Piles and Pile Foundation
Piles and Pile Foundation
Foundations provide support to the structure and transfer the loads from the structure to the soil. But the layer at which the foundation transfers the load is required to have an adequate bearing capacity and suitable settlement characteristics. There are several types of foundations depending on different considerations such as total load from the super-structure, soil conditions, water level, noise and vibrations sensitivity, available resources, time-frame of the project, and cost.
Broadly speaking, foundations can be classified as (i) shallow foundations, and (ii) deep foundations. Shallow foundations are normally used when the bearing capacity of the surface soil is adequate to carry the loads imposed by a structure. On the other hand, different types of deep foundations are normally used when the bearing capacity of the surface soil is not sufficient to carry the loads imposed by a structure. Hence, the loads are needed to be transferred to a deeper level where the soil layer has a higher bearing capacity. Pile foundation is one of the deep foundation types.
Pile foundation is defined as a series of columns inserted into the ground to transmit loads to a lower level of sub-soil. A pile is a long cylinder made up of a strong material, such as concrete. Piles are pushed into the ground to act as a steady support for structures built on top of them. Piles transfer the loads from structures to hard strata, rocks, or soil with high bearing capacity.
The pile is derived from the word ‘Pill’ which means sharp arrows or poles from the ‘Anglo-Saxon’ language and some even name it a spade. The pile plays an important role as a key component in moving the base load through low-bearing soil strata to strata or rocks with high bearing capacity.
In normal soil conditions, pile foundations are used to withstand elevations such as offshore platforms, foundations below groundwater levels, or basic transmission towers. Pile foundations are also used in soft soil to withstand horizontal loads, such as wind power and earth-quake bending forces. Soils which are easy to grow and shrink and which are sensitive to soil moisture change also need a pile foundation. In addition, construction works on water such as jetty and bridge piers, it also needs a pile foundation.
Situations are encountered in the field when the upper soil levels have a very low bearing capacity or when the column loads are very large. Footings in these cases can occupy a large percentage of the area beneath the structure and such footings are neither economical nor safe. Pile foundations are adopted in these cases.
Piles are relatively long and slender members which are used to transmit the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity. The main types of materials used for piles are wood, steel, and concrete. Piles made from these materials are driven, drilled, or jacked into the ground and connected to pile caps. Depending upon type of soil, pile material and load transmitting characteristic, piles are classified accordingly. Piles are also sometimes used to resist heavy uplift and lateral forces.
The use of pile as one of the important elements in the construction of a structure has long begun in the history of civil engineering in the world. Piles are used as supports and tools to move loads from the structure to the ground.
Piles are normally used for (i) to carry structure loads into or through a soil stratum, (ii) to resist uplift or overturning forces, (iii) to control settlements when spread footings are on marginal or highly compressible soil, (iv) to control scour problems on bridge abutments or piers, (v) in offshore construction to transmit loads through the water and into the underlying soil, and (vi) to control earth movements, such as land-slides.
Piles are used (i) when the existence of a suitable bearing layer cannot be obtained, i.e., the soil under the structure is not functioning well or in other words is unable to bear the load caused by the structure when the shallow foundation is used, (ii) when there is inadequate bearing capacity for shallow foundations, (iii) the compressibility of soil causes large sedimentation when the shallow foundation is used, (iv) when the distribution of soil beneath the surface is not uniform, (v) for the purpose of preventing the action of gravity from beneath the surface such as hydrostatic action, (vi) for getting a strong-strata in the excavation work, (vii) for the prevention of uplift forces, and (vii) for reducing excessive settlement. The function of a pile is the same as the function of a column. In which the load of the structure has the ability to transmit from the upper level to the lower level. But in the formation of the piles’ foundation, the bulbs are provided at a lower level in the ground so that there is no concern regarding buckling in the pile foundation.
Use of pile foundation is preferred (i) when the load coming from the user structure is heavy and its distribution is uneven, (ii) the structure is located on a sea-shore or river bed when the foundation is likely to be affected by the scouring action of water, hence they are useful in marine structures, (iii) for the structures in the area where canals, deep drainage lines, etc. are near the foundation, (iv) the top soil has a poor bearing capacity, (v) the construction of the raft foundation or grillage foundation is likely to very costly or is practically impossible, and (vi) the sub-soil water level is high so that the pumping of water from the open trenches for the shallow foundation is difficult and uneconomical.
There are different types, shapes, sizes and materials used for the pile foundation and the selection is dependent on the specific situation. The selection and design of the pile system is required to meet several requirements such as (i) it has an appropriate safety factor for failure of pile structure as well as of soil which supports the pile system, (ii) the quantity of sludge and the sludge difference is not to be large so that it does not affect the service conditions of the structure, (iii) pile resistance for specific soil conditions, and (iv) the safety and stability of the surrounding building structure and its service conditions is to be taken into account. However, there are three key factors in the design and selection of the appropriate pile, namely (i) underground conditions, (ii) the location and type of construction of the structure, and (iii) pile resistance and durability.
A pile foundation is a deep type of foundation. It can be made using long, thin, columnar elements. It can be made using steel, reinforced cement concrete (RCC) and timber as a material. The depth of this type of pile foundation is at least three times its width. The main components of the foundation are the pile cap and the piles. Pile foundations are formed by long, slender, columnar elements typically made from steel or reinforced concrete, or sometimes timber. Fig 1 shows elements of deep foundation.
Fig 1 Elements of a deep foundation
The purpose of a deep foundation is to transmit the structural loads to a stratum which is capable of providing both bearing capacity and acceptable settlements. The deep foundation is also to be capable of resisting vertical compressive, lateral, and uplift loads.
Pile foundation is a foundation system which transfers loads to a deeper and competent soil layer. It is 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 piles carry the load through friction generated along the shaft and the tip resistance because of the end bearing. If the tip of the piles rest on a strong stratum such as rock, dense sand, or gravel, a major portion of the load is carried by the resistance developed at the base of the pile. These piles are called ‘end-bearing piles’ or ‘point bearing piles’ (Fig 2a). Sometimes, it is not possible to drive the pile to rest on a strong stratum at a great depth a major portion of the carrying capacity will then be carried by the shaft because of the adhesion developed between the embedded surface of the pile and surrounding soil. Such piles are called ‘friction piles’ or ‘floating piles’ (Fig 2b).
