Is 13063 -1991 Structural Safety o Fbuildings on Shallow Foundations on Rocks - Code of Practice

8/10/2019 Is 13063 -1991 Structural Safety o Fbuildings on Shallow Foundations on Rocks - Code of Practice

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I ndi an St andard

STRUCTURALSAFETYOFBUILDINGSONSHALLOWFOUNDATIONSONROCKS-

CODEOFPRACTICE

-hfuy 1991

UDC 6241515 : 624121 : 00676

0 BIS 1991

BURE U OF INDI N ST ND RDS

MANAK BHAVAN 9 BAHADUR SHAH ZAFAR MARGNEW DELHI 110002

Price Group 9

( Reaffirmed 1996 )


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Rock Mechanics Sectional Committee, CED 48

.

FOREWORD

This lrdian Standard was adopted by the Bureau of Indian Standards, after the draft finalized bythe Rock Mechanics Sectional Committee had been approved by the Civil Engineering Division

Council.

Shallow foundations cover such type of foundations in which the load transfer is directly on thebearmg strata and whose width is greater than its depth.3 m from natural ground level.

Depth of foundation is normally up to

Since most unweathered intact rocks are stronger and less compressible than concrete, thedetermination of allowable bearing pressure on such rocks may be unnecessary. However, Intactrock masses without weathered zones, joints or other discontinuities are encountered rarely innature. The existence or location of specific discontinuities of foundation rocks often remainunknown until the rock mass is exposed by excavation. Thus, design engineers should be aware

of dangers associated with the hetrogeneity, anisotropy and unfavourable rock conditions. Thisis because overstressing a rock foundation may result in large differential settlements tilts orperhaps sudden failure.

In the formulation of this standard due weightage has been given to the standards and practicesprevailing in different countries and that in our country.

For the purpose of deciding whether a particular requirement of this standard is complied with, thefinal value, observed or calculated, expressing the result of a test or analysis, shall be rounded off inaccordance with IS 2 : 1960 Rules for rounding off numerical values ( revised). The number ofsignificant places retained in the rounded off value should be the same as that of the specified valuein this standard.


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I ndian St andard

IS 13063:1991

STRUCTURALSAFETYOFBUILDINGSONSHALLOWFOUNDATIONSONROCKS-

CODEOFPRACTICE1 SCOPE

1.1 his standard lays down the generalrequirements for stuctural safety of buildingshaving shallow foundations on rock mass.

1.2 The provisions of this code does not coverthe special requirements/conditions for founda-tions under tensile loads.

2 REFERENCES

The Indian Standards listed in Annex A arenecessary adjuncts to this standard.

3 TERMINOLOGY

For the purpose of this standard, the definitionsgiven in IS 2809 : 1972 and IS 1904 : 1986 shallapply.

4 SITE INVESTIGATIONS AND OTHERGENERAL CONDITIONS

4.1 Objective of Site Investigation

The objective of any site investigation shall befor the determination of the followings:

a) The character of ground, identification ofrock types together with their discontinui-ties which tend to influence the behaviourof rock mass [ see IS 11315 ( Parts 1 to11 )I;

b) Character of ground water, measurementof piezometric pressures and location ofthe more permissible zones.

c) Engineering properties of the rocks asdetermined by laboratory and field testing.

4.2 Existing Information

In areas which have already been developed,advantage should be taken of existing localknowledge, records of trial pits and bore holes,etc, in the vicinity and behaviour of existingstructures, particularly those of a nature similarto that of the proposed structures. In suchcases, exploration may be limited to checkingthat the existing ground conditions are similaras in neighbourhood.

4.3 Site Reconnaissance

4.3.1 If the existing information is not suffi-cient or is inconclusive, the site should beexplored in detail so as to obtain a knowledgethe of type, uniformity, consistence, thickness,

1

sequence and dip of the strata and of the groundwater conditions.

4.3.2 The site reconnaissance would help indeciding the future programme of field investiga-tions, that is, to assess the need for preliminaryor detailed investigations. This would also helpin determining the scope of work, method ofexploration to be adopted, field tests to be con-ducted and administrative arrangement requiredfor the investigation. Where detailed publishedinformation on the geotechnical condition is notavailable, an inspection of site and study oftopographical features are helpful in gettinginformation about soil, rock and ground waterconditions. Site reconnaissance includes a studyof local topography, excavations, revines, quar-ries, escarpments, evidence of erosion or landslides, beaviour of existing structures at or nearthe site, water marks, natural vegetation, drain-age pattern, location of seeps, springs andswamps. Information on some of these may beobtained from topographical maps, geologicalmaps, pedological and soil survey maps andaerial photographs.

4.4 Preliminary Exploration

The scope of preliminary exploration is restrictedto the determination of depth, thickness, extentand composition of overlying soil strata, locationof rock and ground water and also to obtainapproximate information regarding strength andcompressibility of various strata. During pre-liminary investigation, geophysical methods areuseful guides.

4.4.1 Geophysical Inv esti gations

Geophysical surveys make use of difference in,physical properties like elastic modulus, electri-cal conductivity, density and magnetic SUS-ceptibility of geological formations in the areato investigate the subsurface. Out of the fourmethods of geophysical surveys, namely,seismic, electrical, magnetic and gravity surveys,only seismic refractive surveys are widely usedin geotechnical investigations.

