shallow foundation-part2

8/16/2019 Shallow Foundation-Part2

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BY Ir. NEOH CHENG AIK E-GEO CONSULTANT SDN [email protected]


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1. Introduction

2. Common Foundation Distress & Causes3. Cases histories & lessons learnt3.1 Case History # 1

.3.3 Case History # 33.4 Case History # 43.5 Case History # 53.6 Case History # 6

4. Q & A5. Concluding remarks


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Footings in difficult sites that need special considerations with proper mitigationsare: footings on slopes, in fill, in weak fractured or laminated bedding rocks. Footingssubject to inundation, flooding, ponding, scouring. Footings in ground that subject tofluctuation of groundwater level, excessive pore water pressure, excessive seismicloading. Problems & solutions?

How to estimate allowable bearing capacity ? Factors & issues : soil/rock types,extent/scope of SI, site geology & soil variability, stiffness of ground beams &superstructure, loading variations, etc. Selection of effective strength (peak, critical orresidual) or undrained strength? Worst cases? WT consideration? FOS=2-4? Methods &scope of design validation?Settlement consideration ? Allowable limits total differential & rate issues ? Factors:

soil type & subsoil profile, WT, stiffness of structure, size of footings, loading variations,construction tolerances, methods of analysis, scope of design validation, etc. Largefootings & raft usually have critical settlement problems.Compared with deep foundations , shallow foundations are more simple to design &construct because detail subsoil conditions (strength & WT) can be obtained with ease &with high degree of certainty. Construction is also very simple & easy to control.

However, foundation distress or failures also sometimes are encountered, especially indifficult ground.Defective design is usually caused by inadequate and/or unreliable SI, inadequatedesign mitigations vs WCGW, inadequate design validation, inadequate case histories,etc. Defective construction is usually caused by inadequate supervision, supervisionby inexperience/unreliable Engineer, inadequate site temporary drainage & measures to

prevent infiltration/ponding, poor construction planning & sequence of works, etc.


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For low-rise buildings or light structures in original or cut ground (not soft orfilled grounds), where shallow foundations are feasible, JKR probes shall becarried out (about 6-20 probes per block of building or at spacing of about 10mto 20m). JP results plus HA/TP/PBT to check soil type/strength & WT aregenerally adequate for design if the following conditions are fully complied :JP results are reasonably consistent & good bearing layers are 3m bgl

.Design bearing pressure


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DeskDesk studiesstudies for scope of project & likely loading. Site plan & site conditions. For softground site, likely has some fill. Refer case histories of D & C of buildings on soft grd &necessary precautions & mitigations. Similarly for other types of site such as fill grd, slope,ex-mining site, etc. Be aware & understand the problems & necessary mitigations vspossible problems for various types of difficult sites, especially the treacherous ones. Siteinspection.

PlanPlan scopescope of of SISI based on established guidelines (REAM GL 6/2004) to get subsoil profile& necessary parameters for the necessary analysis & design as per CP or design criteriarequirements. BEM Circular 4/2005?FoundationFoundation designdesign : select type of foundation based on loading, SI results & CP(BS8004/EC7) & reported case histories of similar nature. Study & evaluate the possiblechan es in subsoil & round conditions. Estimate & check FOS estimated likel

settlement, prepare mitigations vs WCGW at site. Design validation methods & scope.Prepare drgs & construction control requirements.Typical important notes for shallow foundation construction : 1. All footings aredesigned to be at least 1.2m below FGL with ABC=??? kPa . Confirmatory probe shall becarried out at each footing to ensure footings are founded at strata with probe value of notless than ?? blows/ft . No footings shall be in fill or saturated ground unless approved by

Engineer in writing. 2. Excavation for each footing shall be immediately protected by 75mmscreed or within 2 hr after excavation. Concreting for each footing shall be carried out assoon as possible or within the same day with necessary backfilling plus compaction asspecified. Temporary site drainage to prevent ponding shall always be maintained. Anyexcavation for footing subject to ponding or saturation shall be reconfirmed by confirmatoryprobe. Costs for additional excavation/ works shall be fully borne by the Contractor if site

drainage is not satisfactory or the Contractor does not carry out the works with due care,skill & diligence as specified.


