Types of foundation.

1.      Concrete strip FOUNDATION.

ü  They are frequently used.

ü  They consist of continuous mass concrete strips poured in the bottom of the trenches.

ü  These foundations supports load bearing walls which are centered on the concrete strips to spread the pressure from the walls, roofs, and other floor loads evenly.

ü  The concrete strip is usually a uniform width and depth.

ü  The foundation must be wide and deep enough to avoid soil movement that could cause instability

ü  Depending on soil conditions, the max depth may be 900mm.

ü  The concrete must be at least as thick as its projection from the base of the wall.

ü  This ensures that the pressure of the building loads are distributed in the concrete at an angle of 45°.

2.      Deep strip FD

The deep strip foundation is a variation of strip foundations.

Deep strip foundations are usually dug out with a mechanical excavator, which cuts narrow trench that is backfilled with the concrete up to the ground level.

These foundations use more concrete ,but reduce the cost of masonry walls and may remove the use of timber support for the trenches.

3.      Raft foundation

The best solution if the soil has a poor bearing capacity or if the building loads are quite small, because  the cost of digging separate foundations is eliminated. The oversite concrete slab that's forms the ground floor becomes the raft foundations.

The slab can be thickened at the edges with an edge beam and thickened underneath internal load bearing walls.              

 Mesh reinforcement increases the strength of the raft foundation and distributes the pressures of the building loads evenly.

The foundation consisting of a thick R.C.C slab covering the whole area of a mat is known as raft foundation.

Method of construction of Raft Foundation:

1.      In Raft Foundation construction the whole area is dug out to the specified depth and 30 cm more wide than the area to be covered.

2.      The bed is compacted and sprinkled over with water.

3.      Then a layer of lime concrete or lean concrete ( 1: 8 : 16 ) is laid to a suitable thickness to act as a bottom cover.

4.      After this, the reinforcement is laid. The reinforcement consists of closely spaced bars placed at right angles to one another.

5.      Then the cement concrete (1 : 2 : 4 ) is laid and compacted to the required thickness.

6.      The concrete slab so laid is then properly cured

7.      When loads are excessive, thick concrete beams running under the columns can also be constructed.

 

SUITABILITY :

This type of foundation is useful for public buildings, office buildings, school buildings, residential quarters etc, where the ground conditions are very poor and bearing power of the soil is so low that individual spread footing cannot be provided.

4.      Pad foundation

ü  Pad foundations are isolated foundations designed to support columns.

ü  The area of foundation is determined by dividing the column load plus the weight of the foundation by the allowable bearing capacity of the ground.

ü  The thickness of the foundation must not be less than the projection from the column (unless reinforced) and must, in any case I not be less than 150 mm.

ü  The size of foundation can be reduced by providing steel reinforcement towards the bottom of the foundation running in both directions.

5.      Piled foundation.

A pile is basically a long cylinder of a strong material such as concrete that is pushed into the ground so that structures can be supported on top of it.

Pile foundations are used in the following situations:

ü  When there is a layer of weak soil at the surface. This layer cannot support the weight of the building, so the loads of the building have to bypass this layer and be transferred to the layer of stronger soil or rock that is below the weak layer.

ü  When a building has very heavy, concentrated loads, such as in a high rise structure.

ü  Pile foundations are capable of taking higher loads than spread footings.

There are two types of pile foundations, each of which works in its own way.

End Bearing Piles

In end bearing piles, the bottom end of the pile rests on a layer of especially strong soil or rock. The load of the building is transferred through the pile onto the strong layer. In a sense, this pile acts like a column. The key principle is that the bottom end rests on the surface which is the intersection of a weak and strong layer. The load therefore bypasses the weak layer and is safely transferred to the strong layer.

Friction Piles

Friction piles work on a different principle. The pile transfers the load of the building to the soil across the full height of the pile, by friction. In other words, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil.

To visualize  how this works, imagine you are pushing a solid metal rod of say 4mm diameter into a tub of frozen ice cream. Once you have pushed it in, it is strong enough to support some load. The greater the embedment depth in the ice cream, the more load it can support. This is very similar to how a friction pile works. In a friction pile, the amount of load a pile can support is directly proportionate to its length.

SUITABILITY :

This type of foundation is suitable under the following situations ;

·         When the soil is very soft and solid base is not available at a reasonable depth to keep the bearing power within safe limits.

·         When the grillage and raft foundation are very expansive.

·         When the building is very high carrying heavy concentrated loads.