Fig 2 Types of piles
Piles are of several types depending upon material, shapes, and method of installation or functional requirement. The decision to choose a proper type is based on the soil profile and the structure type. Piles are normally placed in groups, the number in a cluster varying from 2 to higher than 60. The piles in the group are joined by a reinforced concrete cap so that the pile cluster can act as a unit. The normal issue in the analysis of pile foundation is that of determining the safe carrying capacity of a group and not the capacity of a single pile.
Foundation piles are normally used for large structures and in situations where the soil at shallow depth is not suitable to resist excessive settlement, resist uplift, etc. The situations where the use of a pile foundation system is of advantage are (i) when the groundwater table is high, (ii) heavy and non-uniform loads from super-structure are imposed, (iii) other types of foundations are costlier or not feasible, (iv) when the soil at shallow depth is compressible, (v) when there is the possibility of scouring, because of its location near the river bed or seashore etc., (vi) when there is a canal or deep drainage system near the structure, (vii) when soil excavation is not possible up to the desired depth because of poor soil conditions, and (viii) when it becomes impossible to keep the foundation trenches dry by pumping or by any other measure because of heavy inflow of seepage. Whenever one of these conditions occur (where pile foundations are suitable for), the foundation engineer has to choose a foundation for the structure among different types of pile foundation.
Surface soils with poor bearing can force engineers to carry their structural loads to deeper strata, where the soil and rock strengths are capable of carrying the new loads. These structural elements are called ‘deep foundation’.
The oldest known deep foundation was a ‘pile’. Originally, piles were simply tree trunks stripped of their branches, and pounded into the soil with a large stone, much like a carpenter hammers a nail into a wooden board. Pile driving machines have been found in Egyptian excavations, consisting of a simple ‘A’ frame, a heavy stone, and a rope.
Pile foundations were in use in ancient times. In its earliest form, a pile foundation consisted of rows of timber stakes driven into the ground. Pile foundations such as these were used by the ancient Aztecs in North America. Roman military bridge builders used a similar technique. Both were early examples of ‘driven’ piles. The Romans made frequent use of pile foundations as recorded by Vitruvius in 59 CE (common era). Pile foundations for ancient Roman dwellings have been found in lake Lucerne. It is reported that during the rule of Julius Caesar, a pile-supported bridge was constructed across the river Rhine. The submerged timber piles of this bridge, which were driven in 900 CE, were found in good condition and were reused.
In 1740, Christopher Phloem invented pile driving equipment using a steam machine which resembles pile driving mechanisms of today. This method evolved by using steam to raise the weights (in lieu of human power) during the late 1800s. The most recent advance in pile placing is the hydraulic hammer. Steel piles have been used since 1800s and concrete piles since around 1900.
The durability of timber piles is shown in the report of the reconstruction of an ancient bridge in Venice in 1902. In the years immediately preceding the turn of the twentieth century, several types of concrete piles were devised. In 1897, the first concrete piles were introduced in Europe, and the Raymond Pile Company drove the first concrete piles in America in 1904. These new concrete piles were designed for 30 tons and over. The diesel hammers were developed in Germany during the second World War.
The early concrete piles were the cast-in-place type. Further development of the concrete pile led to the precast pile and, relatively recently, to the prestressed concrete pile. The need for extremely long piles with high bearing capacity led to the use of concrete-filled steel-pipe piles around 50 years to 60 years ago. More recently, steel H-piles have come into common usage. Their ease of handling, fabrication, splicing, and relatively easy penetration hastened their acceptability in foundation construction.
Driving piles for structure foundations has occurred for centuries. Originally, timber was used for piles. Presently, steel H-Piles and pipe piles are also used. These piles can be expensive but their ability to transfer greater loads has made them economical, particularly in large structures.
Pile driving is the operation of forcing a pile into the ground thereby displacing the soil mass across the whole cross section of the pile. Historically, the oldest method of driving a pile, and the method most frequently used today, is by use of an impact type hammer.
The first hammers known to be used were drop hammers which were used exclusively until the invention of the steam engine, which eventually resulted in steam hammers. Subsequent technological advances have led to the development of air, diesel, hydraulic powered impact hammers, plus vibratory and sonic hammers. Modern day needs for construction have also resulted in different adaptations of the above-mentioned pile driving techniques.
A pile foundation, a kind of deep foundation, can be defined as a slender column or long cylinder made of materials such as concrete or steel which are used to support the structure and transfer the load at desired depth either by end bearing or skin friction. A partial list of some of the definitions unique to the pile driving is given below.
Anvil – The bottom part of a hammer which receives the impact of the ram and transmits the energy to the pile.
Butt of pile – This term is normally used in conjunction with the timber piles. It is the upper or larger end of the pile, i.e., the end closest to the hammer.
Cushion blocks – These are normally plywood pads placed on top of precast concrete piles to eliminate spalling.
Cushion pad – It is a pad of resilient material or hardwood placed between the drive cap insert, or helmet, and drive cap adapter.
Drive cap adapter – It is a steel unit designed to connect specific type of pile to a specific hammer. It is normally connected to the hammer by steel cables.
Drive cap insert – It is the unit which fits over the top of pile, holding it in line and connecting it to the adapter.
Drive cap system – It is the assembled components used to connect and transfer the energy from the hammer to the pile.
Follower – It is an extension used between the pile and the hammer which transmits blows to the pile when the pile head is either below the reach of the hammer (below the guides / leads) or under water. A follower is normally a section of pipe or ‘H’ pile with connections which match both the pile hammer and the pile. Since the follower can absorb a percentage of the energy of the hammer, the contract specifications need the first pile in any location be driven without the use of a follower so as to be able to make comparisons with operations utilizing a follower. In water, the first pile to be driven is to be one sufficiently long to negate the need for the follower. The information from the first pile can be used as base information when using the follower on the rest of the piling. One is to be beware of soil strata which can change through-out the length of a footing. Underwater hammers and extensions to the leads can be used as alternatives to driving with a follower.
