NEPAL NATIONAL BUILDING CODE
NBC 108 : 1994

SITE CONSIDERATION FOR SEISMIC HAZARDS

Government of Nepal

Ministry of Physical Planning and Works
Department of Urban Development and Building Construction
Babar Mahal, Kathmandu, NEPAL

Reprinted : 2064

NBC108V2.RV97 December 1993

NEPAL NATIONAL BUILDING CODE
NBC 108 : 1994

SITE CONSIDERATION FOR SEISMIC HAZARDSThis publication represents a standard of good practice and therefore
takes the form of recommendations. Compliance with it does not confer
immunity from relevant legal requirements, including bylaws
tTsflng >L % sf] ;/sf/ -dlGqkl/ifb\_ sf] ldlt @)^).$.!@ sf] lg0f{ofg';f/ :jLs[tGovernment of Nepal

Ministry of Physical Planning and Works
Department of Urban Development and Building Construction
Babar Mahal, Kathmandu, NEPAL

Reprinted : 2064

NBC108V2.RV97 December 1993

iPrefaceThis Nepal Standard was prepared during 1993 as part of a project to prepare a National Building Codefor Nepal.
In 1988 the Ministry of Housing and Physical Planning (MHPP), conscious of the growing needs ofNepal's urban and shelter sectors, requested technical assistance from the United Nations Development
Programme and their executing agency, United Nations Centre for Human Settlements (UNCHS).
A programme of Policy and Technical Support was set up within the Ministry (UNDP ProjectNEP/88/054) and a number of activities have been undertaken within this framework.
The 1988 earthquake in Nepal, and the resulting deaths and damage to both housing and schools, again
drew attention to the need for changes and improvement in current building construction and design
methods.
Until now, Nepal has not had any regulations or documents of its own setting out either requirements orgood practice for achieving satisfactory strength in buildings.
In late 1991 the MHPP and UNCHS requested proposals for the development of such regulations anddocuments from international organisations in response to terms of reference prepared by a panel ofexperts.
This document has been prepared by the subcontractor's team working within the Department ofBuilding, the team including members of the Department and the MHPP. As part of the proposedmanagement and implementation strategy, it has been prepared so as to conform with the general
presentation requirements of the Nepal Bureau of Standards and Metrology.
The subproject has been undertaken under the aegis of an Advisory Panel to the MHPP.
The Advisory Panel consisted of :Mr. UB Malla, Joint Secretary, MHPP
Director General, Department of Building(Mr. LR Upadhyay)
Mr. AR Pant, Under Secretary, MHPP
Director General, Department of Mines & Geology(Mr. PL Shrestha)
Director General, Nepal Bureau of Standards & Metrology(Mr. PB Manandhar)
Dean, Institute of Engineering, Tribhuvan University(Dr. SB Mathe)
Project Chief, Earthquake Areas Rehabilitation &Reconstruction Project
President, Nepal Engineers Association
Law Officer, MHPP (Mr. RB Dange)
Representative, Society of Consulting Architectural &Engineering Firms (SCAEF)

NBC108V2.RV9Chairman
MemberMemberMemberMemberMemberMemberMemberMemberMember

7 December 1993

ii
Representative, Society of Nepalese Architects (SONA)
Deputy Director General, Department of Building,(Mr. JP Pradhan)

MemberMember-Secretary

The Subcontractor was BECA WORLEY INTERNATIONAL CONSULTANTS LTD. of New
Zealand in conjunction with subconsultants who included :Golder Associates Ltd., Canada
SILT Consultants P. Ltd., Nepal
TAEC Consult (P.) Ltd., Nepal
Urban Regional Research, USA
Principal inputs to this standard came from :Mr. AM Dixit, SILT
Mr. YK Parajuli, TAEC
Mr. JK Bothara, TAEC
Mr. AM Tuladhar, DoB, HMGN
Dr. RD Sharpe, BECA (Team Leader)
Revisions and Updated to this code came from :Mr. Purna P. Kadariya, DG, DUDBC
Mr. Kishore Thapa, DDG, DUDBC
Mr. Mani Ratna Tuladhar, Sr. DIV. Engineer, DUDBC
Mr. Jyoti Prasad Pradhan, Ex.DG, DOB
Mr. Bhubaneswor Lal Shrestha, Ex. DDG, DOB
Mr. Uttam Shrestha, Architect, Architects' Module Pvt. Ltd.
Mr. Manohar Lal Rajbhandhari, Sr. Structural Engineer, Mr Associates
Mr. Amrit Man Tuladhar, Civil Engineer, DUDBC

