Geotechnical behaviour of shell foundations

PRESENTED BY SABNA THILAKAN

Semester III

ME (Found. Engg)

Guided by

Prof. Nisha Naik

4-02-2015

GEOTECHNICAL BEHAVIOUR OF

SHELL FOUNDATIONS

Objective of study

Provide and insight into the geotechnical

behaviour of shell foundations

Showcase its advantages over conventional

foundation structures

Popularize the concept of shell foundations

as an economical replacement to

conventional flat footings

Shells

Structures that derive its strength from its

geometry rather than mass

Three dimensional manifestation of the arch-

action

SHELLS

Commonly found shells in nature

Shells as structures

Used widely as roof structures both in the

past as well as present

Gives maximum strength with minimum

material consumption

Very economical as they can cover large

space

Aesthetics

Shell roofs in Germany showed

remarkable resistance to bombing during

World War II

Some famous shell structures

Shells as foundations

Many buildings in India have been supported on

inverted brick arches

IS 9456-1980 Code of Practice for Design and

Construction of Conical and Hyperbolic Parabolic

Type of Shell Foundations

Shells as foundations

Classification

Shells

Curved

Singly curved

Doubly curved

Folded Plates

Upright Inverted

Doubly curved

Synclastic(Curved in same direction)

Eg. Spherical dome

Anticlastic(Curved in opposite direction)

Eg.Hyperbolicparaboloid

Synclastic and Anticlastic

Shells Used in Foundations

Hypar (Hyperbolic paraboloid)

Made up of two parabolas one over the other

Anticlastic

Also formed as a warped surface using straight line

generatrix

Conical

Simplest form of shell

Pyramidal shell foundation

Square or rectangular in

plan

Combine to form raft

Spherical shell

Complex geometry

Does not possess

straight line property

Difficult to construct

Costly

Elliptical paraboloid shell foundation

Synclastic shape

Used a single unit of

foundation

e.g towers, chimneys

Built with edge beams

iver which columns can

be supported

Inverted Dome Shell

Foundation

Paraboloid and Hyperboloid of

revolution shells

Suited for tower shaped

structures

Literature Survey

Hanna and Abdel- Rahman (1998)

studied behaviour of three types of shells namely,

triangular strip, conical and pyramidal

Compare results with conventional flat counterparts.

Loose, medium, dense sands

Experimental, numerical (PLAXIS) and theoretical

analysis

Special tests on coloured sands

On surface and embedded

Plain

strain

Axisymm

etrical

Three

dimensio

nal

Flat strip Flat

circular

Flat

square

Rise to

half

width =

½

Triangula

r 1

Conical 1 Pyramidal

1

Rise to

half

width =

1

Triangula

r 2

Conical 2 Pyramidal

2

Square flat and Pyramidal model

at ultimate stage

Typical Load-Settlement Curves

for Plain Strain conditions

Comparison

Shell efficiency factor (η )

Settlement factor (Fδ)

Qus is the ultimate load of shell

footing

Quf is the ultimate load of flat

footing

Fδ= Non dimensional settlement factor

δu= Settlement at ultimate load

γ = Unit weight of soil

Ah= area of footing as projected

horizontally

Qu=Ultimate load

Ultimate load higher for shells as compared to flat

foundations

Bearing capacity increases with shell angle θ

Shell gain factor decreases with ϕ

Settlement factor was less for shells as compared to

flat footings

Rupture area was shallower as compared to that of flat

foundations

Calculation of bearing capacity (Abdel,1996)

qu = c Nc + γ Df Nq + γ b Nγ

Nc, Nq and Nγ are bearing

capacity factors as

functions of ϕ and θ

Kurian and Devaki (2005)

Conical,Spherical ,Hypars(a/b=1)

Experimental and FEM analysis

Bearing Capacity

Settlement characteristics

Comparison with flat counterparts (circular and square)

Parameters chosen:

• Interface roughness (µ =0, tan(2/3 ϕ) , 100)

• Type of soil ( c, ϕ and c - ϕ soils)

• Type of loading (vertical loading , horizontal loading, moments)

Huat and Mohammed (2006) FEM analysis

(PLAXIS)

Hypar , conical and

spherical

Load carrying capacity

Effect of edge beams

Effect of depth of

embeddment

Effect of

Embeddment

Effect of edge

beams

Huat et al (2007)

