Architectunl Science Review

The most common melhod used involves the insertion of a plastic coated cardboanl sleeve and is normally used in lhe case ofaugered piles Billlmastic coatings are used in the case of driven piles and a bentonite annulus may be used on either driven or bored piles A slip membrane is constructed at the upper sections of a pile within grOlmd wilh a potential for heave This reduces or eliminateS the uplift forces on the pile The alternati ve to providing a slip mem brane is the installation of tension reinforcement which is often more costly as this involves additional costs ofpile depth and steel reinforcement In the case of bored piles tension reinforcement may be used in conjunction wilh partial sleeving On the other hand in the case of driven piles a slip membrane is the only technically accepted solution

When mini-piles are installed vertically beneath load bearing walls they are subject to compression and if they have high slenderness ratios then their capacities may be limited It is possible to employ steel casings to provide additional strength to mini-piles which carry high loads

The strength of the welds should be considered when si te welding piles especially in situations where the water table fluctuates Furthermore failure due to buckling should be considered when mini-piles have high slenderness ratios eccentric loadings deviations from alignment or lack of straightness In order to reduce the possibility of buckling it has been suggested that the length of mini-piles should be limited to approximately 75 times lheir diamcter (Anon 1986) This is very much a general guide as circumstances may dictate a reduced ratio in cases ofhigh loads and unstable ground extending to the depth of an end-bearing pile or conversely increased ratio in the case of a well supported fric tion pile Because mini-piles generally have high slendershyness ratios good quality control is essential Hence a high level of engineering supervision is always necessary

Some common misllikes in design include shy

I Unforeseen eccentric loadings placed on small diameler piles

2 Realistic slenderness ratios proving unsatisfactory In some circumstances the 1 75 ratio may not be approprishyate An extreme example would be the case ofa 100 mm diameter pile driven through 7 m of soft fill to bear on hard rock In such a situation the pile which may typically carry 40 tN may be offered little lateral support by lhe soft fill In addition it may be subject to aggressive chemical attack thereby weakening a steel cased pile still further

3 The assumption that a particular dynamic driven pile formula is appropriate for all ground conditions The performance ofa driven mini-pile (with relatively lightshyweight driving system) in fll1ll clay is less than that for the same pile driven into dense gravel or soft or weathshyered rocks Energy is absorbed by the clay during lhe driving process and the pore water pressure increases Thc time taken for the pore water pressure to dissipate is normally measured in days In such situations piles should be redriven to check their sets

4 Inclusion ofend-bearing capacity in an auger-bored pile in clay End-bearing capacity usually is taken into acshycount in the design of tradi tionally bored piles whereas in reality spoil fal ls off the bottom of the augers and provides a soft base This can be particularly dangerous

26

Volume 36

when relying on an end-bearing socket in rock Hence it is essential to employ a drilling system which avoids spoil falling to the base of the hole or alternatively a system that ensures a clean base

5 Failure to take full accountoftheeffects ofmade ground particularly in the cases of driven piles The load carryshying capacity of a driven pile is calculated on lhe basis of a set which in tum is influenced by both the end and shaft resistance of the pile during the driving process It is possible for a pile to appear to meet its ultimate resistance within made-ground which may be undergoshying long-term consolidation If the toe of the pile penshyetrates below the made ground the effect of the shaft resistance brings about an artificially low set value which correlates wilh a higher pile load In fact lhe reverse silllation may be true if negative skin friction is brought to bear on the piles in the longer term

6 Misallocation of loadings in partial underpinning schemes

Mini-Piled Underpinning Schemes Mini-piled underpinning schemes may be conveniently

grouped into three types First there is the type which relies upon a direct connection or bond between the mini-pileand the structure itself Secondly lhere is the type where load transfer to lhe structure is achieved by another medium usually a reinforced concrete construction such as a ground beam Thirdly there is the type where the piles are used to stabilize the ground as opposed to the structure