Hammer energy – It is the quantity of energy available to be transmitted from the hammer to the pile.
Leads – It is a wooden or steel frame with one or two parallel members for guiding the hammer and piles in the correct alignment. There are three basic types of leads namely (i) fixed, which is fixed to the pile rig at the top and bottom, (ii) swinging, which is supported at the top by a cable attached to the crane, and (iii) semi-fixed or telescopic, which is allowed to translate vertically with relation to the boom tip.
Mandrel – It is a full-length steel core set inside a thin-shell casing. It increases the structural capacity of the casing so that it can be driven. It helps in maintaining pile alignment and prevents the casing from collapsing. It is removed after driving is completed and prior to placing reinforced concrete.
Moonbeam – It is a device attached to the end of a lead brace which allows a pile to be driven with a side batter.
Penetration – It is the downward movement of the pile per blow.
Pile butt – Pile butt is a member of the pile crew other than the operator and oiler.
Pile gate – It is a hinged section attached to the pile leads, at the lower end, which acts to keep the pile within the framework of the pile leads.
Pile hammer – It is the unit which develops the energy used to drive piles, the two main parts of which are the ram and the anvil.
Pile monkey – It is a device used to position the pile in the leads beneath the hammer.
Pile rig – It is the crane used to support the leads and pile driving assembly during the driving operation.
Ram – It is the moving parts of the pile hammer, consisting of a piston and a driving head, or driving head only.
Rated speed – It is the number of blows per minute of the hammer when operating at a particular maximum efficiency.
Spudding – Spudding is the driving of a short and stout section of pile-like material into the ground to punch through or break up hard ground strata to permit pile driving. It is used extensively in the driving of timber piles.
Striker plate – It is a steel plate placed immediately below the anvil. Also known as an anvil.
Stroke – It is length of fall of the ram.
Tip of pile – It is the first part of the pile to enter the ground.
Piles are inserted into the soil by (i) driving using a pile hammer, (ii) driving using a vibratory device, (iii) jacking the pile downward by reacting against a rigid structure, (iv) drilling a hole (pre-drilling) and inserting a pile into it, and (v) screwed into the ground and injected with a column of grout (continuous flight auger or auger cast shafts).
The continuous flight auger (CFA) piles are also referred to as auger-cast, auger-cast-in place, and auger-pressure grout piles. CFA piles are constructed by using continuous flight augers and by drilling to the final depth in one continuous process. When the drilling to the final depth is complete, the auger is gradually withdrawn as concrete or sand / cement grout is pumped into the hole through the hollow centre of the auger pipe to the base of the auger. Reinforcement, if needed, can be placed in CFA piles immediately after the withdrawal of the auger. The reinforcement is normally confined to the top 10 m (metre).
The CFA piles are also referred to as auger-cast, auger-cast-in place, and auger-pressure grout piles. CFA piles are constructed by using continuous flight augers and by drilling to the final depth in one continuous process. When the drilling to the final depth is complete, the auger is gradually withdrawn as concrete or sand / cement grout is pumped into the hole through the hollow centre of the auger pipe to the base of the auger. Reinforcement, if needed, can be placed in CFA piles immediately after the withdrawal of the auger. The reinforcement is normally confined to the top 10 m to 15 m of the pile. In general, CFA piles are normally 0.3 m to 0.9 m in diameter with a length up to around 30 m. However, piles with larger diameters (up to around 1.5 m) have been used. Typical centre-to-centre pile spacing is kept at 3 pile-diameter to 5 pile-diameter.
Advantages of CFA piles are (i) noise and vibration during construction are minimized, (ii) eliminates splicing and cut-off. Disadvantage is soil spoils need collection and disposal.
Classification of piles – Piles can be classified as per (i) the material used, (ii) the mode of transfer of load, (iii) the method of construction, (iv) the use, and (v) displacement of soil. Each type of the piles has a place in the field of construction, and for some projects more than one type can be satisfactory. Factors which affect the selection of the suitable type of piles for a given project include (i) type, size and weight of the structure to be supported, (ii) physical properties of the soil at site, (iii) depth to a stratum capable of supporting the piles, (iv) availability of materials for piles, (v) number of piles needed, (vi) facilities for driving piles, (vii) types of structures adjacent to the project, and (viii) depth and kind of water, if any, above the ground into which the piles are to be driven.
Classification according to material used – There are four types of piles as per the materials used. These are (i) steel piles, (ii) concrete piles, (iii) timber piles, and (iv) composite piles.
Steel piles are normally either in the form of thick pipes or rolled steel H-section. Pipe steel piles are driven into the ground with their ends open or closed. Piles are provided with a driving point or shoe at the lower end. Epoxy coatings are applied during the pipe manufacture for reducing the pipe corrosion. Sometimes concrete encasement at the construction site is done as a protection against corrosion. An additional thickness of the steel section is normally desired for taking into account the corrosion.
Steel pipe piles are installed by driving pipes to the desired depth and filling them with concrete. A pipe can be driven with the lower end closed with a plate or a steel driving point, or the pipe can be driven with the lower end open. Pipes vary in diameter from 150 mm (millimetre) to 750 mm and the length can reach 60 m. A closed-end pipe pile is driven in any conventional manner, normally with a pile hammer. If it is necessary to increase the length of a pile, two or more sections can be welded together.
An open-end pipe pile is installed by driving the pipe to the needed depth, removing the material from inside, by burst of compressed air, a mixture of water and compressed air, and filling the space with concrete. Since the open-end pipe piles offer lesser driving resistances than closed-end piles, a smaller pile hammer can be used. The use of light hammers is desirable when piles are driven near a structure whose foundation can be damaged by impact of the blows from a large hammer. Open-end pipe piles can be driven to depths which can never be reached with the closed-end pipe piles.