4.4.1.1 Seismic Survey

The seismic survey method makes use of thevariation of elastic properties of the stratawhich affect the velocity of the shock wavestravelling through them, thus providing theuseful information about the subsurface forma-tions. This method has the outstanding advant-ages of being relatively cheap and rapid to apply


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Is 13063:1991

and influencing large volume of rock-mass. Thefollowing information in respect of the rockmass is obtained from these tests:

a

b

Location gnd configuration of bed rockand geological structures in the sub-surface;The efiect of discontinuities in rock mass

can be estimated by comparing the i n situcompressional wave velocity with labo-ratory sonic velocity of intect core obtain-ed from the same rock mass.

RQD percent 1 Velocity Ratio= ( VF/VL ) x 100

where

VF = compressional wave velocityin rock mass, and

VL = compressional wave velocity inintact rock core.

4.4.1.2 For seismic method, Appendix B toIS 1892 : 1979 may be referred.

4.5 Detailed Exploration

Detailed investigations follow preliminary in-vestigation and should be planned on the basisof data obtained during reconnaissance andpreliminary investigations. Detailed investigationincludes the following:

a) Open pit trials,

b) Exploratory drilling,

c) Sampling,d) Laboratory testing, and

e) Field testing.

4.5.1 Open Pit Trial s

The method of exploring by open pit consists ofexcavating trial pits at site and thereby exposingthe rock surface for visual inspection, geologicalmapping, in s i tu field testing and for obtainingrock samples for laboratory testing. Thismethod is generally used for small depthsfissures and zones of weak rocks which wouldbreakup in core barrel.

4.5.2 Exploratory Dri l l ing

The purpose of a drilling investigation is to:

ab

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4

Confirm the geological interpretations,

Examine the cores and bore holes todetermine the quality and characteristicsof the rock mass,

Study the ground water condition, and

Provide cores for laboratory testing andpetrographic analysis.

4.5.2.1 Various methods of exploratory drillinghave been described in IS 1892 : 1979. Follow-ing methods are recommended for exploration

in rock mass:

a Percussion drillingThis method is more suitable in boulder-ous and gravelly strata. As percussiondrilling does not provide us even therepresentative samples, they are notsuitable for careful investigation.

b Core-dril l ingAn object of the drilling is to obtain rockcores for interpretation and testing. It isnecessary to obtain cent percent corerecovery as far as possible. To ensure asuccessful drilling operation, the follow-ing aspects should be remembered:

9 The drilling facilities permitting coresof at least NX size ( 54 mm diameter )and featuring split double tube corebarrels to minimize drilling vibrations.

lt should also include for performingwater pressure tests.

ii)

iii)

iv)

v)

vi)

vii)

4

\

Examination of bore hole walls by borehole cameras should also be considered.

Cores should be photographed as soonas possible and should be carefullymarked, placed in protective wrappingsin the core boxes and properly stored.A systematic method should be usedfor geotechnical logging of cores whichshould include the history of drilling,for example, type of bit, rate of drillingand water test data, core recovery,RQD, weathering and description ofrock.

Each bore hole shall contain one piezometer set at the bottom of thebore hole so that once installed piezo-meters can be observed over an extend-ed period.

If ground water contain harmful che-micals, its sulphate content and pHvalue shall be measured by chemicalanalysis of ground water.

Shot dril l i ng

The system is used on large diameterholes, that is, over 150 mm. Due tonecessity of maintaining the shots inadequate contact with the cutting bit andthe formation, holes inclined over 5 to6 can not be drilled satisfactorily. Thesystem is different from other types ofcore drillings because the coarser cuttingsdo not return to the surface but are accu-mulated in a chip cup immediately abovethe bit and here the chilled shot is used asan abrasive in place of the drill head.

4.6 Sampling

4.6.1 Rock samples commonly used in labora-tory testing are lumped samples, block samples

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and core samples. IS 9179 : 1979 should bereferred to for methods of sample preparation.

4.6.2 Rock lumps are obtained from exposedrock formations or pieces of block samples.However, core Samples may be used as lumpedsamples.

4.6.3 Block Samples

Such samples taken away from the rock forma-tion shall be dressed to a size convenient forpacking to about 100 mm x 75 mm x 75 mm.Many times a large blocks of size 300 mmX 300 mm X 300 mm are ob tamed from siteand specimens of required sizes are prepared inthe laboratory.

4.6.4 Core Samples

Core obtained from core drilling or shot drillingprocess are used as core samples.

4.7 Laboratory Testing

The strength of the rock material may beestimated by means of laboratory tests on intactrock specimens. Most frequently used labo-ratory tests are described in 4.7.1 to 4.7.6.

4.7.1 Poi nt Load St rengt h I ndex Test

This test is very fast and cheap for estimatingthe point load strength index ( Is ). The testmay be conducted on core pieces of length morethan 15 times the diameter. Provisions ofIS 8764 : 1978 shall be complied with for thistest.

4.7.2 Poi nt Load Lum ped St rength I ndex Test

This test is conducted on lump pieces of rockmaterial. The depth of the specimen ( D )between the points should be less than the widthof the specimen but should be more than onethird width of the specimen. Point load lumpstrength index ( IL ) is calculated by the follow-ing formula:

PIt = DW )075 x 2/570

whereIL = point load lump strength index, in

kg/cm2;P = the peak load, in kg, at failure; and

DW)= the cross-sectional area of fracturedsurface, in cmz;

Uniaxial compressive strength qc is related topoint load lump strength index by:

qc 5 15 x IL

4.7.3 U ni axi al Compressiv e St rength Test

This test is conducted on the core samples orblock samples having length about 20 and 30times the diameter/breadth of the sample on the

IS 13063 : 1991

universal compression testing machine. Forcarrying out this test and analysis of the test dataprovisions of IS 9143 : 1979 shall b: compliedwith.

