CE1202 Geotechnical Properties & Construction Methods

EG5111 Advanced Technology, Planning & ProductionAdvanced Foundations1 Piling - IntroductionWhat is piling?Piles can be made from steel or timber although in most housing work piles are made from insitu or pre-cast reinforced concrete. They are used either to transmit loads from the building through soft or compressible ground to firmer strata below (end bearing pile), or to distribute loads into the subsoil along the length of the pile (friction pile).In housing, a concrete beam across the top of the piles distributes the load from the load bearing brickwork into the piles themselves. In framed buildings the piles usually support concrete or steel columns.

Why so popular?20 or 30 years ago piling was comparatively rare for housing (other than medium and high rise flats). Since then, several factors have led to an increase in the use of piled foundations. These include: the increased pressure to re-develop 'brownfield' sites, where strip foundations may not always be appropriate increased costs of 'carting away' and tipping surplus excavation from foundation trenches (particularly in cities) the development and easy availability of smaller piling rigs and piling systems which are, nowadays, cost effective for house foundations greater understanding of piling in general (partly through better building education).

Factors affecting choiceThere are literally dozens of piling companies in the UK each offering a number of different piling systems. In many cases more than one piling system will suit a particular set of circumstances. However, when choosing a piling system there are four main criteria to consider: building load the nature of the ground (ie, the subsoil) local environmental or physical constraints (noise restrictions, height restrictions) cost

What are piers?Pier foundations, sometimes called pad and stem, are not dissimilar to end-bearing piles in their function. However, their construction is very different. Piling is carried out from the surface, by drilling or driving down into the ground. When building piers, individual pits are usually excavated and then backfilled once the piers have been constructed.

2 Piling - The ChoicePile typesThere are basically three types of pile: driven piles which are pre-formed - usually steel or pre-cast concrete (displacement) driven piles which are cast insitu (displacement) bored or augered piles which are cast in situ (replacement) In recent years a new piling system has been introduced by Roger Bullivant. It's called a bored displacement pile and is described later in this section. NB: a displacement pile forces the ground out of the way as the pile is driven. A replacement pile removes it first.

Piles can be supported through their end-bearing or through friction (or a combination of the two) End bearing piles are generally used where rock or dense granular material underlies a softer stratum. Friction piles, where most of the support comes from friction between the pile sides and the soil, are more likely to be used in clay.

Reasons for choosing pilingThe decision to choose piled foundations rather than strip or spread foundations may not always be straightforward. However, if firm ground is some distance below the surface (2 metres or more), piling may be more economical. Other situations which might make piling a preferred choice include: a high water table expensive cart-away costs (very high in some of the larger cities where tips are not close) where soils such as clay are likely to swell or shrink with changes in moisture content where trenches are not very stable and are likely to collapse

Choice of piling system In many cases more than one piling system is appropriate for any given set of ground conditions. Different piling contractors have their preferred systems, often developed in-house over a number of years. Advice, therefore, from two contractors may differ, yet both both may provide suitable solutions. The list below identifies a number of factors to be considered when choosing a piling system. We have tried to keep this section a simple as possible; this web site is mainly concerned with providing an illustrated introduction to house construction - not ground engineering. Click here for a clip showing pre-cast driven piles.

Some factors affecting choice... If the length of the piles is known and the site is readily accessible with no noise or vibration restrictions, driven pre-cast piles may be the most economical choice. Where noise and vibration is a problem augered piles may be the best solution. If piles are close to existing buildings the risk of displacement piles causing soil heave must be taken into account. Shell piles, (driven in sections) can be used where lengths are uncertain and where 'waisting or necking' might occur. On restricted sites there may be problems handling long pre-cast piles and large piling rigs. Ground obstructions can be a problem but steel piles can sometimes be driven through them. Bored or augered piles are normally used for housing in clay soils subject to shrinkage or swelling. In loose granular soils the act of driving can actually help to compact the soil. In clays, driven piles tend to 'whip' as they are driven - augered or bored piles are often preferable.