In construction of foundations which need piles driven to great depths, steel H-piles probably are more suitable than any other type. Steel H-piles can be driven through hard materials to a specified depth for eliminating the danger of failure because of scouring, such as under a pier in a river.
Steel piles can be driven to great depths through poor soils to bear on a solid rock stratum. The high strength of steel combined with the small displacement of soil permits a large portion of the energy from a pile hammer to be transmitted to the bottom of a pile. As a result, it is possible to drive steel piles into soils which cannot be penetrated by any other type of pile.
Concrete piles are produced with reinforced cement concrete (RCC). These piles are either pre-cast or cast in- situ.
Pre-cast concrete piles are produced either in a manufacturing plant or in a casting yard. Reinforcement is provided to resist handling and driving stresses. Pre-cast piles can also be pre-stressed using high strength steel pre-tensioned cables. Square and octagonal piles are cast in horizontal forms, while round piles are cast in vertical forms. After the piles are cast, they are to be cured under damp sand, straw, or mats for the period needed by the specifications, frequently 21 days, if cured under ambient temperatures.
With the exception of short lengths, pre-cast concrete piles are to be reinforced with sufficient steel to prevent damage or breakage while they are being handled from the manufacturing plant / casting beds to the driving positions. The piles are required to contain longitudinal reinforcing steel in a quantity not less than 2 % of the volume of the pile. Lateral steel is to be at least 6 mm diameter round bars, spaced not more than 300 mm apart, except at the top and bottom of a pile, where the spacing is not to exceed 75 mm. The concrete cover over the reinforcing steel is to be at least 50 mm.
Pre-cast piles are to be cast as near the site as far as possible in order to reduce the cost of handling them from the manufacturing plant / casting beds to the pile driver. They are to be transported to the driver. For handling these piles, care is to be exercised for preventing breakage or damage because of the flexure stresses. Long piles are to be picked up at several points for reducing the unsupported lengths. One of the disadvantages of using pre-cast concrete piles, especially for a project where different lengths are needed, is the difficulty of reducing or increasing the lengths of the piles.
Advantages of pre-cast concrete piles are (i) they have high resistance to chemical and biological attacks, (ii) they have high strength, and (iii) a pipe can be installed along the centre of a pile to facilitate jetting. Disadvantages include (i) it is difficult to reduce or increase the length, (ii) large sizes need heavy and expensive handling and driving equipment, (iii) inability to get the piles by purchase department in time can delay the starting of a project, and (iv) possible breakage of piles during handling or driving produces a delay hazard.
A cast in-situ pile is constructed by making a hole in the ground and then filling it with concrete and curing it there. A cast in-situ pile is either cased or uncased. A cased pile is made by driving a steel casing into the ground and filling it with concrete. An uncased pile is made by driving to the desired depth and gradually withdrawing casing when fresh concrete is filled. An un-casted pile can have a pedestal.
There are two methods of constructing cast in-situ concrete piles. These are (i) driving a metallic shell, leaving it in the ground, and filling it with concrete, and (ii) driving a metallic shell and filling it with concrete as the shell is pulled from the ground.
The standard pile is installed by driving a closed end smooth spirally corrugated steel shell connected to a steel piece (which can be removed) at the bottom of the shell, used to drive the shell into the soil. After driving the shell to the desired penetration, the steel piece is pulled outside, and then the shell is to be inspected and filled with concrete. If the shell is damaged during driving, it is preferable to be pulled outside and replaced with a new one. The length of this kind of piles can reach 11 m.
The step-taper pile is installed by driving a spirally corrugated steel shell, made up of sections between 1.2 m to 2.4 m, with successive increases in diameter for each section. A corrugated sleeve at the bottom of each section is screwed into the top of the section immediately below it. Piles of necessary length up to a maximum of 24 m are obtained by joining the proper number of sections at the job site. The shells are available in various gauges of metal to fit different job site conditions. The bottom of shell is closed prior to driving by a flat steel plate or a hemi-spherical steel boot. After the shell is assembled in the desirable length, a step-tapered rigid steel core is inserted and the shell is driven to the desired penetration. The core is removed, and the shell is filled with concrete.
The advantages of cast in-situ concrete piles are (i) the light weight shells can be handled and driven easily, (ii) the length of a shell can be increased or decreased easily, (iii) the shells can be shipped in short lengths and assembled at the job, (iv) the danger of breaking a pile while driving is eliminated, and (v) additional piles can be provided quickly if they are needed. Disadvantages are (i) a slight movement of the earth around an unreinforced pile can break it, (ii) an uplifting force, acting on the shaft of an uncased and unreinforced pile can cause it to fail in tension, and (iii) the bottom of a pedestal pile cannot be symmetrical.
Timber piles are made from tree trunks after proper trimming. The timber used is to be straight, sound, and free from defects. Steel shoes are provided to prevent damage during driving. For avoiding damage to the top of the pile, a metal bond or a cap is provided. Splicing of timber piles is done using pipe sleeve or metal straps and bolts. The length of the pipe sleeve is to be at least five times the diameter of the pile. Timber piles below the water table have normally long life. However, above the water table, these are attacked by insects. The life of the timber piles can be increased by applying preservatives such as creosote oil.
dvantages of timber piles are namely (i) the popular lengths and sizes are available on short notice, (ii) are economical in cost, (iii) are handled easily, with little danger of breakage, (iv) can be cut to any desirable length after they are driven, and (v) can be pulled easily in the case where removal is necessary. Disadvantages of timber piles are (i) can be difficult to obtain piles sufficiently long and straight needed for some projects, (ii) can be difficult or impossible to be driven into hard formations, (iii) difficult to splice them to increase their lengths, (iv) while they are satisfactory when used as friction piles, they are not suitable for use as end-bearing piles under heavy loads, and (v) life can be short unless the piles are treated with a preservative.
A composite pile is made of two materials. A composite pile can consist of the lower portion of steel and the upper portion of cast in-situ concrete. It can also have the lower portion of timber below the permanent water table and the upper portion of the concrete. Since it is difficult to provide a proper joint between two dissimilar materials, composite piles are rarely used in practice.
