CHAPTER IX

STUDIES ON MULTIPLE FUNICULAR SHELL RAFT IN THE FIELD

9.1 General

There are vast areas of soft compressible soil

deposits which pose serious foundation problems. A

feasibility study of using multiple funicular shell rafts

for providing efficient and economical foundations in place

of conventional ones in such sites was considered

worthwhile. With this end in view, a study was made by

building a raft using 110cm x 110cm size funicular shells.

A single room structure was built over this type of

foundation and its performance was monitored for more than

eight years and ten months since 1978.

9.2 Site of the Experimental Study

The study was conducted at Mankompu in Kuttanad,

Kerala State, India. Originally the entire area was below

mean sea level. The typical soil profile at the site

consisted of very soft to soft marine clay deposits of 8 to

10 metres thick underlain by slightly stiffer clays upto 30

to 35m depth. As the entire area is water-logged,

construction of buildings is carried out on reclamations.

Even single storeyed residential buildings on conventional

spread foundation in the area have shown distress by

excessive settlements and cracking. Typical cases of large

deformations on walls and floors of structures due to

foundation problems, are illustrated in Figs. 9.1a to 9.ld.

The piers and abutments of bridges constructed in the region

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are fouI;ded on piles and wells. Buildings with more than

two floors are nearly absent in this area due to the serious

foundation problems encountered. Raft foundation is one of

the types of foundations which can be resorted to under such

conditions. It is proposed to examine whether funicular

shells in the form of multiple units is a suitable and

economical alternative to conventional raft foundations.

For this purpose. a prototype structure on a raft foundation

using four funicular shells with suitable connecting beams

was built and its performance monitored. The soil profile

at the site is given in Fig.6.6.

9.3 Details of the MUltiple Funicular Shell Raft and the

Superstructure

The raft was constructed 60 m below the existing

ground level on the reclaimed soil of sandy loam of grain

size distribution shown in Fig.6.B. A pit of 3m x 3m size

and 60cm deep was excavated at the site for the experimental

structure. A layer of river sand of 10 cm thickness was

spread and compacted. The position of funicular shells were

marked and the sand bed was shaped to receive the conve~

face of the funicular shells. The distance between the

adjacent shells was kept at 20cm to ensure proper filling in

of sand below the shells and between the shells before the

connecting beams are cast. After placing the shells in

position. sand and water were poured in between the shells

and on their sides and properly compacted to ensure perfect

support for the shells. Then the formwork for the

connecting beams and edge beams was made and erected in

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position. The arrangement of shells in position and the

shuttering work with reinforcement for the beams were as

shown in Fig.9.2. The connecting beams were designed to

take up the hogging moments due to the soil reaction. The

edge beams were cast directly on the edge beam of the shell

and were given nominal reinforcements only. Since the

purpose of the experiment was to study the behaviour of the

multiple shell raft units on soft soils, the connecting

beams were slightly over-designed so that they would act

more or less rigidly when the structure built over the raft

was test loaded. The connecting and. edge beams were

concreted with a nominal concrete mix of 1:2:4. After

curing for 28 days, the super structure was constructed on

the edge beams with country burnt bricks of size 20cm x

10cm x 6.5cm in cement mortar 1:4 mix. Four scales

graduated to an accuracy of O.lcm were fitted vertically at

the corners of the structure while the masonry work was in

progress. The walls were constructed to a height of 255cm

above which a reinforced concrete slab of 1Scm thickness

was cast to enable further loading of the structure. A

parapet wall 20cm thick and 90cm high was constructed above"

the roof slab with cement mortar 1:3 mix. The sectional

elevation and plan of the structure are ahown in Fig.9.3.

The inside faces of the parapet walls and the top of the

roof slab were plastered with fine cement mortar so that

the structur~ could be loaded with saturated sand above the

roof. The structure was completed in March 1978 and its

general view is shown in Fig.9.4.

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9.4 Performance of the Foundation

Measurements of soil pressures were attempted but

were not successful over a period of time, presumably due to

entry of some air in the measurement system. Hence the long

term pressure measurements had to be discontinued.

