Foundations

Foundations in Black Cotton Soils

Foundations in Black Cotton Soils:

The following methods are generally adopted to meet the characteristics of foundations in black cotton soils,

 

(1) Foundation loads are limited to 5 tonnes/sq.m if water finds access to the foundations, otherwise it may be about 10 tonnes/sq.m

 

(2) Foundations are taken down to such depths to which the cracks do not extend.

 

(3) Trenches are dug on either side of the foundation and filled with sand  or other material to prevent intimate contact of the black cotton soil with the concrete and masonry of the foundations.

If the thickness of the black cotton soil is only up to 120 cm it should be completely removed and foundation laid on the soil below.

 

(4) For important buildings raft or mat foundations of reinforced concrete are provided.

 

Termite- Soil Treatment for Foundations

Termite Treatment for Foundations

This termite treatment consists in treating the soil under the building and around the foundations with some chemical emulsion which can kill or repell termites. In this termite treatment about 500 mm deep trenches arc made along the external peripherial wall of the building with the help of shovel (width of the trench being equal o the width of the shovel) and 12 mm dia to 18 mm dia holes at 150 mm centres are then made in the trenches close to the wall face with the help of iron rod. The holes should preferably extend upto the top of footing of foundations or to a depth of at least 500 mm whichever is lesser. The holes are then filled with chemical emulsion in water and the hack fill earth is also sprayed with the chemical emulsion as it is returned to the trench thereby creating a barrier of poisoned soil along the external, periphery of the building. The total quantity of the chemical to be used in this termite treatment should be 75 litres per sq. m of the vertical surface of the masonry in foundation.

Termite Trearment

Termites

In case of RCC frame structure, the chemical treatment shall he applied to the soil in contact with column sides and plinth beams along external periphery of the building for a depth of 500 mm below ground level.

Cofferdam – Types of Cofferdam

Cofferdam – Types of Cofferdam

A cofferdam may be defined as a temporary structure that is constructed on a river or a lake or any other water-bearing surface for excluding water from a given site to execute the building operation to be performed on dry surface. The walls of the temporary structure should be practically water tight or at least they should be able to exclude water to such an extent that the quantity of water that leaks inside the enclosed area, can be easily pumped out. Cofferdams are classified according to the type of construction. The type of construction is dependent upon the depth, soil conditions,, fluctuations in the water level, availability of material etc. Cofferdams are advantageously constructed where a large area of site is to be enclosed and the hard bed is at reasonable depth.

Considering the material used in their construction, cofferdams may be divided into the following categories.

(a) Earthen cofferdam
(b)
Rock-fill cofferdam
(c) Single-walled
cofferdam
(d)
Double-walled cofferdam
(e) Crib
cofferdam
(f) Cellular cofferdam (Circular or diaphragm type)

(a) Earthen cofferdam:

It essentially consists of an earthen embankment built around the area to be enclosed. It is constructed in places where the depth of water is not much, say 13 to 18 in. and the velocity of the current is very low. As a precautionary measure, the earth bank is carried about one metre above the water level. The top width of the bank should not be less than 1 in. and the side slopes in a vary from 1 : 1 to 1 : 2. The earth embankment should be built from a mixture of clay and sand or clay and gravel. If the estimated quantity of clay is not easily obtainable, the banks may be constructed with a central clay wall with slopes of sand on either side. In order to prevent the embankment from scouring due to the action of water, side slopes of the bank on water side should be pitched with rubble boulders. If the current of water is such that there is a danger of the earthen embankment getting washed away, canvas bags half filled with material of embankment (mixture of clay, sand or gravel) are stacked one over the other to form the embankment. After the work of construction of cofferdam is over, the water from the enclosed area is pumped out so as to leave a dry surface inside. Excavations can then be performed to the required depth and the work of construction of foundations carried out.

Section of an earthen cofferdam

Section of an Earthen Cofferdam

(b) Rock-fill cofferdam:

If the depth of water to be retained by the embankment of cofferdam is of order of 18 to 3 in., stone or rubble is used for the embankment. This construction is adopted only if the stone is easily available in the nearby areas. The stones are assembled in the required shape of the embankment and the voids are partially filled with earth and stone-chips. The side slope on the water side is protected by pitching.

