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A shallow foundation is a type of foundation which transfers building loads to the earth very near the surface, rather than to a subsurface layer or a range of depths as does a deep foundation. Shallow foundations include spread footing foundations, mat-slab foundations, slab-on-grade foundations, pad foundations, rubble trench foundations, and earthbag foundations.

A spread footing foundation, which is typical in residential building, has a wider bottom portion than the load-bearing foundation walls it supports. This wider part "spreads" the weight of the structure over more area for greater stability.

 

The design and layout of spread footings is controlled by several factors, foremost of which is the weight (load) of the structure it will support as well as penetration of soft near-surface layers, and penetration through near-surface layers likely to change volume due to frost heave or shrink-swell.

 

These foundations are common in residential construction that includes a basement, and in many commercial structures. But for high rise building it is not sufficient.

 

A spread footing which changes elevation in several places in a series of vertical "steps" in order to follow the contours of a sloping site or accommodate changes in soil strata, is termed a stepped footing.

 

Mat-slab foundations

 

Mat-slab foundations are used to distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings. Mat-slab foundations can be constructed near the ground surface, or at the bottom of basements. In high-rise buildings, mat-slab foundations can be several meters thick, with extensive reinforcing to ensure relatively uniform load transfer.

 

Slab-on-grade foundation

Slab-on-grade foundations are a structural engineering practice whereby the concrete slab that is to serve as the foundation for the structure is formed from a mold set into the ground. The concrete is then placed into the mold, leaving no space between the ground and the structure. This type of construction is most often seen in warmer climates, where ground freezing and thawing is less of a concern and where there is no need for heat ducting underneath the floor.

 

The advantages of the slab technique are that it is cheap and sturdy, and is considered less vulnerable to termite infestation because there are no hollow spaces or wood channels leading from the ground to the structure (assuming wood siding, etc., is not carried all the way to the ground on the outer walls).

 

The disadvantages are the lack of access from below for utility lines, the potential for large heat losses where ground temperatures fall significantly below the interior temperature, and a very low elevation that exposes the building to flood damage in even moderate rains. Remodeling or extending such a structure may also be more difficult. Over the long term, ground settling (or subsidence) may be a problem, as a slab foundation cannot be readily jacked up to compensate; proper soil compaction prior to pour can minimize this. The slab can be decoupled from ground temperatures by insulation, with the concrete poured directly over insulation (for example, Styrofoam panels), or heating provisions (such as hydronic heating) can be built into the slab (an expensive installation, with associated running expenses).

 

Slab-on-grade foundations are commonly used in areas with expansive clay soil, particularly in California and Texas. While elevated structural slabs actually perform better on expansive clays, it is generally accepted by the engineering community that slab-on-grade foundations offer the greatest cost-to-performance ratio for tract homes. Elevated structural slabs are generally only found on custom homes or homes with basements.

 

Care must be taken with the provision of services through the slab. Copper piping, commonly used to carry natural gas and water, reacts with concrete over a long period, slowly degrading until the pipe fails. Copper pipes must be lagged (that is, insulated) or run through a conduit or plumbed into the building above the slab. Electrical conduits through the slab need to be water-tight, as they extend below ground level and can potentially expose the wiring to groundwater.

 

Rubble Trench foundation

 

The rubble trench foundation, a construction approach popularized by architect Frank Lloyd Wright, is a type of foundation that uses loose stone or rubble to minimize the use of concrete and improve drainage. It is considered more environmentally friendly than other types of foundation because cement manufacturing requires the use of enormous amounts of energy. However, some soil environments (such as particularly expansive or poor load-bearing (< 1 ton/sf) soils) are not suitable for this kind of foundation.

 

A foundation must bear the structural loads imposed upon it and allow proper drainage of ground water to prevent expansion or weakening of soils and frost heaving. While the far more common concrete foundation requires separate measures to ensure good soil drainage, the rubble trench foundation serves both foundation functions at once.

 

To construct a rubble trench foundation a narrow trench is dug down below the frost line. The bottom of the trench would ideally be gently sloped to an outlet. Drainage tile, graded 1":8' to daylight, is then placed at the bottom of the trench in a bed of washed stone protected by filter fabric. The trench is then filled with either screened stone (typically 1-1/2") or recycled rubble. A steel-reinforced concrete grade beam is poured at the surface to provide ground clearance for the structure.

 

If an insulated slab is to be poured inside the grade beam, then the outer surface of the grade beam and the rubble trench should be insulated with rigid XPS foam board, which must be protected above grade from mechanical and UV degradation.

 

The rubble-trench foundation is a relatively simple, low-cost, and environmentally-friendly alternative to a conventional foundation, but may require an engineer's approval if building officials are not familiar with it. Frank Lloyd Wright used them successfully for more than 50 years in the first half of the 20th century, and there is a revival of this style of foundation with the increased interest in green building.

 

Earthbag foundation

 

The basic construction method begins by digging a trench down to undisturbed mineral subsoil. Rows of woven bags (or tubes) are filled with available material, placed into this trench, compacted with a pounder to around 1/3 thickness of pre-pounded thickness, and form a foundation. Each successive layer will have one or more strands of barbed wire placed on top. This digs into the bag's weave and prevents slippage of subsequent layers, and also resists any tendency for the outward expansion of walls. The next row of bags is offset by half a bag's width to form a staggered pattern. These are either pre-filled with material and delivered, or filled in place (often the case with Superadobe). The weight of this earth-filled bag pushes down on the barbed wire strands, locking the bag in place on the row below. The same process continues layer upon layer, forming walls. A roof can be formed by gradually sloping the walls inward to construct a dome. Traditional types of roof can also be made.

 

 

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.

 

 

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.

 

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

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).

 

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:

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.

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.

The foundation should be stiff enough to have necessary rigidity, since the slightest deflection of foundation can cause considerable bearing troubles.

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.

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.

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.

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.

 

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.

 

(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.

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.

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.

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.

 

Slope stability

 

The field of slope stability encompasses the analysis of static and dynamic stability of slopes of earth and rock-fill dams, slopes of other types of embankments, excavated slopes, and natural slopes in soil and soft rock.[1] Slope stability investigation, analysis (including modeling), and design mitigation is typically completed by geologists, engineering geologists, or geotechnical engineers. Geologists and engineering geologists can also use their knowledge of earth process and their ability to interpret surficial geomorphology to determine relative slope stability based simply on site observations.

 

Earthen slopes can develop a cut-spherical weakness area. The probability of this happening can be calculated in advance using a simple 2-D circular analysis package.[2] A primary difficulty with analysis is locating the most-probable slip plane for any given situation.[3] Many landslides have only been analyzed after the fact. More recently slope stability radar technology has been employed, particularly in the mining industry, to gather real time data and assist in pro-actively determining the likelihood of slope failure.

 

Real life failures