Classification based on mode of transfer of load – Based on the mode of transfer of loads, the pile can be classified into three categories namely (i) end bearing piles, (ii) friction piles, (iii) combined end bearing and friction piles.
End bearing piles (Fig 2a) transmit the loads through their bottom tips. Such piles act as columns and transmit the load through a weak material to a firm stratum below. If bed rock is located within a responsible depth, piles can be extended to the rock. The ultimate capacity of the pile depends upon the bearing capacity of the rock. If instead of bed rock, a fairly compact and hard stratum of soil exists at a reasonable depth, piles can be extended a few meters. These piles are also known as ‘point-bearing pile’. The ultimate load carried by the pile is equal to the load carried by the point or bottom end.
Friction piles (Fig 2b) do not reach the hard stratum. These piles transfer the loads through skin friction between the embedded surface of the pile and the surrounding soil. Friction piles are used when a hard stratum does not exist at a reasonable depth. The ultimate load carried by the pile is equal to the sum of the load carried by the pile is equal to the load transferred by skin friction. Friction piles are also known as floating piles since these do not reach the hard stratum.
Combined end bearing and friction piles transfer load by a combination of end bearing at the bottom of the pile and friction along the surface of the pile shaft, the ultimate load carried by the pile is equal to the sum of the load carried by the pile point and the load carried by the skin friction.
Classification based on method of installation – Based on the method of installation, the piles can be classified into 5 categories namely (i) driven pile, (ii) driven and cast in-situ piles, (iii) bored and cast in-situ piles, (iv) screw piles, and (v) jacked piles.
Driven piles are driven into the soil by applying blows of a heavy hammer on their tops. Driven and cast in-situ piles are formed by drawing a casing with a closed bottom end into the soil. The casing is later filled with concrete. The casing can be or cannot be withdrawn. Bored and cast in-situ piles are formed by a hole into the ground and then filling it with concrete. Screw piles are screwed into the soil. Jacked piles are jacked into the soil by applying a downward force with the help of a hydraulic jack.
Classification based on use – The piles can be classified into 6 categories depending upon their use. These are (i) load bearing piles, (ii) compaction piles, (iii) tension piles, (iv) sheet piles, (v) fender piles, and (vi) anchor piles.
Load bearing piles are used to transfer the load of the structure to a suitable stratum by end bearing, by friction, or by both. Compaction piles are driven into the loose granular soil for increasing the relative density. The bearing capacity of the soil is increased because of the densification caused by vibrations. Sheet piles forms a continuous wall or bulk head which are used for retaining earth or water. They are mainly used for resisting the flow of water and loose soil. Typical uses include cut-off walls under dams, coffer-dams, bulk-heads, and trench sheeting etc. Fender piles are sheet piles which are used to protect water front structures from impact of ships and vessels. Anchor piles are used for protecting anchorage for anchored sheet piles. These piles provide resistant against horizontal pull for a sheet pile wall.
Classification based on displacement of soil – Based on the volume of the soil displacement during installation, the piles can be classified into 2 categories namely (i) displacement piles, and (ii) non- displacement piles.
Displacement piles are driven piles. In the displacement piles, soil is displaced laterally when the pile is installed, hence, the soil gets densified. The installation can cause heaving of the surrounding ground. Pre-cast concrete pile and closed end pipe pile are high displacement piles. Steel H-piles are low displacement piles. In case of non-displacement piles, the soil is removed when the hole is bored, there is no displacement of the soil during installation. The installation of these piles causes very little change in the stresses in the surrounding soil. Bored piles are non-displacement piles.
Selection of foundation – The selection of pile foundation depends on the soil investigation data received from soil exploration bore holes at different depths. Selection of appropriate pile for the desired strength and requirement plays an important role in cost reduction and efficiency.
Factors which affect the selection of pile foundation include (i) soil conditions, (ii) loads from structures, (iii) nature of loads, (iv) number of piles to be used, (v) cost of construction, (vi) type, size, and weight of the structure to be supported, (vii) physical properties of the soil at the site, (viii) depth to a stratum capable of supporting the piles, (ix) possibility of variations in the depth to a supporting stratum, (x) availability of materials for piles, (xi) number of piles needed, (xii) weight and durability of piles, (xiii) facilities for driving piles, (xiv) types of structures adjacent to the project, and (xv) depth and kind of water, if any, above the ground into which the piles are to be driven.
Pile hammers – The function of a pile hammer is to furnish the energy needed to drive a pile. Pile-driving hammers are designated by type and size. The types normally used include (i) drop hammer, (ii) single-acting steam hammer, (iii) double-acting steam hammer, (iv) differential-acting steam hammer, (v) diesel hammer, (vi) vibratory hammer, and (vii) hydraulic hammer. The size of a drop hammer is designated by its weight, while the size of each of the other hammers is designated by theoretical energy per blow, expressed in meter.kilogram (m.kg).
A drop hammer is a heavy metal weight which is lifted by a rope, then released and allowed to fall on top of the pile. Standard drop hammers are made in sizes which vary from around 225 kg to 1,360 kg. The height of drop or fall most frequently used varies from 1.5 m to 6 m. When large energy per blow is needed to drive a pile, it is better to use a heavy hammer with a small drop than a light hammer with a large drop. Drop hammers can strike 4 blows per minute to 8 blows per minute. Drop hammers are suitable for driving piles on projects which need only a few piles and for which the time of completion is not an important factor.
Advantages of drop hammers are (i) small investment in equipment, (ii) simplicity of operation, and (iii) ability to vary the energy per blow by varying the height of fall. Disadvantages are (i) slow rate of driving piles, (ii) danger of damaging piles by lifting a hammer too high, (iii) danger of damaging adjacent buildings as a result of the heavy vibration caused by a hammer, and (iv) cannot be used directly for underwater driving.