NBC108V2.RV97 December 1993

iii
TABLE OF CONTENTS

Preface ........................................................................................................................................................... i

0Foreword ........................................................................................................................................ iv

1Scope ................................................................................................................................................. 1
1.1 Applicability .......................................................................................................................... 1

2Interpretation .................................................................................................................................. 1
2.1 General................................................................................................................................... 1
2.2 Terminology .......................................................................................................................... 1

3Site Considerations ......................................................................................................................... 3
3.1 Consideration of Potential Fault Rupture Hazard .......................................................... 3
3.2 Consideration of Potential Liquefaction............................................................................ 4
3.3 Consideration of Potential Landslides and Slope Instability.......................................... 6

4

Site Investigation............................................................................................................................ 84.1
4.2
4.3
4.4

5General................................................................................................................................... 8
Extent of Site Exploration ................................................................................................... 8
Depth of Exploration............................................................................................................ 8
Allowable Bearing Pressure ................................................................................................ 9

Design of Foundations................................................................................................................... 9NBC108V2.RV97 December 1993

iv
0Foreword
This document sets out some of the factors to be considered during site selection for buildings
in order to minimise the risks to the buildings from both primary and secondary seismichazards.
It also outlines the fundamental requirements for site investigation for the foundation design ofbuildings.
The degree to which each factor should be considered will depend on the importance and sizeof the building under consideration. The document is particularly applicable for State of Art
Design and Engineered buildings.

NBC108V2.RV9

7 December 1993

11

Scope
This standard covers the principles of site selection and site investigation for buildings in boththe mountainous and Terai regions of Nepal.
Conceptual and detailed aspects of the selection and design of building and foundation types are
not dealt with by this Standard, although recommended references are given for good practicein these matters.
1.1

Applicability
In general, the provisions of this standard should be applied to all buildings to beconstructed in Nepal.
The site considerations detailed out in this Standard shall be mandatory for all importantbuildings in Nepal.

The State of Art Design and engineered buildings of all categories should carry an appropriate
level of site investigation and formal reporting at the beginning of the design process and it
shall be incorportated in the permit application documents.
Subject to the mandatory rules-of-thumb and/or advisory guidelines the designer should
considered it as an indication of good practice and apply same as appropriate.
2Interpretation
2.1

2.2

General
2.1.1

In this standard the word "shall" indicates a requirement that is to be adopted inorder to comply with the standard, while the word "should" indicatesrecommended practice.

2.1.2

Words implying the singular only also include the plural unless this isinconsistent with the context.

Terminology
In this standard, unless inconsistent with the context, the following definitions shallapply :BEARING PRESSURE, SAFE means the intensity of the loading that the soil will
carry without undergoing settlement more than permissible for the structure.
CLAY means a type of soil with more than 50 % of the constituent particles smaller
than 2 µm. It is plastic within a moderate range of water content.

NBC108V2.RV97 December 1993

2CLAY, FIRM means a clay which at its natural water content can be moulded bysubstantial pressure with the fingers and can be excavated with a spade.
CLAY, SOFT means a clay which at its natural water content can be easily mouldedwith the fingers and can be readily excavated.
CLAY, STIFF means a clay which at its natural water content can not be moulded bysubstantial pressure with the fingers and requires a pick for its removal from the parentbody.
ENGINEERED BUILDING means any building designed in accordance with theengineered structures provisions of the Nepal National Building Code.
FOUNDATION means that part of the structure which is in direct contact with the soiland transmits load to it.
FOUNDATION SOIL means the soil at the sub-surface which is in direct contact withthe foundation and bears the load due to the structure.
FROST ACTION means the weathering process caused by repeated cycles of freezing
and thawing.
GRAVEL means a cohesionless soil with more than 50 % of the constituent particles
having size greater than 4.75 mm and less than 80 mm.
HAZARD means the probability of occurrence of a destructing phenomena.
IMPORTANT BUILDINGS are those which :contain critical facilities which are essential before and immediately after adisaster (eg, hospitals, fire and police stations, communication centres etc),

by their very purpose have to house many persons at a time (eg, cinema halls,schools, convention centres, etc),

have national and international importance (eg, historical palaces, monuments,etc),are to be used for storage of toxic or explosive materials.

