TECHNICAL REPORT 2

FOUNDATION

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Introduction

Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics to support them.

There are a very wide range of foundation types suitable for different applications, depending on considerations such as:

• The nature of the load requiring support.
• Ground conditions.
• The presence of water.
• Space.
• Accessibility.
• Sensitivity to noise and vibration.

Very broadly, foundations can be categorized as shallow foundations or deep foundations.

• Shallow foundations are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils.
• Deep foundations are necessary where the bearing capacity of the surface soils is not adequate to support the loads imposed by a structure and so those loads need to be transferred to deeper layers with higher bearing capacity.

Types of deep foundations

1. Strip foundation (or footings)

Strip foundations are a type of shallow foundation that are used to provide a continuous, level (or sometimes stepped) strip of support to a linear structure such as a wall or closely-spaced rows of columns built centrally above them.

Strip foundations can be used for most subsoils, but are most suitable for soil which is of relatively good bearing capacity. They are particularly suited to light structural loadings such as those found in many low-to-medium rise domestic buildings - where mass concrete strip foundations can be used. In other situations, reinforced concrete may be required.

Generally, the size and position of strip foundations is typically related to the wall’s overall width. The depth a traditional strip foundation is generally equal to or greater than the overall wall width, and the foundation width is generally three times the width of the supported wall. This results in the load being transmitted at 45º from the wall base to the soil.

The underside of strip foundations should be deep enough to avoid frost action; for example, at least 450 mm unless they are bearing on rock, and at least 1 m on high shrinkage clays.

Deep strip foundations may be necessary where soil with a suitable bearing capacity is deeper.

Wide strip foundations may be required where the soil is soft or of a low bearing capacity, so as to spread the load over a larger area. Wide strip foundations will typically require reinforcement.

2. Pad foundations

Pad foundations are generally shallow foundations, but can be deep depending on the ground conditions. They are a form of spread foundation formed by rectangular, square, or sometimes circular concrete ‘pads’ that support localised single-point loads such as structural columns, groups of columns or framed structures. This load is then spread by the pad to the bearing layer of soil or rock below. Pad foundations can also be used to support ground beams.

They are generally of a uniform thickness, but sometimes the upper face may be sloped or stepped. Their plan shape will depend on the nature of the applied load and the allowable bearing capacity of the layers below. Their thickness must be sufficient to distribute the load across the plan shape. They are generally reinforced on all but the smallest structures, with the reinforcement allowing higher loads to be imposed and the construction of shallower pads which require less excavation and use less concrete.

The arrangement of pad foundations will vary depending on the nature of the structure they are supporting, the loads imposed, the allowable bearing capacity of the layers below and the space available on site. They may be:

a. Plain concrete

Plain concrete pad foundations that do not use reinforcement are an economical solution but only where the applied load will be relatively light. These can also be referred to as footings. The general rule is that the depth of the pad should be equal to the distance from the face of the vertical element to the edge of the pad on both sides.

Pad foundations can be selected as they do not require much excavation, and are generally suitable where the bearing capacity of ground is sufficient at relatively low depths. However, they can be large in plan shape and may not be effective against differential settlement, uplift forces or wind forces.

Example of a plain concrete foundation.

b. Reinforced concrete

The addition of reinforcement allows for relatively wide but shallow pad foundations. In order to make the reinforcing cage easier to construct and place, the pads tend to be designed as a square plan area. The reinforced concrete base is designed to span in one direction, with the main bars longitudinal in the bottom.

Where the width of the base is restricted or where there is eccentric/inclined loading, rectangular pads can be designed.

Reinforced concrete foundation built at site.

c. Combined column foundation

These are where two pad foundations are combined into a longer one and can be used where the outer column is close to a site boundary or existing wall. The purpose is that the balancing effect of the internal column can be incorporated. The plan shape is usually a rectangle.

 Combined column foundation built at site.

d. Continuous pad

This is where the pad foundations are combined together as a single long structural element. This is often the case where the pads and the columns they support are closely spaced. By extending the reinforcement between the pads, differential settlement can be resisted and longitudinal stiffness can be improved.

e. Pad and ground beam

This is similar to a continuous pad but differs in that smaller isolated pads are connected by ground beams. This helps to improve structural rigidity.

 Pad foundation and ground beam for boiler at 55MW power plant project.

3. Raft foundations

Raft foundations (sometimes referred to as raft footings or mat foundations) are formed by reinforced concrete slabs of uniform thickness (typically 150 mm to 300 mm) that cover a wide area, often the entire footprint of a building. They spread the load imposed by a number of columns or walls over the area of foundation, and can be considered to ‘float’ on the ground as a raft floats on water.