A single-acting steam hammer is a freely falling weight, called a ram, which is lifted by steam or compressed air, whose pressure is applied to the under-side of a piston which is connected to the ram through a piston rod. When the piston reaches the top of the stroke, the steam pressure is released and the ram falls freely to strike the top of a pile. Single-acting steam hammers can strike 50 or more blows per minute. These hammers can be open or enclosed. The length of the stroke and energy per blow can be decreased slightly by reducing the steam pressure below that recommended by the manufacturer. The reduced pressure has the effect of decreasing the height to which the piston rises before it begins its free fall.
Advantages of single-acting steam hammers compared with drop hammers are (i) higher number of blows per minute permits faster driving, (ii) reduction in the velocity of the ram decreases the danger of damage to piles during driving, and (iii) the enclosed types can be used for under-water driving. Disadvantages are (i) they need more investment in equipment such as steam boiler or an air compressor, (ii) they are more complicated, with higher maintenance cost, (iii) they need more time to set up and take down, and (iv) they need a large operating crew.
In the double-acting steam hammer, steam pressure is applied to the underside of the piston to raise the ram, then during the down-ward stroke steam is applied to the top side of the piston for increasing the energy per blow. Hence, with a given weight ram, it is possible to achieve a desired quantity of energy per blow with a shorter stroke than with a single-acting steam hammer. The number of blows per minute are around twice as high as for a single-acting steam hammer with the same energy rating.
Advantages of double-acting steam hammers compared with single-acting steam hammers are (i) higher number of blows per minute reduces the time needed to drive piles, and (ii) piles can be driven more easily. Disadvantages are (i) light weight and high velocity of the ram make this type of hammer less suitable for use in driving heavy piles into soils having high frictional resistance, and (ii) this type of hammer is more complicated.
A differential-acting steam hammer is a modified double-acting hammer in which the steam pressure is used to lift the ram and to accelerate the ram on the down-stroke. The ram has a large piston which operates in an upper cylinder and a small piston which operates in a lower cylinder. The lifting of the ram is caused by the difference in the pressure forces acting on the two pistons. The number of blows per minute is comparable with that of a double-acting hammer, while the weight and equivalent free fall of the ram are comparable with those of a single-acting hammer. Hence, this type of hammer has the advantages of both the single-acting hammer as well as the double-acting hammer.
A diesel pile driving hammer is a self-contained driving unit which does not need an external source of energy like a steam boiler or an air compressor. Hence, it is simpler and easily moved from one location to another when compared with a steam hammer. A complete unit consists of a vertical cylinder, a piston or ram, an anvil, fuel tank, lubricating oil tank, a fuel pump, injectors, and a mechanical lubricator. After the hammer is placed on top of a pile, the combined piston and ram are lifted to the upper end of the stroke and released to start the unit operating. As the ram nears the end of the down-stroke, it activates a fuel pump which injects the fuel into a chamber between the ram and the anvil. The continued down-stroke of the ram compresses the air and the fuel to ignition heat. The resulting explosion drives the pile downward and the ram upward to repeat its stroke.
Advantages of diesel hammers are (i) the hammer needs no external source of energy, and the maintenance is simple and fast, (ii) the hammer is light in weight and is economical to operate, (iii) it is convenient to operate in remote areas, (iv) it operates well in cold weather, where it is difficult to provide steam, and (v) since the resistance of a pile for driving is necessary for continuing operation of a diesel hammer, it does not operate if a pile breaks or falls out from under a hammer. Disadvantages are (i) the hammer cannot operate well when driving piles into soft ground, (ii) the number of strokes per minute is less than that of a steam hammer, and (iii) the length of a diesel hammer is slightly higher than the length of a steam hammer of comparable energy rating.
If it is necessary to drive piles below water, there are two methods which can be used. In the first method, when the driving unit is a drop hammer, an open-type steam hammer, or a diesel hammer, the pile is driven until the top is just above the surface of the water. Then a follower is placed on top of the pile, and the driving is continued through the follower. The follower can be made of wood or steel and is to be strong enough to transmit the energy from the hammer to the pile. In the second method, when the driving unit is an enclosed steam hammer, the driving can be continued below the surface of the water, without a follower. It is necessary to install an exhaust hose to the surface of the water for the steam. Also, it is necessary to supply around 1.7 cubic metre per minute of compressed air to the lower part of the hammer for preventing water from flowing into the casing and around the ram.
Loads from structures are transferred to the ground by foundations. Foundations are normally divided into shallow foundations and deep foundations. Spread footings and slabs are examples of shallow foundations which are suitable for strong ground conditions. When ground conditions are weak, deep foundations are used, where piling is the most common one as shown in Fig 3c.
Fig 3 Different types of foundations
Soils are divided into cohesive soil, typically clay, and friction soil, for example sand. Cohesive soil and friction soil behave differently and are hence treated in different ways, meaning that they have different properties describing their strength and stiffness. The parameters describing the strengths are the undrained shear strength ‘s’ for cohesive soil and friction angle ‘a’ for friction soil.
The soil parameter used for deformation calculations is the modulus of sub-grade reaction, in short sub-grade modulus, ‘m’. The sub-grade modulus is a spring stiffness which relates stress to the deformation in the soil and has the unit newton per cubic metre (N/cum). The spring stiffness can be non-linear. As a simplification, for cohesive soil, it is normally assumed being constant along the depth of the pile. The sub-grade modulus for friction soil increases with depth, and the parameter used for stiffness is hence normally a linear sub-grade modulus ‘m’ with the unit N/cum.
Piles can be of different materials such as reinforced cement concrete, steel, or timber. Reinforced cement concrete piles are the most common type of piles used. A pile cap can be cast to a number of piles and form a pile group. The actions in a single pile are then dependent on the other piles, analogous to columns in a frame. Pile groups are normally modelled by the use of frame analysis, also called the direct stiffness method.
