PPT on ANALYSIS AND DESIGN OF AMPHITHEATRE

WRE: Surface and Ground Water Resources (NPTEL)

Cross drainage works.

Why I-section beam is preferred for heavy loading?

Why I-section beam is preferred for heavy loading?

Cross sectional shape I, giving many benefits. It is very good for giving stiffness (less deformation on loading) and to withstand higher bending moments (as a result of heavy loading) on comparison with other cross-sectional shapes of same area. Also, it is very easy to manufacture. It will have more moment of inertia.

STONE MASTIC ASPHALT

 IINTRODUCTION

            Stone Mastic asphalt (SMA), otherwise known as Stone Matrix Asphalt / Split Mastic Asphalt, was developed in Germany in the mid of 1960's and it has spread throughout Europe and across the world in 1980's and 1990's respectively. The excellent performances include resistant to mechanical and temperature deformation, cracking, and particularly rutting, resistant to weathering actions such as aging and low temperature cracking. Durability is excellent even under slow moving heavy traffic. The textured surface increases skid resistance and provides environmental and driving comfort by reduced noise level, and improved visibility in rainy days.

SMA provides a deformation resistant, durable, surfacing material, suitable for heavily trafficked roads. SMA has found use in Europe, Australia and the United States as a durable asphalt surfacing option for residential streets and highways. SMA has a high coarse aggregate content that interlocks to form a stone skeleton that resist permanent deformation. The stone skeleton is filled with mastic of bitumen and filler to which fibres are added to provide adequate stability of bitumen and to prevent drainage of binder during transport and placement. Typical SMA composition consists of 70−80% coarse aggregate, 8−12% filler, 6.0−7.0% binder, and 0.3 per cent fibre. The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional dense graded asphalt (DGA) mixes. Improved binder durability is a result of higher bitumen content, a thicker bitumen film and, lower air voids content. This high bitumen content also improves of flexibility. Addition of a small quantity of cellulose or mineral fibre prevents drainage of bitumen during transport and placement. The essential features, which are the coarse aggregate skeleton and mastic composition, and the consequent surface texture and mixture stability, are largely determined by the selection of aggregate grading and the type and proportion of filler and binder.

SMA is characterized by a stone-on-stone structure. SMA uses a high proportion of larger stones or aggregate that contacts each other. This skeleton of larger stones resists heavy loads by transmitting them to the pavement below. If the under laying pavement is sufficiently strong then the SMA will resist the heavier loads effectively. (A surfacing cannot compensate for a weak pavement).

3.     PERFORMANCE CHARACTERISTICS OF SMA

The development of modern pavement technology is needed to accelerate significant improvement of pavement quality of highways, airport runways and urban roads.

SMA meets the following demands upon an asphalt pavement:

·         Good stability at high temperatures

·         Good flexibility at low temperatures

·         High wearing resistance

·         High adhesive capacity between the stone granules and the bitumen

·         A mix with no tendency to separate

·         Good skid resistance

·         Reduced water spray

·         Lower traffic noise

Good stability at high temperatures

SMA mix has a self-supporting stone skeleton of crushed high quality coarse aggregate, which provides an increase in internal friction and shear resistance and hence its extremely high stability.

Good flexibility at low temperatures

SMA mix has a binder rich mastic mortar which has superior properties over dense graded asphalt in resisting thermal cracking.

High wearing resistance

SMA mix has low air voids, which make the mix practically impermeable, and provide satisfactory ageing resistance, moisture susceptibility and durability.

High adhesive capacity between the stone granules and the bitumen

With the increase of the amount of filler, cellulose fibres are added as stabiliser. The three dimensional structure of cellulose fibre assists the bitumen to maintain a high viscosity, thickens the bituminous film and improves the bitumen/aggregate adhesion.

A mix with no tendency to separate

            An efficient stabilisation of the mastic in order to prevent its segregation from the coarse particles.

Good skid resistance

Because of the macro-texture of the road surface and the use of coarse aggregates with a high Polished Stone Value, SMA pavement achieves a better level of skid resistance.