Section of Rockfill Cofferdam

Section of Rock-fill Cofferdam

(c) Single-walled cofferdam:

This type of cofferdam is used in places where the area to be enclosed is very small and the depth of water is more, say 4.5 to 6 m Timber piles known as guide piles are first driven deep into the firm ground below the river bed. Depending upon the velocity of the current of the water in the river, the centre to centre spacing of the piles may vary between 1.8 to 4 m. Longitudinal runners called wales are then bolted to the guide piles at suitable distance apart. Steel or wooden sheet piles are then driven into the river bed along the wales and are secured to the wales by bolts. The sheets on the two faces arc braced by trussed arrangement of struts. This helps in increasing the stability of walls against the water pressure. Half-filled bags of sand stacked on the inside and the outside faces of the sheets help in increasing the stability of cofferdam. After the cofferdam is constructed, the water in the enclosed area is pumped out and the construction work is taken up.

Single Walled Cofferdam

Single Walled Cofferdam

(d) Double-walled cofferdam:

For cofferdams required to enclose larger areas in deep water, single wall type becomes uneconomical as larger sections of trussed struts would be necessary to resist the water pressure. Double-walled cofferdam is provided in such situations. Its construction is essentially the same as that of a single-walled cofferdam except that in place of one wall, a pair of walls with a gap in between is used all along the boundary of the space to be enclosed. This type of cofferdam can be used in depth of water up to 12 m. As the depth of water increases, the wall should be made wider in order to make it stable against over4urning and sliding. The distance between the two walls depends upon the depth of water. The thickness of wall should be equal to the depth of water up to 3 m. For greater depths of water, the thickness of wall should be 3 m. plus ½ the depth of water in excess of 3 m. At their top, the two faces of the walls are connected by steel rods spaced at close intervals. To prevent the leakage from the ground below, the sheet piles are driven to a good depth in the bed.

Double Walled Cofferdam

Double Walled Cofferdam

(e) Crib cofferdam:

In deep waters where it is difficult to penetrate the guide piles or sheet piles into the hard bed below, crib cofferdam is used. In this type of construction, the sheet piles are supported by a series of wooden cribs. A crib is a framework of horizontal timbers installed in alternate courses to form pockets which can be filled with earth or stones. The length and breadth of each crib depend upon the depth of water and the current of flow. The framework of the cofferdam (made from, logs of wood) is prepared on ground and then floated to the site where the cofferdam is to be constructed. The layers of sand and the other loose material overlying the impervious hard bed is dredged out. Crib is then sunk to the position, the bottom of each crib is given a shape to fit in the variation in the surface of bed rock. The space inside the crib is then filled with stone or any other material, so as to make it stable against sliding and overturning. Timber or steel sheet piles are then driven around the crib.

Crib cofferdam

Crib cofferdam

 

(f ) Cellular cofferdam:

This type of cofferdam is mostly used for de-watering large areas in places where the depth of water may be of the order of 18 to 21 m. Cellular cofferdams are mostly used during the construction of marine structures like dams, locks, whares etc. Cellular cofferdam is made by driving straight web steel sheet piles, arranged to from a series of inter-connected cells. The cells are constructed in various shapes and styles to suit the requirements of site. Finally the cells are filled with clay, sand or gravel to make them stable against the various forces to which they are likely to be subjected to. The two common shapes of the cellular cofferdam are,

(i) Circular type cellular cofferdam.
(ii)
Diaphragm type cellular cofferdam.


(i)
Circular type cellular cofferdam:

The circular type of cellular cofferdam has the advantage that each cell may be filled completely to the top before starting the construction of the next cell without causing any distortion to the shell of the cofferdam, Thus, when one cell is completely filled up it can be used for placing crane or other equipment required for the construction of other cells. In addition, each cell acts as a self-supporting independent unit and in case one of the cells collapses due to scour or interlock damage or some other reason, it does not produce any adverse effect on the neighbouring cells it is found that the interlock stresses reach their maximum permissible value when the diameter of cell is about 21 meter. Hence in case, from design consideration it is necessary to have effective width of the cofferdam more than 21 meter, diaphragm type of cofferdam must be used.