Piles can be categorized according to how they transmit load, which depends on the type of soil or rock by which they are surrounded with. The conceptual illustrations of an end bearing pile is shown in Fig 4a and that of a shaft bearing pile in Fig 4b. With shaft bearing piles, the largest part of the load is transferred to the surrounding soil at the contact surface between the pile and the soil. The shaft bearing pile is also called friction pile, where the word friction describes the friction along the shaft and has nothing to do with friction soil. Both piles in cohesive soil and friction soil can function as shaft bearing piles, but piles in cohesive soil have a more pronounced shaft bearing functionality.
Fig 4 Types of piles and distances between pile groups
Shaft bearing piles in friction soil and cohesive soil are treated differently in design. Unlike shaft bearing piles, the end bearing piles transmit the load mainly through the pile end. This describes a pile resting on bed rock. In practice, piles function by a combination of the two categorizations. However, when calculating the distribution of forces in a pile group using the frame analysis method, this categorization is not relevant and all piles are assumed to be end bearing, meaning that all resistance is gained at the pile end.
In design, the resistance of single piles, and the resistance of the pile group as a whole, is to be verified, as well as the resistance of the pile cap. Pile groups can fail in two conceptual ways, either by failure of a single pile or by failure of a block of piles. The governing failure mode depends on the distance between the piles. For a relatively small spacing, the piles and the soil enclosed by the piles act like a rigid body, meaning that the block failure is the governing failure mode, whereas the opposite is true for larger spacing. This is shown in Fig 4c. Since there are requirements on minimum distances between piles, the block failure is normally not governing. The resistance of single piles is also to be based on multiple failure criteria, i.e., failure of the pile itself and failure of the surrounding soil. A pile group is to be designed so that it can withstand combinations of vertical and horizontal loads as well as bending moments applied to the pile cap.
The design process for pile groups consists of several steps as summarized in Fig 5. The first step is to determine the input data such as loading, geotechnical conditions, and geometrical restrictions. Then, the design process includes choosing the number and type of piles, followed by an iterative process to determine an efficient arrangement of the piles. The iterative process considers and calculates the pile capacity in relation to the section forces obtained for a number of different load cases. Once the piles are installed and their final position is surveyed, the designer is to confirm that the capacity of the pile group is sufficient by repeating the calculations using the actual positions. If the calculations are not confirmed, an additional pile, followed by a recalculation, is needed.
Fig 5 Overview of the design process for pile groups
The design of the pile group is highly influenced by the type of the applied load, i.e., different combinations of vertical and horizontal forces as well as bending and torsion. As there are several numbers of load combinations, it is normally not obvious which arrangement is the most suitable.
A structure can be optimal in different aspects, formally referred to as objectives. Objectives can, for example, be to minimize displacements, section forces, or the cost of the pile group. The purpose of structural optimization is to find the structure which preforms the task in the best way with respect to the objective. For assessment of the design, an objective function is used. The function evaluates every possible design within constraints of the function. The constraints can be divided into design constraints, behavioural constraints, and equilibrium constraints.
Design constraints can, for example, be geometrical limitations of the pile group. The behavioural constraints, on the other hand, represent constraints on the response of the pile group under a certain load condition, such as limitation of displacements and section forces. The equilibrium constraints demand that the pile group is stable.
There are several types of optimization algorithms which can be used for structural optimization. One way of optimizing a structure considering several possible combinations of design variables is to use Monte Carlo simulation.
Monte Carlo simulation – The number of possible combinations of the variables used in an optimization can be huge depending on how they are constrained. Instead of testing all possible combinations, a limited number of combinations from a suitable distribution, gained by a random number generator, can be evaluated. This is called a Monte Carlo simulation. A Monte Carlo simulation runs a model repetitively. Each time random realizations of the input variables are generated, resulting in virtual outcomes for the output variables. The suitability of each virtual outcome can then be evaluated in relation to the objective function. A Monte Carlo simulation can be carried out for input variables which have different types of probability distributions. If all values for a variable have equal probability, the distribution is uniform.
The simulation is particularly useful for predicting outcomes of complex systems, but can also be used in optimization. The Monte Carlo simulation relies on the concept of random number generator. This means that the different trials are independent. Disadvantages of a random generator are that all possible combinations are not tested and some are tested more than once, and that the outcome of an analysis varies from one attempt to another.
Flow of forces in pile groups – The design of the pile group is to reflect the loads. All piles can be loaded with vertical forces. Horizontal force, on the other hand, can be taken by the inclined piles. The more the piles are inclined, the higher is the capacity, simply derived from geometry. Horizontal forces can also be taken as moment in both inclined and vertical piles by using the lateral resistance from the surrounding soil. In order to handle bending moment, there is to be lateral resistance of the soil or pairs of pile forces in the pile group. These pairs can be formed by piles whose pile force extensions does not intersect at one point. Since piles are long, their bending resistance is low. This is improved by lateral soil resistance.
Suitable design of piles because of different loading is shown in Fig 6a, assuming no lateral resistance from the soil. The first pile group can only resist forces applied at the pile centre and the second one is not stable for horizontal force. The third arrangement can handle all types of loads. If lateral resistance of the soil is assumed, all pile groups are be able to handle horizontal force and bending moment.
Fig 6 Static actions of piles and external forces acting on the pile
Load centre and pile centre – When considering several load cases, a method which makes use of the load centre and the pile centre is frequently used. The magnitude of the generated forces in the piles is said to depend on the relationship between the load centre and the pile centre. An optimal pile centre is defined as the state when external forces acting on the pile centre cause only translations and no rotations, as shown in Fig 6b. Hence, the pile centre is normally to be located as close to the load centre as possible for avoiding pile forces because of the moment acting at the pile centre.
The location of the pile centre depends on the stiffness of the pile group, i.e., geometric and material properties. If all the piles have equal stiffness and the surrounding soil is not considered, the pile centre is a theoretical intersection of the extensions of the neutral axes of the piles, as in the left illustration in Fig 6a. When the stiffness of the soil is also taken into account, the pile centre becomes a bit more complicated to determine. It is not always be optimal to locate the pile centre at the exact same position as the load centre. This is because it can be advantageous to use the effect of bending moment to reduce pile tension.
