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International Journal of Civil, Structural,

Environmental and Infrastructure Engineering

Research and Development (IJCSEIERD)

ISSN 2249-6866

Vol. 3, Issue 3, Aug 2013, 57-76

© TJPRC Pvt. Ltd.

BI-LAYER FOOTING FOUNDATION SYSTEM FOR MULTISTOREY BUILDING –

TOWARDS A NEW CONCEPTION AND PRACTICES

GOKUL K BAYAN

Applied Civil Engineering, Principal Scientist, CSIR-North East Institute of Science & Technology, NEIST-Branch

Laboratory, Itanagar, Arunachal Pradesh, India

ABSTRACT

The aim of civil engineering activities is to develop structures with proper technologies to provide more safety,

comfortable and economy for its users. This paper deals with the technical matters regarding a new conception of design

and construction of structures pertaining to shallow foundation as Bi-layer footing foundation system. This system is

proven to offer more safety in high earthquake prone area and also generates 60% more economy in construction for its

substructures. Bi-layer footing foundation system has two footing slab across the vertical axis of a single column or a

group of columns in combination. Hence it demands two kinds of safe bearing capacity of subsoil layers – one is untreated

site occurrence and other is site built-up subsoil treated with some engineering processes.

Similarly, the design and construction procedures are also varies a little from what it is for the conventional

practices. For enhancing its understanding, two examples are provided herein to describe two different occasions and

situations for constructional development. In consistence to such conception, modern geotechnical engineers have

improved their understanding of soil behaviour for such purposes and sharpened their analytical tools and developed

suitable technologies also. However, full advantage of such modern developments has not been taken up to improve design

practices particularly not only in India but also most of the developing countries of our globe. This is a matter of real

understanding for now-a-days. To proclaim such matter of understanding this paper has been produced and published.

KEYWORDS: Conventional, Footing, Shallow, Surface, Bi-layer, Foundation, Bearing Capacity, Safety, Economy

INTRODUCTION

Foundation, Bearing Capacity of Subsoil for the Designer, Owner, etc – In General

Foundations are buried elements of any super-structure, which are equally important as those of superstructures. If

substructure fails then no longer the very carefully designed and executed superstructure built over it cannot withstand in

reality. Hence, substructure bears the key point value of sustainable development for any superstructure. On the other hand,

sustenance of such substructure is wholly depends upon the subsurface sounding of the project site or soil properties

actually encountering the subsoil from geological as well as geotechnical point of view. Thus suitable studies of subsoil

pertaining to any project site proclaim its importance in construction project.

It is obvious that foundation design practices have undergone vast improvement from principally empirical base to

knowledge base. The modern geotechnical engineers have improved their understanding of soil behaviour and sharpened

their analytical tools and developed the suitable technology accordingly. However, full advantage of these developments

has not been taken to improve design practices not only in India but also in almost all developing countries of our present

globe. Because, most of the common owner i.e. layman of Building residing in underdeveloped country like India, are still

providing resistance to adopt new improved process and its technology for real existence. Of course, there are a few who

want to achieve better engineered quality of their substructures only from economic gaining point as well as safety aspects.

58 Gokul K Bayan

The negative habits exist because, the existing gap between a geotechnical engineer and a designer of Structures together

with the Architecture’s rigidity. Similarly, on the other hand even the attitude of most of the expert designers who use to

assume in most cases the basic inputs (real data) pertaining to the relevant properties of subsoil supporting the foundation

which in fact, underscoring such newer and scientifically more reliable modern development. This is a matter of real

understanding, the lack of which becomes the basic reason of failure of substructures more particularly in multistorey rcc

building in weak subsoil. Because of such drawback some of those owners become victim and losses their money and

properties before maturity.

Studies of subsoil of any project site proclaim its importance in construction project. It is obvious that almost all

construction projects in private or even Government sector/ individual housing project, the owner neglects the importance

of study of the subsoil of their proposed building project site beyond ground level and allowed their construction go ahead

for which quite sometimes some of such owner become victim and losses their earnings, provided the subsoil conditions

are found not to perfectly sound. Looking to the above hindrances to element the drawbacks exist among common designer

and owner, for simple assessment and easy method of recognizing safe bearing capacity for design purposes of its

foundation has been prepared, exposed as an out put of innovative R&D work and published in a section of this paper.

Therefore, this paper will deal in details about introduction of a new foundation system for multistorey rcc

building of low to medium height (say from 2-storey upto 12 storeys height) to be placed on weak subsoil (SBC <150

kN/m2) of high earthquake prone area (i.e. area having Basic horizontal seismic coefficient =0.08) besides normal

situation. The new system is known as Bi-Layer Footing Foundation System (BL-FFS). This BL-FFS also implies towards

meticulous development of surface foundation for multistorey buildings (Bayan, 2011.a & b) and road foundation (Bayan,

2012.a&b). For comprehensive understanding of this new footing foundation system (FFS) two worked out practical

examples have been incorporated herein. Out of which one is already executed for retrofitting (for 4-storey rcc building

(Bayan, 2011.c)). And the other practical example (for 7-storey rcc building (Bayan, 2011.d)) is preparing to start its

construction in which the BL-FFS for multistorey rcc building in design aspect will demonstrate its field practices.

Origin of the New Conception Having Bi-Layer Footing Foundation System

Generation of original conception about introduction of Bi-layer Footing Foundation System (BL-FFS) is

developed out of the suitably implemented solution pertaining to the problem encountered to a proposed 4-storey rcc

building, belongs to an owner, who is an Associate Professor, Rajiv Gandhi University, Arunachal Pradesh. The problem

was entrusted to the author of this paper to retrofit his incomplete 4-storey height building (Bayan, 2011.c) at the earliest,

which experiences damage in 6.0 m span beam during/ after construction of its 2nd

storey height of the building during

2010-11. To suit the situation of retrofitting of an old building foundation system built with Isolated footing foundation

poorly tied with Tie beam network, BL-FFS has been introduced and successfully retrofitted and completed the proposed

building.

In general, the analysis and design of BL-FFS, the details of which are dealt in later pages, is completely a new

process of constructing shallow foundation not only in India but also most of the urban cities of our present globe. Hence,

it is a fresh subject. There are rarely found published knowledge which proclaims constructed structures on BL-FFS in

possession.

