Deep foundations
Deep foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths 3 m below finished ground level. Deep foundations can be used to transfer the loading to deeper, more competent strata at depth if unsuitable soils are present near the surface. They are used when there are weak (“bad”) soils near the surface or when loads are very high, such as very large skyscrapers.Deep foundations derive their support from deeper soils or bedrock.
Common Types of Deep Foundations are:
1.Pile foundations
2. Caissons
3.Basement foundations
4. Buoyancy rafts (hollow box foundations)
Piles
Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity. Piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.
Application and Uses for pile:
Piles are commonly used during a variety of construction projects includingforBuildings, Industrial Buildings, Bridge Building, Pile-supported pipelines and Slope stabilization.Pile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface. The main components of the foundation are the pile cap and the piles.
Pile foundations are frequently needed because of the relative inability of shallow footings to resist inclined, lateral, or uplift loads and overturning moments. They are used in areas of expansive or collapsible soils to resist undesirable seasonal movements of the foundations.
Pile caps are thick slabs used to tie a group of piles together to support and transmit column loads to the piles.
Piled foundations can be classified according to :
1- the type of pile
(different structures to be supported, and different ground conditions, require different types of resistance) and
2- the type of construction
(different materials, structures and processes can be used).
Types of pile:
· End bearing piles
· Friction piles
· Settlement reducing piles
· Tension piles
· Laterally loaded piles
· Piles in fill
Piles are often used because adequate bearing capacity cannot be found at shallow enough depths to support the structural loads. It is important to understand that piles get support from both end bearing and skin friction. The proportion of carrying capacity generated by either end bearing or skin friction depends on the soil conditions. Piles can be used to support various different types of structural loads.
End bearing piles
End bearing piles are those which terminate in hard, relatively impenetrable material such as rock or very dense sand and gravel. They derive most of their carrying capacity from the resistance of the stratum at the toe of the pile.
Friction piles
Friction piles obtain a greater part of their carrying capacity by skin friction or adhesion. This tends to occur when piles do not reach an impenetrable stratum but are driven for some distance into a penetrable soil. Their carrying capacity is derived partly from end bearing and partly from skin friction between the embedded surface of the soil and the surrounding soil.
Settlement reducing piles
Settlement reducing piles are usually incorporated beneath the central part of a raft foundation in order to reduce differential settlement to an acceptable level. Such piles act to reinforce the soil beneath the raft and help to prevent dishing of the raft in the centre.
Tension piles
Structures such as tall chimneys, transmission towers and jetties can be subject to large overturning moments and so piles are often used to resist the resulting uplift forces at the foundations. In such cases the resulting forces are transmitted to the soil along the embedded length of the pile. The resisting force can be increased in the case of bored piles by under-reaming. In the design of tension piles the effect of radial contraction of the pile must be taken into account as this can cause about a 10% - 20% reduction in shaft resistance.
Laterally loaded piles
Almost all piled foundations are subjected to at least some degree of horizontal loading. The magnitude of the loads in relation to the applied vertical axial loading will generally be small and no additional design calculations will normally be necessary. However, in the case of wharves and jetties carrying the impact forces of berthing ships, piled foundations to bridge piers, trestles to overhead cranes, tall chimneys and retaining walls, the horizontal component is relatively large and may prove critical in design. Traditionally piles have been installed at an angle to the vertical in such cases, providing sufficient horizontal resistance by virtue of the component of axial capacity of the pile which acts horizontally. However the capacity of a vertical pile to resist loads applied normally to the axis, although significantly smaller than the axial capacity of that pile, may be sufficient to avoid the need for such 'raking' or 'battered' piles which are more expensive to install. When designing piles to take lateral forces it is therefore important to take this into account.
Piles in fill
Piles that pass through layers of moderately- to poorly-compacted fill will be affected by negative skin friction, which produces a downward drag along the pile shaft and therefore an additional load on the pile. This occurs as the fill consolidates under its own weight.
General advantages of piles:
· Material of pile can be inspected before it goes into the ground.
· Stable in "squeezing" ground
· Not damaged by ground heave when driving adjacent piles
· Construction procedure unaffected by ground water
· Can be readily carried above ground level, especially in marine structures
· Can be driven in very long lengths
General disadvantages of piles:
· May break during hard driving causing delays and replacement charges, or worse still may suffer major unseen damage in hard driving conditions
· Uneconomical if amount of material in pile is governed by handling and driving stresses rather than by stresses from permanent loading
· Noise and vibration during driving may cause nuisance or damage
· Displacement of soil during driving piles in groups may damage adjacent structures or cause lifting by ground heave of adjacent piles
· Cannot be driven in very large diameters
· End enlargements not always advantages
· Cannot be driven in conditions of low headroom
Choice between different types of driven preformed piles:
Timber
Suitable for light loads or temporary works.Unsuitable for heavy loads.Subject to decay due to fluctuating water table.Liable to unseen splitting or brooming if driven too heavily.
Concrete
Suitable for all ranges of loading. Concrete can be designed to suit corrosive soil conditions. Readily adaptable to various sizes and shapes. Disadvantages: additional reinforcement must be provided for handling and driving stresses; liable to unseen damage under heavy driving; delay between casting and driving.
Steel
Suitable for all ranges of loading. Can be readily cut down or extended. Cut off portions have scrap value and they can be used for extending other piles. Can be driven hard without damage.Can be driven in very long lengths by welding on additional lengths. Some types have small ground displacement. Structural steel bracing can be readily welded or bolted on. Resilience makes it suitable for jetty or dolphin structures. Disadvantages: subject to corrosion in marine structures and requires elaborate paint treatment and/or cathodic protection; long and slender piles liable to go off line during driving.