4.7.4 Brazilian Test

It is an indirect method of estimating the tensilestrength of rock material. In this, a cylindricalrock specimen lying on its sides is loadeddiametrically with compression load so as tobring about a uniformly distributed tensile stressover the vertical, central and diametrical plane.This test should be conducted according to theprovisions of IS 10082 : 1981.

4.7.5 D i rect Shear St rength Tests

These tests may be conducted without normalstress or with normal stress on shear planes.

These tests are conducted for shearing throughintact rock specimen as well as along the weakplanes, for example, joints surfaces, beddingplanes, etc.

4.7.6 UI t ra Sonic Int erferomet ry Tests

These tests are conducted on core sample tomeasure the time of travel of p-wave in rockmaterial and then the p-wave velocity and elasticmodulus of rock material is calculated. It is aquick and cheap test for determining the elasticmodulus of intact rock material.

4.8 In Sit u Tests

Rock strength tests are performed because rockmass may contain various discontinuities andplane of slippages which reduces its strength toa fraction. Following in s i tu tests on Rockmasses are recommended.

4.8.1 Plate Load Test

Plate load test shall be performed on thefoundation rocks according to IS 1888 : 1981and a curve between pressure on plate versus its

settlement is obtained. Using pressure-settle-ment curve the allowable bearing capacitycorresponding to the permissible maximumsettlement of foundation can be evaluated asper 8 of IS 12070 : 1987. Tbis curve is also usedto calculate the settlement of the footing for theactual bearing pressure on them.

NOTE - Horizontal plate load test may be coo-ducted where vertical plate load test is not feasible.

4.8.2 Bl ock Shear Test

Block shear test shall be conducted on thefoundation rocks in s i tu as per IS 7746 : 1975to evaluate the shear characteristics of the rockmass. These tests shall be carried out along thesurface parallel to the base of the footing. Incase of laminated/jointed rock mass, these testsshall also be conducted along the laminationplanes or joint planes to assess the shearingcharacteristics along the planes. Concrete block

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IS 13063 : 1991

shear test shall also be carried out on the in sit ufoundation rocks to assess the shear characteris-tics of joint between the rock mass and theconcrete.

.

4.8.3 Pr essure M et er Test

Pressure meter test allows for a direct deter-mination of the strength of rock mass including

discontinuities and weathering for design offoundation on weak rocks.

4.8.4 Uniaxial Jacking Test as per IS 7317 : 1974may be conducted on the two opposite walls ofa drift or gallery to determine the modulus ofdeformation of rock mass.

4.8.5 Seismic refractive surveys are conductedto delineate the rock profile and to measure thevelocity of p-waves in rock mass.

4.9 Choice of Method

4.9.1 The choice of method of exploration andthe tests depends upon the nature of ground,topography, importance and size of buildingand cost.

4.9.2 In case of lightly loaded foundation ( wallfootings up to three storey actual bearingpressure on foundation up to 20 t per squaremetre ) may be less than safe allowable pressurefor weakest rock and size of foundation in suchcases would be decided by other considerations.

In such cases, it would be adequate to ensurethat any type of rock other than boulder existsat foundation level.

4.9.3 Where rock is exposed over large area ofbuilding prior to construction or will be exposedafter excavation, visual inspection of the rocksurface provides a very reliable estimate of thetype and quality of rock and the discontinuitiesof rock mass can be mapped from the exposedsurface. The number of drill holes and tests tobe conducted will depend upon the nature andtype of rocks exposed and the size and import-

ance of the building.

4.9.3.1 Number of dri ll holes

a) In massive crystalline rocks, drilling maybe omitted altogether, except in case ofvery large/important buildings in whichcase one hole for every 1 000 m2 of areawith a minimum of 3 holes shall bedrilled.

b) In moderately jointed and foliated rocks,one drill hole for every 300-500 ma ofarea shall be drilled. A minimum of 5drill holes shall be drilled in case offoundations for very large, heavy andimportant buildings.

c) In heavily jointed, sheared and weatheredrock mass, one drill hole for every 200-300 m2 area shall be drilled.

4.9.3.2 D ept h of hol es:

a The depth of drill hole below the foun-dation level should be equal to half ofeffective width of the foundation systemin case of massive and sound rocks.

b

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e

In moderately jointed and foliated rocks,the depth of drill holes below the founda-tion level should be equal to the effectivewidth of the foundation system.

In heavily jointed and weathered rocksthe depth of drill holes below the founda-tion level should be equal to twice theeffective width of the foundation system.

Drill holes shall extend inside the rockmass to a minimum depth of 3 m.

In the case of solid rock below jointedrock the drill hole shall extend below thefoundations as per (b) and (c) above asappropriate or half the effective widthinto solid rock whichever is shorter.

4.9.4 Where the rock is not exposed on thesurface, one hole shall be drilled for every 300-500 m2 of the area to know about the subsurfaceformations. If the depth of rock is irregularlyvarying with the adjacent holes, more holes shallbe drilled to fairly estimate the rock profile.The criteria for depth of hole shall be sameas 4.9.3.2.

4.9.5 If the foundation rock happens to be limestone and dolomite associated with ground water

flow, there is a likely chance of solution cavitiesunderneath. In such cases, drill holes shall bedrilled up to depth of at least one and half timethe effective width of the foundation below thefoundation level.

4.9.6 If the foundation rock happens to beconsolidated sand rock; silt stone or clay shalewith likely chance of getting saturated, necessaryexploration and testing shall be conductedassuming it to be the soil.