3 Pre-cast Piles (with insitu ground beam)On this site, formerly a power station and docks, the nature of the ground varied. In some parts of the site, mostly away from the water's edge, strip foundations were acceptable. In others, in particular along the quayside, piles had to be used. Here, there was a sloping stratum of rock (mudstone and sandstone) some 6 to 18 metres below the ground's surface. The material above the rock included soft clay, silt, fill, and a number of man made obstructions from previous use of the site.

Pre-cast reinforced concrete piles were chosen for this site; it was the engineer's view that these offered the fastest and cheapest solution. The piles were driven into the ground by a crane mounted drop hammer - vibration etc was not an issue as the site was fairly isolated. A wooden insert under the driving head helped cushion the piles and prevent cracking in the pile itself. Where obstructions in the ground prevented the pre-cast piles from being driven into the ground or forced them out of position, steel piles were driven alongside.

In one or two places the piles had to be jointed (ie, to make them longer). Have a close look at the photo on the left - the metal plates on the pile ends were joined using steel pins. The excess lengths of pile were removed by a hydraulic 'crusher' mounted on the back of a JCB. Further 'trimming' was carried out by compressed air tools to expose the steel bars in the piles.

The photo on the left shows a steel pile nearing its 'set'. This had already been determined by the engineer, taking into account the building load and the nature of the ground. If 10 hammer blows produced downward movement of not more than 5mm in the pile the 'set' had been reached.

The plan on the left shows part of the pile cap and ground beam layout. The reinforced ground beam was poured monolithically (ie, at the same time - from the Greek meaning 'single stone') with the pile caps and floor slab. In the right hand picture you can see the exposed steel of the piles and some blockwork walls - these walls are in fact formwork designed to contain the concrete when it is poured.

The left hand photograph shows the view from the adjacent scaffolding. The line of the ground beam can clearly be seen - the intermittent projections or the pile caps. The two concrete cylinders are large inspection chambers (to be topped with cast-iron covers). The right-hand photograph was taken a few days later and shows the steel in position, and ready for concreting.

The concrete for the ground beam and pile caps was delivered to site ready-mixed and placed with the help of a track mounted excavator. To ensure the beam was dead level (for the steel framed superstructure) a spinning laser ('Laserplane') provided a benchmark- by deducting the reading on the staff from the known height of the laser, the exact level, or height, of the beam can be worked out.

4 Augered PilesContinuous Flight Auger

Augered piles (replacement piles) are suitable for many types of ground, particularly clays. Piles can be formed up to 750mm in diameter (depending on type of rig) with safe working loads up to 3500kN (350 tonnes). A 200 mm diameter pile has an SWL of about 600kN. Pile lengths of up to 25 metres can be constructed. The insitu concrete pile is reinforced, the exact details of the reinforcement will depend on the nature of the loading.A standard detail comprises a single,centrally positioned, reinforcement bar; reinforcement cages will be required to withstand horizontal or bending moment loadings.

CFA piles are formed using hollow stem augers boring techniques (in most cases this will produce spoil 'arisings' which require disposal). Once the correct depth is reached, concrete is injected down the hollow stem of the auger. As the auger is extracted, the concrete fills the void, thus forming an insitu pile. The reinforcement is positioned once the auger has been removed.

Sectional Flight Auger

In principle this system is the same as the one above. It differs in that the augers are in short lengths (or sections) thus permitting the rig to be used in confined spaces with limited headroom, ie, inside buildings. Additional augers can be added to achieve the correct depth. Solid stem or hollow stem augers can be used. The former are easier and cheaper, but only if it is considered that the bore will stay open at the correct cross sectional area. Hollow stem augers are essential where the side wall material is unstable and might collapse. Reinforcement is usually installed immediately after concreting the bore. A starter bar cage can also be installed in the top of the pile to connect to the ground beam or pile cap above.

5 Continuous Helical Displacement Piles This piling system is quite unusual in that it is a bored displacement pile; most bored piles are replacement piles - in other words the ground is removed before concreting takes place. The advantage of this pile is that there is minimal 'cart away' and, unlike most displacement piles, it is quite and vibration free.