Intensive scientific efforts have been put forwarded to develop it. Both field and some laboratory experiences with

25 years base knowledge drawn through geotechnical engineering and as well as earthquake resistant structure designing

habit made it possible for use in common practice with simplest technology. Thus it was tried to develop the BL-FFS and

Bi-Layer Footing Foundation System for Multistorey Building–towards a New Conception and Practices 59

to overcome the present problem in hand pertaining to having weak subsoil at problematic site under high earthquake

prone zone of our country. Subsequently BL-FFS has come out based on economic aspect and engineered construction

habit useful for mediocre to rich people.

It is also propounded that this BL-FFS can replace the deep foundation needed by conventional practice to some

extent for high rise buildings and may save more than 60% budget money kept for development of substructures of any

nature of proposed multistorey building project.

BI-LAYER FOOTING FOUNDATION SYSTEM

Definition and Working Process of Bi-Layer Footing Foundation System

Bi-Layer Footing Foundation system (BL-FFS) is completely a new concept of foundation system as sub-

structural development. It comprises the construction of surface and isolated shallow foundation in combination at the

same location of a column, not only in India but also in most of the urban cities of our present globe. Hence, it is a fresh

subject. There are rarely found published knowledge besides Bayan (2011.a&b) work which proclaims constructed

structures in BL-FFS in possession. BL-FFS is useful from the economic aspect as well as from the simplest engineered

construction habit point of view for low to medium rise of multi-storey buildings (2-12 storeys height). It is also

propounded that this BL-FFS can replace the deep foundation needed by conventional practice to some extent and may

save about 65.4% budget money kept for development of substructure for any proposed building project (Bayan, 2011.a).

The simplest definition is: The foundation system comprising two structural foundation level slab at different

depth connected by a single column, which possesses more efficient rate of transmission power of building load from

shallowest to shallow depth into structural soil in-terms-of more safety and economic aspect of engineering development,

is called Bi-layer footing foundation system. This BL-FFS has two footing slab across the vertical axis of a single column

or a group of columns in combination. It implies two footing slabs at two different levels of subsoil below a single column

base to transmit the building load by using two different SBC holding subsoil layers. Obviously, the surface foundation

slab demands higher value of SBC to produce perfect interaction. Figure 1 gives an illustration representing two process of

the concept of BL-FFS.

Figure (a): Idealised 60% Economic BL-FFS Figure (b): Idealised > 60% Economic BL-FFS

Figure 1: Illustrations Demonstrating All About a System of Two Processes of the Overall Concept of BL-FFS

In consistence to the above idea, 1st level of footing slab placed at a suitable greater shallow depth below column

base, which is made always as per conventional design practice of Isolated Footing Foundation (IFF) is to bear a smaller

part of the total building load i.e. 1/3 or < 1/3 of total loads of the proposed structure as per the conception of BL-FFS.

However, the major part of the total building load i.e. the remaining 2/3 or > 2/3 of the total factored load and moment of a

column or whole building, is normally allowed to bear by 2nd level footing slab placed at shallowest or surface level i.e.

just below Normal Ground level or occasionally at elevated level which may be at Plinth level of the Building for

60 Gokul K Bayan

economic point of view, below column base. Hence this 2nd level foundation system is also termed as Surface Footing

Foundation (SFF). In this regard, the detail design procedure of which is found after Bayan (2011.a&b). Of course, a brief

note of the same are stated hereunder as:

For instance, in consistence to the above idea, 1st level of footing slab placed at a suitable shallow depth that

depending upon the prevailing site condition, below column base, which is made always as per conventional design

practice of IFFS. Obviously, this part of load shall be allowed to carry by such IFFS should not exceed 1/3 (or < 1/3) of the

total factored load and moment of a column or whole building. Hence, as a First step work of design, it is better to fix the

total number of such IFF which will carry 1/3 of the total loads and moments. It is neither essential nor compulsory that all

columns of proposed building should have been built on its IFF as 1st layer (Figure 1). Conception proclaims that only the

peripheral columns (i.e. outer line columns/ wall) of the building may have IFFS and 1-2 in the intermediate location as per

demand [Figure 1(a&b)]. These entire IFFS is to be designed based on the normal field SBC of the site only. In this

regards, for instance, a practically designed 1st level foundation system of the BL-FFS for 7-Storey height building (Bayan,

2011.d) is shown in Figure 2 (a).

Figure 2(a): 1st Layer Footing Foundation of a 7-Storey Building Having Bi-Layer Footing Foundation System

On the other hand, the 2nd

level of footing slab is made based on a new ideally developed level of foundation level

slab holding all columns over it of the proposed building. Thus 2nd

level of footing slab placed at surface level coinciding

the ground/ basement floor level where from almost all remaining columns base will start to erect. It is neither made

always as per conventional design practice of combined footings nor Raft and Mat footing foundation. It is planned looking

like the normal floor slab the beams of which are building under the engineered subsoil of more SBC but its structural

analysis for rcc framed beam-column network is differ from what is for normal rcc framed building structures. Thus this

footing is propounded to name as ‘Surface Footing Foundation’ (SFF). The column having IFF below it will work as the

structural column in a heavier medium (i.e. subsoil of the site in this case) and offers resistance to sway of the SFF. On the

other hand, the depth of main beam of this SFF foundation system will work as ‘Skirt’ for the ground improvement

technology which is always needed to implement in this BL-FF system i.e. to achieve the improved desirable SBC. Fig

2(b) is depicting the 2nd

layer footing foundation of the same 7-storey building proposed to construct under BL-FFS

(Bayan, 2011.d).


Figure 2(b): 2nd

Layer Footing Foundation i.e. Surface Foundation of the Bi-Layer Footing Foundation System

From shear force (SF) and bending moment (BM) point of view, the part of total load to be allowed to carry by

such SFF should fixed at 2/3 or slightly more of the total factored loads and moments of all columns. Consequently, such

SFF is to be designed based on the engineered SBC i.e. improved SBC only. For comprehensive understanding about such

conception two practical examples are described in brief herein this paper. However, the present study will not deal in

detail about how to achieve the targeted improved SBC for designing the BL-FFS but may be found after Bayan (2013.b).