4.9.1 Sel ecti on of Tests

4.9.7.1 For foundations on massive rocks, sub-jected to mainly vertical loads, only the testsfor point load strength index or uniaxial com-pressive strength may be required.

4.9.7.2 For foundations on massive rocks sub-jected to heavy horizontal loads, concrete blockshear tests and in sit u shear test shall be conduc-ted to check stability against sliding.

4.9.7.3 For foundations on moderately jointedrocks, the tests for points load strength index

or uniaxial compression strength and insitu

shear test shall be carried out.

4.9.7.4 For foundations on very weak andweathered rocks, plate load test or pressuremeter test and in sit u sheer test shall be carriedout.

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4.10 Presentation of Geological Data

4.10.1 In summary, the following geotechnicaland geological *informations are consideredimportant in the stability of the building foun-dations on rock mass:

ab

d

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efg

h

Rock type and origin;Orientation of discontinuities in the rockmass;

Spacing of discontinuities in the rockmass;Condition of discontinuities includingroughness separation, continuity, weather-ing and infilling ( gauge );Ground water conditions;

Major faults;

Properties of the rock material that isuniaxial compressive strength, point loadlump strength index; andDrill Core Quality ( RQD ).

For evaluation and presentation of the abovedata, IS 11315 ( Parts 1 to 11 ) shall be referred.

4.10.2 Communication between the engineeringgeologist and the design engineer will be greatlyenhanced, if the data is presented in standardformat. The followings are recommended:

a) Bore hole should be presented in wellexecuted geotechnical logs.

b) Mapping data should be presented asspherical projection or surface pro-jections.

c) Longitudinal and cross-sections of struc-tural geology at the site should form anintegral part of a geological report.

d) For every important structures considera-tion should be given to constructing ageological model of the site.

e) A summary of all the geotechnical andgeological data including the ground waterconditions should be entered in the inputdata sheet for rock mass classificationpurpose.

4.11 General Stability of Area

4.11.1 When slopes develop instability, there islittle that can be done to protect an existingbuilding in the affected areas. Area subject toland slips, mass movements and unstable slopes,shall, therefore, be avoided.4.11.2 Ifthere happen to be an cld slip zonenear the site of the buildings, detailed stabilityanalysis of the area shall be carried out andworst probable slip zone shall be demarcated.No buildings shall be constructed in the areawhich is 200 m or less away from the boundaryof the maximum probable slip zone.

4.11.3 If the area happen to be hilly terrain,before planning and approving a building com-

IS 13063 : 1991

the slopes shall be carried out and maximumamount of the load that can be placed by wayof buildings construction and their occupants onparticular area shall be worked out and notified.Regulations shall be promulgated to effectivelycontrol construction in sensitive areas.

4.11.4 While carrying out the stability analysisof the zone where building complex is proposed,change in the subsurface water conditions dueto development of township shall be accountedfor.

4.11.5 Mass movements of the ground are liableto occur from causes independent of the pre-sence of the building. These include excavation,saturation, mining subsidence, land slips onunstable slopes and creep on slopes. Thesefactors shall be taken into account in the designand expert advice shall be sought, whereverrequired.

4.11.6 On uphill side of a building on a slopingsite, drainage requires special considerations.The natural flow of water shall be diverted awayfrom the foundation.

4.11.7 No trees which grow to a large sizeshall be planted within 8 m of the foundationsof buildings.

5 GENERAL CONSIDERATIONS FORDESIGN SAFETY )

5.1 Loads on Foundation

5.1.1 Foundations shall be proportioned for thefollowing combination of loads:

a) Dead load, live load, earth pressure,water pressures/snow load in areas whereit is encountered.

b) Dead load, live load, earth pressure,water pressures/snow load where it isencountered with wind or seismic load.

5.1.2 Dead load also includes the mass ofcolumn/wall, footings, foundations, the overly-ing fill including frost, if encountered, ignoringthe mass of soil displaced by foundation system.

NOTE - There are a number of states in Indiawhere snow will be encountered during winter orperenially over underlying bed rock on which thefoundation system is intended to be constructed.

5.1.3 Live loads from the floors above, asspecified in IS 875 : 1964 shall be taken intoaccount.

5.2 Allowable Bearing Pressures

5.2.1 Pressures coming on the rock due tobuilding foundation shall not be more than thesafe bearing capacity of rock-foundation systemtaking into account the effect of eccentricity.The effect of interference of different founda-


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Is 13063 1991

5.2.2 The total settlement of the foundation(s)shall rot be more than permissible ( recom-mended/allowable/tolerable ) settlement ( Sper )

value that is

where

s=

S < Sper

Calculated maximum average settle-ment due to imposed load under thefooting; and

Sper = Permissible value of total settlement.

5.2.3 Differential settlement ( AS ) and/or tiltof the building ( T ) or/and part of a building

all be not more than the recommended valuesASper and Tper respectively.

AS < A Sper

and T < Tper

whereAS = Calculated maximum value of

differential settlement;

ASper = Permissible value of differentialsettlement;

T = Estimated value of maximumTilt; and

Tper = Permissible value of Tilt.

5.2.4 The recommended values of permissiblesettlements, differential settlements and angulardistortion ( tilt) are tabulated in Table 1. Highervalues of maximum settlement can be adoptedin case of highly weathered and disintegratedrock masses.

5.2.5 For satisfying the conditions of 5.2.1,5.2.2 and 5.2.3, load combinations as describedin 5.1.1 shall be considered.