LANDSLIDE means the downward and outward movement of slope-forming
materials.
LIQUEFACTION means the phenomenon exhibiited when relatively loose, saturated
sandy soils lose a large proportion of their strength under seismic shaking.
ORDINARY BUILDING is one which is not an important building (eg, a residential,
general commercial or ordinary office building, etc)
NBC108V2.RV97 December 1993

3ROCK MASS means those naturally occurring mineral aggregates in-situ with all theirplanes of discontinuities.
ROCK MATERIAL means a piece of rock detached from the rock mass and boundedby natural planes of discontinuities or induced fractures.
SAND means a cohesionless soil with more than 50 % of its constituent particles having
a size greater than 75 µm and less than 4.75 mm.
SAND, COARSE means a cohesionless soil with 50 % or more of its constituent
particles having a size greater than 2 mm and less than 4.75 mm.
SAND, FINE means a cohesionless soil with 50 % or more of its constituent particles
having a size greater than 75 µm and less than 0.425 mm.
SAND, MEDIUM means a cohesionless soil with 50% or more of its constituent
particles having a size greater than 0.425 mm and less than 2.0 mm.
SILT means a fine-grained soil with 50 % or more of its constituent particles having asize greater than 0.2 µm and less than 75 µm.
SOIL means a mineral aggregate or product of rock weathering lying at the place offormation or deposited after being transported by different natural means oftransportation.
SOIL, COARSE GRAINED means a soil with more than 50 % of its materials grains
larger than 0.75 µm.
SOIL, FINE GRAINED means a soil with more than 50 % of its material grains
smaller than 0.75 µm.
TERAI means the predominant alluvial plain to the south of the Siwalik range. This is
one of the five physiographic divisions of Nepal.
WATER TABLE means the locus of points in soil water at which the pressure is equal
to atmospheric pressure.
WEATHERING means the group of processes, such as the chemical action of air andrain water and of plants and bacteria and the mechanical action of changes oftemperature, whereby rocks on exposure to the weather change in character, decay, andfinally crumble into soil.
3

Site Considerations
3.1

Consideration of Potential Fault Rupture Hazard
A surface fault rupture occurs when an earthquake fault breaks the earth's surface and it
may result in several centimetres to several metres of differential ground

NBC108V2.RV9

7 December 1993

4
displacement. This instantaneous ground displacement may occur along anapproximately linear path that may extend for several tens of kilometres. If the fault
traverses houses, buildings or infrastructures and lifelines, it may damage or destroy thefacilities.
Therefore, while selecting the site for an important building or structure, it shall beensured that the building is not located within a distance of 500 m from the surface traceof a known active fault.
Figure 3.1 depicts the principal active faults so far identified within Nepal and can bereferenced for this purpose.
Areas closer than 500 m to the trace of an active fault should be used for activities
unlikely to be severely affected by surface faulting. These include use as grassland,
forest, gardens, parks, etc.
3.2

Consideration of Potential Liquefaction
Liquefaction of subsurface soil occurs when groundwater saturated, loose, granular soil
is exposed to strong earthquake shaking for a relatively long duration. It commonly
results in sand boils, fissuring of the ground, settlement of the ground surface and lateral
spreading of the ground surface. These phenomena can cause different forms ofdamage to structures and properties lying across the soil failure areas.
The plains of the eastern Terai experienced wide-spread occurences of liquefaction
during the Bihar-Nepal earthquake of 1934 and the Udaypur earthquake of 1988.
Substantial damage resulted from this process. The most significant and spectacular of
the phenomenon was the generation of numerous sand craters and upwelling of water intube wells and dug wells.