Wood piles:
Advantages
• The piles are easy to handle and relatively inexpensive.
• Sections can be joined together and excess length easily removed.
Disadvantages
• The piles will rot above the ground water level. Have a limited bearing capacity.
• Can easily be damaged during driving by stones and boulders.
• The piles are difficult to splice and are attacked by marine borers in salt water.
Steel piles (Rolled steel section):
Advantages:
• The piles are easy to handle and can easily be cut to desired length.
• Can be driven through dense layers and carry heavy loads.
• Can be driven hard and in very long lengths.
• Can be successfully anchored in sloping rock.
• Small displacement piles particularly useful if ground displacements and disturbance critical.
• The lateral displacement of the soil during driving is low (steel section H or I section piles) can be relatively easily spliced or bolted.
Disadvantages:
• The piles will corrode.
• Will deviate relatively easy during driving.
• Are relatively expensive.
Prefabricated concrete piles (reinforced) and pre stressed concrete piles:
Advantages:
• Do not corrode or rot and are easy to splice- Relatively inexpensive.
• The quality of the concrete can be checked before driving.
• Stable in squeezing ground, for example soft clays and peat.
• Can be driven in long lengths. Can be carried above ground level, for example, through water for marine structures.
• Can increase the relative density of a granular founding stratum.
• Can be re driven if affected by ground heave. Construction procedure unaffected by ground water.
Disadvantages:
• Relatively difficult to cut.
• Displacement, heave, and disturbance of the soil during driving.
• Can be damaged during driving. Replacement piles may be required.
• Sometimes problems with noise and vibration.
• Cannot be driven with very large diameters or in condition of limited headroom.
Driven and cast-in-place concrete piles : Permanently cased (casing left in the ground) or Temporarily cased or uncased (casing retrieved)
Advantages:
• Can be inspected before casting can easily be cut or extended to the desired length.
• Relatively inexpensive - Low noise level.
• The piles can be cast before excavation.
• Pile lengths are readily adjustable.
• An enlarged base can be formed which can increase pile capacity.
• Reinforcement is not determined by the effects of handling or driving stresses.
• Can be driven with closed end so excluding the effects of GW
Disadvantages:
• Heave of neighbouring ground surface, which could lead to re consolidation and the development of negative skin friction forces.
• Damage piles consisting of uncased or thinly cased green concrete due to the lateral forces set up in the soil, for example, necking.
• Limitation in length owing to lifting forces required to withdraw casing, nose vibration and ground displacement may a nuisance or may damage adjacent structures.
• Cannot be driven where headroom is limited.
• Relatively expensive and have limited length.
• Time consuming. Cannot be used immediately after the installation.
Bored and cast in -place (non -displacement piles):
Advantages:
• Length can be readily varied to suit varying ground conditions - No risk of ground heave.
• Soil removed in boring can be inspected and if necessary sampled or in- situ test made.
• Can be installed in very large diameters - End enlargement up to two or three diameters are possible in clays.
• Material of piles is not dependent on handling or driving conditions.
• Can be installed in very long lengths.
• Can be installed with out appreciable noise or vibrations.
• Can be installed in conditions of very low headroom.
Disadvantages:
• Susceptible to "necking" in squeezing ground.
• Concrete is not placed under ideal conditions and cannot be subsequently inspected.
• Water under artesian pressure may pipe up pile shaft washing out cement.
• Enlarged ends cannot be formed in cohesionless materials without special techniques.
• Cannot be readily extended above ground level especially in river and marine structures.
• Boring methods may loosen sandy or gravely soils requiring base grouting to achieve economical base resistance.
• Sinking piles may cause loss of ground I cohesion-less leading to settlement of adjacent structures.
Large Diameter Piles or Drilled Shafts
Disadvantages:
• Larger, heavier equipment is generally required for construction.
• The quality and performance of drilled shafts are sensitive to construction procedures, so both experienced construction personnel and careful inspection are required.
• Cylinder piles are sometimes quite difficult to drive.
Caisson
Is a cylinder or hollow box that is sunk into the ground to a specified depth by auguring a deep hole into the strata. The cylinder or box is then back filled with concrete, thus creating the foundation. This type of foundation is most often used when constructing bridge piers and other such foundations that will be beneath bodies of water since the caissons can be floated to the correct locations and then sunk in place using concrete.
This type of foundation will keep the soils underneath the building or structure from moving vertically. Since soil will settle over time, the building or structure on top of the soil will also settle. This can cause major structural damage. Since a caisson foundation is drilled into the earth and large concrete t filled cylinders are placed within the ground rather than on top, the settlement of the soil will not cause many difficulties for the building or structure.
Applications and Uses for Caissons
Caissons are commonly used during a variety of construction projects including for building and bridge construction. They can be used to hold back water and other moisture when founding pilings or peering a foundation especially when a construction project makes it necessary to drill into river bed material or below the water line during construction. Steel caisson pipe provides a durable and reliable solution for permanent and temporary foundation construction. The three most popular styles of steel caissons include rolled and welded steel pipe, spiral weld steel pipe and DSAW type manufacture.
Concrete Caissons
A 10″ or 12″ diameter holes are drilled into the earth and embedded into bedrock 3 to 4 feet. Usually used for the structural support for a type of foundation wall, porch, patio, monopost, or other structure. Two or more “sticks” of reinforcing bars (rebar) are inserted into and run the full length of the hole and then concrete is poured into the caisson hole. A caisson is designed to rest on an underlying stratum of rock or satisfactory soil and is used when unsatisfactory soil exists