·         When it is necessary to construct a building along the sea shore or river bed

Methods of Excavation 

There are number of excavation methods which are used for deep foundation construction such as :

a)      full open cut method,

b)      bracing excavation,

c)      anchored excavation,

d)     island excavation methods,

e)      zoned excavation,

f)       top down construction methods etc.

These excavation techniques are discussed.

Full Open Cut Method

It divided into two major types including sloped full open cut as shown in Figure 2 and cantilever full open cut as illustrated in Figure 3.

The former is assumed to be economical since the side of excavation would be sloped and does not need any support to held foundation wall. However, if the slope is considerably gentle or the excavation is largely deep, it will costly.

The latter needs retaining wall to support foundation wall soil and prevent collapse of foundation wall but it neither require backing nor slopes. Therefore, it cannot categorically be claimed that which method is more cost effect. The economical method may be distinguished based on analysis, design, and evaluation results.

Bracing Excavation Method

Bracing excavation (as shown in Figure 4) is the placement of horizontal struts in front of retaining wall to held excavation wall material pressure. Bracing system consist of wale, strut, center posts, end braces, and corner braces.

Earth pressure transfer to horizontal struts through wale, and the purpose of corner and end braces is to reduce wale span without increasing strut number. Center posts prevent the failure of struts due to their own weight.

Anchored Excavation Methods

In this technique, anchors as shown in Figure 5 are installed to counter act against earth pressure. Configuration and components of anchored excavation technique are illustrated in Figure 6.

Bonded portion of the anchor provides anchoring force that works against earth pressure whereas unbonded part of the anchored transfer pressure to the anchor head. Anchor head transfer loads to the retaining wall.

The anchoring force is greatly based on the soil strength. The higher the soil strength the greater the anchoring forces. This technique is not suitable for clay and granular soil with high ground water table.

Lastly, it require short time to complete excavation with great efficiency and suitable for large areas and shallow depth.

Fig.5: Anchored Excavation Method

Fig.6: Configuration of Anchors and Different Parts of the System

Island Excavation Methods

In this method, the center of excavation area is dug and excavated material placed close to the retaining wall to create a slope. After that, the major part of the structure would be constructed at the center of the excavation. Then, the sloped soil will be excavated and struts will be placed between retaining wall and the main structure.

Finally, the struts will be removed and remaining parts of the structure will be constructed. Sometimes, it might be required to use anchored or braced technique to removed slopes soil material, specifically when the excavation is too deep.

Fig.7: Island excavation method explained, this method is suitable for sizable excavation area

Zoned Excavation Methods

Diaphragm walls are used as a retaining wall in the zoned excavation method. Deformation of the longer span wall would greater than short span wall as explained Figure 8.

Fig.8: Deformation of Longer Span Wall Compared to Shorter Span Ones

So, the deflections of longer span walls are declined by dividing excavation area into small area to decrease wall deformation and settlement as shown in Figure 9.

Fig.9: Dividing total excavation area into two smaller area

The excavation will begin in area B while area A would be left to support the wall of area B. then struts in area B would be installed and excavation starts in area A. This process will continue in stages till the whole excavation is completed.

It can be clearly observed that the load on diaphragm wall would be considerably large and hence deflection would great if the area had not been divided into smaller area.

Top Down Excavation Methods

In this method, construction begins from the top to the bottom of excavation and superstructure construction starts after the construction of the first slab is completed.

So, slabs are constructed after each stage of excavation is finished. The slabs play the same role as struts in holding earth pressure.

Construction process order include retaining wall construction, pile construction under column of superstructure, placing columns on piles, and installing formwork for the first slab at the top then other slabs of the would be constructed after each excavation.

This technique would need short construction time, but the cost is higher compare to other methods. Another advantage is that, construction area safer since slabs are stronger than struts.

Fig.10: Top Down Construction

Fig.11: Comparison between top down construction and bottom up construction

Factors Affecting Selection of Excavation Methods

o    Allowable construction period

o    Construction budget

o    Availability of construction equipment

o    Existence of adjacent excavation

o    Condition of adjacent buildings

o    Type of foundation of neighboring structure

o    Construction site area

Setting Out Building Foundations on Ground

Setting out of building foundation trenches is the process of laying down the excavation line and centerline on the ground based on the foundation plan. The setting out process is also called as ground tracing that is performed before commencing the excavation process.

Procedure for Setting Out Building Foundation

The basic steps involved in setting out the foundation trenches are:

1.      The initial step is to mark the corners of the building. After which, the lengths of the sides are checked by diagonal measurements.

2.      The axial lines (center lines) of the trenches are marked with the help of profiles, sighting rails, strings, and pegs.

3.      The trench positioning is controlled by outline profile boards. Profiles are set 2m away from the outline so that they do not interrupt the excavation process.