Circular Type Cellular Cofferdam

Circular Type Cellular Cofferdam

(ii) Diaphragm type cellular cofferdam:

This Consists of a series of diaphragm of steel sheet piles connected as shown in the image below.  The straight diaphragm wails are connected to each other by steel piles arranged in the form of arches on either sides. The radius of the connecting arcs is generally made equal to the distance between the straight diaphragm walls. With this arrangement, the tension in the arcs and cross wails remain equal. After the cells are driven to the required depth, they are filled with earth, sand, gravel or other filling material. In this type of cofferdam, as the diaphragm which separates the two cells is a straight wall, it is necessary to fill adjacent cells at approximately the same rate. If this is not done, the unbalanced pressure from the fill will distort the diaphragm (cross-walls) which may result in the failure of the interlocks. In this respect, the circular type cofferdam has the advantage over the diaphragm type cofferdam because in the former, it is not necessary to fill the adjacent cells at the same time. This type of cofferdam has the advantage that the effective width of the cofferdam can be increased to desirable limits without increasing the interlock stresses.

Diaphragm type Cellular Cofferdam

Diaphragm type Cellular Cofferdam

Composite Piles

Composite Piles

Composite Piles are those piles of two different materials are driven one over the other, so as to enable them to act together to perform the function of a single pile. In such a combination, advantage is taken of the good qualities of both the materials. These prove economical as they permit the utilization of the great corrosion resistance property of one material with the cheapness or strength of the other.

The different stages in the construction of a composite piles having a timber pile at its lower part and precast concrete pile above are shown below. This type of composite pile is used with the object of achieving economy in the cost of piling work.

Composite Piles

Composite Piles

Another type of composite piles commonly used consists of a steel pipe or H-pile at the bottom and cast-in-situ concrete pile at the top. This type of composite pile is recommended in cases where the designed length of the pile works out to be greater than that available for the cast-in-situ type of pile.

Deep Foundations

Deep Foundations

In case, the strata of good bearing capacity is not available near the ground, the foundation of the structure has to be taken deep with the purpose of attaining a bearing stratum which is suitable in all respects. In addition there may be many other conditions which may require deep foundations for ensuring stability and durability of a structure. For example, the foundation for a bridge pier must be placed below the scour depth, although suitable bearing stratum may exist at a higher level. The most common forms of construction pertaining to deep foundations are
(a) Piles
(b) Cofferdams
(c) Caissons

Foundation Concrete

Foundation Concrete

The type of foundation concrete and the proportion of the ingredients used in its making depend upon the nature of the structure, quality of the materials used and the site conditions. Since lime is fairly cheap, lime concrete is generally used for foundations in dry-subgrade. Lime concrete is produced by mixing one Cu. m of wet ground lime mortar with 25 cu. m of ballast. The proportion of ingredients in lime mortar may be 1 lime : 2 sand or 1 lime : 1 surkhi : 1 sand or 1 lime : 2 surkhi. The ballast may be of brick, stone or shingle. The size of the ballast is generally restricted to 40 mm.

For moist subgrade with high sub-soil water (usually 15 in. or less below the foundation level) cement concrete should always be used. In such situations the foundation concrete should not be leaner than 1:4:8 (1 cement 4 sand : 8 stone ballast). For less important work in dry sub-grade (subs oil water level below 1.5 m from foundation level) a leaner cement concrete such as 1:8:16 may be adopted. In normal practice, however, cement concrete 1:5:10 is recommended for lean concrete.

The mixing of concrete can be done either by hand or in mechanical mixer. In case of hand mixing of concrete, the concrete should be mixed on a clean dry and water-tight platform. Concrete should be laid (and not thrown) in layers not exceeding 15 cm in thickness. Each layer should be thoroughly rammed and consolidated before the succeeding layer is laid.