The fIrst type is used for stabilizing walls and concrete slabs Walls normally are stabilized by stitch piling melhods which usually invol ve the construction of raking piles installed at07 m to I m centres and are raked alternately in opposite direcshytions to minimize theeffect ofbending moments (Fig 2) With most foundations a rake angle of 10 to 20 to the vertical is required to allow the mini-pile to pass beyond the centre-line at the base of the footing If the depth of footing varies around the building then the rake angle should also be varied but it

Fl00f finIsh and screed

Fill --_

Figure 2 Raked mini-piles installed through existing foundations

Number 1

should not exceed 20 An important consideration in such stitch piling schemes is the size and quality of the walls and footings These factors determine the bond or load transfer achieved between the pile and the structure as well as the ability of the footing to span between the piles The materials and quality of the footings must be consistent throughout and free from cracks which are often present in the case ofa settled structure Ifinconsistencies are found then the bond of the pile to the structure together with the ability of the footing to span bctween pile positions may be questionable

Riling piles should not be installed in clay soils which are liable to shrink or swell as such volumetric changes may impose bending forces on the piles Again steel casings can be used to provide additional tensile strength in mini-piles which are likely to be subject to bending However a small reinshyforcedcageor tubularreinforcernent within the concrete of the pile itself is preferable

Driven mini-pile casings must terminate benealh a footing or alternatively should be cut off to the correct level in order to allow an adequate bond with the struclllre Concrete should have high workability using 10 mm aggregate in the smaller sizes of mini-pile with cementcontent greater lhan 300kgm

Mini-piles used to reinforce domestic or light industrial floor slabs are normally 75 to 150 mm in diameter and spaced at bctween I and 15 m centres The spacing of the piles is determined by the quality and thickness of the existing slab and a spacing of I to 12 m is normall y used in the case ofa 100 mm slab of good quality concrete Piles are positioned to within 300 mm from the edge of the slab If a floor slab has cracked for example as a result ofexcessive settlement then replacementof the slab normally is preferred However minishypiles can be used to stitch the slab by positioning them up to 300 mm from the crack along its length When lhe thickness of a slab is less than 100 mm or is constructed of poor quality concrete it is not practical to rely on the tensile stresses generated in the concrete and in such situations it usually is recommended that the slab is replaced Additional support to the slab may be provided by forming inverted conical cavities beneath the slab at each pile position prior to the pile being placed (Fig 3) These subsequently are filled with grout when the piles are poured

