BLT 1003 Construction Substructure (1)

8/13/2019 BLT 1003 Construction Substructure (1)

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Construction Substructure

Foundations and Excavations

Site Preparation

Once the survey work has been done on a site, prior to the building work commencing the site has tobe stripped of all vegetation and top soil from under any part of the building or associated paths,roads and hard paving. Care must be taken with any trees as they may have Tree PreservationOrders (TPOs) on them, damaging a preserved tree can result in fines of up to !",""" per tree.

Top soil can be defined as#

$t this stage the site consists of what is called sub%soil, can be many different materials. The ne&tstage is to establish what is called the Reduced Level,this is the level or group of levels at which thefloor construction, paving formation and foundation e&cavation commences. This level will define thelimit of what is called the Bulk Excavation. This work is controlled by means of either sight rails or

more commonly now by the use of rotating laser%levelling e'uipment.

Once at the reduced level we can now start to commence the foundation work, foundations havebeen defined as#

ifferent types of sub%soil have different bearing capacities the designer takes this into account whencalculating si*es. +n the case of poor loadbearing capacity he designs a foundation that is wider to

spread the load over a large area thus reducing the load per unit area.

Strip Foundations

The simplest of foundation type used is the StripFoundation. The centre line of the foundation ismarked on the ground in sand or spray paint andan e&cavator digs out a given trench width to acontrolled depth all around the building and forany load bearing internal walls re'uired.

The minimum width of this trench is determinedby the width of the wall sitting on top of it and thethickness of the un%reinforced concrete strip

placed in the bottom of the trench.

epending on the nature of the sub soil that isbeing e&cavated and the depth, the sides of thee&cavation may need some form of support. Thissupport will allow the trench to be filled withconcrete without the concrete being

contaminated with e&cavated material and allow the safe working of men within the trench, cleaningup the bottom or placing level pegs to control the depth of concrete.

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p

d-"

proection p/ depth d/ min 0-"mm

loads act at -" and thus through base offoundation


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The concrete would then be placed in the trench carefully and compacted and levelled ready toreceive the substructure brick or blockwork.

$ny changes in level in the foundation must be planned, their height must not be greater than thethickness of the concrete to be used, and the concrete must overlap by twice the depth of theconcrete foundation or 1""mm which ever is greater. +n certain circumstances where traditional stripfoundations are inappropriate, the ade'uate bearing strata is at a low level, we may resort to using amethod known asTrench Fill or Deep Strip Foundations.

Trench Fill or Deep Strip Foundations

+n this method the same setting out is used, but the e&cavation is much deeper down to an approved

level. The width of the e&cavation is kept to a minimum, but often the very nature of the material thatyou are e&cavating throughmeans that the trench endsup wider than re'uired.

+n order for this method tobe economical the concreteis normally poured into thetrench as soon as it is downto the re'uired depth, nocleaning up of the bottomtakes place as this wouldre'uire men to go down

into the trench.

The e&cavation would beover 0.!m in depth andthus the sides of the

e&cavation would need to be supported, incurring e&tra costs.

+f neither of the previous two methods solve the problems posed by the ground conditions then wemay choose the following#

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mass concrete fillto suitable level

as dug, often undulating sidesof the e&cavation

e&cavation to a

suitable bearing level

ground level

concrete to agreed level

2ot less than ! & or1""mm minimum

Dmass concretefoundation

3rick or block work


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Reinforced Wide Strip Foundations

+n poor load bearing conditions it is possible to reduce the load on the sub strata by widening thefoundation, the effective width of an un%reinforced foundation is - "either side of the wall. +f we wishto make the foundation wider than that we must ensure that the wall cannot shear through thefoundation.

+n order to make full use of the e&tra width we insertwhat is called steel reinforcement in the lower part ofthe concrete strip and this then counteracts the shearand spreads the load over the whole of the surfacearea of the strip and thus sub strata.