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For footing design using large ABC (>200 kPa), design shall becomprehensive including checking ground movement, possible changes insubsoil & ground conditions, etc. Also, more elaborate designvalidation/site tests such as plate bearing tests, check WT, HA or BH, etc.,shall be specified for construction stage. Instrumentation & some

monitoring to check ground movement shall be specified. Footings Footings ononnewlynewly filledfilled groundground shallshall bebe avoidedavoided unlessunless extensiveextensive analysisanalysis & &mitigationsmitigations areare carriedcarried outout toto addressaddress thethe problemsproblems of of changeschanges in in moisturemoisturecontent,content, collapsecollapse settlement,settlement, etcetc for for longlong termterm basisbasis. ... Why?What to do if results of confirmatory probes are substantially different from

e es gne va ues o o assess encoun ere ou ers a s a owdepths? In case WT is above footing level during the excavation? Casehistories?For footings near slopes/walls or on unstable ground such as soft/filledgrounds, etc., ground movement estimation during construction & inservice based on adequate & reliable SI shall be carried outWhat to do if the positions of stumps are deviated excessively (>75mm)?When there is a failure, the foundation designer or supervisor could beaccused of professional negligence, which is generally defined as failure of the engineer to possess and to use the necessary due care, diligence &skill of a normal competent engineer. Standard of care?


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Setting up & marking of column positions or wall alignment shall be by qualified personneland independent checked by surveyor. In case of doubt, consult the designer.Confirmatory probes shall be carried out at visually weak spots first to ensure the shallowfoundations are on suitable & stable levels as designed for. If results are too much differentfrom the designed level, the designer shall be consulted. Few PBT/TP shall be carried out tocheck & verify bearing capacity & deformation as part of design validation scope.Excavate to the approved designed levels and inspect the subsoil conditions, which ifabnormal or significantly different from the expected designed conditions such asencountering WT or boulder, etc., design revision shall be carried out, after consultation withthe designer. The base of excavation shall be immediately (or within 2 hrs after excavation

wetting/ponding/swelling.Fixing reinforcement, formwork & concreting for each footing shall be efficiently planned &executed soonest possible and within the same day of excavation unless otherwiseapproved by the Engineer. In case rain is encountered in the process of the excavation, allsoftened soil shall be removed & more screed shall be added. Backfilling with suitablematerial & compaction shall be carried out soonest possible or as directed by the Engineerafter the concrete have adequately hardened.Temporary site drainage to keep the site dry and to prevent localized ponding shall bealways carried out & maintained with necessary temporary drains. Refer Eurocode 7.Due to limitations of SI & geologic interpretations, there is bound to have some deviationsfrom the anticipated designed subsoil conditions. Hence, site supervisors shall haveadequate experience & knowledge to recognize the significant design deviations at site sothat necessary adjustments to the design can be made.


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ImportantImportant for for designersdesigners to to bebe awareaware of of possiblepossible signssigns of of distressdistress & & thethecommoncommon causescauses. .Terminology,Terminology, definitiondefinition for for failuresfailuresProblems/failures (effects) invariably & mainly are caused by Technical TechnicalShortfallsShortfalls as the result of OVERSIGHTSOVERSIGHTS or human factors (90%, Peck)such as unawareness, ignorance,…. 10% due to lack of technology.DefinitionDefinition of of buildingbuilding foundationfoundation distress/failuresdistress/failures

* Deformation causing unacceptable distress to superstructure. * Any unacceptable defects or deformation of substructure