Geotechnical Aspects for Development of BL-FFS

Traditional practices for constructing a house of 1-2 storey height never demands prospective geotechnical

investigation to recommend its SBC for design and execution, the reason of which is that such buildings are normally

constructed over good land. However, good lands are very scarce now-a-days. Consequently, it generates a common

mistake for all that soil investigation is a neglecting subject falls under Building or other projects development. Thus

Common people are acquainted with the popular Architectural Thumb rule process for the same. Overall, such conception

leads to the generation of failure of building substructure in modern days also because good land sites are very very scarce

now-a-days. Lands available are almost weak soil, which need improvement for achieving its desirable designed SBC.

On the other hand, conventional practices so developed during past 60 years, proclaim many procedures for

subsoil investigations, which are assumed to be needed for high rise and heavy loaded buildings only. Thus, modern

development so appeared and prevailing with such complex activities also found creating similar mistake for layman. On

the other hand, the changing of traditional building construction practices prevailing among laymen are still producing

failure risk.

There are numerous processes (i.e. field and laboratory tests) to know the behaviour of the subsoil which

altogether is known as subsoil sounding of a project site. However, all are not essential for design of a surface type

foundation system. The most important test is to be carriedout at field is that of the integrity test. Such test will allow

engineers to know all about, whether the subsoil successions pertaining to the proposed building site have any void in

soilmass which is generated out of dynamic liquefaction effects by high earthquake shocks or not, at least within a depth of

3 times the width of the proposed building in plan or maximum width of the SFF slab. Besides this, the most important test

needed is the field Plate Load Test (PLT). If such field PLT is not possible to carryout in the proposed site then, laboratory

Model PLT becomes the only alternative. To carry out the Model PLT, measuring the field density below foundation level

62 Gokul K Bayan

at site is compulsory to evaluate. Similarly, collections of disturbed and undisturbed soil samples are also needed

sufficiently to complete the general aspects of the geotechnical investigation at laboratory work level.

With the collected soil samples routine tests are to be carriedout in the laboratory except some special kinds of

tests. The special tests are to evaluate and to fix the exact shear parameters and SBC perfectly both for existing field

condition and for improved engineered condition of the structural soil masses. In this regard pertaining to field condition,

following special tests are to be carried out meticulously:

Large Box Direct Shear test (LB-DST) (Figure 3), under fully saturated drained condition & Small Box Direct

Shear test (SB-DST) in same condition to fix the shear parameter first – for theoretical evaluation of the SBC.

Model PLT test by using the same subsoil at field density in the LB-DST machine with water table at just below

footing test plate level – for practical evaluation of the SBC.

Figure 3: Shows the Large Box Direct Shear Test Machine and Conduction of a Model-PLT

Before fixing a recommended SBC, Factor of Safety (FoS) of the subsoil is needed to be evaluated from the tests

data of the model PLT carriedout by using same subsoil as of the proposed building site. It is essential because it will

describe the geological condition under which the subsoil is originated. Hence, it shall be found out by plotting the load

intensity data against total settlement both in linear graph and in Log-Log graph papers (Figure 4 (a&b). The result of the

linear graph will give ultimate load bearing capacity of the subsoil, where as the Log-log graph with its corrected data after

plotting gives yield stress (i.e. apparent SBC) of the subsoil pertaining to the same subsoil. Thus, the ratio of ultimate stress

divided by yield stress reveals the FoS of the subsoil of the proposed site which may be expressed as equation 1 below:

1.EqsametheofstressYield

PlateTestofstageFailuringUltimateatStressFoS

This FoS is independent of size of the test plate used in carrying out the model test or field test. However, it varies

based on the conditions i.e. site/ engineered conditions of model test, because of the alteration of its inter-granular faces

most particularly the faces of silt and clay particles, as and when it is placed in the Model box. This is why the field PLT is

the best to establish a perfect FoS of the subsoil system. Thus by using evaluated shear parameters and result of corrected

data of PLT will yield final SBC, which may be fixed from higher value provided, evaluated FoS is used in practice. Figure

4 represents an exact result related to a project site. The two SBC evaluated in this process are to be used in designing both

the IFF and SFF system. It will be worth mentioning that the same FoS evaluated under site condition shall be used to

finalise the improved SBC to be derived out of proposed method of soil improvement system and utilise the same for

design and developing SFF.


Figure 4: Evaluation of FoS and Recommendation of SBC of a Proposed Building Site as Per New Concept

Finally geotechnical investigation report for recommendation of SBC, shall comprise the value of unrestrained

SBC (i.e. Field SBC) and skirted SBC (i.e. Engineered or improved SBC) of the subsoil pertaining to the proposed

building site considered for designing as BL-FFS. It is worth mentioning that the contribution of group of granular pile

system used for one of the soil improvement technique, normally use to reduce the ultimate settlement of the foundation

system (Bayan, 2003) but such granular system will not contribute much more towards increment of improving SBC

required for design of SFF. The suitable method finalized after many trials, is being developed for BL-FFS is ‘Honey comb

cell structures’ below confined SFF system (Bayan, 2013.a&b).

Earthquake Response Properties Needed for Development of BL-FFS and Related Safety

Development of BL-FFS is mainly targeted to draw less earthquake effect on buildings. Till present, the effect of

earthquake on superstructure is a little known and technologies developed accordingly. However, the effect of earthquake

on individual foundation and its overall substructure is still in deeming light. Hence, study of earthquake effects on

substructure is needed first and subsequently it is found as second matter of study on whole building. Through this paper, it

is tried to draw a thematic idea over the action of earthquake force on substructures. In this regard the Time-period of the

building is used as per some well known empirical formulae, which is in practical use for analysis of superstructure,

besides consideration of the basic horizontal coefficient. The details of such study are beyond the scope of this paper.

