Tag: Foundation

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.

 

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).

Raft or Mat Foundations

Raft or Mat Foundations

In case of soils having low bearing capacity, heavy structural loads are usually supported by providing raft or mat foundations. Also if the structure is vulnerable to subsidence on  being located in mining area or due to uncertain behaviour of its sub-soil water condition, raft or mat foundations should be preferred. Raft or Mat Foundations provides an economical solution to difficult site conditions, where pile foundation cannot be used advantageously and independent column footing becomes impracticable.

Raft or mat foundations consists of thick reinforced concrete slab covering the entire area of the bottom of the structure like a floor. The slab is reinforced with bars running at right angles to each other both near bottom and top face of the slab. Sometimes it is necessary to carry the excessive column load by an arrangement of inverted main beams and secondary beams, cast monolithically with the raft slab.

Raft Foundation

Raft Foundation

Method of Construction:

The raft slab generally projects for a distance of 30 cm. to 45cm. on all the sides of the outer walls of the structure  as such the area of excavation is slightly more than the area of the structure itself. The excavation is made to the required depth and the entire excavated area is well consolidated. This surface, when dry, provides the base upon which the raft or mat slab is laid. All the precautions that are necessary to be observed during the reinforced concrete construction are strictly adhered to and further construction is started only after the curing of the raft has been fully done.

Combined Footings

Combined Footings

Combined footings are constructed in a way that the centre of gravity of the supporting area coincides the line of centre of gravity of the two column loads. These are usually in the form of rectangular or trapezodial in shape. Rectangular shape is adopted where loading condition is such that either the two columns are equally loaded or the interior column carries greater load. On the other hand, in case of trapezodial footing, no such condition is applicable.

Combined Footings

Combined Footings

Grillage Foundations – Steel & Timber Grillage

Grillage Foundation – Steel & Timber Grillage

Grillage foundation is used when heavy structural loads from columns, piers or stanchions are required to be transferred to a soil of low bearing capacity.  Grillage foundation is often found to be lighter and more economical. This avoids deep excavation and provides necessary area at the base to reduce the intensity of pressure within safe bearing capacity of soil. Depending upon the material used in construction of grillage foundation can be broadly divided in the following two categories.

(a) Steel grillage foundation
(b) Timber grillage foundation

(a) Steel grillage foundation:

Steel grillage foundation consists of steel beams also known as grillage beams which are provided in single or double tiers. In case of double tier grillage foundation, the top tier is laid at right angles to the bottom one. The grillage beams of each tier are held in position by 20 mm spacer bars with 25 mm diameter pipe separators. The beams are suitably spaced so as to provide facility for the placing and compacting of concrete between them. A minimum clearance of 8 cm is considered most suitable. In any case, the distance between the flanges of the beams should not be more than one and half to two times the flange width with a maximum of 30 cm. If the beams arc spaced more distance apart, there is a danger of the concrete filling not acting monolithically with the beams, and as such, may result in the failure of the foundation. In order to protect the beams against corrosion, a minimum cover of 10 cm is kept on the outer sides of the external beams as well as above the upper flange of the top tier, Cover of concrete under the lower beam should not be less than 15 cm.

Steel Grillage Foundation

Steel Grillage Foundation

Method of construction of Steel Grillage Foundation:

Excavations are carried to the designed depth and the bed is well levelled. This foundation bed is covered with a layer of rich mixture of concrete not less than 15 cm in thickness. This is well compacted so as to make the layer of concrete an impervious bed. Grillage beams of the designed dimensions are then placed on this bed of concrete at specified distance apart using separators. The upper surface of the grillage beam flanges is brought in a horizontal plane and rich cement grout is poured all round the lower flanges of the beams so as to secure the beams to the concrete bed. The concrete is then placed between and around the beams. If another tier is required to be provided. it is kept at right angles to the already laid tier and the entire space is filled with concrete.

(b) Timber grillage:

Where the soil encountered is soft and is permanently water-logged building walls can be economically supported by suitably designed timber grillage foundation. Timber grillage foundation can be safely used for light buildings by limiting the loading on the soil to 5.5 tonne/sq.mt. In this type of construction, the concrete block usually provided below the wall footing is replaced by timber platform.

Timber Grillage Foundation for Wall

Timber Grillage Foundation for Wall

The timber platform consists of planks usually 8 cm to 10 cm. thick, arranged in two layers, one longitudinal and the other across the wall extending beyond the footing base by about 45 cm to6O cm on either side. In the lowermost layers, the planks are 5 cm to 10cm thick depending upon the loading and site conditions. The two layers of planks are separated by rectangular sections of timber spaced at not more than 38cm. centre to centre, the depth of the sections being 0.75 times the width.