New membrane after piles formed

-------------------------- iu _ 1 _

o-~Floor stab

~~~~

Grouting Invened conical seating formed in fill and grout

Clearance hole

- Casing cut off level

through floor slab shy Pile formed to top of for pile casing floor slab in one pow

Figure 3 Mini-piling ofoar slab which is settling

The second type of underpinning system involves the transshyfer of load via some form of concrete construction such as ground beams This technique is often tailor designed to suit individual site requirements The most common form of this type of underpinning involves the construction of pile pairs each side of the wall supporting a needle beam which in 1lIm

Mardi 1993

supports that section of the wall (Fig 4) The spacing of the pile pairs is determined by the load on the structure capacity of the piles and the ability of lhe struclllre to span between needle beams Needle beams are typically spaced between 07 and 15 m and are generally 300 to 600 mm square They may be installed either above or below an existing footing conshystruction below the footing being more costly but allowing a greater span between needle beams

Dry mortar packing Needle beam

D~ I [gt~gt-I

Sleel stool on temporary ~ -J~~ packing

Figure 4 Underpinning with vertical miru-piles and needle beams

A natural development from the above involves the installashytion ofhighercapacity pile pairs and needle beams supporting a wall beam which in tum supports the struCIllre The wall beam normally is constructed wilhin the wall by systematishycally removing brickwork and installing sacrificial mechanishycal props Reinforcement then is fIxed and concrete poured in the normal way Needle and wall beam design is generally in accordance with BS 8 110 1985 however special limiting considerations to deflection are required for longer spans

In si tuations where access is restricted to one side of the wall then piles supporting a cantilever beamlarger diameter piles supporting a hammer-head construction or vertical and raking pile pairs may be used The cantilever system is more easily analysed and designed from a technical point of view The system involves a compression pile constructed immediately adjacent to the wall with a tension pile positioned typically at between 05 and 15 m in from the wall This system is mainly suited to a bored type of pile as it is diffICult to predict tensi le forces generated by driven piles

The hammer-head type of underpinning requires special consideration ofdeflection indeed it is the denection brought about by the bending moment which limits the minimum pile size to 200 mm in the most lightly loaded struCIllreS although a 300 mm diamctcr pile is preferred In silllations where solid ground bearing slabs are available to accommodate the lateral forces then lh is type of system becomes more attractive

Systems inVOlving vertical and raking piles again bring technical complications ofdeflection although the direction of the deflection is reversed Similar conditions to the above apply provided that the lateral forces can be accommodated

The third type of system employs root piles to stabilize the soil It is designed to form a three-dimensional soil structure and is used when excavation is not possible Small diameter (75-250 mm) root piles are installed in cased boreholes Reinforcement usually a single steel bar is placed in the

27

1 0 j( ~ or

Itr 11lIl l ( l1 CIlllIg loundations should only be made when Imiddot 1 ) I bull Iconccmed are known to be high In fact there y kw ca= when the foundations of domestic dwellshyf ltall lx relied upon to fonn a structural member of an IJIIlkrpinning system

As the suuctural integrity of any building which is to be underpinned must be preserved during the underpinning procshyess any existing damage such as cracks should be docushymented and their cause investigated Cracks should be kept under surveillance by the use oftell-taleor measuring devices Other factors such as suuctwalloadings partial underpinning considerations geometry of the slrUCture access to the walls access to the working area and environmental considerations influence the design of an underpinning scheme and as such require investigation

The scope of investigation worlcs may need to be revised during the courseof the investigation as further information is revealed which allows a better prediction of the form of underpinning required A re-design of the underpinning sysshytem may be necessary in the lightofnew information revealed Assumptions made during the design of an underpinning scheme should be confirmed on site during the course of the works

In remedial underpinning it is essential to diagnose correctly the reason why ground movement has occurred Causes of such movement includeshy

I Volumetric changes in clay soils In this respect the presence of large tree andor leaking drainage can accenshytuate these changes

2 Long-term volume increases of expansive clay soils for example heave on the site of previously wooded areas

3 Over-stressing of marginal ground 4 Consolidation of flUed siteS for example backfilled