NBC108V2.RV97 December 1993

5NBC108V2.RV97 December 1993

6The site selection for an important engineered building in the Terai and on alluvial riverbanks within the mountainous terrain shall be preceded by examination of theliquefaction susceptibility of the sub-surface ground. Necessary mitigation measures
should be taken to minimise the potential risk. Sites located on ancient lake deposits(the Kathmandu Valley, for instance), shall be similarly investigated.
3.3

Consideration of Potential Landslides and Slope Instability
Earthquake-induced landslides can occur due to the strong ground shaking caused by anearthquake. All major earthquakes in mountainous terrain result in increased instancesof landslides. The vast majority of these are rock falls, although more coherent
landslides, such as debris slides and soil slumps, also take place.
Clearly, areas that are susceptible to landslides from storm damage, river undercutting
and quarrying are also susceptible to strong ground shaking from an earthquake.
Buildings may be destroyed by landslides because they are located on the body of thelandslide body, or by the impact and covering of the building by debris derived from alandslide generated uphill from the site. The building and associated earthworks mayalso contribute to the instability of potential landslides.
Locating an important building on sloping ground in the mountainous terrain should bepreceded by examination of the hill slope stability conditions and identification ofnecessary structures for counteracting the effects of the adverse conditions.
On a sloping ground, the location of all buildings should meet the requirements shownin Figure 3.2, unless special slope stability measures are taken.

NBC108V2.RV9

7 December 1993

7Figure 3.2

LOCATION OF BUILDING IN SLPING TERRAIN

Face offooting
Top ofslope

Toe ofslope

H

Face of structure
H/3 But need notexceed 40' max.

H/2 but need not exceed 15' max.

NBC108V2.RV97 December 1993

84Site Investigation
4.1

GeneralAll site investigations should address the following basic questions :Is there any danger of inherent natural susceptibility of the land to the processof sliding and erosion?

Will the construction adversely affect the existing conditions and trigger
landslide, erosion, land subsidence, pore pressure generation due to blockage ofor otherwise the sub-surface flow of water; will the construction adversely affectthe water table?

What will be the extent of settlement of the building?

Is the sub-surface capable of taking the load due to the proposed construction?

Is there any other natural/geological process likely to threaten the integrity ofthe building? (eg, changes in a river course, flooding, failure of an irrigation
canal ?)

What are the possible engineering solutions for ensuring stability of the building
foundation in view of the identified conditions?

Answers to some of these questions can be found by examining the existing local
knowledge, records of any past exploration in the adjacent area and the behaviour ofexisting similar structures. If satisfactory answers to the questions can not be found, it
will be necessary to undertake additional site investigation including subsurface
exploration, in-situ and laboratory testing, geophysical surveys and testing, probing, etc.
4.2

Extent of Site Exploration
The extent of exploration will depend upon the geological and geomorphological natureof the terrain, and on the importance of the building and the scale of the project. Largescale engineering geological mapping of the site and previous practices of constructionin the adjacent area could form the basis for deciding the extent of exploration for an
important building.

4.3

Depth of Exploration
The depth to be explored should be adopted based on the geological conditions at thesite eg the depth and type of subsurface soil, depth of weathering, the depth of groundwater fluctuation, the depth of frost action, etc. Generally, the exploration should becarried out to the depths at which the induced stress due to the loads from theconstruction dissipates substantially and the strata can carry the load without
undesirable settlement and with an acceptable factor of safety against failure. Past
experience and performance of adjacent buildings could be of prime importance inNBC108V2.RV97 December 1993

9deciding about the depth of exploration which shall not be less than one and a half times
the estimated width or the lower dimension of the footing below the foundation level.
In hilly areas, exploration up to the depth of sound bed rock will generally be sufficient.
For the analysis of liquefaction susceptibility, 20 m should suffice as the depth ofexploration.
4.4Allowable Bearing Pressure
The allowable bearing pressure should be calculated as per current good engineering
practice.

5.

Design of Foundations
The design of foundations shall be carried out in accordance with good engineering practice.
Compliance with the following established codes/standards may be taken as the basis of goodengineering practice :NBC108V2.RV97 December 1993

10The following Indian Standards :INDIAN STANDARDSINDIAN STANDARDS(1) IS :1892-1979 Method of practice for subsurface investigation for foundations (firstrevision)

IS:2720 (Part II)-1973 Part II
Determination of water content (secondrevision)

IS:2131-1981 Method for standard
penetration test for soils (first revision)

IS:2720 (Part III)-1980 Part III
Determination of specific gravity :Section 1 Fine grained soils (firstrevision)
Section 2 Fine, medium and coarse
grained soils (first revision)

IS:2132-1972 Code of practice for thinwalled tube sampling of soils (first revision)
IS:4434-1978 Code of practice for insitu
vane shear test for soils (first revision)

IS:2720 (Part IV)-1975 Part IV Grainsize analysis (first revision)