Simple Raft Foundations

This 'uite simply is the use of a large slab of reinforced concrete, which has the effect of spreadingout the load of the building uniformly over the whole area of the footprint. The resulting loads on thesupporting ground is thus lower and therefore the building can be constructed on poorer bearingcapacity ground than a comparable strip foundation building could.

PILED FOUNDATIONS.

2ot a new concept, used for thousands of years % prehistoric man used 3irch piles in his lake dwellings.4e now use steel and concrete, the basic principles are the same. The idea of using piles is to produce auniform, strong and stable base for a building. This can be the timber and reed hut of prehistoric man or aconcrete and glass office building in any city. These are the same three basic criteria for all foundationwork.

0. UNIFORM% $ny building regardless of si*e, shape etc. needs a uniform strength foundation, so that itdoes not e&hibit the same tenancies as the 56eaning Tower of Pisa5.

7teelreinforcing

bars

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7teelreinforcing

mesh or barsteel


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!. STRONG% The base needs to be able to support the imposed load of the building, so we do not get abuilding with four basement levels instead of one.

1. STABE% $ skiing hotel on the side of a mountain needs a stable foundation, easier for the skiers tofind ne&t year if it does not slide down the mountain.

Three distinct and different functions for the pile to perform % but what is a pile8One definition is #%

The way in which piles transmit the structural loads to the lower subsoil classifies them into two mainPerformance related areas,

END BEARING piles and FRICTIONA BEARINGpiles.

a. END EARIN!piles do ust this, the forces are transmitted vertically downwards to a firm strata.

b. FRI"TIONAL EARIN!piles rely on the pile being gripped by the surrounding strata and the forcesbeing dissipated through that strata.

9orces transferred verticallydown through the pile andstraight into the surrounding

sub%strata

7trong sub%strata capableof absorbing the verticallytransferred forces

T!pica" End Bearin# $i"eArran#e%ent

Typically loose filledmaterial or soft sub%strata incapable ofsupporting normalfoundation loadsoverlying a strong sub%strata at an economiclevel forpiling

+mposed load ofbuilding or structure

:round level

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4ithin these two basic classifications we can then further divide up the piles into two types based on theprocess used to construct them. DIS$ACEMENT $IESand RE$ACEMENT $IES&the former aredriven displacing the soil, the latter are bored out and the ground replaced with concrete and steel.

'( DISPLA"E#ENT PLIES& piles that displace soils, including pre%cast concreter pre%formed, partiallypre%formed and driven insitu piles. Timber pile would also come under this category though not oftenused e&cept in sea defences and in developing countries.

:round level

+mposed load ofbuilding or structure

9orces transferredthrough the pile and outinto the surrounding sub%strata

7ub%strata gripping thepile and absorbing the

transferred forces

T!pica" Frictiona" $i"eArran#e%ent

Typically loosefilled materialor soft sub%strataincapable ofsupportingnormalfoundationloads

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Pre%formed pile forcedinto the ground

T!pica" Disp"ace%ent$i"e

:round level

Typically loosefilled material orsoft sub%strataincapable ofsupporting

normalfoundation loads

:round displacedhori*ontally by the pile

being forced into theground

:round heave atsurface caused by pilebeing forced into theground


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i. Precast "oncrete Piles% :enerally up to !"m in length, ranging from !;- % -!" mm in diametercapable of supporting loads ranging from 1-" k2 %0!-" k2 depending on the bearing strata. They areend bearing piles, there can be little or no frictional bearing, since the driving process tends to mould anycohesive soils round the pile forming a slip plane. Concrete piles are mostly made up from a range ofstandard length sections, these can then be ointed on site as the pile is being driven. The sections canbe made of concrete containing various types of cements which can along with chemical treatment canproduce piles that can withstand attack in corrosive ground conditions. They can however be used insome soils to form friction piles depending on the nature of the soil.

