construction and same has been successfully used in field. Distress in Lightly
Loaded Structures Founded on Expansive Soils If the load is placed on the
expansive soil is more, the selling is arrested. When the imposed loads are
light, the swelling is more pronounced. It is interesting to note that it is rare
that heavily loaded structures have problem with swelling soils while it is the
lightly loaded single and two storeyed buildings which experience maximum
distress.
Causes for Distress in Lightly Loaded
Structures
The following are the
causes for distress in lightly loaded structures founded on expansive soils:
- The
construction of building on marshy area and water table is observed at a
shallow depth below the ground level.
- There
is no flagging/plinth protection around the building.
- Growth
of vegetation is observed around the building.
- Sump
tank and sewage pipes are very close to the foundation.
- Wastewater
and rainwater are disposed directly on the ground very close to the
foundation.
- Cracks
at plinth, sill, lintel levels and differential heaving of flooring,
shifting of walls, extensive cracks are observed in internal and external
walls of the building. It is due to the high swelling and shrinking
characteristics of expansive black cotton soil in the foundation region.
- The
presence of chloride and sulphate contents in fine aggregate are very high
compared to the permissible values aggregate could have affect the
concrete durability, which in turn have results severe corrosion of
reinforcement in various members.
Measures for Rehabilitation of Distressed
Structures Founded on Expansive Soils
The following
restoration measures as suggested below to counteract the dual problem of
swelling and shrinking behavior of expansive clay (Rama Rao M, et al (2004),
Sivapullaiah, et al (2005), Prabhakar, et al (2005):
- Construction
of additional one or two floors above the existing building should be done
so that the loading on the foundation would be more than the existing
swelling pressure.
- The
plinth beam should be separated from the natural ground by leaving an air
gap of 8 to 10cm between the plinth beam bottom and natural ground. If the
gap is not provided the plinth beam have at least to be designed for
upward pressure due to soil swelling.
- A
flexible water proof apron (plinth protection) of about width 2.0m shall
be provided all round the building.
- Installation
of horizontal/ vertical moisture barriers around the perimeter of the
building.
- The
internal non-load bearing walls with wide multiple cracks and dislocations
shall be removing completely and rebuilt. Before dismantling, the complete
roof should be supported by either steel or timber props.
- Flooring
shall be redone after removing existing filled up soil up to about 1.5 m
from the floor level and replacing the same with well-compacted
non-expansive materials placed in layers not exceeding 30cm thickness.
- The
sewer pipes with leak proof joints close to the foundation shall be beyond
the foundation media.
- Providing
sump tank far from foundation region.
- Plantation of trees, plants and hedges
within 3m distance around the building should be avoided. This because of
extensive watering of plants close to the building contributes to
swelling.
- Discharging
rainwater collected from roof at a distance from the structure.
Conclusion
Adequate geotechnical
investigations are imperative for the characterization of expansive soil. By
evaluating the properties of expansive soils accurately, it is feasible to
choose the proper foundation technique with a good constriction quality. The
distress in the lightly loaded structures is essentially due to high swelling
and shrinking characteristics of expansive black cotton soil in the foundation
media. The light loaded structures founded on expansive soils must be designed
in such way to observe that the load coming on the structure is sufficiently
more than the swelling pressure of the expansive soil. It should be ensured
that there is no presence of high level of chloride and sulphate contents in
fine aggregate using during construction, if not that may lead to the corrosion
of reinforcement.
Under-Reamed Piles: Safest Foundation Solution
for Black Cotton soil
Swelling and shrinkage
due to the property of volumetric changes of expansive soil like black cotton
soil results movement of the ground producing cracking of the order of
(sometimes) 15 to 20 cm wide and 2.5 to 4 m deep. This property is very
dangerous for the structures supposed to be founded on it. Many techniques are
discussed previously in various post published in this blog. Some of these are
provision for reinforced concrete ties or bands all around the main walls of
the building, removing entire black
cotton soil where possible or commencing construction during dry seasons.
It is found that under-reamed piles provide
an ideal solution to foundation in black cotton soil or other similar types of
expansive soils.
