53.-Brick Wells for Deep Foundations of Bridges in Pakistan

ABSTRACT

In 1968 IIVO mulli-span higlllvay brid­ges were compleled over rivers in West Pakistan. The first complete bridge ai lhelum eomprised Iwenly-/lVo spal/s of 147 fI w"ile lhe bridge aI BahalV­alpur "as Mell'e spans of 160 fI. The bridge piers arefoul/ded 011 rei/iforced brick walled caissol/s 25 fI in diameler sunk lo a deplh of 115 fi below f100d levei. This lVas considerably cheaper Ihan olher forms of foul/dalion. The desigll and eonsrluclion of lhe lVells and inherent problems connected with Ihis Iype of foundalion are described.

1. INTRODUCTION

by G. O. KEE DOllovan H. Lee & Partners, Londofl SWJ

L'Ulilisalion des Briques dans les Fondations d'un Pont lmpm'tant

En 1968, deux ponls de grande com­munication à plusieurs arches on! été achel'és sur des fteul1es dans te Pakistan Oues/ . Le premier pont, à Jhe/um, comprenail 22 arehes de 45 melres tandis qu'à Bahawalpur, le pont com­prenail 12 arches de 49 melres. Les piles de ponls prennenl place dans des caissons de 7·60 m de diametre, à parais ren/areées en brique el qui sonl e"fouis à uI/e profondeur de 35 In au­dessous du niveau du jfew1e. Ce type de fondalions élail considérablemenl moins coúteux que d'au/res. L'étude et la constructiolJ des murs ainsi que les problhnes inhérenls à ee Iype de fondalion sonl décrils.

Heavy brick masonry is no longer used to any extent in Great Britain, but in some overseas countries it is found to be a suitable rorm af construction for certain purposes. Large-diameter brick wells have been constructed for bridge foundations in West Pakistan for at least 100 years and are still in many cases the most satisfactory way of providingdeepfoundations in fine sand. The bridges which were opened in 1969 had this traditionaI form of founda­tions, and the reason why it was chosen arose from the particular si te conditions.

Ziegelschiichle for das Fundamenl­maue,'we,'k von B,.ücken in Pakistan

1m Jahre 1968 wurden zlVei lI1ehrbogige StrajJenbriicken über F/üjJe in West­pakislan ferliggestelll. Die zuersl "ollendele Brücke bei JheJum beslehl aus 22 Abschnitten vonfast 45 In Eill­zel/lil/ge, wlihrend die Brücke bei Bahawalpur zw6/f Abschl/ille mil je rund 49 m Llinge hal. Die Brüekenp­fei/er sind auf Senkkiislen aus bewehr­tem Ziegelmauerwerk mil 7,60 m Durchmesser, die 35m Ullter den hoch­sten Wasserstand abgesenkl worden waren, begrlilldet. Das war bedeutend billiger aIs andere Fundamentierungs­Arlen. El1twurf llnd Konstruktion der Mauern sowie spezielle Probleme dieser Fundamentbauweise sind besch­rieben.

, ,

Probably nowhere in the rest of the world are there such large depths of seasonaI scou r of the river beds as in the Punjab. These exceptionaI conditions are due to the e!fect of very Iarge peak floods-caused each year by the monsoon rains and the melting of the snows in the Himalayas- on the great depths of fine sand and silt transferred from upstream over a considerable period. In the case of the two bridges lhe maximum scours at the bridge piers were calculaled to be 63 fI and 75 fI below high flood leveI, necessilating lhe underside of the foundations to be 90 ft and 115 fI respectively below flood leveI.

FIGURE I- The bridge over the River Sutlcj at Bahawalpur, comprising twelve spans of 160 fI. (The bridge aI lhelum is similar

but has twenty-two spans of 147 ft).

In addition at one site the stone for concrete aggregale had to be brought nearly 500 miles whereas the bricks were made Iocally. Tn lhese circumstances brick wells were the natural soIution being both the traditionaI and locally understood form of construction.

2. DESCRIPTION

The wells were 25 ft ex!. dia., with 3 ft 9 in. thick walls, and were sunk by building up 20 ft of brickwork on a concrete kerb and grabbing out the sand from inside the well. A further 20 ft of brickwork was then added and the wells sunk again, and so on unliI their full depth was

323

obtained using concrete or sand-bag kentledge where necessary to overcome the side friction and make the wells sink.