They are suitable where:

• Floor areas are small and structural loadings are low, such as in one or two-storey domestic construction.
• A basement is required.
• Ground conditions are poor and strip or pad foundations would require significant excavation, for example on soft clay, alluvial deposits, compressible fill, and so on.
• Settlement, or differential settlement is likely.
• Where it may be impractical to create individual strip or pad foundations for a large number of individual loads. In very general terms, if strip or pad foundations would cover 50% or more of the floor area, then a raft may be more appropriate.

Raft foundations can be fast and inexpensive to construct, as they tend not to require deep excavations compared to strip or pad foundations and they may use less material as they combine the foundation with the ground slab. However, they tend to be less effective where structural loads are focused on in a few concentrated areas, and they can be prone to erosion at their edges.

They are generally constructed on a compacted hardcore base (perhaps 100 mm thick). A layer of blinding concrete may then be laid to allow formation of the raft (typically 50 mm) with a waterproof membrane above.

Types of raft foundation include:

• Solid slab raft, sometimes referred to as a plain raft, and including; flat rafts, mats, wide toe rafts, slip plane rafts, blanket rafts, and so on.
• Slab beam raft.
• Cellular raft.
• Piled raft.

The concrete raft tends to include steel reinforcement to prevent cracking, and may incorporate stiffening beams or thickened areas to provide additional support for specific loads, for example, below internal walls or columns (which may require punching shear reinforcement). Beams may stand proud of the raft, either above or below it, or may be 'hidden' beams, formed by reinforced areas within the depth of the raft itself. These thickened areas are particularly useful where there are poor ground conditions, as the required thickness of the raft itself might otherwise be uneconomic.

Typically, a thickened reinforced area is created at the perimeter of the raft to form an edge beam supporting the external walls of the building. A concrete toe often supports the external leaf of the wall.

Steps to build a shallow foundation (example of reinforced pad foundation at site).

Steps:
1. Excavation work for foundation at designated place. The volume excavated are normally bigger than the foundation appropiately.
2. A lean concrete is then poured onto the ground. The main function of the lean concrete is to provide a uniform surface for foundation and to prevent the direct contact of foundation to the soil. Then, form work is installed after measuring work done.
3. Next, reinforcement work is carried out based on the design. Usually steel wire and welding work would be used for steel bars binding work.
4.  Concreting work is carry out after the reinforcement work and form work installation work.
5. Lastly, the foundation concrete would left for curing work for 3 days. Normally the worker will watering the foundation for curing work.

Types of deep foundations

1. Pile foundations

Pile foundations are deep foundations. They are formed by long, slender, columnar elements typically made from steel or reinforced concrete, or sometimes timber. A foundation is described as 'piled' when its depth is more than three times its breadth.

Pile foundations are principally used to transfer the loads from superstructures, through weak, compressible strata or water onto stronger, more compact, less compressible and stiffer soil or rock at depth, increasing the effective size of a foundation and resisting horizontal loads. They are typically used for large structures, and in situations where soil is not suitable to prevent excessive settlement.

Piles may be classified by their basic design function (end-bearing, friction or a combination) or by their method of construction (displacement (driven) or replacement (bored)).

End-bearing piles develop most of their friction at the toe of the pile, bearing on a hard layer. The pile transmits load direct to firm strata, and also receives lateral restraint from subsoil.

Friction (or floating) piles develop most of the pile-bearing capacity by shear stresses along the sides of the pile, and are suitable where harder layers are too deep. The pile transmits the load to surrounding soil by friction between the surface of the pile and soil, which in effect lowers the bulb of pressure.

Driven (or displacement) piles are driven, jacked, vibrated or screwed into the ground, displacing the material around the pile shaft outwards and downwards instead of removing it. These piles are useful in offshore applications, are stable in soft squeezing soils and can densify loose soil.

Bored (or replacement) piles remove spoil to form a hole for the pile which is poured in situ. They are used primarily in cohesive subsoils for the formation of friction piles and when forming pile foundations close to existing buildings. They are more popular in urban areas as there is minimal vibration, they can be used where headroom is limited, there is no risk of heave and where it may be necessary to vary their length.

Screw piles have a helix near the pile toe so they can be screwed into the ground. The process and concept is similar to screwing into wood.

Micropiles are used where access is restricted, for example underpinning structures affected by settlement. They can be driven or screwed into place. Micropiles can also be used in combination with other ground modification techniques where complex site conditions and design specifications are present.

Pile walls can be used to create permanent or temporary retaining walls. They are formed by placing piles directly adjacent to one another. These can be closely-spaced contiguous pile walls or interlocking secant pile walls; which depending on the composition of the secondary intermediate piles can be hard/soft, hard/firm or hard/hard secant walls. Sheet piles is one of the type of pile walls which are commonly use in our country.

The selection of sheet piling is dependent on factors, such as:

• The type of work, for example. whether it is permanent or temporary.
• Site conditions.
• The required depth of piles.
• The bending moments involved.
• The nature of the structure.
• The type of protection required.