On the left you can see a small part of a much larger drawing showing the pile layout. The whole contract comprised nearly 1000 piles, all with a safe working load of 300kN (approx. 90 tonnes). On the right you can see some of the more important notes which qualify the piling plan.

In most ground conditions this is an ideal alternative to continuous flight augers (CFA). During boring the ground is compacted as the rotary head 'drills' into the ground. At the appropriate depth concrete is pumped under pressure down the hollow shaft to the boring head while the shaft is reverse rotated and withdrawn from the bore.

Reinforcement in the form of a cage and/or single bar is lowered into the bore when the concrete operation is complete. Reinforcement projects from the top of the pile to form a strong connection with a pile cap or ground beam.

Once the piles are complete the ground beam or pile caps can be cast. The example shown here is a continuous ground beam. The photo on the left shows the shallow excavation along the line of the piles for the ground beam. On the right you can see the weak concrete blinding (laid to form a clean, level working surface) and the reinforcement cage partly in position.

Test piles are loaded to assess their loadbearing capability. The test rig comprises three steel beams anchored to four deep corner piles. A hydraulic jack loads the test pile under the centre of the beam. On this site a safety factor of 2.5 was required. So, if the design load is say, 300kN, the test pile should carry 750kN. Click here to see a video clip

6 Insitu Ground BeamsThis page shows how a ground beam can be formed. The first stage is to cut the piles to the right length and dig a trench between them. This has been blinded with a thin layer of concrete to provide a level, clean surface for the next stage - building the reinforcement cages. Before constructing the ground-beam cage the the integrity of the piles is checked.

The steel cage is made from a series of preformed and bent reinforcement rods. the size of the rods is calculated by an engineer. The ground beam, when it is complete, will take the loads from the walls and distribute them into the piles (you can just see the top of the pile in the right-hand photo). The load from the beams is not carried by the ground below the beam.

The yellow plastic is a proprietary permanent formwork. This prevents concrete from being wasted. On some sites this formwork, or shuttering, is built in blockwork. If there is danger of ground swell, for example in clay soils, a special type of formwork can be used which incorporates a collapsible layer. This is often required under the beam (in between the piles) as well as at the sides. Click here for another example

7 Piers

8 Foundation Case StudiesCase Study One

Roger Bullivant Ltd provided its pre-cast house foundations package, consisting of segmental pre-cast piles, pre-cast ground beams and suspended pre-cast floors to two major house developers on the same site in Cheddar, Somerset. The developments were undertaken on a greenfield site which suffers from high ground water table problems. The majority of ground levels needed to be raised by as much as 750mm across the area. It was also anticipated that localised flooding of the site during work would be experienced. A solution to overcome all of these difficulties was needed to enable work to proceed, as conventional foundations would be difficult and costly to construct.The use of segmental pre-cast piles, Tee-beam system and suspended pre-cast floors, helped overcome the poor ground conditions. The segmental pre-cast piles provided a cost effective solution which eliminated both excavation and spoil disposal.

Sacrificial probe piles were installed prior to final negotiations to determine the actual pile lengths so that a fixed price package could be agreed. In total 560, 175 x 175mm square segmental pre-cast piles were installed to a maximum depth of 7m. Each pile was capable of carrying loads to 350kN and comprised 3m and 4m segments with single T16 bar reinforcement. The piles were cropped to the required level and either pre-cast caps or cast in-situ concrete caps positioned on top. Approx. 2000 metres of pre-cast Tee-beams were installed, together with 2500metres of pre-cast beam joist floors.

Both projects were completed using purpose-built piling rigs, designed to undertake work of this nature where piles ranging in size from 150 200mm are required, with loads from 100 400 kN. The rigs use 2 3 tonne hydraulic hammers to drive in the piles. As the gross weight of each rig is only 17 tonnes, it can be mobilised onto site without the need for police escorts or movement orders. The size and weight of the rig also reduces on-site problems resulting from noise and vibration.