* Classification: structural & non-structural

* Settlement/deflection/angular distortion: extent, rate & amount* Cracks: pattern, width, depth & length* Unacceptable tilt or distortion* Excessive or intolerable limits of foundation deformation (refer to ACI or Bjerrum

or Burland, etc.)CommonlyCommonly & & invariablyinvariably technicaltechnical shortfallsshortfalls or or causescauses for for buildingbuilding foundationfoundation

distress/failuresdistress/failures is is duedue toto excessive ground movement beyond tolerable limits asthe result of bold design or inadequacies of structural stiffness/designs byinexperienced/unqualified designer or by not qualified checker, poor workmanship by unreliable/ unqualified Contractor/workers, works not incompliance with spec due to lack of quality supervision. All these causes aremainly oversights or human factors: unawareness or ignorance/inexperience,carelessness, false economy, greed, etc. In fact Prof. Peck (1981) also concluded

that 90 % of failures are not due to lack of technology, but oversights that couldhave been avoided.


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CommonCommon foundationfoundation distressdistress for for foundationsfoundations inin typicaltypical DifficultDifficult GroundsGrounds (fill, slope,soft ground, ex-mining site, etc.). Mechanism? Soil-structure interaction behaviour,common problems & Common Mitigations?

TypesTypes of of causescauses for for buildingbuilding foundationfoundation distress/failuresdistress/failures: uniform verticalsettlement of whole rigid/stiffened structure (no structural damages exceptincoming services), Differential movement of the whole structure (tilting; no damageexcept incoming services) & differential movement of part of the structure (distress,cracks, etc). Need of stiffened foundation on unstable ground?

ypesypes oo oun a onoun a on movemenmovemen r gger ng causes con r u ng causes; ec n cacauses & procedural causes; human factorsObjectivesObjectives && methodsmethods of of failurefailure investigation??investigation??Well known case histories: Leaning Tower of Pisa, etc. Lessons learnt?ProfessionalProfessional ethicethic: always be aware & awake. Always exercise with due care & skill

in design & construction. How? Professional negligence?GeotechnicalGeotechnical engineeringengineering notnot aa perfectperfect sciencescience. Why? SOTA/P? Erratic subsoilprofile & properties (despite detail SI) coupled with imperfect methods of designcan be the main causes for the problems even when adequate FOS is applied byexperienced designers. Hence, adequate design validation is always specified byexperienced designers. Muar Flat research case history is an enlightening one.


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SK Raub: be cautious for inconsistent results with extremelocalized hard layer; too good result may not be good.SK Mersing: Footing design based on JP results withoutknowing site conditions & soil type can be dangerous. JPresults in sandy subsoil are highly sensitive & affected byfluctuation water table or tidal effects.

ampo u p e a : resu s can e very m s ea ng nnewly filled ground. Reasonably compacted fill is usuallyunsaturated with void>5%-10% & Su=75-150kPa, but Su canbe drastically reduced to Su=40 kPa upon saturation due toinfiltration by surface runoff or capillary action from WT.

Fraser Hills bungalow (micropiles to footings)Mentakap Sch Extension (piles to strip footings)Kuantan Class G Quarters (piles to footings)Cheras Mosque in limestone formation (combination of footings, hand dug caissons & piles)Segambut Condo in granite formation hill (piles plus footings)


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Wh excessive tilt? No

much differentialsettlement?How to stabilize thedeformation?Prof. Burland’sproposal.


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Potential failures & Problems of footings founded on rocksPotential failures & Problems of footings founded on rockswith discontinuities. Mitigations & Solutionswith discontinuities. Mitigations & Solutions ??


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Housing Project, about 100 units of medium cost 2-storey terrace housesconstructed over newly filled ground of about 10m deep max. Less than 10units are fully on cut areas. Project site: rolling terrains, metasedimentary

rocks.Immediate after filling with some compaction, JKR probes plus HA & testpits were carried out and shallow foundations (ABC=75kpa to 125 kPa) areused to support the RC framed structure. Compacted subsoil up to about

= - ,

Silt=39-49%, Clay=18-21%). WT below 2m. 3 field density tests at about1.5mbgl show degree of compaction=89 to 92% of BS Std compaction &M/C=18-20%. ABC is based on JKR probe value for each column & JKRprobe correlation chart.Before the buildings are fully completed some serious structural distress ofcracks, tilt & differential settlement are noted.What are the possible causes for the distress? What shall be the scope ofinvestigation to verify the hypothesis of distress?Scope of SI for foundation design adequate (BEM 4/2005)?