In order to reduce the pile forces because of the applied moment, the horizontal locations of the piles are to be placed as far from the pile centre as possible, and hence increasing the moment of inertia of the pile group. However, if the piles are placed far from the applied loads, other problems can appear, such as assuring sufficient stiffness and capacity of the pile cap. A compromise between both phenomena is normally preferable.
Symmetrical pile group – Forces acting on the piles arise from structural weight and different types of varying loads, such as traffic, wind, and thermal effects (taking an example of a bridge). One can argue that a bridge is loaded fairly symmetrically when there are traffic loads, braking loads and thermal effects in both directions and wind from both sides. Hence, it is reasonable to design symmetrical pile groups. The symmetry can be around one axis or two, depending on the loading stuation. Examples of unsymmetrical loading are centrifugal forces acting on a curved bridge and earth pressure at an abutment.
Regulative requirements on pile groups – When designing a pile group, several regulations as per the standards and / or codes are to be considered. In addition to the regulations, there are normally requirements for an economical and sustainable structure. The contractor who constructs the pile group can also have requirements on the arrangement of piles, because of the practical reasons at the construction site.
Geometric requirements – The most important geometric requirements are minimum distance between the piles, minimum distance between the pile and the edge of the pile cap, and maximum pile inclination. There are two main reasons for having a minimum distance between piles namely group effect, and risk for collision when installing the piles. Group effect is a concept where the piles stand so close to each other that the stress field around a pile affects the pile next to it. Risk for collision is present when piles stand close to each other and their positions deviate during installation. By ensuring sufficient distance between the piles, this is avoided. The requirements on minimum distances have a considerable impact on the size of the pile cap.
The standards and / or codes normally gives the minimum distances. For piles leaning from each other, the minimum distance at the pile cap is 0.8 meter. Parallel piles are more likely to collide hence have stricter requirements which depends on the diameter and the length of the pile.
Ultimate limit state requirements – The capacity of the pile group is restricted to the minimum of the structural resistance of the individual piles and the geotechnical resistance of the soil. The failure mode which governs depends on the length of the piles, whereas the geotechnical capacity governs short piles and structural capacity governs long piles. Practically, majority of the piles can be seen as long piles, which undermine the relevance of the geotechnical capacity. However, for tension forces the geotechnical capacity can be decisive and is to be verified. The structural resistance of the piles includes, for example, moment and normal force capacities and the combination of these. As Fig 7 shows, the conceptual behaviour for combined normal force and bending moment is quite different depending on whether the pile is made of steel, timber, or reinforced cement concrete.
Fig 7 Conceptual cross section capacity for piles
For steel and timber piles, a simplified elastic interaction formula expressed as N(Ed)/N(b,Rd) + M(Ed)/M(c,Rd) = 1 maximum can be used in preliminary design. Here ‘N(Ed)’ is the normal design force in piles, ‘N(b,Rd)’ is the compression buckling capacity of a pile, ‘M(Ed)’ is the design bending moment in piles, and ‘M(c,Rd)’ is the elastic moment capacity of pile.
The design bending moment is the resultant from the moments Mx and My acting around the x- axis and y-axis. Only the size of the resultant, and not the direction, is considered using MEd = under root [(Mx)square + (My)square].
The design moment capacity can be in another direction than the imposed bending moment, a simplification on the safe side. The moment capacity for a quadratic pile is normally lowest for bending around the diagonal. Other effects such as local buckling is not considered in this formula.
For reinforced cement concrete, the interaction of normal force and moment is not linear. For a given moment, an increase in normal force can be favourable up to a certain point, and thereafter unfavourable.
Irrespective of material, buckling of piles is to be considered. For steel and timber, this is done by calculating a buckling reduction factor, and for reinforced cement concrete by determining second order effects. In both approaches, the stiffness of the soil is to be considered. The tensile capacity can be checked using the expression N(Ed)/N(t,Rd) = 1 maximum.
There are requirements, other than ultimate limit state, which can have a decisive impact on the pile group. Serviceability limit state, fatigue, and accidental events are examples of such requirements. If there are on-going settlements in the area, negative skin friction needs special treatment.
Practical requirements – For determining an optimal arrangement of the piles, the designer is not only to concentrate on the technical aspects but also be aware of the practical part of installing a pile-group. By considering the practical aspects at an early stage, the work can be facilitated at the site and safety for the manpower can be assured and both time and money can be saved.
The overall intention is to utilize all piles as much as possible, but there are also reasons not to. If the pile group is designed with high extent of utilization, deviations occurring at installation can have a large impact on time and cost. If the piles are either deviating in position, direction, or inclination, or are completely eliminated, the pile group cannot work and a new pile is to be installed. This is both time consuming and expensive. It is much more expensive to supplement the pile group by adding a pile afterwards, than to design a robust pile group from the start.
Depending on the type of pile and dimension of the pile, different equipment and machines are needed. If the pile group consists of several different pile sizes, more equipment is needed, which increases the cost. It is also time consuming to change the equipment. It seems advantageous to install one pile with large dimensions instead of several small piles, but one is not to forget that they need different machines. The size, weight, and cost of the piling machines increase with pile size. Hence, several aspects need to be considered when choosing quantity and dimensions of the piles.
For avoiding having to constantly move the machine back and forth, the piles not yet installed are frequently stored on the ground between the already installed piles. Hence, arranging piles in straight rows facilitates the handling of the piles and reduces the risk for hitting the already installed piles when picking up a pile with the crane.
The more the pile is inclined, the more efficiently it handles horizontal forces. However, since there are limitations of the pile crane and safety requirements for the working environment, when designing a pile group the piles are not to have a larger inclination than 4:1. It is easier to install a vertical pile than an inclined one, as gravity improves the precision of a vertical pile but impairs the precision of an inclined pile. The risk of the machine tipping is a severe safety issue which increases when inclining piles more than 4:1.
In short, the important aspects are (i) construction of robust pile groups, (ii) use of same pile dimensions wherever possible, (iii) arrangement of piles in ‘grids’ with straight rows and columns, and (iv) limitation of inclination for safety reasons.