In case of lime concrete, the curing should start (by keeping the concrete damp with moist gunny bags, sand etc.) after 24 hours of its laying and should be continued for a minimum period of 7 days. The masonry work over the foundation lime concrete should be started only after 7 days. In case of cement concrete, however, the masonry work over the foundation concrete may be started after 48 hours of its laying. The curing of the cement concrete, which starts 24 hours after its laying, is continued along with the masonry for at least 10 days.

Excavation of Foundation in Water Logged Sites

Excavation of Foundation in Water Logged Sites

Excavation of foundation in water logged sites poses a great problem for the site engineer. There are various methods of dealing with the situation which depend upon the depth of excavation, depth of water table and many other factors. Following methods are generally adopted while digging foundation trenches in water-logged sites.

(1) By constructing drains:

This method is generally adopted in shallow foundations in water-logged ground. In this method, drains of suitable size are constructed by the sides of the foundation trench. The drains collect sub-soil water from the sides and the enclosed area and convey it into a shallow pit or sump well. From the sump, the water is continuously bailed or pumped out. This is the cheapest method of draining excavated area and can be easily adopted by deploying unskilled labour and by using simple equipment.

Dewatering of Foundations by Constructing Drains

Dewatering of Foundations by Constructing Drains

(2) By constructing deep wells:

In coarse soils, porous rock or in sites where large quantity of sub-soil water is required to he drained out, 30 to 60cm diameter wells are sometimes constructed at 6 to 15 m centres all round the site. for temporary drainage of the ground. The water collected in the wells is pumped out continuously . This method can be adopted for depths of excavation up to 24 m.

(3) Freezing process:

This process is suitable for excavations in water-logged soils like sand, gravel and silt. It is advantageously used for deep excavation such as foundation for bridges etc. specially when excavation is to be made adjacent to an existing structure or near some waterways. The process consists in forming a sort of coffer darn by freezing the soil around the area to be excavated. Freezing pipes encasing smaller diameter inner pipes are sunk about one metre centre to centre along the periphery of the area to be excavated. The layout of the pipes should preferably be such that the area enclosed is circular in plan. Freezing liquid is then supplied to the freezing pipes by refrigeration plant. This makes the ground around the pipes to freeze and form a thick wall of frozen earth around the area to be excavated. This process can be used up to 30 m depth of excavation.

(4) By chemical consolidation of soil:

In this method, the soft water-logged soil is converted into a semi-solid mass by forcing chemicals like silicates of soda and calcium chloride into the soil. This method is used for small works.

(5) Well point system:

This is a method of keeping an excavated area dy by intercepting the flow of ground water with pipe wells driven deep into the ground. The main components of a well point system are : (i) the well points, (ii) the riser pipe, (iii) the header pipe and (iv) the pumps.

The well point consists of a perforated pipe about 120 cm long and 4 cm in diameter. This pipe has a ball-valve to regulate the flow of water and a screen to prevent the mud from entering into the pipe. The well point tube, is connected to 5 to 75 cm diameter pipe known as riser pipe and is sunk into the ground by jetting.

In the process of jetting, water is forced down through the well point at the rate of 20 to 25 litres per second. The water jet dislodges the surrounding soil and enables the well point to be sunk to the desired depth. After the well point has been sunk to the required depth, the water jet is allowed to run for some time (to ensure washing all sand or silt ‘out of the hole) till the return water from the hole is quite clean. Thereafter the water jet is closed and the annular space formed around the well point (by jetting action of water) is filled with coarse sand and gravel to form a filter zone around the well point. The filter zone prevents the entry of fine particles of the surrounding soil into the well point and avoids clogging of well point screen. The filter sand around the well point should be filled up to water table. The depth of the hole above the water table is filled with tamped clay to act as a clay seal to minimize air getting into the well point through the sand filter.

Well Point System

Well Point System

The well points are suitably spaced (normal spacing being 100 cm c/c) so as to enclose the whole area to be excavated. The riser pipes at their upper ends are connected to a header pipe which in turn is connected to a high capacity suction pump.