It is well known that for high rise buildings (storey height > 3 storeys in weak subsoil) deep foundations are

normally recommended to overcome problems associated with weak subsoil. Deep foundations are built as the integral part

of the Column base system. Since the members of the deep foundation are resting on a denser medium and ends at a much

stiffer subsoil, hence, it creates the scope of transmitting more earthquake shocks much faster to superstructures built on it

and carrying big magnitude than that of through subsoil. Consequently, earthquake shocks/ oscillation disseminates to

beam and floor very quickly through stiffer medium (i.e. deep foundation here) rather than basement/ subsoil ground base

floor. It means the excitation of building members of superstructure and substructures varies extensively. Hence, high

scopes of risk increase for causing damages due to earthquakes.On the other hand, if it is possible to reduce the length of

rigid integrity of the foundation system (i.e. deep foundation here) upto its possible minimum depth (Shallowest depth or

close to surface) and concentrate it at surface of the ground or elevated ground, then definitely the scope of the high risk

damage in the building so caused by the earthquake shocks will be reduced sufficiently. Hence, it is propounding this new

64 Gokul K Bayan

foundation system i.e. BL-FFS. Ultimately we can proclaim that it is safer than any type of foundation revealed by

conventional practices till date. However, the matter of load bearing buildings i.e. structure built in burnt clay brick is

different (Kalita & Bayan, 1990.b).

It is will be very good if it can develop some damping effects deriving from surface foundation system and its

associated substructures, then the time period of oscillation of the superstructures of building will definitely reduce and

oscillate uniformly and hence, the economy as well as more safety in superstructure will reveal. Therefore, it is aimed to

generate damping effect at surface foundation level. To achieve this goal, the 2nd

layer of the BL-FFS is placed on surface

level of the building with introduction of some unique methods of soil improvement processes (Bayan, 2013.a&b and

2012.a&b).

It is evident to state that, during 1990 three forced vibration tests were carriedout in a refinery site (Bayan, 1990.a)

which was published during 1992 (Bayan and Kalita, 1992) and 2013 (Bayan, 2013.c). The study of such dynamic

properties of the subsoil at that site reveals a notable remark. The remark is that existence of sand layer in a geological

deposit of subsoil having clayey-silt nature generates damping effect over the structure which is rocking at a particular

oscillation based on some frequencies. This yielding response of the subsoil is the outcome, which can be used in

development of BL-FFS. To avail this response efforts have been made to introduce the 2nd

layer of BL-FFS with skirted

subsoil of granular mass, which is in fact provides binary benefits as below:

In the first, it will provide facilities to improve SBC as per design requirement out of the weak subsoil to be

improved. The improvement technique can be tuned upto the desired design requirement value of SBC.

And in the second, it will provide definitely a kind of damping effect against earthquake shocks/ oscillation

transmitting from substructure to superstructures, which will contribute from such substructure of BL-FFS.

Since these aspects of BL-FFS is initially developed and try to use in reality, therefore the details of such aspect is

still beyond limit of the scope of this paper. It will be dealt in a separate publication on due course of time.

Analysis and Design of Structures

Analysis of Building Superstructure

To find out the Shear Force (SF) and Bending Moments (BM) to be acted upon the foundation, there needs to

analyse the superstructure first. To get such values, lot of exercises starting from load counting upto static and seismic

analysis, are to be carriedout within the prefixed dimensions of room sizes and column-beam frame-work of proposed

building which is done by adopting the conventional practices. For carryout the seismic analysis, seismic coefficient

considered is maximum which is stipulated in BIS:1893-1975.

For instance, the result of the seismic analysis carriedout for moment resistant frame portal SP-2 related to 4-

storey rcc Building (Bayan, 2011.c) considered for retrofitting is shown in Figure 5. In this regard, it will be worth

mentioning that, the building has two major foundation level located below and above ground floor level as it is situated in

a part of a hillock flange (Figure 5&10). It was done by the owner without having knowledge of earthquake effects over

structures. Fortunately it was analysed for the building for such situation and the BM and SF are evaluated accordingly for

the present purposes of retrofitting the building.


Figure 5: Represents the Sectional Side Elevation of the 4-Storey Building Showing Two Foundation Levels and

Seismic Analysis of the Superstructure Possessing Column-Beam Moment Resistant Frames

Analysis of Substructure

After finding the vertical load and moments subjected to the base of columns the foundation connecting the

columns are to be analysed properly. Looking to the available situation of the columns some of them/ all columns are in

one line is to be combined and analysed accordingly, which is needed for planning a combined footing for retrofitting

purposes. For instance the bending moments and shear forces for a group of such combined footing to be placed as the top

layer of the BL-FFS is shown in the Figure 6.

On the other hand the situation of a fresh building with BL-FFS the analysis is to be carried out as described

earlier. For instance the bending moments and shear forces for a surface footing foundation to be placed as the top layer or

surface layer of the BL-FFS is shown in the Figure 7

Analysis for Finding the Economy in Development of BL-FFS over Conventional IFF

After achieving the safety, the next aspect of engineering building design implies to its economy in construction.

The gaining of economy i.e. the budgetary money saved in relation to the development of whole substructure is also

studied through an actual structure planned and designed for execution. Let the building plan as a whole is to be depicted,

for instance, which is used as the 2nd

example in this publication for comprehensive understanding is shown in Figure 8

(Bayan, 2011.d). It has 3-blocks of foundation system. However, Figure 2 (a&b) shows only Foundation Block-1. For

finding the gaining in economy it was analysed all the 3-Blocks. In this regard the two kinds of SBC i.e. field SBC and

improved SBC developed by skirted wall and honey-comb-cell particles (Baya, 2013.b), are 107 kN/m2 and 158 kN/m

2.

Similarly, the axial loading and BM are given in Table-1, which also gives the designed sizes and reinforcement

specifications of the respective classes of columns.

Figure 6: Analysis of Combined Footing for 5-Columns to be Placed as 2nd

Layer of the BL-FFS

66 Gokul K Bayan

.

Figure 7: Analysis of Main Beam of Surface Footing Foundation Slab Having 8-Columns in a Row Runs along

Intermediate Line to be Placed as 2nd

Layer of the BL-FFS

Now, as per conventional practice, let it be assumed first that all foundation of columns is Isolated footing

foundation (IFF) in one hand. On the other hand, as per new conception of BL-FFS the foundations of columns have two

different natures as shown in Figure 2(a&b). In consistence to achieve the total cost of development of the substructure by

considering both the foundation systems, it is revealed after comparison of material estimate so involved in complete

development of the substructure having concrete, steel and earthwork works upto ground floor level is very encourageous.

The final opinion is a gaining of 65.42% over the conventional system. Table 2 gives the real account for this example. The

gaining in economy reveals a brilliant % of money saving. Following are some reasons behind revealing such brilliant

savings:

Firstly, the rcc slab of surface footing foundation replaces the pcc work involves in making the ground floor as a

whole.