4.      The offsets are measured from axial lines and the frontage lines are placed in their correct position relative to local requirements.

5.      The cross walls positioning is performed by measuring along the main walls and squared from these walls as required. The total width of trenches must be carefully outlined during this process.

Requirements in Setting Out Foundation

The setting out play out must establish the following requirements:

1.      The size of the excavation

2.      The shape of excavation

3.      The direction

4.      The width of the walls

5.      The position of the walls

 The following points should be observed while setting out trenches:

1.      In order to set out foundation plan, nails, pegs, profiles, strings, and lime are used.

2.      In order to correctly determine the position of trenches, the sight rails have to be properly erected at the corners of the building.

3.      Accurate center lines or axial lines can be determined and marked by using a theodolite.

4.      To the nails or pegs on the profiles, strings are tied and stretched to achieve horizontal control of dimensions.

5.      At a distance of 1 meter from the edges of excavation vertical reference pillars are erected. Hence vertical control is achieved during building construction.

6.      A standard datum is previously determined and marked by the surveyor, based on which the levels on the site are obtained. The depth of trenches and other levels should also be regulated by measurements from this point.

7.      Before placing the concrete into the trenches, the bottom must be properly rammed and compact.

8.      The width is marked by means of lime powder when the excavation is performed by hand. These markings give accurate cutting.

9.      Centreline is marked when the excavations are performed by a machine.

Methods of Dewatering Excavations at Construction Site

o   Dewatering of any excavated area is done in order to keep the excavation bottom dry, to prevent the leakage of water or sand and to avoid upheaval failure. Dewatering could turn out to be a herculean task if one doesn’t adopt the right method.

o   The different methods available for dewatering of excavations at constructions sites are not necessarily interchangeable as each one has a narrow range of applications therefore adopting the right method of dewatering for a particular ground condition is always a critical and a difficult decision to make.

o   Minor amount of water can always be pumped out by creating a sump but when other factors like continuous seepage, excessive smudge come into play one has to resort to a bit of sophistication.

Methods of Dewatering Excavations at Construction Site

There are four important dewatering methods one should be aware of:

1.      WellPoint method of dewatering,

2.      Educator wells,

3.      Open sump pumping and

4.      Deep WellPoint method

5.      WellPoint Method of Dewatering Excavations

1) A series of wells of required depth are created in the vicinity of the excavated area from where the water has to be pumped out. The wells are arranged either in a line or a rectangular form where the wellpoints are created at a distance of at least 2m from each other.

2) Riser pipes or dewatering pipes are then installed into those closely spaced wells which on the surface are connected to a flexible swing pipe which is ultimately appended to a common header pipe that is responsible for discharging the water away from the site. The purpose of using a flexible swing pipe is just to provide a clear view of what is being pumped and the purpose of header pipe is to create suction as well as discharge the water off the working area.

3) One end of the header pipe is connected to a vacuum pump which draws water through notches in the wellpoint. The water then travels from the wellpoints through the flexible swing pipe into the header pipe to the pump. It is then discharged away from the site or to other processes to remove unwanted properties such as contaminants.

4) The drawdown using this method is restricted to around five to six meters below the wellpoint pump level. If a deeper drawdown is required, multiple stages of wellpoints must be used.

Eductor Wells Method of Dewatering Excavations

The method is very similar to the wellpoint method of dewatering; the only difference lies in the usage of high pressure water in the riser units instead of vacuum to draw out water from the wellpoints. The method uses the venturi principle which is the reduction in fluid pressure that results when a high pressure fluid flows through a constricted section of a pipe.

  A high pressure supply main feeds water through a venturi-tube just above the well-screen, creating a reduction in pressure which draws water through the riser pipe. The high pressure main feeds off the return water. The biggest advantage of using the eductor system is, the water table can be lowered from depths of 10-45 m if multiple pumps are operated from a single pump station. This method therefore becomes economically competitive at depth in soils of low permeability.

Open Sump Pumping Method of Dewatering Excavations

This is the most common and economical method of dewatering as gravity is the main playing force. Sump is created in the excavated area into which the surrounding water converges and accumulates facilitating easy discharge of water through robust solid handling pumps.

Its application is however confined to the areas where soil is either gravelly or sandy. Since the bottom of the sump is situated at a level lower than that of the excavation bottom, it will abridge the seepage way along which groundwater from outside seeps into the excavation zone and as a result the exit gradient of the sump bottom will be larger than that on the excavation surface.

If the excavation area is large, several sumps may be placed along the longer side or simply use a long narrow sump which is called a ditch.