quarries 5 Biological decay 01 organic material in and consolidation

of domestic landfill 6 Particle removal of granular soils by groundwater lbis

may be due to changes in the groundwater regime or leaking services

7 Mass removal of soils by rain wash 8 Local landslip for example slippage into local excavashy

tions which have been inadequately backflUed or mobishylization of local ground by load increases or percolating water

9 Vibrations causing loose granular soils to settle 10 Migrating underground cavities such as abandoned

mineworkings 11 Subsidenceofsolution holesforexampleswallow holes

in chalk

When ground movements are the result of subsidence due to subsurface mining then underpinning will prove ineffective as long as subsidence continues unless a form of jacking system is employed

Underpinning by Mini-Piling There are numerous methods of underpinning which are

used to provide additional support to a structure but here attention is focused on the use of mini-piling It is generally accepted that mini-piles have a diameter of 300 mm or less

24

Volume 36

Piles with diameters less than 150 mm are increasingly used in remedial work in connection with underpinning walls and repair of floor slabs

The use of mini-piling as a method of underpinning is becoming increasingly popular According to Bruce (1988) this reflects the increasing amount of redevelopment which is going on in urban areas In particular upgrading of existing structures frequently requires underpinning of foundations and problems ofaccess complex soil conditions and environshymental considerations often combine to limit construction operations so thal mini-piling represents the best solution Mini-piles can be inslaUed in virtually aIllypes of soil condishytions and can be installed in situations where either access or head room are limited

One of the major advantages of using mini-piles as an underpinning system especially when compared with tradishytionally excavated underpinning methods is the reduction in contract time and therefore in cost Traditionally excavated underpinning methods are labour intensive as well as time consuming mainly because they involve high volumes of material transfer (excavated spoil out concrete and other materials in) An approximate indication of present-day cost comparisons of underpinning for domestic and light industrial buildings is given in Fig I Furthermore it is generally easier to define contract costs in relation to piled methods because if unsuitable ground conditions are discovered at a greater depth than was anticipated at the time of the investigation then the relative increase in costs ofa piled system in dealing with this situation is much less than those of a traditional scheme Yet another advantage is that mini-piled systems as remarked above can be installed in a variety of ground conditions including unstable soils soils containing boulders and soils wi th a high water table

Cost per linear metre(C)

300 400 500 600

10

sect 1e 20

~ 15 ~

~ 30 2

0

50

Figure 1 Cost comparison between mini-piled and excavated underpinning

The main considerations which have to be laken into account wilh a piled underpinning solution are the ground conditions access to lhe areas requiring underpinning and the configurashytion of walls which are to be underpinned along with their

mh I

lnKlural integrity For example ifsuit~blc bearing ground is 11fIscnt ata relatively shallow deJth then small diameter piles middot111 be installed at close cenues at relatively short depths_ By nrast where piles have to extend to an appreciable depth Ihen it is normaUy more economical to utilize more highly Inad~~l larger diameter mini-piles which are spaced more (Iely In theeaseofclosely spaced piles the foundations and walls if in good condition may be able to span the piles or needle beams without suffering damage When piles are Widely spaced a wall beam is required to support the load Ix~ween the piles The existing suuctural integrity influences lhe spacing of piles and beam construction Restricted access 10 one side of a wall limits the type of scheme to vertical and raking pairs of piles pairs of tension and compression piles supporting cantilever beams or hammer-head connections to larger diameter vertical mini-piles All ofthese solutions bring with them technical complications the simplest form being the cantilevered method of support

Ground conditions tend to lend themselves to either a bored ora driven steel cased pile method However wall configurashytion and access may dictate a particular method Structural integrity of the building may limit the solution to a bored type of pile in what normally would be driven-pile ground a hollow-stem continuous flight auger or other rolary drilling techniques being used In the case of a cantilevered scheme util izing tension piles a bored pile normally is preferred to generate the necessary tensile capacity

Some Design Considerations A mini-pile derives support from its base (Q) and the friction

on the shaft (QJ Hence the ultimate load carrying capacity (Q) of a mini-pile can be obtained from