Triangular shell

Upright and inverted

Effect of shell thickness t and shell angle θ

Lab tests

Field tests

Numerical tests ( 2D and 3D using LUSAS)

Field test set up

Endalkachew (2009)

Conical shells and flat circular footing

Numerically modeled in PLAXIS

Soil used was red Clay

Mohr coulomb model was used in FEM

Azziz et al (2011)

Hypar shell footings

Best shell configuration for foundation

Easy construction because of straight line

property

Economical, Savings in construction costs

Rinaldi (2012)

Studied upright

and inverted shells

Modeled in PLAXIS

deflection

soil stresses

contact pressures

Triangular

modelsCylindrical

modelsPlaxis modeling

SummaryAdvantages of shell foundations over conventional flat

foundations

shells are simple to construct (especially hypar, conical)

Greater load capacity

Greater stability

Minimum material consumption.

Lesser construction costs.

Inverted shells have higher load capacity than upright

ones

Advantageous in developing countries with high

material-labour costs

Conical foundation suitable for chimney and tower like

structures.

Easier to construct than roof

Disadvantages

Limited shapes as foundations

Difficult to prepare formworks (esp. spherical )

Skilled labour required

Soil subsidence can pose problems

Uncertainties when dealing with soils with large

volume changes

References

Abdel-Rahman, M.M, and A.M.Hanna (1990) Experimental Investigation of Shell Foundations

on Dry Sand. Canadian Geotechnical Journal, J.35,pp.847-857.

Abdel-Rahman, M.M, and A.M.Hanna (1990) “Ultimate Bearing Capacity of Triangular Shell

Strip Footing on Sand”.Journal of Geotechnical Engineering ASCE, 116(2):1851-1863

Abdel-Rahman, M., (1996).” Geotechnical behavior of shell foundations”. Ph.D Thesis,

Department of Civil Engineering, Concordia University, Montréal, Canada.

Ángel E. C., William C., Yoermes G, José Á.(2011).”A look at half a century of shells

foundations, methods of calculation and associate research in Cuba” Revista Ingeniería de

Construcción ,Vol. 26 No3, pp 245-268

Fernando N., Sendanayake E., Sendanayake D., De Silva N., (2011) “The Experimental

Investigation of Failure Mechanism and Bearing Capacity of Different Types of Shallow

Foundations” Department of Civil Engineering, University of Moratuwa.

Hanna, A.M. and M. Abdel-Rahman, (1998). “Experimental investigation on shell foundations on

dry sand”. Can. Geotech. Journal., 35: 828-846.

Huat, B.B.K. and Mohammed, T.A. (2006). “Finite Element Study Using FE Code (PLAXIS) on

the Geotechnical Behaviour of Shell Footings”.Journal of Computer Science. USA: NY. 2(1):

104-108.

.

References

Huat B. B. K.,Mohammed T. A., Abang Ali A. A. A. and Abdullah A.A,(2007), “Numerical and Field

Study on Triangular Shell Footing for Low Rise Building”, International Journal of Engineering and

Technology,l. 4- 194-204

IS 9456 – 1980, Code of Practice for Design and Construction of Conical and Hyperbolic

Paraboloidal types of Shell Foundations

Kurian, N. P. & Jeyachandran, S. R. (1972). “Model studies on the behaviour of sand under two and

three dimensional shell foundations”. Indian Geotechnical Journal, Vol. 2, No. 1, pp. 79-90.

Minami K.,’Foundation Construction Using Hollow Shells to Minimize Settlement,” Part 1, General

Concepts, Trans., Arch. Institute of Japan, No. 37, pp 52-57, Tokyo

Nainan P. Kurian and V.M. Jayakrishna Devaki, (2005), “Analytical studies on the geotechnical

performance of shell foundations,” published on the NRC Research press web site at http://cgi.nrc.ca

on 19 May 2005

Rinaldi R.(2012).”Inverted Shell Foundation Performance In Soil”.A Thesis in the Department of

Building, Civil & Environmental Engineering. Concordia University, Montreal, Quebec, Canada

Varghese P.C.,(2009) .”Design of Reinforced Concrete Foundations”.Published by Ashok K Ghosh,

PHI Learning Private .pp 27