A wide range of equipment is available for piling, including:

• Percussion drivers: Hammers driven by steam, compressed air or diesel.
• Hydraulic drivers: Hydraulic rams push piles into the ground.
• Vibratory drivers: Piles are vibrated into the ground.
• Rotary augers: Used to screw replacement piles into the ground.

Steps to build a deep foundation (Example of a driven end bearing piles at site).

a. Pegging of pile points at site based on a designed pile point drawing plan.

b. Demobilization of piles at site. A lorry send a group of piles to site and a crane carry out the demobilization work of piles at suitable place near to the pile point.

c. Next, the piling machine started to carry a pile and drove the pile at the pegged point.

 d. Lastly, the pile is driven onto a hard layer or until it set.

 
2. Diaphragm wall

A diaphragm wall is a structural concrete wall constructed in a deep trench excavation, either cast in situ or using precast concrete components. Diaphragms walls are often used on congested sites, close to existing structures, where there is restricted headroom, or where the excavation is of a depth that would otherwise require the removal of much greater volumes of soil to provide stable battered slopes.

Diaphragm walls are suitable for most subsoils and their installation generates only a small amount of vibration and noise, which increases their suitability for works carried out close to existing structures. In addition, floor slab connections and recessed formwork can be incorporated into the walls.

The walls generally range in thickness from 500 - 1,500 mm and can be excavated to depths of over 50 m. Excavation is typically carried out using rope-suspended mechanical or hydraulically-operated grabs. Specific ground conditions or greater depths may require the use of hydromills – hydraulically-operated reverse circulation trench cutters – to penetrate into hard rock by ‘cutting’ rather than ‘digging’. Hydromills can achieve depths of up to 80 m.

The excavation stability is maintained by the use of a drilling fluid, usually a bentonite slurry. This is a controlled mixture that has thixotropic properties, meaning that it exerts a pressure in excess of the earth and hydrostatic pressures on the sides of the excavation. The walls are constructed, using reinforced or unreinforced concrete, in discrete panel lengths generally ranging between 2.5 - 7 m. Purpose-made stop ends can be used to form the joints between adjacent panels, with a water bar incorporated across the joints. More complicated arrangements such as ‘L’ or ‘T’-shaped panels can be constructed where additional bending moment capacity or wall stiffness is required.

Precast concrete diaphragm walls have the same advantages but are less flexible in terms of design. The units are installed in a trench filled with a special mixture of bentonite and cement with a retarder added to control the setting time. Ground anchors are used to tie the panels or posts to the retained earth to provide stability.

The high cost of diaphragm walls can make them uneconomic unless they can be incorporated into part of a building structure. As such, they are suited for deep basements, underground car parks and rail stations, tunnel approaches, underpasses, deep shafts for tunnel ventilation, pumping stations, and so on.

3. Caisson

A caisson is a box-like structure commonly used in civil engineering projects where work is being carried out in areas submerged in water. Such projects might include:

• Bridge piers.
• Abutments in lakes and rivers.
• Break water and other shore protection works.
• Wharves and docks.
• Large water front structures.

Caissons differ from cofferdams in that cofferdams are removed after completion of the work, whereas caissons are built to remain in place as a part of the completed structure.

Caissons can be made of materials including timber, steel, masonry and reinforced concrete, and may be constructed onshore then floated to the required location, where they are sunk into place, enabling access to the bed and excavation of foundations to the required depth.

They are particularly suitable for the construction of underwater foundations or where the water is deep, as they are strong enough to withstand significant vertical and horizontal loads, as well as lateral forces such as waves.

Box caisson

This is a watertight timber or reinforced concrete box with a closed bottom and an open top. The caisson is cast and cured on land and then sunk into place, or it can be rested on top of a pile formation. Sand, concrete or gravel is used to weigh down and sink the caisson. This is most suitable for areas where the bearing strata is reasonably level and no excavation is required, although it is possible for some dredging to further level the base if required to avoid the tilting of the caisson once in place. This type of caisson is generally relatively economical but may not be suitable if the bearing strata requires compacting and/or leveling.

Open caisson

This is a timber, steel or concrete box that is open at both the bottom and the top. The walls are heavy and made with sharp edges that facilitate the sinking process. There are three different types of open caisson:

i. Single wall                                      ii. Cylindrical                     iii. Open with dredging wells

Example of an open caisson.

Pneumatic caisson

Pneumatic caissons are closed at the top but open at the bottom, with the water forced out using compressed air, creating a working chamber which is airtight in order for excavation to be carried out. This is suitable when it is not possible to excavate wet ground in the open.

Although this method is suitable for difficult locations, such as depths ranging from 25-40 m, it is a complex, slow and expensive procedure.