ii.Precast "oncrete "omposite Piles% +n this type a pre cast concrete shell is driven into the ground toa predetermined level once this segmental shell is in place the steel mandrill is with%drawn and areinforcement cage and concrete introduced to fill the shell producing a composite pile. $n advantage ofthis type is that lengths up to


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ii.Rotar( ored Piles) Traditionally done with a crane or lorry mounted auger, often using a steelcasing to maintain the bore whist working. This auger is used to cut the hole, it is raised and lowered toremove the spoil until the depth is achieved the auger is then removed, the concrete and steel introducedand the casing removed. 6arge diameter auger piles can be formed up to a diameter of 1.. 9rankie 7.$. of 3elgium,Cementation (now ?avaerna 9oundations)were the first to take a licence for use in the@.?. This process produces a helical typecast insitu pile with a high bearing capacity. +t

produces no spoil as the auger head actuallycompacts the soil as it is screwed into theground and re'uires no casing. +t is 'uiet andproduces little or no vibration. 7teel andconcrete are introduced down the hollowstem. Piles of up to !!m can be achieved withdias. from 1


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this system is eminently suitable for medium and long span industrial and commercial units, thusenabling this type of development to make use of cheap poor 'uality reclaimed land. The ne&t 'uestion iswhat type of pile is suitable for the proect. 9actors that will influence this decision will include the natureof the ground, depth to suitable bearing strata, any shallow bands of peat or other compressiblematerials, the loadings of the buildings, access to and on site and the pro&imity and state of surroundingbuildings. 7ee table of depths and properties.

SOIL STAILISATION AND I#PRO.E#ENT

4hen deciding on foundation types for any building the information gained from a detailed siteAsoilinvestigation is essential. This will decide whether the ground is suitable or not for strip or raftfoundations to be used. +f it is not suitable for these type of foundations, it may not be bad enough towarrant the use of piles due to cost. There are some other alternatives as set out below# %

0) o not build at all on this plot of land.

!) +f the problem strata is at a level that is accessible, it is possible to e&cavate and replace the poorstrata with granular fill which is compacted in layers. 2ot generally economic over depths of ."" m.

1) Surchargethe ground. Place on top of the ground a pile of material which should be at least the

weight of the building. 6eave it in place for a previously calculated time period, so that the ground cansettle and be able to support the weight of the pile of e&cavated material and thus the weight of theeventual building. Bas its problems, time can be a factor, finding the e&cavated material and thendisposing of it.

) Dynamic onsolidation. This is an e&tremely cost effective techni'ue of ground improvement onlarger sites. The techni'ue involves the controlled dropping of heavy steel tampers(weights), usuallyfrom standard crawler cranes. 4eights of up to 0- tonnes are normally used and drop heights of up to!"m. The techni'ue was originally developed in the early 0="Ds by a 9rench engineer, 6ouis Eenardand has been widely available in the @? since 0=1. The treatment can be effective up to 0!m indepth and provide safe ground bearing capacities in the range of 0""k2Am! to !""k2Am!. :roundconditions suitable for improvement by this method include, natural granular soils, essentially granularmade ground(filled ground), and occasionally refuse depending on structure.

@sually several tamping passes are re'uired to achieve the re'uired effect, and carefulmonitoringAtesting of the works forms an essential part of ground improvement. On a typical proectthere may be up to four passes, with drops for the first pass on a 0"m grid, and subse'uent passeson closer spacings, say m and -m. There could be up to 0" drops on each tamping position. Theobective of the early tamping passes is to improve the deeper layers, while subse'uent passes aim

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to improve successively higher layers. 9inally the uppermost layer would be compacted using avibrating roller.

-) !i"ro#compaction and !i"ro#displacement. >ibro%compaction consists of inserting a vibratingprobe suspended from a crane into soft sand, the resulting hole is then filled up with more sand andvibrated until the sand has reached the re'uired bearing capacity.