Secondly, the number of Isolated footing foundation is reduced sufficiently by introduction of main beam (Figure

7)

The poor number of Isolated footings used in BL-FFS are designed only for 1/3rd

or less loads and moments of

the whole building system.

Hence, a brilliant saving in economy ranging fro 60% to > 60% yields for the building owner.

Figure 8: Shows the Front Elevation and Foundation (BL-FFS) Layout Plan of the Proposed 7-Storeys Building


Table 1: Gives the Total Designed Information of Column sizes, its Loads and BM Confined by Foundation Block -1

Sl. Column marked as Total axial Design Unit B.M at base of Sizes of Column

Total nos. of

No. Columns = 53 nos. Load on column columns & Rinfc. Bars

1 D1,G1, .. 966.78 kN Mx=55.1 kN-m 350x350; 4-20mm and

Corner (=13 Nos.) My=92.01 kN-m 4-16mm dia HYSD bar

2 F1, D2,.. 1925.38 kN Mx=110.24 kN-m 400x500; 6-20mm and

Interior (=4 Nos.) My=184.02 kN-m 4-16mm dia HYSD bar

3 G3, G4, ... 2549.11 kN Mx=110.24 kN-m 400x500; 8-20mm and


4 J3, J4, … 2352.24 kN Mx=115.56 kN-m 400x500; 8-20mm and

Interior (= 8 Nos.) My=184.02 kN-m 2-16mm dia HYSD bar

5 L6, J5, .. 2142.12 kN Mx=115.56 kN-m 400x500; 6-20mm and


6 F4,F5,… 1509.19 kN Mx=103.77 kN-m 350x450; 6-20mm and

Side (=14 Nos.) My=92.01 kN-m 2-16mm dia HYSD bar

BI-LAYER FOOTING FOUNDATION SYSTEM IN PRACTICAL EXAMPLES

Retrofitting of 4-Storey Building

The Problem and the Efforts

In consistence to the above strategy, a problem that happened during construction of 2nd

floor height of a proposed

4-storey building was offered for investigation and necessary retrofittings. Accordingly, the problem was undertaken,

investigated in the light of both geotechnical aspects for substructures built up and structural strength of the columns,

beams & slabs so constructed upto 2nd

floor level. Design of the 4-storey building structure was designed as per

conventional practice. Of course, a new foundation system has to introduce for substructures, for which analysis and design

were carried out and executed in the site for its future sustenance (Bayan, 2011.c).

Table 2: Gives a Total Comparison about Material Estimates Pertaining to Total Volume of Concrete, Steel &

Earth Work in Development of Overall Substructure

Type of foundation Vol. of Concrete Cement Sand Stonechip Steel Soil Filling/Ring

used for 655.56 m2 in Cu. M in Bags in Truck in Truck in kN cutting for improving

1.Conventional M10 116.415 in base 696 12 22

Isolated footing M15 465.384 in Structre 4077 47 86 196.67 1501 1417

Total materials = 581.799 4773 58 108 196.67 1501 1417

M10 44.56 in base 266 5 9

2. Bi-Layer M15 172.55 in Structre 1512 17 32 178.69

Footing Foundn.M20 94.48 in Structre 1054 9 17 1673 957

Total materials = 311.59 2832 31 57 178.69 1673 957

% Economy gained by concrete over convensional = 46.44 Total p.c. gained over

% Economy gained by Steel work over convensional = 9.14 conventional practice

% Economy gained by earth work (cutting + Filling) … do=9.84 = 65.42

The building is situated in the flange of Polocolony hillock at Naharlagun Township i.e. capital complex of

Arunachal Pradesh (AP). The owner of the building came to me and entrusted the job for retrofitting. In consistence to my

efforts beyond engineering planning, designing & guidance for proper execution, I have normally performed a parallel job

educating contractors, masons, etc in this line. The subject of education are all about the dissipation of knowledge

pertaining to building bye-laws, its rules and regulation to achieve the best performance in reality such that victim related

to building collapse should be reduced to zero during occurrence of natural hazards like earthquakes. This project contains

68 Gokul K Bayan

necessary detailing all about the design and reinforcement specification of all rcc members for achieving the following:

Investigation of the problem and recommendation of suitable foundation system for such old structures as

retrofitting measures.

Retrofitting of all old members of the super structures built upto 2-storey level and then completion of the

remaining construction.

Overall sustenance of the building under high earthquake shock pertaining to seismic Zone-V as it is situated in

Hilly terrain with different level of Foundation (Figure 5). Hence, the building is proposed to be a 100%

earthquake resistant building.

To meet the above demands, efforts have been made in accordance to the design guidance stipulated in different

BIS: Code of practices, which are followed obediently. Hence, the proposed 4-Storey rcc building is designed to sustain

during High Earthquake shocks of its life period of time which is expected undoubtedly if construction is found to be in

order.

Investigation at Site

Geotechnical Investigation

There is limited scope for conducting detail subsoil investigation to explore various geotechnical properties at site

for achieving a thorough picture about the subsurface sounding on already constructed building plot. The plot is found tight

with its planning. The room height inside the building at ground floor is also low (2.9m). Hence, no field test particularly

SPT, DCPT or even SCPT are possible to carryout.

On the other hand, there is no geotechnical information made available from the site before construction. Looking

to the above situation 3-pit trenches were advanced upto a depth between 1.5-2.5 m and collected disturbed and

undisturbed soil samples to analyse the same in the laboratory. During investigation it was recognized that though there is

no normally standing GWT as a high land in hilly terrain, yet there are sufficient water seepages found which generates

water accumulation beneath foundation level and saturated the subsoil underneath during monsoon period.

With the collected soil samples routine tests were carriedout in the laboratory except some special kinds of tests.

The special tests performed are as per tests described in earlier pages. Result of all the tests carriedout in the laboratory

were foundout and utilise in design purposes.

Finally geotechnical investigation report recommends the value of unrestrained SBC of the subsoil, which is 135

kN/m2

(i.e. Field SBC) and the type of foundation for accommodating the retrofitting provision of the old foundations is a

unique form of Foundation system introduced first time in India as Bi-layer combined footing foundation. Table-3 gives

the details all about the geotechnical properties of the building site. Obviously, the retrofitting jobs have been done as per

design.