Selection of Type of Foundation

The selection of a particular type of foundation is often based on a number of factors, such as:

1. Adequate depth

The foundation must have an adequate depth to prevent frost damage. For such foundations as bridge piers, the depth of the foundation must be sufficient to prevent undermining by scour.

2. Bearing capacity failure

The foundation must be safe against a bearing capacity failure.

3. Settlement

The foundation must not settle to such an extent that it damages the structure.

4. Quality

The foundation must be of adequate quality so that it is not subjected to deterioration, such as from sulfate attack.

5. Adequate strength

The foundation must be designed with sufficient strength that it does not fracture or break apart under the applied superstructure loads. The foundation must also be properly constructed in conformance with the design specifications.

6. Adverse soil changes

The foundation must be able to resist long-term adverse soil changes. An example is expansive soil, which could expand or shrink causing movement of the foundation and damage to the structure.

7. Seismic forces

The foundation must be able to support the structure during an earthquake without excessive settlement or lateral movement.

Based on an analysis of all of the factors listed above, a specific type of foundation (i.e., shallow versus deep) would be recommended by the geotechnical engineer.

Foundation Failures

Foundations provide the support and resistance of the loads of the structures above. They serve as structural systems that transfer loads to the soil below and that provide stability, including resistance to overturning, sliding, and uplift, for the overall structure. Due to the importance of their structural system to the overall structure, it is imperative that their structural integrity is maintained for the overall structure to function. However, in some cases, foundations can fail. We are now going to explore the different types of causes that can affect the failure of foundations, in order to prevent and remediate the failures.

Below are the causes of foundation failures:

Uneven loading

The uneven distribution of loading from the superstructure can induce uneven stresses at different locations of the foundation. This can cause differential settlement at locations where vertical structural elements, such as columns and walls, directly transfer the superstructure loads to the foundation.  Differential settlement can eventually lead to cracks at the foundation.

Overloading

Overloading from the superstructure can also create foundation failure. Foundations can fail by cracking when the design moment and/or shear is above its moment and/or shear capacity. Failure can also occur when there are large concentrated or point loads, which can induce large punching shear onto the foundation, and when there is over designing of bearing pressure.

Different properties of soil at the foundation interface 

Different parts of the foundation can rest on different properties of soil. For example, one part of the foundation can sit on clay, while another part of the foundation can sit on rock. When all design checks are adequate for one part of the foundation due to that part resting on good soil and when checks fail for another part of the foundation due to bad soil properties at the other part of the foundation, the whole foundation can fail.

Ground investigation will need to be used to determine these different soil properties. The foundation structure will need to be designed in consideration of the different soil stiffness and soil properties.

Insufficient soil compaction 

The soil fill underneath the foundation might not be compacted properly and to its required degree of compaction. Since the soil is not compacted properly, air voids can be created within the soil, in which soil and water can displace in and out of. There will then be movement within the soil, which causes swelling and contracting. The swelling and contraction of the soil can cause pressure to the foundation that the soil supports.

Air voids within the soil can cause loose soil or soil with low density, which lacks adequate strength to support the foundation. Poor compaction equipment can also lead to foundation failure.

Therefore, it is best to compact the soil beneath the foundation to its required compaction degree before concrete placement of the foundation, in order to reduce soil displacement, to increase subgrade reaction and soil density, and to reduce differential and overall settlement of the foundation.

Uneven moisture levels of soil beneath the foundation

Similar to the above scenario, uneven moisture levels of the soil can cause soil swelling and contraction at specific parts of the foundation. This can lead to stress at intersecting locations where the soil is swelling and contracting and where the soil is not.

Changes in moisture levels of soil beneath the foundation
Moisture levels of the soil can change due to varying humidity levels, rainy weather, or poor drainage conditions, which can cause soil to swell (or heave) and contract, therefore leading to cracks. Similar to insufficient soil compaction, the voids within the soil can be filled up by water or other fluids, which can create pressure onto the soil particles from the fluid.
 However, when there are dry periods, the water evaporates from the soil and leaves from the voids within the soil. This can cause soil shrinkage.
 Moreover, when there are cracks within the foundation, water seepage can also occur.

Vibration from adjacent construction
Vibration from nearby construction can displace soil particles underneath the foundation. This can then create air voids within the soil, which can loosen up the soil and lower the soil density. The lower the soil density, the lower the soil strength for the support of the foundation. This will then cause foundation failure.

Transpiration
If there are trees adjacent to the foundation, the trees can evaporate the water from the soil into its roots and into the atmosphere. This can cause changes in the moisture level of the soil.

Conclusion

              The foundation is the base and one of the most important part in the construction of any building or structure. Foundation act as a main part to carry the structure load above of it and transfer the load into the ground to prevent the structure or building from collapse. The type of foundation must be wisely chose and designed based on the type of soil at site to maximise its function. To prevent the case of foundation failure, soil investigation work must be carry out carefully.