N E P A L  N A T I O N A L  B U I L D I N G  C O D E 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 N E P A L  N A T I O N A L  B U I L D I N G  C O D E NBC 108 : 1994 SITE CONSIDERATION FOR SEISMIC HAZARDS This 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 तत्कासिन श्री X को सरकार (मन्त्रिपरिषद्) को मिति २०६०४११२ को निर्णयानुसार स्वीकृत Government of Nepal Ministry of Physical Planning and Works Department of Urban Development and Building Construction Babar Mahal, Kathmandu, NEPAL Reprinted : 2064 i
Preface
This Nepal Standard was prepared during 1993 as part of a project to prepare a National Building Code for Nepal. In 1988 the Ministry of Housing and Physical Planning (MHPP), conscious of the growing needs of Nepal'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 Project NEP/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 or good practice for achieving satisfactory strength in buildings. In late 1991 the MHPP and UNCHS requested proposals for the development of such regulations and documents from international organisations in response to terms of reference prepared by a panel of experts. This document has been prepared by the subcontractor's team working within the Department of Building, the team including members of the Department and the MHPP. As part of the proposed management 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 :

Member Representative, Society of Nepalese Architects (SONA) Deputy Director General, Department of Building, (Mr. JP Pradhan) Member-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 ii
Preface . 0 1 2 3
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S Design of Foundations .
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Foreword
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 seismic hazards. It also outlines the fundamental requirements for site investigation for the foundation design of buildings. The degree to which each factor should be considered will depend on the importance and size of the building under consideration. The document is particularly applicable for State of Art Design and Engineered buildings. 1
Scope
This standard covers the principles of site selection and site investigation for buildings in both the 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 practice in these matters.
1.1
Applicability
In general, the provisions of this standard should be applied to all buildings to be constructed in Nepal. The site considerations detailed out in this Standard shall be mandatory for all important buildings 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.
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Interpretation
2.1
General
2.1.1 In this standard the word "shall" indicates a requirement that is to be adopted in order to comply with the standard, while the word "should" indicates recommended practice. 2.1.2 Words implying the singular only also include the plural unless this is inconsistent with the context.
2.2
Terminology
In this standard, unless inconsistent with the context, the following definitions shall apply : 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. CLAY, FIRM means a clay which at its natural water content can be moulded by substantial 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 moulded with the fingers and can be readily excavated. CLAY, STIFF means a clay which at its natural water content can not be moulded by substantial pressure with the fingers and requires a pick for its removal from the parent body. ENGINEERED BUILDING means any building designed in accordance with the engineered structures provisions of the Nepal National Building Code. FOUNDATION means that part of the structure which is in direct contact with the soil and transmits load to it. FOUNDATION SOIL means the soil at the sub-surface which is in direct contact with the 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 a disaster (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) ROCK MASS means those naturally occurring mineral aggregates in-situ with all their planes of discontinuities. ROCK MATERIAL  means a piece of rock detached from the rock mass and bounded by 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 a size greater than 0.2 µm and less than 75 µm. SOIL means a mineral aggregate or product of rock weathering lying at the place of formation or deposited after being transported by different natural means of transportation. 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 and rain water and of plants and bacteria and the mechanical action of changes of temperature, whereby rocks on exposure to the weather change in character, decay, and finally crumble into soil.
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Site Considerations
Consideration of Potential Fault Rupture Hazard
3.1 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 This instantaneous ground displacement may occur along and displacement. approximately 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 the facilities. Therefore, while selecting the site for an important building or structure, it shall be ensured that the building is not located within a distance of 500 m from the surface trace of a known active fault. Figure 3.1 depicts the principal active faults so far identified within Nepal and can be referenced 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 of damage 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 in tube wells and dug wells. The site selection for an important engineered building in the Terai and on alluvial river banks within the mountainous terrain shall be preceded by examination of the liquefaction 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 an earthquake.  All major earthquakes in mountainous terrain result in increased instances The vast majority of these are rock falls, although more coherent of  landslides. 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 the landslide body, or by the impact and covering of the building by debris derived from a landslide generated uphill from the site.  The building and associated earthworks may also contribute to the instability of potential landslides. Locating an important building on sloping ground in the mountainous terrain should be preceded by examination of the hill slope stability conditions and identification of necessary structures for counteracting the effects of the adverse conditions. On a sloping ground, the location of all buildings should meet the requirements shown in Figure 3.2, unless special slope stability measures are taken. Figure 3.2
Face of
Site Investigation
4.1
General
All site investigations should address the following basic questions : . Is there any danger of inherent natural susceptibility  of the land to the process of sliding and erosion? . Will the construction adversely affect the existing conditions and trigger landslide, erosion, land subsidence, pore pressure generation due to blockage of or otherwise the sub-surface flow of water; will the construction adversely affect the 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 of the 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 of existing 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 nature of the terrain, and on the importance of the building and the scale of the project. Large- scale engineering geological mapping of the site and previous practices of construction in 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 the site eg the depth and type of subsurface soil, depth of weathering, the depth of ground water fluctuation, the depth of frost action, etc.   Generally, the exploration should be carried out to the depths at which the induced stress due to the loads from the construction 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 in
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deciding 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 of exploration.
4.4
Allowable 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 good engineering practice :