Case Study Two

Roger Bullivant Ltd provided pre-cast concrete piling for the construction of a new housing development in Burnham-on Sea, Somerset. The project involved foundation piling on a section of a new development of timber-framed homes. Three of the planned houses presented a particular challenge, as they were situated in close proximity to a hedgerow, with the added complication of a ditch around the perimeter where the ground was soft and wet. The plots were also very close to existing adjacent housing which was being developed and therefore any work had to be carefully contained.The contractor (RB) proposed the installation of pre-cast segmented piles together with pre-cast Tee beams. The system offers a cost-effective solution to installations in close proximity to existing buildings or structures and, in addition, overcomes problems of poor ground conditions without the need for soil disposal or excavation.

The area of the site was prepared to a reduced level of 650mm below floor slab level. Investigations of the soil revealed eight distinct layers; these varied from topsoil, firm clay, clayey and laminated silt of various types, down to fine-medium sand. This dictated that piling would need to be to a depth of approx 17m. As the ground compaction was good, no hardcore was needed for piling platform.The segmental pre-cast concrete piles were installed with a top-driven hydraulic hammer rig to depths ranging from 1 5.5 1 6.4m . The pre-cast piles were 250mm square capable of loadings up to 600kN. The segments were each 4m with single bar reinforcement to reduce costs and minimise wastage.

Case Study Three

This housing development was being constructed on the site of a former sand quarry which had been filled before the Second World War. A school was then built on the site, incorporating underground air raid shelters constructed from very thick, high-density concrete. The site over the disused shelters had subsequently been developed by the addition of extra classrooms and playgrounds etc.The exact location of the air raid shelters, the degree of fill and precise depth of the underlying bedrock were all unknown. It had been identified that on an adjacent piece of land, homes constructed post-war had needed to be demolished because of major subsidence.

The developers had already completed the first six houses on one section of the site, using vibrated concrete columns and very deep foundations. As these initial properties had sold immediately, they were looking for an alternative solution that would speed up the construction programme and also avoid the high cost of such deep foundations. The solution comprised a combination of steel tubular piles and pre-cast concrete piles both followed by pre-cast pile caps, pre-cast tee-beams and pre-cast concrete floor slabs. The combination of both pile types enabled the differing conditions on the site to be accommodated, with only minimal need for spoil removal. On the section of the site where it had been identified that air raid shelters had not existed, the ground conditions were sound and pre-cast concrete piles 200mm to a depth of 6m were used. On the more difficult parts of the site, 170mm diameter steel piles were used, installed down to 8m and in some cases down to 16m. The use of steel piles ensured that they could be driven through any concrete structures forming part of the old shelters. Where any major obstructions were identified, local excavations were carried out to assess and re-plan the piling as required.

Installation was followed by pile cropping which ensured a sound connection between the pile and the pre-cast cap. Over 1500metres of pre-cast reinforced concrete tee-beams were then installed directly onto the pile caps to carry the wall and floor loads.

Case Study Four

This housing development is on a site formerly used as a tramway depot. The site was bounded by two roads, a railway and a nearby underground line. Vibration was perceived as a major constraint, as was the need to minimise traffic, and keep noise to a minimum. Continuous helical displacement piles were used and the whole project (nearly 500 piles) was completed in four weeks.

The piles were founded into clayed silt at depths down to 18.50 metres. The piles had designed working loads of up to 300kN. The first test pile was toed into boulder clay at a depth of 17.60 metres and gave a settlement of 15mm at 1200kN. A second test pile, founded in clayed silt at a depth of 15.50 metres provided an ultimate load of 600kN, giving a safety factor of 2.

9 Repair Systems - Roger Bullivant This repair system (shown in isometric on the left, and in section on the right) is suitable for many types of shallow foundation stabilisation, especially where access is restricted. A series of piers, at centres of up to 1.5ometres, transfer the loads from the wall down to a firmer stratum. Loadings of up to 100kN per metre can be achieved, individual piers are generally rated at about 50kN. a 300mm by 300mm pocket in the brickwork is removed to make way for the reinforced concrete 'knuckle'. This system is less disruptive than traditional piling, it's ideal for restricted situations, and possibly most important of all, it only requires access from one side of the wall - occupants do not have to be moved out.