The wells were reinforced with mild steeI to cater for any tensions which could develop if they became gripped by the ground near the top while the base was free; aIs o in the final position they had to be designed for maximum scour occurring at high fiood times, and aIso to allow for some seismic forces. After sinking, the wells were plugged with concrete and filled with sand. The top of the wells were generally corbelled out to take the concrete cap and the bridge piers.

Although wages are Iow in Pakistan, Iabour is not cheap; the rate of working is very sIow and lhe output is low. However, in the construction industry, manual

324 Brick Wells for Deep Foundations of Bridges in Pakistan

I:il.i" wOill..!::!.V.!!... __

PrCltr~'Hd Gird~r' ot 9ft Oin . cn.

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8ft lin.

Circular Pi~r - --

35ft " in.

K~rb

19ft Oio. 8ft l in.

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Normol B~d L~v~I .. .....,. hA'C I h I ... Jsl-) ,..,.:_.;:- • XW::= i I ---- ---- - -+-

R~inforC«<l !rlck-walled I Op~n CailSOo ,/

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FIGURE 2- Section through wells at Bahawalpur.

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G. O. Kee 325

FIGURE 3- Bricklaying in progresso

FIGURE 4--Reinforcement in wells.

labour is usually cheaper than mechanized methods and imported plant is in short supply.

The standard of bricklaying was not high for this sim pie work. A team of twelve masons could raise a well 3 ft during a particularly good day, and this meant about 800 bricks per mason in a 12-h day, but gencrally the output was much less.

General labourers earn about 3 rupees (f 0'25) a day, bricklayers about 8 rupees and much of the general carrying is done by nomadic women coolies. Donkeys are the general method of transporting bricks for short distaoces, but camels or butfalo carts are also used at Bahawalpur, a near-desert regiDo.

Although Bahawalpur is a brick-producing area there were great difliculties in obtaining suflicient suitable bricks even though the strength specified was only 1800 Ibf/in'. Ali the local brickworks were checked for quality hefore the contract was placed and a limited number were approved as a source.

The bricks were made from local silty clay in 'bull's trench' type kilns of about 100 000 to 125000 bricks

capacity. A two-man team of a 'mixer' and a 'moulder' make the bricks. The mixer selects the silt and by adding water l11ixes it to a consistency sllitable for moulding. He then kneads it and rolls it into lumps which he passes to the moulder. The moulder forms each brick in a metal mould . The bricks from the mould are Ieft for 10 to 14 days to be dried and hardened by the sun. If the mixer makes the mix too dry, it is diflicult for the moulder to fill the mould satisfactorily and there are often cracks in the bricks because the mixer has folded the mix without it binding on itself.

If lhe mixer makes the mix too wet , the bricks 'run ' after being moulded. The resu lts are badly shaped brick or bricks out of square. Neither of these two faults ever appear to deter a good team on piecework who will still produce upwards of 100 bricks per hour without appa­rently noticing lhe diversities of their material.

When the bricks have been sun dried to such hardness that they can be handled without breaking, they are placed in the kiln usually by donkey labour. The kiln is fired usually with a mixture of coai and wood to start with and then by wood alone. Coai (of poor quality) is found in small quantities in one area in North Pakistan and is expensive. Hence wood firing is generally preferred. Firing takes about 2 weeks and there is no systematic temperature control. The result is that bricks nearest to lhe firing are over-burnt and those at the outer edge of the kiln are under-burnt.

It should be realized that although 50 % of the kiln output might be suitable for bridge masonry, when the cracked and mis-shaped bricks have been rejected, the figure is probably nearer 33 %. Each brickmaker usually had two kilns running and since they could sell the whole of their output, inc1uding under-bllrnt and over-burnt bricks, for normal building construction, great difficulty was experienced in trying to get them to improve their produc!.

The cost of bricks delivered to the si tes was 60 rupees (1:5·00) per thousa nd and each brick was individually examined for shape--although not very critically- and a high proportion were rejected. However, as stone was so diflicult to obtain there was a ready market for rejects, in fact many thousands of over-burnt bricks were broken down for use as the sub-base of the roads. In the twenty­three wells at Jhelum over 5 million bricks were used, and at Bahawalpur over 31 million bricks were used for the thirteen wells.