Q Q + Q (1)

In the case of augered piles in clay Q cannot be relied on as cuttings often fall to the bottom of the hole when withdrawing the augers However if other techniques are used to ensure a clean base then in clay

Q cNAb + acAs (2)

where

c undrained shear strength of the clay

N bearing capacity factor normally laken as 9 for piles in clay

a adhesion factor given by cjc (between 095 for soft clays and 045 for stiff clays) c being the undrained adhesion between the pile shaft and clay

As shaft area of pile and

Ab base area of pile

In the case of firm to stiff clays (shear suength greater than 40 lcPa) the maximum resistance to sliding developed on the shaft is normally the larger component of the ultimate bearing capacity On the other hand when mini-piles are installed in soft clays loose sands or poorly compacted tills then no contribution from the shaft friction should be included In fact if soft clays or poorly compacted fills are for one reason or another undergoing consolidation then they can impose negashytive skin friction on the shaft of the pile In such cases it may be necessary to assign a separate value of skin friction for the shaft affected by down-drag When piles are arranged in closely spaced groups they act as a single unit and conseshy

Mardll993

quently the ultimate load ofthe groupofpiles is not necessarily the same as thatofa single pile times the number ofpiles in the group Various empirical formula have been proposed for the efficient design of piled groups (Poulos 1987)

As far as granular soils are concerned the ultimate end bearing capacity (Q) of a pile can be obtained from

Q yzNAb (3)

where yz is the effective overburden pressure atlhe depth of the pile and N is a bearing capacity factor Meyerhof suggested that lhe ultimate skin friction (Q) may be derived from

Q fsAs (4)

where fs = Kyz tan Ii (K is the coefficient of effective earth pressure 05 for loose sands to 10 for dense sands y z is the average effective overburden pressure over the pile length 1)

is the angle of friction between the pile and the soil and is generally laken as 075 for driven concrete piles and 09 for cast-in-place concrete piles)

Driven piles are normally used in cases where soft unstable ground overlies granular materials or rocks Load bearing characteristics either are calculated in accordance with a recognized form of the Hiley formula or other empirical formulae appropriate for the more recently developed pile driving systems The Hiley formula is used for the convenshytional system involving a weight acting on either the top or bottom of the pile dropped over a given height while the more recent formulae developed by contractors are normally used for either hydraulic or pneumatically powered hammers A generally accepted version of the Hiley formula for a bottom driven cased pile isshy

R 270w (J + h) (5) bull 127 + s

where R = ul timate pile capacity (IOnnes)

w weight of hammer

h = height of drop (maximum of 20 m) and

s = set per blow (mm)

In the case of vibrated driven piles adequate resistance is generally regarded as having been mobilized if pile penetrashytion is IO mm or less during IO s ofvibration This is normally checked over a penetration of 100 mm

The suuctura1 load which a pile carries when used for underpinning depends on the nature ofthe ground movements If they were caused by subsidence then the piles are designed to carry the total suuctural loading including both the live and dead loads When settlement is responsible for ground moveshyment then the piles can be designed to carry that part of the load which is necessary to cause the seulement to cease lbis is the policy of Abbey Underpinning and Foundations Ltd

Estimation of the vertical and horiwntal movements of piles can be done by a number of theoretical methods Often however accurate calculations for underpinning of domestic dwellings are not required as the systems employed tend to be slandardizedOn the other hand calculationsarecarriedout for a non-standard underpinning system

In soils with a potential for swelling the soil can be kepi separate from the main body of the pile by several method

This is the policy of Abbey UnderpiJUling and Foundations Ltd

25

1 0 j( ~ or

Itr 11lIl l ( l1 CIlllIg loundations should only be made when Imiddot 1 ) I bull Iconccmed are known to be high In fact there y kw ca= when the foundations of domestic dwellshyf ltall lx relied upon to fonn a structural member of an IJIIlkrpinning system

As the suuctural integrity of any building which is to be underpinned must be preserved during the underpinning procshyess any existing damage such as cracks should be docushymented and their cause investigated Cracks should be kept under surveillance by the use oftell-taleor measuring devices Other factors such as suuctwalloadings partial underpinning considerations geometry of the slrUCture access to the walls access to the working area and environmental considerations influence the design of an underpinning scheme and as such require investigation