5.2.6 For satisfying the conditions laid downin 5.2.1 and 5.2.2, safe bearing pressures shallbe estimated in accordance with IS 12070 : 1987.

When wind loads or seismic loads are consider-ed, the safe bearing pressure may be increasedby 25 percent and 33 percent respectively. Thesafe bearing pressure shall not exceed the allow-able pressures for the grade of concretelmaso-nry of foundation slab or grade of concretelaid over the rock surface, whichever is lower.

5.2.7 The allowable bearing pressure should,then, be determined by the following steps:

a) Proportion footing making use of the safebearing pressures value determined as per5.2.5;

b) Calculate differential settlements offootings;

c) Calculate angular distortions;d) Compare the above values with those

given in Table 1, and

e) If the comparison is not satisfactory,revise the allowable bearing pressure andrepeat the steps (b), (c) and (d) until thecomparison is satisfactory.

5.2.0 auses of Settlements

For safety of building foundations, the engineer-in-charge should be well familiar with all causesof settlements. Foundations on rocks may settledue to combination of the following reasons:

a) Elastic compression of the foundationand the underlying bearing strata;

b) Ground movement on slopes due toerosion, creep or land slides;

c) Increase in ground water may soften thejoint fill material, causing slippage alongthe joints;

d) Other causes such as adjacent excavation,mining, subsidence and undergrounderosion; and

e) Rock like schist, soft shale, etc, weatherrather quickly and may cause post con-struction settlements within its life time.

Table 1 Maximum and Differential Settlements of Buildings on Rock Mass( lause 5.2.4 )

lz.Type of Structure Maximum Differential Settlement Angular Distortion

Settlement Isolated Raft Isolated RaftFooting Foundation Footing Foundation

mm mm mm(1) (2) 3) 4) 5) 6) 7)

i) For steel structure 12 003 3L :003 3L l/300 l/300ii) For reinforced concrete structures 12 -001 5L *OOZL l/666 l/500

iii) For plain bricks block walls inmultistoreyed buildingsa) For Lx/Ha < 3 -000 25L - l/400 -

For Ll/Ha z 3 :1 *OOo33L - l/300

iv) For water towers and silos 12 - 0.002 SL - l/400NOTE - The values given in the table may be taken only as a guide and the permissible settlement anddifferential settlement in each ease should be decided as per requiremets of the designer depending uponimportance of structure.

L - denotes the length of deflected part of wall/raft or centre to centre distance between columns.2H - denotes the height of wall from foundation footing.

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5.2.9 Causes of Di fferential Settl ements

Some of the causes of differential settlembnt areas follow:

a) Non-uniform pressure distribution fromfoundation to rock mass;

b) Overlap of stress distribution in rock mass

due to loads of adjacent structures;c) Geological and physical non-uniformity of

rock mass under the foundation;

d) Slippage along the weaker planes of rockmass;

e) Water table fluctuations at constructionsite;

f) Non-uniform structural disruptions ordisturbances of foundation rocks dueto freezing and thawing;

g) Movement of the rock mass due to ins-tability of general slopes;

h) In soluble rocks, solution cavities maybecome larger with time and may lead todifferential settlements; and

j) Seepage of water in foundation of a partof a building.

5.2.10 The foundations shall normally be SOproportioned that no tension is created at thefoundation plane.

5.2.11 If tension is created under the founda-tion, the maximum tensile stress shall be lessthan three fourth of the 7 day modulus ofrupture of foundation concrete or of concretelaid between the foundation and rock surfacewhichever is less. For this purpose, tensilestrength of rock or cable anchors, if any, shallnot be considered.

5.2.12 If tension is created under the base, themaximum bearing pressure shall be calculatedon the modified base area which remain incompression only.

5.3 Stability Against Overturning and Sliding

Stability of foundation against overturning andsliding shall be checked and factors of safetyshall conform to the requirements specifiedin 5.3.1 and 5.3.2.

5.3.1 Sliding

5.3.1.1 The factor of safety against sliding ofstructures shall not be less than 15 when deadloads, live loads, earth pressures, water pressures

and uplift pressures are considered together withwind load or seismic force. When only deadload, live load, earth pressure, water pressureand uplift are considered; the factor of safetyagainst sliding shall be not less than 175.

5.3.1.2 For buildings founded on sloping rocksurface, the factor of safety against sliding shall

IS 13063 : l Bf

be calculated along the rock surface. Thefactor of safety against sliding may be improvedby anchoring the foundation to the deeperstrata of rock.

5.3.1.3 The adhesion and th: coefficient offriction between the concrete and the rock massunder dry and submerged conditions and also

of foundation rocks along weak shear zones andbedding planes shall be determined by in situblock shear tests at site. For preliminary designthe value given in Table 2 may be used.

Table 2 Values of Adhesion and Cosfacientof Friction

Clause 5.3.1.3 )

Sl Material Adhesion Coefficient ofNo. kg/cm2 Friction

i) Massive and sound rock 5-10 0.80-l-Oii) Fractured, jointed rock l-3 0.80

5.3.2 Overturning

The factor of safety against overturning shall b=not less than 15 when dead loads, live loads,earth pressures and unlift are considered to-gether with wind load or seismic forces. Whenonly dead load, live load, earth pressures anduplift pressures are considered, the factor ofsafety shall be not less than 2.

5.4 Depth of Foundation

5.4.1 The minimum depth of foundation b:lowthe natural ground surface shall be 05 m.

5.4.2 In case of massive intact and unweatheredrocks, the foundation can be laid over the rocksurface after chipping off the top surface forpreparation of a proper seat for the foundation.