Structural Investigation

In absence of any drawing, there is difficult to study the overall plan and specification of the built-up structure

upto 2nd

floor level. Hence, detail measurement survey was carriedout. After measurement the different dimensions of the

building room, columns, beam and slab were found out and are given in a drawing form. Some of which are shown in

Figure 9.


It was observed during the time of investigation in February 2011 that the sizes of all columns, beams and slabs

are inadequate. Span of the beams are regular in X-direction and almost irregular in Y-direction (Figure 9). Maximum size

of column having 6.0m span is 290x355 mm and minimum size is 250x300mm. On the other hand, maximum size of the

structural beam in section found for 5.0m & 6.0 m span is 320 mm x 450 mm.

Figure 9: Shows the Building Plan at First Floor Level and Foundation Layout Plan for Retrofitting of the Sub-

Structure of the Existing 4-Storey rcc Building

Substructure constructed was of Isolated footing foundation placed at different foundation level even for Ground

floor also (Figure 5). The footings are poorly connected by tie beams in X-direction. Along Y-direction there is no tie beam

for 6.0 m span bay (Figure 5&9). Maximum size of the isolated footing foundation found is 1.5 m x1.5 m. These footings

are placed at various depth levels from 1.5m upto 2.8 m (Figure 5).

It was informed that the age of the construction is about 1 year and all members were constructed with M15 grade

of concrete and steel grade Fe 500. In such condition, structural investigation reveals that all the beams having 6.0m span

execute vertical shear cracks at three different locations. They are at centre of the beam i.e. at about 3.0m from the column

support and almost ¼ of the beam span length at both ends. Such cracks also executing by the 6.0m beam at 2nd

floor level

also.

Consequent to both of geotechnical and structural investigations, it is proclaimed that the cracks so developed in

the 6.0m beam may be due to differential settlement occurred at Isolated footing foundations level only, which are not

proper in sizes and connected by tie beams or due to inadequate proportioning the footing foundation as the SBC and

compression index of the site are found very poor (Table 3).

Table 3: Result of Various Geotechnical Properties of the Subsoil of 4-Storey Building Site

Location of

Undisturbed

Soil

Samples

Depth

of

Sample

‘D’ in

Metre

Field

Bulk

Density

‘’ in

kN/m3

Field

Moisture

Content

‘W’ in

%

Sieve Analysis (p c Passing) Size in mm

Type

of Soil

Compre-

ssion

Index

Cc

Shear Para-

Meters from

SBC. of

Sub-Soil 10.0

(IS:

10)

4.75

(IS:

480)

1.18

(IS:

120)

0.60

(IS:

600

micron)

0.3

(IS:

300

micron)

0.075

(IS:

8= 75

micron)

SB-

DST*

LB-

DST*

Sample-1 West side

1.5 195 17.6 99.1 93.4 81.9 69.9 57.9 30.9 CL 0.981 C=0.50

=41 C=0.10

kg/cm2

=26.5

degree

From

model PLT,

135

kN/m2 & From

Analysis,

92 kN/m2

Sample-2

Hill side 1.8 185 22.5 100 99.8 95.8 89.9 83.2 61.6 CI 0.815

C=0.48

=31

Sample-3

East -side 1.3 185 12.5 97.6 88.9 68.5 57.4 46.4 27.7 SM quick

C=0.35

=30

70 Gokul K Bayan

Analysis and Design of Structures

Analysis of Building Structure

To find out the shear force and moments to be acted upon the foundation, structural analyses have been done

starting from load counting upto the static and seismic analysis. For carryout the seismic analysis, seismic coefficient

method was considered which is stipulated in BIS:1893-1975. For instance, the result of the seismic analysis carriedout for

moment resistant frame portal SP-2 of this 4-strorey building is shown in Figure 5. In this regard, it will be worth

mentioning that, the building has two major foundation level (Figure 5) located below and above ground floor level.

Analysis of Combined Footing as 2nd

Layer of BL-FFS

After finding the vertical load and moments subjected to the base of column the foundation connecting the

columns are to be analysed properly. Looking to the available situation of the columns some of them is in one line and

others are combined and analysed accordingly. For instance the bending moments and shear forces for a group of such

combined footing to be placed as the top layer of the BL-FFS is shown in the Figure 6. By using such method all kinds of

foundation structures were analysed and designed. Detail specifications of the foundation are shown in working drawings

which are beyond scope of this paper.

Figure 10: Shows the Progress Taken Place for Retrofitting as 2nd

Layer and the Building View during May 2011

Execution of Bi-Layer Footing Foundation as Retrofitting of Old Structures

For the purpose of retrofitting of building foundation for this 4-storey rcc building, initial conception of BL-FFS

has been adopted and designed accordingly (Figure 10). The design is carriedout by assuming 1/3 of total loads of the

column is supposed to be supported by existing small sized old Isolated footing foundation system in position, i.e. the 1st

level footing placed at shallow depth level with field SBC 135 kN/m2 and the remaining 2/3 loads of the column is

considered to be carriedout by 2nd

level combined footing (Figure 9&10) to be placed at shallowest depth i.e. closed to

ground floor level with higher or improved SBC. Thus all combined footings were designed with improved SBC evaluated

as 180 kN/m2. The method of improving the SBC from 135 kN/m

2 (unrestrained) to 180 kN/m

2 is done at site by

replacement of in-situ silty-clay layer by gravelly-sand soil with proper mechanical compaction by hand ramming where

there is no use of skirting method is adopted. After designing, the execution were done in the old structures as shown in

Figure 10.

Construction of 7-Storey Building with BL-FFS

The Proposal and Action Plan

An Executive Engineer, WRD, Govt. of A.P by profession has entrusted the job for planning, designing and

detailing of (G+6) storey rcc building to be constructed at his proposed building plot in the ridge of a low rise hillock

situated at Doimukh township, Naharlagun i.e. capital complex of Arunachal Pradesh (AP). The global location of this

building plot is 27008'47.05" N; 93045'13.26" E; Elevation of the Site: 460 Feet above MSL. His initial aim is to achieve


economy in construction and safer building apartment for private residential complex for occupation.