Deep Well Method of Dewatering Excavations

Just like the wellpoint method, wells are drilled around the excavated area, but the diameter of wells in this case varies between 150-200mm. By creating deep wells around the vicinity, the ground water is made to fall into them under the influence of gravity.

As a result, the ground water level in the surroundings would decline. According to the type and arrangement of pumps, the depths of the wells could reach up to 30m. This method is generally adopted when a heavy amount of water from the ground has to be drawn out.

Casings of diameter fitting to wells are installed in order to retain the wells. Additionally, well screens and filters (between sidewalls and casing) are used which serve as a filtering device therefore not letting the unwanted sediments enter into the well. The water thus accumulated is pumped out using a submersible pump or a centrifugal pump.

It is prudent to assess ground-permeability conditions beforehand since the whole process of accumulation and pumping takes quite a bit of time. This may cause settlement in the nearby areas and hence a different technique might need to be adopted.

Timbering

Timbering consists of providing timber planks or boards and struts to give temporary support to the sides of the trench.

When the depth of trench is large, or when the sub-soil is loose, the sides of the trench may cave in. The problem can be solved by adopting a suitable method of timbering.

Timbering of trenches, sometimes also known as shoring .

Methods of timbering:
1. Stay bracing.
2. Box sheeting
3. Vertical sheeting
4. Runner system
5. Sheet piling. 

1.      Stay bracing. 

This method (Fig. 2.31) is used for supporting the sides or a bench excavated in fairly firm soil, when the depth of excavation does not exceed about 2 metres. The method consists of placing vertical sheets (called sheathing) or polling boards opposite each other against the two walls of the trench and holding them in position by one or two rows of struts. The sheets are placed at an interval of 2 to 4 metres and generally, they extend to the full height of the trench. The polling boards may have width of about 200 mm and thickness of 44 to 50 mm. The struts may have size 1OO x 100 mm for trench upto 2 m wídth and 200 x 200 mm for trench upto 4 m width.

 FIG. 2.31  STAY BRACING.

2. Box sheeting. 

This method is adopted in loose soils, when the depth of excavation does not exceed 4 metres. Fig. 2.32 (a) shows the box like structure, consisting of vertical sheets placed very near to each other (sorne times touching each other) and keeping them in position by longitudinal rows (usually two) of wales. Struts are then provided across the wales.

Another system of box sheeting, shown in Fig. 2.32(b), is adopted for very loose soils. In this system, the sheeting is provided longitudinally, and they are supported by vertical wales and horizontal  struts [Fig. 2.32 (b)]. If the height is more, braces are also provided along with struts.

FIG. 2.32 BOX SHEETING.

3. Vertical sheeting. This system is adopted for deep trenches (upto 10 m depth) in soft ground. The method is similar to the box sheeting [Fig. 2.32 (a)] except that the excavation is carried out in stages and at the end of each stage, an offset iS provided, so that the width of the trench goes on decreasing as the depth increases. Each stage is limited to about 3 m in height and the offset may vary from 25 to 50 cm per stage. For each stage, separate vertical sheeting, supported by horizontal wailings and struts are provided (Fig. 2.33).

4. Runner system. This system is used in extremely loose and soft ground, which needs immediate support as excavation progresses. The system is similar to vertical sheeting of box system, except that in the place of vertical sheeting, runners, made of long thick wooden sheets or planks with iron shoe at the ends, are provided. Wales and struts are provided as usual (Fig. 2.34). These runners are driven about 30 cm in advance of the progress of the work, by hammering

                     FIG. 2.33 VERTICAL SHEETING.             FIG. 2.34 RUNNER SYSTEM.


5. Sheet piling. This method is adopted when (i) soil to be excavated is soft or loose (ii) depth of excavation is large (iii) width of trench is also large and (iv) there is sub-soil water. Sheet piles are designed to resist lateral earth pressure. These are driven in the ground by mechanical means (pile driving equipment). They can be used for excavating to a very large depth.

Measures taken for Foundation of Timbering Trenches:

Precautionary Measures in Construction of Deep Open Foundation:

Sometimes, it becomes necessary to take the foundation deep in the soil. Excavation of deep trenches creates problems, specially when the soil is loose or granular or soft or of mixed variety or the water table is high. The walls of the foundation trench which are kept vertical cave in and crumble. Precautionary measures for protection of the trench walls are necessary.

i. To protect the trenches, timbering is done. However, in cases when the space at site is available, the walls of the trenches may be made sloped instead of keeping them vertical.