What remediations?


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Single storey terrace houses of column load mainly about 3.5 to5.5 ton on 1m to 1.8m fill over soft ground with ABC=50 kPaspread shallow foundations. ABC is solely based on JKR proberesults & correlation chart.

Serious cracks over the building (unsuspended ground floors,walls, beams & columns) after about 3 years of completion.Scope of SI for design adequate?

Possible causes for the problems?How to identify & assess the problems? Scope of SI required?How to remedy? Any further potential problems?


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More than 100 unit of 2-storey link houses are to be built over soft peaty flatlandProposed construction: remove top 2m to 3m of peat and the hydraulic fill the

site with fine to medium silty sand of about 1.5m to 2.5m over the OGL. Raftfoundation (9.6mx56m for 8 units with total load 2150 ton) is proposed. You are required to assess the feasibility of the proposal. Estimate settlement(immediate, primary & secondary settlement) & check FOS of bearing failure.

design validation?The site is flat coastal area of about 9 acres. Site vegetation: grass, ferms andsome bakau trees. OGL=0.8m to 1.8m mainly around 1.0m. Site geology:Quaternary alluviums of very soft ground, No bedrock up to 40m deep (basedon 5 BH results). Typically subsoil consists of saturated, deposited soft tovery soft peaty & silty clay or clayey silt with some localized sandy layers.Top layer: 1m to 2.8m of peaty soil (to be removed). Very soft layer: bulkdensity 16-17 kN/m 3, M/C=28-55%, Cc=0.4-0.5, e 0=0.9 to 1.5, CR=0.2 to 0.25,Cv=3-8 m 2 /yr with av Cv=4.5 m 2/yr.

Subsoil profile & properties (see generalized subsoil profile)


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An additional block of 2-storey school building (10mx60m building area,column loads mainly 28 to 32 tons) is to be constructed for an existingschool that has several blocks of buildings (1 to 3 storey, some on piles &some on footings). No signs of building distress are noted.Results of JKR probe: @ 0.3m=12-16 blows, @0.6m=20-27 blows, @0.9m=30-35 blows, @1.2m=36-48blows, @1.5m=42-56 blows, @1.8m=48-59 blows,@2.1m=59-66 blows, @2.4m=69-77 blows, @ 2.7m=78-95 blows, @3.0m=93blows, @3.3m=133 blows, @3.6m=299 blows, @3.9m=345 blows, @4.2m=350-400 blows/155mm, @4.5m=345-400 blows/125mm, @4.8m=380-400

, . .Questions:Is the scope of SI adequate? Why?If you are required to design the foundation based on given JP results, whichtype of foundation (spread/strip/raft or pile) you would like to recommend?

Why? What methods & scope of design validation you would like torecommend?If shallow foundation is suggested, what is your estimated safe bearingcapacity? Any settlement problems?If the site is prone to frequent inundation up to about 200mm of water for upto 2 hours, what shall be your answers to the above questions?


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1. Can shallow foundations be designed & constructed at (a) steep slopes, (b) ex-mining area, © filled ground, (d) soft alluvial soil? What are the problems? How toassess the problems? What are the risks and uncertainties? Possible mitigations?

2. Building platform over soft ground usually has a fill of 1m or more. What are thepotential problems for shallow foundation in such soil? How to assess & evaluatethe problems? What are the normal scope of SI required to procure the relevantproperties & info? How to assess & check to assess & check ground movement is within tolerablelimits? What are the critical info required? How to obtain? Methods of analysis?Design validation? Awareness of the problems & reliable info for the analysis arevery important. Uncertainties in info & analysis methods ? How to account for theseif shallow foundation is used?