The project ‘Design of 7-Storey rcc Building with BL-FFS (Bayan, 2011.d) was completed pertaining to the

conception as shown in Figure 1(a), which contains necessary detailing all about the design and reinforcement

specifications of members including Bi-Layer footing foundation system. To meet the above demands, efforts have been

made in accordance to the design guidance stipulated in different BIS: Code of practices, which are followed obediently.

However, the only exception is about the introduction of BL-FFS as new foundation system, the design of which has a

little difference. Hence, the proposed 7-Storey rcc building is designed to achieve safer situation during High Earthquake

shocks of its life period with more economic gaining.

Geotechnical Investigation

The building plot is an open land. Obviously, to establish the new conception of the subsoil investigation method

described earlier was followed and carriedout. It was ascertained at field that there is no void exist within the successions

of the subsoil layers upto a depth of 15 m from the proposed foundation level i.e. the site has integrity among its

constituent layers.

Looking to the above situation no field PLT was carried out as the whole soil profiles has granular materials only.

Therefore, disturbed (D) and undisturbed (UD) soil samples were collected to analyse the same in the laboratory. During

investigation it was recognized that though there is no normally standing GWT as a high land in hilly terrain yet there is a

pond found adjacent to the site, hence, ground water effect on the foundation is considered.

Table 4: Result of Various Geotechnical Properties of the Subsoil of 7-Storey Building Site

Sl.

no.

Location

of

undisturbe

d soil

Samples

Depth

of

sample

‘D’ in

metre

Field

Bulk

density

‘’ in

gm/cc

Natural

mois-

ture

content

‘W’ in

%

Sieve analysis (p. c. passing) size in mm

Shear parameters at

Wet density1.85 &

moisture content 16.5%

40

mm

20

mm

10.0 (IS:

10)

4.75 (IS:

480)

2.36 (IS:

240)

1.18 (IS:

120)

0.60 (IS:

600

micron)

0.3 (IS:

300

micron)

0.075 (IS:8

=75

micron)

Cohesion

‘c’ in

kg/cm2

Angle of

internal

friction ‘’

in degree

1.

Mixed

Sample from site

1.2 1.85 9.2 100 90.2 84.2 90.9 75.2 69.3 53.9 40.1 25.89 0.10 29.75

F.M = 3.64

2.

UD Sample

from

site

2.0 1.95 7.4 100 100 90.1 87.3 81.4 75.2 62.2 47.3 18.5 0.25 35.0

F.M = 2.38

With the collected soil samples routine tests were carriedout in the laboratory except some special kinds of tests

carriedout, which were described in some earlier pages. Result of all the tests carriedout in the laboratory were found out

and utilised in design purposes. Some of the results were tabulated and are given in Table 4 & Table 5 and shown in Figure

4. On the other hand, in fixing the shear parameters, the role played by Large Box Direct Shear Test (LB-DST) is

prominently optimum and hence, its value is used in calculating the SBC by using standard equations given in the relevant

BIS:Code of practices. To perceive such important consequences Table 5 gives the evaluated shear parameters out of both

kinds of test done in DST machines. Therefore, in this BL-FFS SBC plays a vital role. Henceforth it must be evaluated as

given herein rather assuming. Assuming SBC or finding SBC through conventional process, where FoS is normally taken

as per general guidance stipulated in BIS: code of practices, which should be avoided. For instance, in this present case a

comparison has been made in evaluated SBC and calculating SBC using shear parameters of Table 5 are given in Table 6

which reveals the economy with safety.

72 Gokul K Bayan

Analysis and Design of Structure

Analysis of Building Structure

Analysis and design of superstructures were performed as described earlier. In this regard, the seismic analysis of

column-Beam frame of portal MP-8 of the proposed 7-storey building along X-direction is shown in Figure 11.

Table 5: Result of 2-Kinds of Direct Shear Test Revealing Shear Parameters of the Subsoil in the Laboratory

Sl.

No.

Location of

Undisturbed/

Remoulder

soil Samples

Depth

of

Sample

‘D’ in

Metre

Various Properties of Direct Shear Test

Small Shear Box Saturated Drained

Test Large Shear Box Saturated Drained Test

‘’

in

kN

/cu

m

as

test

ed

‘W’

in

% a

s

test

ed

No

rm

al

stre

ss

in N

/mm

2

Sh

ea

r st

ress

in N

m2

‘C’

in N

/mm

2

‘’i

n d

egre

e

‘’

in

kN

/cu

m

as

test

ed

‘W’

in

% a

s

test

ed

No

rma

l st

ress

in N

/mm

2

Sh

ear

stre

ss

in N

/mm

2

C’

in N

/mm

2

‘’i

n d

egre

e

1

Mixed

Sample

from site

1.2-

2.3

197 17.1 0.000 0.000

0.0

26

36

.0

185 16.5 0.000 0.00

0.0

1

29

.75 197 17.1 0.021 0.043 185 16.5 0.007 0.014

207 16.9 0.098 0.103 185 16.2 0.038 0.029

236 16.6 0.204 0.206 186 16.0 0.086 0.059

252 16.4 0.396 0.272 187 15.7 0.136 0.088

2.4 16.1 - - 188 15.3 0.206 0.128

2 UD Sample 2.3 183 7.4 0.025 35 - - - - - -

Table 6: Comparison between Evaluated (Model test) and Calculated (as per BIS:6403-1981) SBC of Same Subsoil

Having Evaluated FoS 3.5

Process Field SBC

in kN/m2

Skirted SBC in

kN/m2

Upward Soil Pressure on

Foundation Structure

Using soil parameters 88.7 146 99.4 in kN/m2

for IFF and

154.2 in kN/m2 for SFF From Model PLT test 107 158

Analysis of Surface Foundation Structure as 2nd

Layer of BL-FFS

After finding the vertical load and moments subjected to the base of column, the foundation connecting the

columns are analysed properly. Thus IFF and SFF have been designed according to the procedures described earlier which

are shown in Figure 1(a&b) and Figure 8.

However, the analysis of beam combining many columns in a line has followed different aspects. Figure 7

represents such type of loading diagram of main central beam of SFF pertaining to foundation Block-1 of the proposed 7-

storey building having 8 numbers of columns. It includes some action of moments coming from 1st layer foundation.