The scope of investigation worlcs may need to be revised during the courseof the investigation as further information is revealed which allows a better prediction of the form of underpinning required A re-design of the underpinning sysshytem may be necessary in the lightofnew information revealed Assumptions made during the design of an underpinning scheme should be confirmed on site during the course of the works

In remedial underpinning it is essential to diagnose correctly the reason why ground movement has occurred Causes of such movement includeshy

I Volumetric changes in clay soils In this respect the presence of large tree andor leaking drainage can accenshytuate these changes

2 Long-term volume increases of expansive clay soils for example heave on the site of previously wooded areas

3 Over-stressing of marginal ground 4 Consolidation of flUed siteS for example backfilled

quarries 5 Biological decay 01 organic material in and consolidation

of domestic landfill 6 Particle removal of granular soils by groundwater lbis

may be due to changes in the groundwater regime or leaking services

7 Mass removal of soils by rain wash 8 Local landslip for example slippage into local excavashy

tions which have been inadequately backflUed or mobishylization of local ground by load increases or percolating water

9 Vibrations causing loose granular soils to settle 10 Migrating underground cavities such as abandoned

mineworkings 11 Subsidenceofsolution holesforexampleswallow holes

in chalk

When ground movements are the result of subsidence due to subsurface mining then underpinning will prove ineffective as long as subsidence continues unless a form of jacking system is employed

Underpinning by Mini-Piling There are numerous methods of underpinning which are

used to provide additional support to a structure but here attention is focused on the use of mini-piling It is generally accepted that mini-piles have a diameter of 300 mm or less

24

Volume 36

Piles with diameters less than 150 mm are increasingly used in remedial work in connection with underpinning walls and repair of floor slabs

The use of mini-piling as a method of underpinning is becoming increasingly popular According to Bruce (1988) this reflects the increasing amount of redevelopment which is going on in urban areas In particular upgrading of existing structures frequently requires underpinning of foundations and problems ofaccess complex soil conditions and environshymental considerations often combine to limit construction operations so thal mini-piling represents the best solution Mini-piles can be inslaUed in virtually aIllypes of soil condishytions and can be installed in situations where either access or head room are limited

One of the major advantages of using mini-piles as an underpinning system especially when compared with tradishytionally excavated underpinning methods is the reduction in contract time and therefore in cost Traditionally excavated underpinning methods are labour intensive as well as time consuming mainly because they involve high volumes of material transfer (excavated spoil out concrete and other materials in) An approximate indication of present-day cost comparisons of underpinning for domestic and light industrial buildings is given in Fig I Furthermore it is generally easier to define contract costs in relation to piled methods because if unsuitable ground conditions are discovered at a greater depth than was anticipated at the time of the investigation then the relative increase in costs ofa piled system in dealing with this situation is much less than those of a traditional scheme Yet another advantage is that mini-piled systems as remarked above can be installed in a variety of ground conditions including unstable soils soils containing boulders and soils wi th a high water table

Cost per linear metre(C)

300 400 500 600

10

sect 1e 20

~ 15 ~

~ 30 2

0

50

Figure 1 Cost comparison between mini-piled and excavated underpinning

The main considerations which have to be laken into account wilh a piled underpinning solution are the ground conditions access to lhe areas requiring underpinning and the configurashytion of walls which are to be underpinned along with their

mh I

lnKlural integrity For example ifsuit~blc bearing ground is 11fIscnt ata relatively shallow deJth then small diameter piles middot111 be installed at close cenues at relatively short depths_ By nrast where piles have to extend to an appreciable depth Ihen it is normaUy more economical to utilize more highly Inad~~l larger diameter mini-piles which are spaced more (Iely In theeaseofclosely spaced piles the foundations and walls if in good condition may be able to span the piles or needle beams without suffering damage When piles are Widely spaced a wall beam is required to support the load Ix~ween the piles The existing suuctural integrity influences lhe spacing of piles and beam construction Restricted access 10 one side of a wall limits the type of scheme to vertical and raking pairs of piles pairs of tension and compression piles supporting cantilever beams or hammer-head connections to larger diameter vertical mini-piles All ofthese solutions bring with them technical complications the simplest form being the cantilevered method of support