3 . Ground settlement surely affects the buried utilities & services. Building foundation designers.

4. What are the differences between engineered fill & non-engineered fill (uncontrolledfill)? How shear strength of newly compacted fill can be reduced by subsequentsaturation? How saturation can occur? Why it is important to be aware &understand the properties & behaviour of fill plus all sorts of ground movementsthat can affect foundations & buried services? Possible problems & relevantproperties that shall be investigated, studied and assessed before design thebuilding foundation on fill ground? Boulder problems in fill for foundationconstruction? Collapse settlement? What mitigations against the normal problemsare necessary?

5. Awareness & understanding of the potential problems of building foundations invarious types of grounds can be learnt through reported case histories, etc. Adequate

& reliable SI & info are equally important to address the problems effectively. What ismeant by adequate & reliable SI?


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Shallow foundations such as isolated/strip footings, raft, etc., are generallycost-effective for low-rise buildings and light structures instable ground.Some localized weak spots can or may be treated economically.Important scope of design checks shall include 4 aspects, namely FOS vsshear/sliding failure >2, settlement within tolerable limits by the structures,checks & mitigations vs WCGW especially possible ground movement &distress plus adequate scheme of design validation.Estimation of ultimate bearing capacity & settlement shall be based onadequate SI and reliable methods/practice plus past experience or reportedcase histories, especially for difficult /unstable sites.

Difficult & unstable grounds such as filled & soft grounds, erratic subsoilwith localized soft strata, structures near slopes/ponds/rivers, etc., shall bethoroughly assessed for the long term bearing capacity and groundmovement effects if shallow foundations are to be used. Reported casehistories of similar nature shall be referred & studied. Adequate designvalidation & monitoring shall be also included.Foundation distress/failures are usually due to excessive foundationstructure movement as the result of unstable ground or excessive soilmovement caused by oversights, ignorance & carelessness of designer orlack of adequate/reliable information (SI). Contents of BEM 4/2005?Difficult & unstable site shall be avoided if possible unless the design isbased on extensive investigation & assessments.


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Types of causes of failures: Technical Shortfalls as the result of humanerrors/oversights (90%), inherited risks due to limited/inadequacies in CP/SOTA,circumstances of uncertainty and variability in soil properties & analysis methodsor inherited uncertainties/variability of soil properties/design methods, etc (10%).

Building foundation failures are not just accidents nor acts of God. They aremainly due to technical shortfalls as the result of oversights or human errors

such as unawareness, ignorance, carelessness, greed or false economy. Hence,the most effective mitigations against building foundation failures are to refer tomany reported case histories of similar nature, to engage experienced designersplus independent design audit by BEM accredited checker or IEM/ACEMaccredited experts. Mitigation is cheaper & better than cureMitigation is cheaper & better than cure.

e aware un erstan t e common pro emat c s tes, nc u ng so t groun ,filled ground, ex-mining site & sloping ground, etc. How problems can arise &effective methods of mitigations can be best learnt from reported case histories.The mechanism & behaviour of ground or soil movement for building foundationsin problematic sites shall be adequately investigated & assessed. Methods ofprediction of ground behaviour are very empirical. Heavily based on engineering engineeringprinciples and SI results plus experiences or reported case histories. Tests &principles and SI results plus experiences or reported case histories. Tests &

instrumentation to verify critical design assumptions & performance are veryinstrumentation to verify critical design assumptions & performance are verynecessary. Scope of SI required to procure the relevant properties & info for thenecessary. Scope of SI required to procure the relevant properties & info for theanalysis & assessments? Adequate & reliable SI are critical for reliable analysis.analysis & assessments? Adequate & reliable SI are critical for reliable analysis.

What can go wrong will go wrong unless effective mitigations are in place.Effective mitigations depends on adequate & reliable SI. There is no right way todo wrong things.


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shallow foundation-part2

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