Hence, upward pressure will increase due to the existence of 1st layer foundation besides the reaction transferred from

Secondary beam into the main beam. On the other hand, the reaction transferred by the end part of secondary beam will be

0.93 times of middle beam is also considered. In addition to the above followings are to be considered as under:

The beam will get soil reaction from the slab immediately below it i.e. from beam rib, Skirt wall & Cantilever part

and

The reaction of Footing slab of the Isolated foundation placed as Layer-1 of this Bi-Layer FFS.

After finding SF and maximum BM, all beams of the proposed SFF pertaining to whole building were designed

(Figure 8). On the other hand, the seismic analysis of different MP and SP of superstructures were done as per procedure

shown in Figure 11. Final reinforcement specifications of all the designed members are given in corresponding working

drawings for execution. However, Table 1 explains the same for all members comprises Block – 1.


Figure 11: Represents the Sectional Side Elevation of the 7-Storey Building Showing Seismic Analysis of the

Column-Beam Moment Resistant Frames of the Superstructure along MP-8

Economy and Execution of Bi-Layer Footing Foundation System

For the purpose to fulfil the owner’s demand of having more economic building, it has already been described in

earlier pages that the BL-FFS so used reveals economy besides more safety and comforts. The material and cost analysis

carried out for foundation Block-1 are given in Table 2. Based on the cost analysis given in Table 2, it is proclaimed that

the new form of foundation system i.e. BL-FFS is about 60% more economy than the conventional Isolated footing

Foundation system. On the other hand, regarding execution of the proposed building, it is in final stage for preparation of

commencing the construction work at the earliest.

DISCUSSIONS

Consequences of non-investigation of proposed building site and assumption of SBC leads to failure risk on

structures, which is dealt herein details. It proclaims the importance of the recommendations of results of geotechnical

investigations and their uses in design and execution for any building sites. Otherwise, case of retrofitting may arise in the

life of built up structures as it is reflected in 4-storey rcc building stated as 1st example. In consistence to the above

findings, the executed process of the retrofitting solution generates a new conception of foundation system, which is

termed as Bi-Layer Footing Foundation System – as described herein.

A new and easy procedure of recognizing SBC for BL-FFS was also developed and established briefly herein,

such that the expert engineer can follow it perfectly. Similarly, structural analysis which is needed for design of building

are also dealt meticulously and described and established as far practicable limit. It is also analysed more elaborately all

about the cost involved in developing substructures in terms of material estimates which yields better economy for using

BL-FFS. At the end, it was found out that using the granular soil material just below the surface foundation i.e. 2nd

level of

foundation as structural soil layer bringing more damping effects on the superstructures during earthquakes. Hence, using

of BL-FFS offers the building user more safety, economy and comfort.

CONCLUSIONS

The present innovative R&D study carried out on foundation systems for multistorey building may be concluded

as:

74 Gokul K Bayan

For any building project any form of geotechnical investigation on the proposed site is mandatory to carryout, as

good site becomes scarce.

Ignoring the evaluated SBC and settlement criteria no structure reveals risk free habitation, not only for BL-FFS

but also for any multistorey building, this may offer threatening risk for occurring failure hazard.

Bi-Layer FFS offers safer stay in multistorey rcc building provided perfect plan, analysis, design and execution of

2nd

level foundation i.e. surface foundation level are done more technically particularly in high earthquake prone

zone.

A general construction habit may be grown up with Bi-layer FFS especially in earthquake prone region because it

offers more damping effects and yields more than 60% economy in development of its substructure.

Retrofitting of foundation system of old multistorey building is possible by adopting the concept of BL-FFS.

ACKNOWLEDGEMENTS

The author extends his thanks gratefully to Dr. R.C. Barua, Acting Director, North East Institute of Science and

Technology (CSIR), Jorhat, Assam; India (formerly RRL-J), for his kind permission to publish this paper. Further, the

author extends his thanks gratefully also to the organisation, M/S Design-Tech Engineering Developer (DgT-ED)

(Formerly, Design-tech Pvt. Ltd). Assam, an NGO, for their kind permission to use their design data related to both

building projects.

REFERENCES

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Better Foundation soil. International Journal of Scientific & Engineering Research, (ISSN 2229-5518); Impact

Factor: 1.4. Vol-4, Issue-6, June, pp 574-579.

2. Bayan Gokul K (2013.b): Improved Bearing Capacity for 2nd Layer of Foundation Subsoil in Bi-layer Footing

Foundation System – A Concept of Surface Foundation. Phoon, K. K., Chua T. S., Yang, H. B. & Cham, W. M.

(editors).Accepted for oral presentation and publication in International Symposium on Advances in Foundation

Engineering (ISAFE 2013) 5-6 December 2013, Singapore.

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Geostructure; Q. Yang et al (Eds): Constitutive modeling of geomaterials, SSGG; Beijing, China [© Springer-

Verlin Heiberg], pp 391-397.

5. Bayan, Gokul K (2012.b): Road foundation – a new avenue for its stable and everlasting aspect”. B. Indraratna,

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Hanoi 2011; Phung (Edt.); Construction Publishing House (ISBN 978-604-82-000-8); Vietnam; pp 387-398.

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Kerala, pp 999 – 1002.

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storeys & completion of the remaining construction of his rcc Building at Polo-colony; Papum Pare Dist., Report

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Footing Foundation System at Doimukh; Papum Pare Dist., Technical report of M/S Design-Tech Pvt. Ltd. Report

No.DgT/ Nabam/ rcc Building/ 7s-2011 July; Naharlagun; Arunachal Pradesh; India pp 1-166.

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11. Bayan G. Kakati and Kalita U.C. (1992): Detail analysis and design of foundation for vibratory machine in weak

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214-218.

12. Bayan G Kakati (1990.a): Geotechnical Investigation for Foundation of 1600 KVA Diesel Generator sets inside

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Regional Research Laboratory (CSIR); June, 1990; Jorhat, Assam, India, pp 1-42.

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in North-eastern India. Journal of Masonry International, June, London; Vol-3, No-3, pp 86-89.

14. Bayan G. Kakati, Borthakur B C, Nambiar MKC, Barua P, Biswas A, and Kalita U C, [1990.c], “Geotechnical

Investigations for Digboi Refinery Modernisation Project”, Technical report No. An/112/C/1989, Applied civil

engineering division, Regional Research Laboratory (CSIR), Jorhat; Assam; India; pp 1-392.

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