Reduced water spray

Because of its greater texture depth, there is less water spray, and at night there is fewer glares reflected from the road surface and better visibility of road markings.

Lower traffic noise

SMA road surfaces generally offer lower levels of noise due to the texture properties.

4.     COMPOSITION OF SMA

Stone Mastic Asphalt is characterised by its high stone content which forms a gap-graded skeleton-like stone structure. The voids of the structural matrix are filled with high viscosity bituminous mastic. The high stone content of at least 70% ensures stone-on-stone contact after compaction. The required degree of mastic stiffness is achieved through the addition of crushed sand.

SMA mixes have a bitumen content of minimum 6.5%. The bitumen in the gap-graded mix is stabilised during the mixing process, intermediate storage, transportation, surfacing and compaction through the addition of cellulose fibre stabilising additive.

Addition of cellulose fibre does not chemically modify bitumen, but rather enhances physical property of the finished product by allowing the use of higher bitumen contents. It tends to thicken or bulk the bitumen so that it does not run off the aggregate prior to compaction. The content of cellulose fibre is 0.3% by weight of mixture. If the technological requirements of SMA are fully met, good results can be obtained by just using standard bitumen and a cellulose fibre drainage inhibitor.

  

4.1 Materials Used

1.      Coarse and fine aggregate

2.      Bitumen

3.      Fibre

4.      Filler

4.1.1 Coarse and fine aggregate

The aggregates are crushed by using jaw pressure to get different size of aggregates varying from 16 mm to 75 micron. The coarse aggregate must be hard, durable, and roughly cubical in shape when crushed. Qualities of aggregates were check through various tests like Impact Value Test, Crushing Value Test, Los Angel’s Abrasion Value Test, Flakiness and Elongation Index Test.

4.1.2 Bitumen

Bitumen  act as a binder in SMA mix. Different grade of bitumen are used in different mix like hot-mix or gap-graded mix or dense-graded mix. For preparation of SMA mix we used 60/70 bitumen.

4.1.3 Fibre

Fibres are used as stabiliser in SMA mix. Fibres help to increase the strength and stability and decrease the drain down in SMA mix. There are different types of fibres are used in SMA mix like cellulose fibre, polymer fibre, natural fibre and mineral fibre.

4.1.4 Filler

Filler is used in SMA mix for better binding of materials. Rock dust, slag dust, hydrated lime, hydraulic cement, fly ash, mineral filler and cement are used as filler in SMA mix, also we can use the fine aggregate below 75 micron as filler.

5.     ADVANTAGES

·         20-30% increase in pavement life over conventional pavements

·         Good aggregate interlock

·         Low permeability

·         Improve in skid resistance due to the high percentage of fractured aggregate to motoring public particularly on wet pavement.

·         Surface texture characteristic may reduce sound from the tyre and pavement contact as well as water spray and glare.

·         Strength and stiffness derived from binder and aggregate structure

·         Relatively high binder contents provide good Durability

·         Durability (longer in-service life) of SMA should be equal to, or greater than, DGA and significantly greater than OGA.

·         It provides a textured, durable and rut resistant wearing course.

·         Surface texture characteristics are similar to OGA, so noise generated is lower than DGA but slightly higher than OGA.

·         It can be produced and compacted with the same plant and equipment as for normal hot mix DGA using procedural modifications.

·         SMA can be used on heavily trafficked roads where good deformation resistance is required.

·         Surfacing may reduce reflective cracking from underlying cracked pavements due to its flexible mastic.

·         At the end of its service life it is 100% recyclable.

6.     DISADVANTAGES

·         SMA mix requires higher mixing temperature.

·         Potential construction problem with SMA mixtures are drainage and bleeding.

·         Storage and placement temperatures cannot be lowered to control drainage and bleeding problem due to the difficulty in obtaining the required compacted.

·         Increased material cost associated with high bitumen and filler content.

·         Increased mixing time and time taken to add extra filler may result in reduced productivity.

·         Possible delays in openings (the road) as SMA should be cooled to 40°c to prevent early flushing of the binder to the surface.