Ground conditions tend to lend themselves to either a bored ora driven steel cased pile method However wall configurashytion and access may dictate a particular method Structural integrity of the building may limit the solution to a bored type of pile in what normally would be driven-pile ground a hollow-stem continuous flight auger or other rolary drilling techniques being used In the case of a cantilevered scheme util izing tension piles a bored pile normally is preferred to generate the necessary tensile capacity

Some Design Considerations A mini-pile derives support from its base (Q) and the friction

on the shaft (QJ Hence the ultimate load carrying capacity (Q) of a mini-pile can be obtained from

Q Q + Q (1)

In the case of augered piles in clay Q cannot be relied on as cuttings often fall to the bottom of the hole when withdrawing the augers However if other techniques are used to ensure a clean base then in clay

Q cNAb + acAs (2)

where

c undrained shear strength of the clay

N bearing capacity factor normally laken as 9 for piles in clay

a adhesion factor given by cjc (between 095 for soft clays and 045 for stiff clays) c being the undrained adhesion between the pile shaft and clay

As shaft area of pile and

Ab base area of pile

In the case of firm to stiff clays (shear suength greater than 40 lcPa) the maximum resistance to sliding developed on the shaft is normally the larger component of the ultimate bearing capacity On the other hand when mini-piles are installed in soft clays loose sands or poorly compacted tills then no contribution from the shaft friction should be included In fact if soft clays or poorly compacted fills are for one reason or another undergoing consolidation then they can impose negashytive skin friction on the shaft of the pile In such cases it may be necessary to assign a separate value of skin friction for the shaft affected by down-drag When piles are arranged in closely spaced groups they act as a single unit and conseshy

Mardll993

quently the ultimate load ofthe groupofpiles is not necessarily the same as thatofa single pile times the number ofpiles in the group Various empirical formula have been proposed for the efficient design of piled groups (Poulos 1987)

As far as granular soils are concerned the ultimate end bearing capacity (Q) of a pile can be obtained from

Q yzNAb (3)

where yz is the effective overburden pressure atlhe depth of the pile and N is a bearing capacity factor Meyerhof suggested that lhe ultimate skin friction (Q) may be derived from

Q fsAs (4)

where fs = Kyz tan Ii (K is the coefficient of effective earth pressure 05 for loose sands to 10 for dense sands y z is the average effective overburden pressure over the pile length 1)

is the angle of friction between the pile and the soil and is generally laken as 075 for driven concrete piles and 09 for cast-in-place concrete piles)

Driven piles are normally used in cases where soft unstable ground overlies granular materials or rocks Load bearing characteristics either are calculated in accordance with a recognized form of the Hiley formula or other empirical formulae appropriate for the more recently developed pile driving systems The Hiley formula is used for the convenshytional system involving a weight acting on either the top or bottom of the pile dropped over a given height while the more recent formulae developed by contractors are normally used for either hydraulic or pneumatically powered hammers A generally accepted version of the Hiley formula for a bottom driven cased pile isshy

R 270w (J + h) (5) bull 127 + s

where R = ul timate pile capacity (IOnnes)

w weight of hammer

h = height of drop (maximum of 20 m) and

s = set per blow (mm)

In the case of vibrated driven piles adequate resistance is generally regarded as having been mobilized if pile penetrashytion is IO mm or less during IO s ofvibration This is normally checked over a penetration of 100 mm

The suuctura1 load which a pile carries when used for underpinning depends on the nature ofthe ground movements If they were caused by subsidence then the piles are designed to carry the total suuctural loading including both the live and dead loads When settlement is responsible for ground moveshyment then the piles can be designed to carry that part of the load which is necessary to cause the seulement to cease lbis is the policy of Abbey Underpinning and Foundations Ltd

Estimation of the vertical and horiwntal movements of piles can be done by a number of theoretical methods Often however accurate calculations for underpinning of domestic dwellings are not required as the systems employed tend to be slandardizedOn the other hand calculationsarecarriedout for a non-standard underpinning system

In soils with a potential for swelling the soil can be kepi separate from the main body of the pile by several method

This is the policy of Abbey UnderpiJUling and Foundations Ltd

25