5.4.3 In case of jointed, sheared and partiallyweathered rocks, the base of the foundationshall be kept at least 50 cm inside the rocksurface, so that the upper portion of highlyweathered rock miss is avoided. This wouldalso take care of the error in judgem:nt aboutthe depth of rock surface.

5.4.4 In case of rock of very low strength, highlyweathered rocks ( for example: clay shales, sandrock and soft silt stone in SHlVALZKS ), thedepth of foundation shall be decided in accor-dance with provisions of IS 1904 : 1986, on-sidering the foundation material to be as soil.

5.4.5 A foundation on any type of rock massshall be below the zone significsotly weakenedby root holes or cavities produced by barrowing

animals or worms. The depth shall also beenough to prevent the rain water scouring belowthe footing.

5.4.6 Where there is excavation, ditch or slop-ing ground adjacent to the foundation which islikely to impair the stability of a building, eitherthe foundation of such building shall be carried

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IS 13063:1991

6.5 Foundations Adjacent to Sloping Ground

Where a foundation is to be constructed adjacentto the sloping ground, the following conditionsshall be satisfied.

6.5.1 The frust;m of bearing rock under the -~0 transferred from higher foundation and theconstruction of the lower foundation shall bedone prior to construction of higher foundation.

NOTE-In hills, there is bad practice to cons-truct the walls of a building directly over the

6.7 Detailing of Reinforcement

retaining walls which are constructed in drv

6.7.1 The requirement of IS 456 : 1978 shall be

RandomRubble Masonry. Since the deformability

complied with.

of dry Random Rubble Masonry is very high incomparison to that of the wall masonry as well asthe foundation rock mass, they normally give wayleading to cracking of the houses and sometimesresults in total collapse of the building. Suchpractice should be discontinued.

3BFIG. 3 FOUNDATION ADJACENT TO

SLOPING GROUND

6.6 Foundation at Different Levels

6.7.2 In case of strip footings, the main rein-forcement is provided along the width offootings. Some longitudinal reinforcement inthe footing is desirable to assist in bridging oversoft spots associated with heterogeneous condi-tions in rock masses. The longitudinal rein-forcement in strip footing shall be not less than:

a) 015 percent ( in case of mild steel ) or

012 percent ( when high steel deformedbars or welded wire fabric are used ) ofthe total cross-sectional area on each face( top and bottom ) of the RCC footing.

b) 25 percent of the main reinforcement.

7 TREATMENT OF ROCK DEFECTSWhere foundations of the adjacent buildings areto be located at different levels, the following 7.1 Open Vertical Jointsconditions shall be satisfied ( see Fig. 4 ).

7.1.1 More or less vertical joints from one to6.6.1 The minimum horizontal distance between several centimetres are sometimes encountered

the adjacent foundations shall be such that the even in unweathered rocks. These joints mayloads from foundation at higher level are not either be open or clay filled. Such joints shalltransferred to the other foundation at lower level be cleaned out to a depth of four to five timesfor which the lower foundation shall be outside their width und filled with cement grout, athe bearing frustum ( making an angle of 60 mixture of one part of cement and one part ofwith horizontal ) and also the weak planes sand with enough water to permit pouring of

9


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IS13063r1991

grout into the joints ( see Fig. SA ). Largespaces, wider at top are commonly filled with

thsn 20 percent of the total area, the joint

dental concrete.be washed and cleaned for depth equal to3 times the width of the joint and shall be

7.1.2 If the foundation rocks comprises of up with the ( I:1 ) cement sand groutnearly vertical joints so wide that they constitute Fig. 6 ).. . . ,. .

shall2 tofilled

see

an appreciable rraction of area ( more than20 percent ), the excavation is usually deepened 8.1.2 If the area of weak seams under the

until thejoints

are no longer within the base orthey narrow down to acceptable limits ( within foundation is more than 20 percent of the total20 percent of base area j.

base area, treatment as per 8.2 hall be donesee Fig. 7 ).

OENTAL CONCRETE

L-1: 1 CEMENT-SAND

SLURRYFIG 5A STEEPLY DIPPING OPEN JOINTS

7.2 Open Horizontal Joints 8.2 Fooodation Rocks of DifferentComoressibilities

Many rocks contain nearly horizontal joints Lwhich gets opened up due to relief in vertical When foundation spans rock material of diffe-stress. If such open joints are located beneath rent compressibility, the following conditions

the foundations as shown in ( Fig. 5B), the rock should be complied with.mass above the open joints shall be removed, if 8.2.1 The design of the foundation shall beeconomically feasible; otherwise, the open space carried out for bearing pressures inversely pro-of the joints shall be filled with cement sand portional to the compressibility of the strata.grout. The foundation section shall be designed for

I I I I I

FIG 5B OPEN HORIZONTAL JOINTS

7.3 Solotion Cavities

Solution cavities require detailed attention. Inbedded lime stone, cavities are more likely tooccur in some horizons than in others. Explora-tion shall be carried out to locate the horizonwhere unfavourable conditions exist. If thearea of cavities is less than 20 percent of thebase area of foundation, the safe bearing pressureshall be reduced as prescribed. in IS 12070 :1987.

8 SPECIAL TREATMENTS

8.1 Weak Seams Under the Foundation

8.1.1 When inclined seams filled with softmaterial are located on the excavated rocksurface under the foundation base covering less

10

FIG 6 WEAK SEAMS UNDER FOUNDATION LESSTHAN 20 PERCENT OF AREA

one and half times the moments and shearforces so calculated. Alternatively, the designof foundation can be carried out considering nobearing on weaker strata.