·         Needs more carefully monitoring the composition at the mixing plant.

·         Moisture seeping from the SMA surface for long periods after rain.

·         White fines on the surface of the pavements.

·         Premature rutting

·         Stripping of asphalt layers below the SMA surfacing.

·         Potholing

7. APPLICATIONS

Stone Mastic Asphalt has proved superior on heavily trafficked roads and industrial applications:

·         with high lorry frequency

·         intense wheel tracking

·         at traffic lights

·         at intersections

·         on highways

·         on gradients

·         on bridges

·         in bus lanes

·         at bus-stops

·         in car parks

·         in harbours

·         on airport runways

·         on un/loading areas

Types of Bridge

Bridge is basically a structure that is built over a road, river, railroad/railway etc so that people can cross from one side to another. The basic concept or idea behind constructing a bridge is to cross large bodies of water or land. Bridge is a mean to join the two afar areas mostly over masses water or land so that it become easy for people to move across from one side to another quite easily. There are so many types of bridges which are based on the techniques used in construction.

Different Types of Bridges 

Below are some of the different types of bridges:

Beam Bridge: A beam bridge is basically drawn from the log bridge. Its construction relies on low steel beams, concrete and box grinders. It is said that the construction of beam bridge is the technically the easiest and uncomplicated among all the other types. Some of the bridges of this type include highway overpasses, flyovers or walkways. In this type of bridge a flat beam is supported on its both ends on piers.


Truss Bridge: This type of bridges is constructed by joining straight elements. These elements are often joined by means of pin joints. There is an abundance of forests in United States and hence of wood as well. Due to this fact, a lot of truss bridges were made in the past with timbers iron rods. Timbers were used as a source of compression and iron rods were used handle the tension. In the course of history, the truss bridges became popular from 1870s to 1930s. One such popular bridge is Deck truss Railroad Bridge constructed on the Erie Canal.


Arch Bridge: Arch bridges are called arch bridge because of having a shape similar to arches. These bridges are normally constructed with weight equally distributed into the ropes or chains at both the ends. The oldest arch bridge that still exists is the ‘Mycenaean Arkadiko Bridge’ constructed in Greece somewhere around 1300BC. Although, Greeks and Etruscans were familiar with the arches, Romans were known to be the first who discover the art of constructing an arch bridge. Present day arch bridge are the modified from as they have become compression arch suspended deck bridge which rely on light but stronger tensile construction material.

Suspension Bridge: A suspension bridge is a bridge that hangs from steel cables which are supported by towers on each end. Technically, the load of the bridge is transformed into the stretchiness in the cables. Some of the popular suspension bridges include the Golden Gate Bridge of United States, the Humber Bridge of England and the Tsing MA Bridge of China.

Cable-stayed Bridge: These are almost similar to suspension bridge in their structure but with few exceptions. The major differences between the two exist in the quantity of steel cable used. In the cable-stayed bridge, the towers used to support cables are relatively shorter and require less amount of cable as compared to suspension bridge. Cable-stayed bridge has two different versions: the harp design and fan variant design. Cables are connected to several points in harp design while in fan variant, the cables are attached to the tower. In United States, Cable Bridge has the reputation of first of this type. Other popular bridge is Centennial Bridge.


Cantilever Bridge: These are the bridges which are constructed in such a way that they stand out in the direction of horizontal-axis in space. These bridges are supported just on one end. The bridges for low traffic are simply based on beams whereas the bridges for heavy traffic are comprised of box grinders or trusses. Two of the most popular cantilever bridges include the Quebec Bridge of Canada (1800 feet long) and Oakland Bay Bridge of Sans Francisco (1400 feet long).

Bridges are surely a great way to reach the places which people never think of by any other simple mean. Bridges not only connect far-off lands but also provide opportunity to the mankind to explore different aspect of new technology. A bridge may be a an inspiration of a man who saw a block of wood floating on a water surface or perhaps the urge to come in contact with people living far-off ends. Whatever the reasons, the bridge is surely a great way to defeat the physical hindrances.

Expansive soils and their formmation

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.