>ibro%displacement involves the introduction of granular columns into the ground and thus increasingthe bearing capacity. This is done by inserting the vibrating pokers into the ground and forming avertical void in the ground which is surrounded by consolidated strata by virtue of the vibro action.7ingle si*ed stone usually "mm is then introduced into the void and compacted in layers to formstone columns. +n reasonably stable ground a top fill method may be used as shown below# %

4here the ground is more likely to collapse when the poker is removed, a bottom feed type of systemhas been developed, this eliminates the problem of the void collapsing before the stone is introduced.The only way vibro%displacement could be used in this type of ground prior to this system beingdeveloped was to use the wet system, where the void was kept open by using ets of water to fill upthe hole until the stone is introduced.


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) Fet grouting. This method use a probe which is lowered into a pre%formed hole and then by the useof compressed air and water under pressure washes out a cavity in the soft soils. This cavity is thenfilled with a cement grout that mi&es with the sub%soil and sets and produces a solid mass.

Co%p"ex Ra+t Foundations

9oundation choice depends upon# %

Total loading of the building,

2ature and bearing capacity of the subsoil.

+f we are considering a raft type of foundation, it generally means that there is some doubt as to thesuitability of the GnormalD pad type foundation that one would use in the case of framed buildings. Themost economical type of foundation for framed buildings is generally the pad type, providing that theground conditions are uniform and of sufficient bearing capacity.

$ raft has the ability to take all of the building live and dead loads and combine them then spreadthem out over the whole of the area of the raft. Thus if a framed building measuring 0""m & 0"m,weighing 0"""tonnes was supported on 0" columns the with pad foundations !m & !m, the totalweight on each column thus pad would be 0""tonnes. 7pread over the s' m of the pad, it wouldneed the ground bearing capacity tobe at least !-tonnes per s' m. +f we used a raft, the groundbearing capacity needed could be reduced. The same 0"""tonnes is still spread over 0" columns, butthen each of the columns is effectively supported by one tenth of the area of the raft, area of raft /0""" s' m, one tenth / 0"" s' m. Thus the ground under the raft would need to be capable of

Pad9oundation 7teel or concrete column

Concrete base

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supporting 0tonne per s' m.

+t is thus possible to build on ground that is not capable of supporting high point loadings that wouldresult from the use of pad foundations. 4e can also use rafts where a piece of ground does not haveuniform load bearing capacity, if -H of the site is capable of supporting !tonnes per s' m then theload could be carried on this and the rest of the site could be capable of supporting only .!-tonnes

per s' m.

The higher the load and the more comple& the building, the more comple& and rigid the raft must be.4hen using rafts for framed buildings, we ensure that the loads on each column are capable of beingtransferred into the raft without the columns punching through the raft. there would need to be a largeamount of shear steel in this area or a large cross%sectional area to the base of the column. Columnbases were often splayed where they met the raft, thus increasing cross%sectional area.

7olid rafts are used to good effect with framed structures and can be of the region of ""mm inthickness, fairly heavily reinforced, be simple to construct and thus they can compare with such aspiling or other foundation techni'ues in cost terms.

@p to ""mm thick reinforcedconcrete raft

Columns with haunched

bases bearing on the raft

ConcreteupstandAwall

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7olid Iaft 9oundation


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3eam and slab rafts are then used where ground conditions are more variable, and the raft needs to

be more structurally rigid. 4e can use downstand beams or upstand beams which ever is the mostappropriate method for that particular situation.

$ cellular raft can also be used, this is the above upstand raft with a built%in top and thus becomescellular. The cell depth usually ends up sufficient to use this void as parking or some other use. Theraft then is very deep and thus very rigid.

3eam and 7lab Iaft

Columns bear on

intersection ofbeams

@pstand ordownstand beams

Ieinforced concrete slab and beams

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Cellular Iaft

Columns bear onintersection of beams

BLT 1003 Construction Substructure (1)

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