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IS 13063 1991

.

WE K SEAM

FIG. 7 WEAK SEAMS UNDER FOUNDATION MORE THAN 20 PERCENTOF BASE ARIIA

8.2.2 The maximum bearing pressure under thefoundation shall also not exceed the allowablebearing pressure for the weaker strata.

8.2.3 The minimum reinforcement spanning thethe weaker strata shall be not less than05 percent of the cross section of foundation.

c)

8.3 Foundation on Steeply Dipping Rock Surface

The foundations on steeply sloping ( more than15 with horizontal ) rock surface shall normallybe avoided. If it is obligatory to do so, thefollowing safety measures shall be adopted intheir design:

a) The foundation shall be embedded to acertain depth (d) inside the rock such thatthe prescribed factor of safety in slidingalong the foundation plane are obtained.In these calculation, the support provid-ed by the rock on down slope face offooting is accounted for ( see Fig. 8 ).Shear strength of rock/cable anchors,even if provided, is not considered inthese calculations;

ANCHORS

d) Foundation level benches may be pro-vided if possible at site. In such casesalso the sliding stability shall be checkedas if no benching was there; and

e) Wherever provisions of anchors is

necessitated for increasing the stability ofthe structure, caution may be exercised toensure that the sufficient length of anchorsare embedded in the massive and freshrock to mobilies rock action, and anchorsextend well beyond slumpmass and com-putations for the ma of slumpmass bealso taken into consideration.

8.4 Foundation on Moderate Slopes

At locations, where the Frock surface is gentlydipping ( less than 15 with horizontal ), therock footing shall be taken deep enough ( notless than 30 cm > inside the rock such that

desired factor of safety in sliding along the rocksurface is achieved having considered the bear-ing resistance on down slope face. It shouldalso be checked that even without the bearingsupport on down slope face of foundation, thefactor of safety against sliding in worst loadingcondition is not less than 110 ( see Fig. 9 ).

size and number and depth of the anchorsshall be worked out considering therequired shearing strength of anchors,foundation size and the foundation rock;

Depth (d) shall not be less than 60 cm inany case;

d360cm

FIG. 8 FOUNDATION ON STEEPLYDIPPING SLOPES

b) The stability of foundation withoutbearing resistance on the down slope faceshall also be checked. To ensure thedesired factor of safety against sliding,rock/cable anchors may be provided. The FIG. 9 FOUNDATIONS ON MODERATE SLOPES

11


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IS 13063 1991

8.5 oundation on a Bench

If a foundation is provided over a bench whosebedding planes, or weaker planes are steeplydipping and getting exposed on the slope face,the stability of the rock mass alongwith thesuperimposed loads by the foundation, shall be

checked along the deepest exposed weak planeand it shall be ensured that:

a) Desired factor of safeties are achieved.If not, rock anchors may be provided toachieve it ( see Fig. 10 ).

1

shall be removed up to a distance and filled upwith concrete so that the soft material is notencountered up to the thickness equal to halfthe width or twice the thickness of the footingwhichever is larger.

8.6.2 If removal of soft material, as requiredin 8.6.1, loosen or dislocate the top rock massor the thickness of top hard rock is less than thethickness of footing, the top rock shall beremoved and the foundation shall be laid on thelower hard strata ( see Fig. 11B ). In suchcase, the side support from top hard strata shallbe ignored in calculations.

Y-ROCK/CABLE ANCHORS

FIN. 10 FOUNDATION OF STEEPLY DIPPINGSTRATA ON A BENCH

b) If rock anchors are required to make thewedge stable, factor of safety againstsliding shall also be checked ignoring theshearing strength of rock anchors and itshall be not less than 110 in worstprobable condition.

8.6 Foundation on Rock Shelf

8.6.1 If a foundation is so located that the bear-ing strata is underlain by soft material and theminimum depth of the hard rock under the baseof footing is more than the thickness of footing( see Fig. 11A ). In such cases, the soft material

_-L__Ax 3 B/2

xz, 20

FIG. 11A FOUNDATION ON STEEPLYDIPPING CLAY SEAM

A**FIG. 11B FOUNDATION ON MODERATELY

DIPPING CLEAR SEAM

8.7 Undulating Rock Surface

If the rock surface profile is highly undulatingdue to solution cavities or any other reason( see Fig. 12 > the loose material from the un-dulations shall be removed up to the depth thatremaining surface area of loose material be notmore than 20 percent of total area and shall bebackfilled with lean concrete having allowablebearing stress more than maximum bearingpressure under the footing.

8.8 Foundation on Soft Rock Over Laid onSteeply Dipping Hard Rock

The footing shall be SO located that the minimumthickness of bearing strata over the hard rockshall be more than one third of the base widthof foundation and also more than one metre.Otherwise piles, bearing on hard strata, shallbe provided ( see Figi 13 ).

8.9 Foundation on Different Rock Masses

If a building is so located that part of it restson hard rock and remaining part on soft rock

DESIGN LEVELOF FOUNDATION LEAN CONCRETE

LINE OF 80 PERCENT SURFACEAREA OF ROCK

FIG 12 FOUNDATION ON UNDULATING ROCK SURFACE


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MINIMUM CUSHION

X>lm

. x B/3

SOIL OR VERYSOFT ROCK HARO ROCK

FIG. 13 STEEPLY DIPPING JOINT PLANE

( see Fig. 14 ), then a settlement joint shall beprovided in the foundation at such location that

IS 13063 : 1991

the minimum depth of soft rock under the baseof one footing is not less than one third of thewidth of the footing and the other footing restsolely on the hard rock.