Details of Well point System

Details of Well point System

After all the well points are installed and connected, the suction pump is put into operation. Due to suction, the ball valve in the well point gets closed and the ground water is drawn in through the well point screen. The water from the well point is sucked up through the riser pipes, flows through the header pipe and is finally discharged away from the site of the work.

This method can be successfully adopted for depth of excavation up to 18 m. Since the suction pump is normally not used to lift water above 6 m depth, in’ deep excavations, where it is necessary to lower water table to a greater depth, multi- stage system of well point is used..


(6) By constructing sand drains:

Sand drains prove very effective in marshy soils. Soil becomes marshy by the process of deposition of thick layers of clays and silts mixed with organic matter by the passage of time. Marshy soil is thus subjected to capillarity and has a high pore water pressure. When this type of soil is subjected to load, its wet soils contents are gradually pushed out on either side and this results in subsidence of the ground. To avoid this, sand drains are made in the ground. The diameter of the sand drains normally varies between 300 mm to 450 mm and their centre to centre spacing may vary from 3 to 6 metre The hole for making the sand drain can be made by driving steel pipe casting into the ground. The drain holes are driven deeper than the marshy layer possibly up to an underlying rock or firm base. The marsh in the pipes is removed by means of jets. Selected type of sand is then filled into the pipes and the pipes are withdrawn leaving vertical sand piles in the ground. A thick layer of sand (sand blanket) is spread over the entire area to be consolidated. When the sand layer is subjected to load, the water from the muck of the marshy soil gets squeezed into the vertical sand drains.

Sand Drains

Sand Drains

By capillary action, the water from the sand drains rises up and is fed into the sand blanket from where, it can be drained out. The objective of consolidation of soil by this method is to develop increased soil resistance to superimposed loads usually consisting of earth fills in highway or airport construction.

(7) Electro-Osmosis:

Well point system is rendered ineffective in very fine sands, silts or clay, because such soils tend to hold the water by capillary action and offer great resistance to percolation. It has been established that if a direct current is passed through a soil of low permeability, its rate of drainage is greatly increased. In the process of Electro-Osmosis, steel rods forming the positive electrodes are driven in to the soil midway between the well-points, which are made to act as negative electrodes. When electric current is passed, the ground water flows towards the negative electrode (well-points) and is pumped out. This requires very expensive equipment and hence it is rarely used.

Setting out of Foundations

Setting out of Foundations

Before Commencement, of the excavation of trenches for foundation, a setting out plan is prepared on paper. The setting out plan is a dimensioned ground floor plan, usually drawn to scale of 1:50. The plan is fully dimensioned at all breaks and openings. One of the methods of  setting out of foundations is to first mark the centre line of the longest outer wall of building by stretching a string between wooden pegs driven at its ends. This serves as the reference line for marking the centre line of all the walls of the building. The centre line of the wall, which is perpendicular to the long wall, is marked by setting up a right angle. Right angle is set up by forming triangles with sides 3,4and5units long. If we fix the two sides of the right angles triangle to be 3 m, and 4 m, then the third side i.e. the hypotenuse should be taken a 5 m. The dimensions should be set out with a steel tape. The alternative method of setting out right angle is by the use of theodolite. This instrument is also helpful in setting out acute or obtuse angles. Small right-angled Projections are usually set out with mason’s square.

Setting out of Foundations by Masonry Piers

Setting out of Foundations by Masonry Piers

The method of  Setting out of Foundations described above is not so reliable for important works as there is likelihood of the wooden pegs being pulled up or displaced. In an accurate method, the centre lines of the building walls arc carefully laid by means of small nails fixed into the head of the wooden pegs driven at the quoins. In case of rectangular buildings, the diagonal from the opposite corners are checked for their equality. Small brick walls, pillars or platforms are constructed 9ocm clear of the proposed foundation trench. The platforms are about 15 cm wider than the trench width and are plastered at top. The tops of all platforms or pillars should be at the same level preferably at plinth or floor level of building. The strings are then strenched over the nails in the pegs and the corresponding lines are marked on the wet plastered platforms top by pressing the stretched string on the plastered surface by a trowel. The outside lines of the foundation trench and the plinth lines are marked on the wet plastered platform top in the similar manner.