The angle of slope should not be more than the angle of repose of the soil. This will increase the cost of earthwork in excavation and back-filling of the trenches; but may still be economical in view of the high cost of timbering materials and construction difficulties.

ii. When the trench is very deep, and sufficient space for making the walls slope at desired angle is not available, timbering of the trenches will have to he resorted to; however, in cases, a combination of timbering and sloping of the walls may be done according to the suitability of the site.

iii. In case of foundations of depth beyond 4.0 m, timbering of trenches in any of the methods may not be considered suitable. In such cases, the trench may be protected by piling. Sal bullah or timber or precast R.C.C. piles are driven close enough to touch each other. The length of the piles need be minimum 1.5 times, if possible 2 times, the proposed depth of excavation.

The piles are driven first in rows and then excavation is done and simultaneously the wall is braced with horizontal bracings and struts. In cases of excavation of basement, horizontal strutting may not be possible, inclined strutting may have to be resorted to with proper anchorage.

iv. According to the situation a combination of piling and sloping of the sides may be adopted.

The closed pile walls always have gaps, providing passage for water to come out. If water only comes out, it would not be harmful; but if mud comes out which means flow of soil it is a sign of danger. Pumping of water from the trench must be stopped immediately and the temporary closed pile walls must be checked for stopping the leakage.

However, it is very difficult to achieve complete sealing of the leakage and it is not desired also. The soil flow should be stopped allowing seepage of water. Release of water will reduce lateral thrust on the closed pile wall from the +++arth outside the wall.

Sheet piling would have been the best answer; but it is very costly. The length of the piles need be 2 times the proposed depth of excavation. After construction, it becomes very difficult to extract the sheet piles and, in cases, they are left in situ which increases the cost.

The diaphragm walls which are considered a substitute to sheet piles and are left in position are also very costly. The height of the diaphragm wall need be 2 times the’ proposed depth of excavation. Both sheet piles and diaphragm walls are required to be provided with horizontal supports which make them costlier.

FOUNDATION MAINTENANCE

1. Pay Attention to Foundation Cracks

Regardless of the size, foundation cracks can cause long-term damage to your home foundation. A “minor crack” is not determined by size, but by the severity of the crack. Some details to consider when you first notice the crack are:

• The date you observed the crack and any changes over time

• Temperature

• Rain exposure

• Possible leaks in the homeIt’s important to monitor the cracks and look for changes in width and length. If the cracks increase in length or become wider, it’s best to call a specialist for a foundation inspection.

A structural engineer can examine the crack and determine if your house is safe or hazardous to live in.

2. Keep Soils Consistently Moist

Movement of the soil puts un wanted stress on your foundation. To avoid the constant soil expansion and contraction process, it is essential to utilize a soaker hose system. During the dry months, the soaker hose will help prevent the top layer of soil from drying out too quickly, which will prevent extensive evaporation. During the wet months, it will also prevent the soil from swelling. In addition, using the soaker hose system can help keep the moisture at a consistent level and prevent extensive evaporation.

3. Keep Water Away from the Home

Storm water runoff and sprinkler water can cause severe damage if the water is near the perimeter of the home’s foundation. Homeowners should routinely check for standing water that could be near the foundation wall or slab and confirm that the home’s downspouts have extensions to direct water away from the foundation.

4. Be Aware of Tree Placement

Tree placement is critical, as trees can be harmful and costly if they're too close to the foundation. Large trees can absorb up to 150 gallons of water per day. If trees are near a home’s foundation with limbs extending over the roof, damage to the foundation in that area could occur due to thirsty tree roots that can cause the soil in the area to shrink. To prevent soil shrinkage and other negative impacts, homeowners should consider installing a root barrier.

Trees that soak up a lot of water can dry out the soil around your home, causing foundation damage.

5. Be a Smart Landscaper

Homeowners should keep the ground elevations of any landscaped flowerbeds and brushes located close to the home at least three to four inches over the finished elevation of the wall. The landscaped areas should be sloped away from the foundation for proper drainage and to help prevent moisture intrusion from entering through the foundation.

6. Maintain a Consistent Temperature

Keeping home temperature consistent can prevent construction materials such as concrete, wood and drywall from contracting and expanding. Often, thermal fluctuations are the sole cause of cracks and foundation damage.

7. Get a Plumbing Test

Don’t delay in getting a plumbing test. Small leaks can slowly deteriorate the soils beneath the home over time. An inexpensive plumbing test may save you thousands of dollars in the long run.

Following these recommendations and tips will help protect your home foundation and decrease the chances of costly damage and repairs. All homeowners should consider an annual inspection as part of an on-going foundation maintenance program. To learn more, contact a professional foundation engineer.