The settlements at the four corners and the centre

of the structure were measured using a precision level with

which readings up to O.Olcm can be read on a scale and upto

O.001cm by estimation, with scales held on to the structure

and a permanent bench mark established 15m away from the

structure. On 15 th August 1978, the structure was test

loaded by pumping in water on to the roof top in stages of

1Scm height of water at a time. The maximum load on the

foundation with the tank full of water was estimated to be

19.5t. This load was maintained till 1st of llovember 1978

when the water was emptied and river sand was refilled with

an average density of 1.75t/m3 and water pumped upto the top

of the fill. Then the load was estimated to be 23t. This

load was 1.44 times the ultimate combined capacity of the

four shells in the unit. The unit had almost uniform

settlement. The settlements of the structure since th~

start of its construction are given in Fig.9.S. The shells

are intact even today after nearly nin~ years since the

completion of the structure. The performance of the

structure had been very satisfactory from the point of view

of settlements and structural stability. The load of 23t

corresponds to an average contact pressure of 4t/m 2• For a

three or four storeyed building with room· dimensions in the

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range of 3 to 4 metres, the mUltiple funicular shell raft

may prove to be an economical and efficient foundati.on as

in the case of mUltiple funicular shell footing units. By

properly designing the connecting and edge beams, the shells

may be put to its optimum use.

9.5 Special Features of Multiple Funicular Shell Rafts

The self weight of the multiple funicular shell

raft is less than that of a conventional raft, designed to

transmit the same pressures to the soil.

If there are some minor loca\ised settlements in

the foundation, the readjustments of the shells can take

place without necessarily producing undesirable

defortnations.

The multiple funicular shell raft foundation may

prove to be more economical and efficient than the

conventional raft or pile foundations for three or four

storeyed buildings with room dimensions of 3 to 4 metres.

Incidentally, this study has shown that rafts using

funicular shells can be a suitable proposition in

prefabricated constructions for large housing projects and

townships on soft soils. However, there is need for

practical construction of large sized structures and their

monitoring since the size effects can lead to larger total

settlements and consequently the differential settlements

may become higher.

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FIG.9 .1a. CRACKS ON A WALL DUE TODIFF ERENTIAL SETTLEMENT OFFOUNDATIONS

131

FlG.9.1b. HORIZONTAL CRACK ON WALL ATWINDOW SILL LEVEL

132

FIG.9 .le. CRACKS ON A VERANDAH PARAPETWALL NEAR THE CENTRE OF THESPAN BETWEEN TWO COLUMNS.

133

FIG. 9 .ld. CRACKS ON THE FLOOR DUE TODIFFERENTIAL SETTLEMENTS OFFOUNDATION SOILS UNDERBASEMENT FILL

134

FIG. 9·2. DETAILS OF REINFORCEMENT FORCONNECTING BEAMS AND EDGE BEAMSOF MULTIPLE FUNICULAR SHELL RAFTUSED IN FI ELD TEST.

135

SOIL

FUNICULAR

Scale

dge beam

Pressure cell

RCC Roof slab20cm thick brick

wall

Connecting beam

..,.Scale

1""""-20cm thick parapetof brickwork

ScaleftC---t-----~-.lH-Funicular shell

G.L.

-

"" E

/,-,<-

I~('rl7"-"\ I'-' -"

i- -

1Eu

aen

] 1'"i:.-.

IIEu

Lf1Lf1

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- ..··:·f ..... ~ ,.4, b:~~-:.

E l·Wi\<,:,........••..........:...'.?•.., ~, :".'" ,; •. :<i: SOF Tu 1 Sectional Elevationa~

fEu

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vr~---------255cm ~

Plan(Foundation leve I)FIG9.3TEST STRUCTURE ON MULTIPLE

SHELL RAFT IN THE FIELD

136

FIG. 9.4. STRUCTURE BUILT ON MULTIPLE FUNICULARSHELL RAFT IN FIELD.

137

y

)' N~MBER OF YE~RS SINCE START OF CONSTRUCTION6/ - 8 91 2 3 4 5 6 7

~1\~\

~

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------ ----r---.

---"w00

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°30«320

10

o1

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:= 8wIf) 9

)101112

FIG.9.S GRAPHS OF LOAD vs NUMBER OF YEARS AND NUMBER OF YEARS

vs SETTLEMENT OF THE STRUCTURE BUILT ON MULTIPLE FUNICULARSHELL RAFT