Before starting excavation, the strings are stretched between the outside lines of the foundation trench marked over the platform top and the cutting lines are marked on the ground by lime powder. If necessary, the lines may be marked by a daghbel or pick-axe.

Machine Foundation

Machine Foundation

The design of machine foundation involves careful study of the vibration characteristics of the foundation system. Relevant data required for the design and construction of the machine foundation of machine should be obtained from the manufacturer of the machine, prior to the start of design. All parts of machine foundation should be designed for maximum stresses due to the worst combination of vertical loads, torque, longitudinal and transverse forces, stresses due to temperature variation and the foundation dead load. In case, the machine foundation layout is partly built up of beam and column construction, straight bars should be provided both at top and bottom of the beams and the spacing of the stirrups should be close. The main foundation block should have the designed thickness and should be reinforced both at top and bottom, even if the reinforcements are not required from design considerations.

The general principles of machine foundation design are given below:

  1. The mass of the foundation block should be adequate to absorb vibrations and also to prevent resonance between the machine and the adjacent soil. This can be achieved by increasing the weight of foundation block in proportion to the power of the engines. Some authors suggest that for each break horse power of multicylinder engines, a minimum of 725 kg. weight of foundation should be provided for gas engines, 565 kg. for diesel engines and 225 kg. for steam engines. For single cylinder engines, the above value should be increased by 40 to 60%. As a thumb rule, the weight of the foundation should be at least 2½ times the weight of the whole machine.
  2. To avoid the possibility of differential settlement, the machinefoundation should be so dimensioned that the resultant force due to the weight of the machine and the weight of the foundation passes through the centre of gravity of the base contact area.
  3. The foundation should be stiff enough to have necessary rigidity, since the slightest deflection of foundation can cause considerable bearing troubles.
  4. To avoid transmission of vibration from the machine to the adjoining parts of the building, a gap should be left around the, machine foundation to isolate it from the adjoining parts of the building.
  5. As far as possible, overhanging cantilevers should be avoided. However, in situation where it is not possible to avoid cantilever projections, they should be designed for strength and rigidity against vibrations.(6) All units of machine foundation should be provided with reinforcement running both ways along the surface of the concrete block. The concrete cover to the reinforcement should not be less than 75 mm at the bottom, 50 mm on sides and 40 mm at the top. In case of foundation for steam turbo-generators, cover for the reinforcement at bottom, side and top of base slab should not be less than 100 mm.
  6. The amount of reinforcement in foundation units should not be less than 2 kg per cu. m of concrete for impact type or reciprocating type of machines, 50 kg per cu. m of concrete for rotary type of machines and 100 kg per cu. m of concrete for steam turbo generators.
  7. M 150 to M 200 grade of concrete can be used in the foundations and as far as possible, the-entire block should be concreted in one operation without construction joints.

Foundations on Sloping Ground

Foundations on Sloping Ground

To avoid sloping foundation bed or excessive depth of excavation at the top end, stepped foundation is necessary to be provided in a considerably sloping ground. Foundations on Sloping Ground is achieved by cutting the portion of the foundation trench in steps. The steps should not preferably be more than the depth of the concrete bed and each step should be a multiple of the depth of one brick so as to fit in with the brick courses. The lap of concrete at each step should never be less than the vertical thickness of the concrete.

In some cases, the bottom of the footings of different walls of the same structure may be at different levels. The following limitations (as given by I.S.I.) are necessary to be observed in deciding the depth of footings in such circumstances.
(1) The distance between the lower edge of the footing to the sloping surface should not be less than 1 m for soils and 60 cm for rocks.
(2) In clayey soils, the line drawn between the lower adjacent edge of the upper footing and the upper adjacent edge of the lower footing should not have a steeper slope than 2:1 (i.e two horizontal : one vertical).
(3) In granular soils, the line drawn between the lower adjacent edges of adjacent footings should not have a slope steeper thin 2:1 (i.e. two horizontal : one vertical).