This system, the pier and beam system, is slightly more complex than the one above and is used where lateral, as well as vertical, restraint is required. A typical situation might be where poor quality underground brickwork requires lateral restraint between supports. The longitudinal ground beam can be seen in the graphic on the left. As in the above example the piers can be driven or augered.

This system requires the installation of pairs of piles, one in compression and one in tension, as shown in the graphics. A reinforced concrete beam on top of the piles cantilevers into the wall to provide support. This piling repair method can be economic where the bearing stratum is deeper than 1.5metres. Pile sizes range from 90mm to 250mm diameter; the piles themselves can be drilled, driven or augered. Again, the work can be carried out from one side.

Angle piles can be single or double as shown in the left-hand graphic. Piles are normally installed both sides of the foundation although they can be installed from one side if there is suitable lateral restraint. Permanently cased steel driven piles, or solid or hollow stem augered piles, are then installed through the pre-drilled hole with the casing terminated at the underside of the existing foundation. The pile is then concreted and reinforced up through the existing foundation. This is a fast piling system with high load capability.

10 Rafts Raft foundations were sometimes used as far back as the 1920s and 1930s. This example is a house designed in 1936 - the site was a drained marsh. In the 1940s and 1950s raft foundations were quite common, particularly beneath the thousands of prefabricated pre-cast concrete or steel buildings erected during the years following the Second World War. Most of these houses were built on good quality farm land where the soil was generally of modest to high bearing capacity. Rafts (or foundation slabs as they were sometimes called) were often used because they were relatively cheap, easy to construct and did not require extensive excavation (trenches were often dug by hand). In 1965 national Building Regulations were introduced for the first time (London still had its own building controls), but these did not contain any 'deemed to satisfy' provisions for raft foundations (as they did for strip foundations) - consequently each had to be engineer designed. As a result they quickly fell out of favour.

In modern construction rafts tend to be used: Where the soil has low load bearing capacity and varying compressibility. This might include, loose sand, soft clays, fill, and alluvial soils (soils comprising particles suspended in water and deposited over a flood plain or river bed). Where pad or strip foundations would cover more than 50% of the ground area below the building. Where differential movements are expected. Where subsidence due to mining is a possibility.

Flat slab rafts (right hand graphic) offer a number of advantages over strip foundations, no trenching is required, they are simple and quick to build, there is less interference with subsoil water movement, and there are no risks to people working in trenches. Detailing needs careful thought, - the example on the right for instance may be subject to frost attack around the edges, the edges themselves are exposed, and there is the risk of cold bridging around the perimeter. They are generally suitable for good soils of consistent bearing capacity.Flat slab rafts (ie no perimeter or internal beams - see below) have been recommended in some mining areas. These rafts will flex if ground movement is considerable so the superstructure needs to be designed accordingly.

Shallow rigid rafts for 1, 2 and 3 storey housing can be cheaper than piles. On poor ground the raft must be stiff enough to prevent excessive differential settlement. This usually requires perimeter and internal ground beams to help stiffness and minimise distortion of the superstructure.Some, overall, settlement of the house will inevitably occur but differential settlement should be kept within acceptable limits. In this country rafts have to be designed on a one-by-one basis, in other words there are no 'deemed to satisfy' provisions in the Building Regulations as there are with strip foundations. In practice, engineers are advised to consider local practice with regard to raft design. Some typical dimensions of the various elements are shown in the graphic on the right (upper) The lower graphic shows the nature of the perimeter and internal beams.

On filled sites rafts can, depending on the fill depth, be a cost effective alternative to piling. They can also be used on sloping sites as an alternative to stepped strip foundations. A well compacted (in shallow layers), graded granular fill can form a suitable base. Designing the fill and the raft is obviously specialist work and many speculative house builders would probably prefer 'tried and tested' stepped strip foundations.

2007 University of the West of England, Bristolexcept where acknowledged