The morta r was I to 3 using the local river sand from selected areas which were clean and relatively free from si!t. Thc ccrnent varied considerably and care was necessary as to the factory from which delivery was accepted. In spite of this the mortar strength was very consisten! a! abou! 2000 Ibf/in'-

The bricks were generally soaked in pits for at leas! I h before laying and the work was cured for up !o 2 weeks with damp bessian. This was necessary as the summer temperatures sometimes rose to 46°C.

The stresses which develop during sinking cannot be accurately estimated and the wells were reinforced against them using I ft 6 in. x 3 ft O in. horizontal bands of reinforced concrete every 30 ft with I-in-dia. vertical rods and i -in . horizon!al rods on both faces. In recen! years there have been one or two cases of large wells splitting during sinking bu! no such difliculties were experienced bere.

326 Brick Wells for Deep Foundations of Bridges in Pakistan The brickwork was designed for various combinalions

of slresses due lo:

(a) Ali verlical loads, bolh dead and live.

(b) Wind on bridge.

(c) Forces due lo lhe current flowing against the piers and wells at time cf maximum scour.

(d) Lateral pressure of the ground.

(e) Seismic efrecls, taken as a horizontal force of W /20 for Ihis region.

The calculaled slresses are given in Table I.

TABLE l -CALCU LATED STRESSES

lhe/um Sutlej

max. mino max. mil1.

Max. wind and water but no Iive load (lbf/in2) 157·1 35·8 96·4 52·9

FulJ load including seismic forces (Ibf/ in 2) 205·3 12·4 200·6 14·5

Estimated max. teosion which could develop during sinking (tonr) - - 167 -

The design stresses in the brickwork were taken as 170lbf/in' plus 50 % extra when seismic forces were added, so the sections were jusl acceptable. The lension during sinking was assu med lo be laken by lhe fifty I-in.-dia. vertical bars aI 12000 Ibf/in' max.

I! will be apprecialed Ihal lhe sinking of Ihese wells presenled many problems. The mass of brickwork, as opposed lo thinner concrele weUs which are somelimes used , was mosl helpful in providing weighl lo make lhe wells sink but when Ihey were abouI Iwo-Ihirds down ir was generally found neeessary lo slarl adding kenlledge of sand-bags Of concrete blocks, and in some cases up to 1000 tons had to be added and lhe waler levei inside lhe weU pumped down lO reduee the buoyaney and make lhe sand aI lhe bottom ' boil '. Under these eonditions lhe weU would somelimes hold up and lhen slarl to sink rapidly and Ihere was always lhe risk of it moving oui of plumb. The specifieation only aUowed for lhe weUs lo be up lo I in 60 oul of plumb, and out of posi lion aI the 10p by 6 in. , so the conlractor had lo be eonlinuously correcting lhe lilt by eccenlric kentledge and grabbing from one side or lhe olher.

GeneraUy lhe wells were sunk wilhin lhe limits per­mitted, or perhaps only a lillle outside Ihem excepl for one abutmenl well which sluek when passing lhrough a lough band of c1ay and lilted unlil il was aI a slope of I in 16. As Ihis weU was aI ils design deplh il had lo be sunk below Ihal levei aI lhe eonlraclor's expense and while lhal was done 400 lons of kentledge was plaeed on

FIGURE 5-Corbelling ai top of well to take capo

FIGURE 6-Kentledge to assist sinking.

a braekel buill ofr lhe cap giving some 14 fI of eccen­lricily and aI lhe same lime lhe ground oulside lhe weU on lhe high si de was exeavaled 30 ft deep. Evenlually lhe well was broughl back to wilhin 12 in. of ilS correel position and this was Ihen accepled.

At Jhelum, where lhe flood season could lasl up lo 5 months, and during thal time little or no work could be done on lhe river bed , speed of sinking was mosl criticai and wells were sunk in aboul 3 working months. AI SuUej, where lhe wells were deeper, Ihere was a much longer season and aboul the quiekesl well look 4 months to sink.

Although lhe work described above may nol bear muc~ resemblance lO lhe usual presenl-day masonry wilh high slresses and carefully eonlrolled qualily, these conlracls are typical of many bridge foundalions in lhe area and represenl a traditional form of conslruction which is likely lo be used lo provide e!fieient and economic foundalions for many years lo come.