IS:4968 Method for subsurface sounding forsoils :IS:4968 (Part I)-1976 Part I Dynamic
method using 50 mm cone without
bentonite slurry (first revision)
IS:4968 (Part II)-1976 Part II Dynamic
method using cone and bentonite slurry(first revision)
IS:4968 (Part III)-1976 Part III Staticcone penetration test (first revision)
IS:8763-1978 Code of practiceundisturbed sampling of sandsfor

IS:9214-1979 Method of determination ofmodules of subgrade reaction (K-value) of
soils in field(2) IS:2720 Methods of tests for soils:
IS:2720 (Part I)-1972 Part I Preparationof dry soil samples for various tests (firstrevision)

IS:2720 (Part V)-1970 Part V
Determination of liquid and plastic limits(first revision)
IS:2720 (Part X)-1973 Part X
Determination
ofunconfinedcompressive strength (first revision)
IS:2720 (Part XIII)-1972 Part XIII Direct
shear test (first revision)
IS:2720 (PartDetermination
properties

XV)-1965 Part XVofconsolidation

IS:2720 (Part XXVIII)-1974 PartXXVIII Determination of dry density ofsoils in place by the sand replacement
method (first revision)
IS:2720 (Part XXIX)-1975 Part XXIX
Determination of dry density of soils inplace by the core cutter method (firstrevision)

contd.../NBC108V2.RV9

7 December 1993

11INDIAN STANDARDSINDIAN STANDARDS

IS:2720 (Part XXXIII)-1975 Part XXXIII
Determination of the density in-place by
the ring and water replacement method

IS:2911 (Part 1/Sec 1)-1979 Concretepiles Section 1 Driven cast in-situ piles(first revision)

IS:2720 (Part XXXIV)-1972 PartXXXIV Determination of density of soils
in-place by rubber-balloon method

IS:2911 (Part I/Sec 2)-1979 Concretepiles Section Bored cast insitu piles (firstrevision)

IS :2720 (Part XXXIX/Sec1)-1977 PartXXXIX Direct shear test for soils
containing gravel, Section 1 Laboratorytest(3) IS:1498-1970 Classification andidentification on soils for generalengineering purposes (first revision)(4) IS:401-1982 Code of practice forpreservation of timber (third revision)(5) IS:6403-1981 Code of practice fordetermination of allowable bearing pressure
on shallow foundations (first revision)(6) IS:1888-1982 Method of load tests on soils(second revision)(7) IS:8009 (Part I)-1976 Code of practice forcalculation of settlement of foundations:
Part I Shallow foundations subjected tosymmetrical static vertical loads(8) IS:1080-1980 Code of practice for designand construction of simple spreadfoundations (first revision)(9) IS:2911 (Part IV)-1979 Code of practice fordesign and construction of pile foundations: Part IV Load test on piles Section Boredcast in-situ piles(10) IS :2911 Code of practice for design andconstruction of pile foundations.

(11) IS:2911 (Part 1/Sec 3)-1979 Code ofpractice for design and construction of pilefoundations: Part I Concrete piles, Section
3 Driven precast piles (first revision)(12) IS:2911 (Part III)-1980 Code of practice fordesign and construction of pile foundations:
Part III Under-reamed pile foundation (firstrevision)(13) IS :2911 (Part III)-1980 Code or practicefor design and construction of pilefoundations: Part II Timber piles (firstrevision)(14) IS:2974 Code of practice for design andconstruction of machine foundations
IS:2974 (Part I)-1969 Part I Foundations
for reciprocating type machine (firstrevision)
IS:2974 (Part II)-1966 Part II
Foundations for impact type foundations(drop and forge hammer foundations)
IS:2974 (Part III)-1975 Part III
Foundations for rotary type machines(medium and high frequency) (firstrevision)
IS:2974 (Part IV)-1968 Part IV
Foundations for rotary type machines oflow frequency

contd.../

NBC108V2.RV97 December 1993

12INDIAN STANDARDSINDIAN STANDARDS

IS:2974 (Part V)-1970 Part V
IS:9556-1983 Code of practice for design
Foundations for impact type machinesand construction of diaphragm walls
other than hammers (forging andstamping press: pig breaker, elevator and (15) IS:9214-1979 Method of determination ofhoist tower)
subgrade reaction (K value) of soils in thefield
IS:3955-1967 Code of practice for designand construction of well foundations

NBC108V2.RV97 December 1993