8.10 Stepped Foondation for Moltistorey Frames

In case a multistorey building is to be construc-ted on hill slope ( see Fig. 15 ) the followingcriteria shall be adopted:

a) The stability of the rock slopes alongwiththe load on it due to the building shallbe ensured;

b) The building frame shall be so plannedthat column points are located on innerside of the rock benches;

.bc) The columns shall be well anchored into

the foundation rocks to provide completefixity at base. The minimum depth of

SOIL OR XalrnVERY SOFT ROCK

/

HARDx3 B/3

ROCK

FIG. 14 FOUNDATION ON DIFFERENT ROCK MASSES

TIE BEAM CIRCULAR OR

ROCK ANCHORS

/ MAY BE REQUIRED

ACEPLA SNAP


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IS 13063 1991

column foundation shall be at least onemetre below the excavated terrace level;

d) The beams shall be provided between the

e

f

columns at plinth level as to increase thestiffnels of the RCC frame of thebuilding;

The frame may be anchored to rock masswith the help of horizontal anchors ateach beam level, if required, to stabilizethe terraces or the frame itself; and

The building frame shall be designed forthe probable differential settlement due tonon-uniformity of foundation rock. Thecommon practice in such cases is toprovide stiff column with slender beamsso that the beam column joints behavesas hinge joints.

8.11 Raft Foundations Adjacent to Hill Slope

If a raft foundation is to be constructed adja-cent to hill slope, if possible, the building shallbe so planned that heavier part of the buildingare located on the up hill side part of the raftto provide better stability ( see Fig. 16 1.

8.12 Foundations on Slopes of Tallos Deposits

In case of buildings up to two storeys arerequired to be built on steep hill slopes dippingmore than 15, wall and column construction

HEAVY WEIGHTON HILL SIDE

7

FIG. 6 RAFTFOUNDATION ADJACENT TOHILL SLOPES

will not be possible. At such sites, flexiblestructure which can withstand small groundmovements is best suited. Still type foundationshown in Fig. 17 is one of that type. Theseconsist of wooden or steel framed structureswith fibre glass, aluminium or G.C. steel roofingand walls clad with PVC sheets. The roofs areconstructed of wooden planks. The depth offoundation in loose tallus deposit shall beat least 2 m.

FLOOR BEAMS WITH

WOODEN FLOOR

S CLADDEDPVC SHEETS

200 mm WOODEN OR

100mm STEEL PIPE

FIG. 7 STILL FOUNDATION

. 14


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IS13063:1991

ANNEX Amm 2

. LIST OF REFERRED INDIAN STANDARDS

IS No.

456 : 1978

875 : 1964

1888 : 1982

1892 : 1979

1904 : 1986

28 9 : 1972

6066 : 1984

7317 : 1974

7746 : 1975

8764 : 1978

9143 : 1979

Title

Code of practice for plainand reinforced concrete( third revisionCode of practice for designloads ( other than earth-quake ) for building andstructures: Part 2 Imposedloads ( second revision

Method of load test on soils( second revision

Code of practice for sub-surface investigation forfoundations ( first revisionCode of practice for designand construction of founda-tions in soils: general require-ments ( thtrd revision

Glossary of terms andsymbols relating to soilenginnering ( first revision

Recommendations for pres-sure grouting of rock foun-dations in river valleyprojects ( jirst revision

Code of practice for uni-axial jacking test for defor-mation moulds of rock

Code of practice for in situshear test on rockMethod of determinationof point load strength indexof rocksMethod for the determina-tion of unconfined com-pressive strength of rockmaterials

IS No.

9179 : 1979

10082 : 1981

11315

Title

Method of preparation ofrock specimen for laboratorytesting

Method of test for deter-mination of tensile strengthby indirect tests on rockspecimens

Methods for the quantitativedescription of discontinui-ties in rock mass

11315 Orientation( Part 1 > : 1987

11315( Part 2 > : Spacing1987

11315 Persistence( Part 3 > : 1987

11315( Part 4 ) :

Roughness1987

11315( Part 5 ) :

Wall strength1987

11315( Part 6 ) :

Aperture1987

11315( Part 7 ) :

Filling1987

11315( Part 8 ) : 1987

Seepage

11315 Number of sets( Part 9 ) : 1987

11315 Block size( Part 10 > : 1987

11315( Part 11 > : 1985

Core recovery and rockquality designation

12070: 1987 Code of practice for designand construction of shallowfoundation on rocks

15
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Copyright .

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Revision of Indian Standards

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i Dot : No. CED 48 ( 4463 )

Amendments Issaed Since Publication

Amend No. Date of Issue Text Affected

BUREAU OF INDIAN STANDARDS

Headquarters :

Manak Bhavan, 9 Bahadur Shah Zafar Marg, New Delhi 110002Telephones : 331 01 31, 331 13 75 Telegrams : Mahaksanstha

( Common to all Offices )

Regional Offices :

Central : Manak Bhavan, 9 Bahadur Shah Zafar MargNEW DELHI 110002

Eastern : l/14 C.I.T. Scheme VII M, V.I.P. Road, Maniktola

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Is 13063 -1991 Structural Safety o Fbuildings on Shallow Foundations on Rocks - Code of Practice

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Transcript of Is 13063 -1991 Structural Safety o Fbuildings on Shallow Foundations on Rocks - Code of Practice