jamuna river 230 kv crossing, bangladesh. part 2: construction

Jamuna River 230 kV crossing,Bangladesh—Pt. 2: Construction

J.A. Chandler, M.A., F.I.C.E., L.W. Hinch, F.I.C.E., R.I. Fair, B.Sc, M.I.E.E.,D.A. Hughes, B.Sc, A.M.I.C.E., A.M.I.E.E., J. Peraino, B.A., F.A.S.C.E., and

Prof. P.W. Rowe, D.Sc, Ph.D., B.Sc, F.I.C.E.

Indexing terms: Cables and overhead lines, Power transmission and distribution

Abstract: The Jamuna River crossing is part of the 230 kV east-west interconnection project which intercon-nects the east and west networks of the Bangladesh Power Development Board. The purpose of the intercon-nection is to export cheaper (gas-generated) energy from the eastern grid to the western grid, to improve systemsecurity, and, in the future, to permit west-east transfer of energy from future nuclear and coal-fired plants in theWest. Part 1 of the paper describes the design of the crossing which consists of ten equal transmission-line spansof 1220 m, carried on 111 m high towers, with the two circuits arranged in delta formation. The eleven towersare supported on reinforced concrete caissons sunk some 100 m into the bed of the Jamuna River which isabout 12 km wide at the site of the crossing and has a maximum recorded flow of 92 300 m3/s. Part 2 deals withthe method of sinking the caissons, the erection of the towers and the conductor stringing, and the constructionproblems encountered.

1 Introduction

A construction contract for the crossing, including thecaisson foundations, transmission towers and conductors,was awarded in November 1979. Founding of the last ofthe 11 caissons was completed on 30th April 1982. Theerection of the 11 towers was completed on 9th October

© 1985: This paper is being published simultaneously in the Proceedings of theInstitution of Civil Engineers and the Proceedings of the Institution of ElectricalEngineers. Copyright in the paper is held jointly by the ICE and the IEE

Paper 3472C (P7, P8), first received 11th April and in revised form 12th July 1984Messrs Chandler and Hinch are with Rendel Palmer & Tritton, Consulting &Designing Engineers, 61 Southwark St., London SE1 ISA, England. Messrs Fairand Hughes are with Merz & McLellan, Consulting Engineers, Amberley, Kill-ingworth, Newcastle upon Tyne NE12 0RS, England. Mr. Peraino is with RaymondTechnical Facilities Incorporated, Consulting Engineers, 2 Pennsylvania Plaza, NewYork, NY 1001, USA. Prof. Rowe is with the Department of Civil Engineering,University of Manchester M13 9PL, England

centrestationlevel

-86.34- i-88.5

12.2 1.2

12:2d i a -1.83

design scourJfiJOL-60.5

poorly gradedmedium gravel

' K=O.O93m/s

design scourlevel -48.3

poorly gradedmedium gravelK = 0.093 m/sfine gravel

-filter K=0.0Um/s

-78.8fine gravelfilter K=0.0Km/s-91

caissons3,4,5,7,8and9

Fig. 1

caissons1,2and10Typical sections through caissons

1982 and conductor stringing was completed on 30thNovember 1982.

2 Caisson construction

Typical sections of the caissons, giving founding levels,river levels etc., are shown in Fig. 1.

2.1 Preliminary workAlthough borings had been taken in the project area, nonehad been taken at the site of each caisson. Further boringswere made at each caisson site, some of which were used asground-water observation holes during the sinking of thecaissons. No obstructions were found, nor any variationsin soils from those classified in previous borings. Becauseof the numerous sources of error associated with use of thestandard penetration test (SPT), referred to in the second

.17.0

caisson

6

design scourtaL

poorly gradedmedium gravelK = 0.093m/s

fine gravelfilter K=0.0U m/s

-86.2fine gravelfilterK=0.0Um/s

12.2

waterequilisingpipes

design scourlevel

53.5

poorly graded, medium gravelK=0.093m/s

poorly gradedcobbles andmedium gravel

= 0.04 m/s

caisson11

Note: Levels of different caissons vary, but were controlled during sinking to within approximately 0.15 m of the levels shown

IEE PROCEEDINGS, Vol. 131, Pt. C, No. 7, NOVEMBER 1984 319

paragraph of Section 8.1 of Part 1, a programme of staticcone penetrometer testing was carried out at each caissonsite before, during and after caisson sinking as follows:

(a) 4 tests to a depth of 107 m before any sinking com-menced

(b) 4 tests to a depth of 6 m below the bottom of thecaisson when the design scour level was reached

(c) 4 tests to a depth of 6 m below the bottom of thecaisson when the full design depth was reached.

The purpose of the tests was to assess the degree to whichthe ground surrounding the caisson had been disturbedduring sinking.

2.2 Cone penetrometer test dataConsidering that cone penetration testing to 107 m depthwas clearly a major application of the method for animportant structure, it was disappointing to find that theapparatus delivered to site had no electrical transducer atthe tip. The cone-tip resistance was recorded mechanicallyat ground level. Dividing the tip bearing stress by the verti-cal static effective stress for each test elevation andground-water level at each site gave the bearing capacityfactor N . Typical ranges of values are shown in Fig. 2.

bearing capacity factor Nq

0 10 20 30 40 50 60 70 80

approximatescour level

approximate 2510 Equivalent $'

20

30

40

50E

£ 6 0

aa.

* 70

80

90

100

Fig. 2 Cone penetrometer test data# caisson Ix caisson 7+ caisson 10O caisson 11

The majority of these data indicate values of (f>' (angle ofinternal friction) of the material between 28° and 26° (usingthe mean of theoretical Nq against <£' relations checkedagainst centrifuge pile test data), for elevations below scourlevel. The outer range was 31° to 24°. The expecteddecrease in <f>' with increase in depth and confining press-ure is evident.

Any friction between the rod and the sleeve of the pen-etrometer over these considerable depths could only meanthat the true <p' values would be smaller still. The rate ofpenetration of 0.16 m/min is not likely to have caused gen-eration of pore pressure and the fluctuations in the mea-sured load between start and stop of cone advance do not

indicate serious error. Using mean values for comparison:CPT = 27°, bearing of shoe = 31° and SPT = 36°.

During the contract, the excavated sand, and the sam-pling from further SPT work, showed that the sands werehighly micaceous throughout the depths to foundationlevel. The presence of mica increases the compressibility,and it is probable that the sand was compressed furtherbelow the sleeve of the penetrometer, before bearing failureunder the cone, than in the case of normal sand. Thiswould tend to decrease the local vertical stress and thebearing pressure, and indicates a lower value of </>' thanthat at shear failure.

Given the deposition of the sand through water in thepresence of mica, the deposit must be loose for the givenconfining pressure, and the original shear tests in this con-dition gave 0' = 31°, and this remains the most likely pro-perty value for the normally consolidated deposit. But thepresence of the mica, which had not been previouslyreported at depth, has increased the compressibility some 3times above that for normal loose sands, and, after allow-ance for the overconsolidation applicable when scouroccurs, the compressibility and caisson lateral deflectionscould be about twice those chosen on the basis of the orig-inal site investigation, even after the SPT data had beenproperly discounted. This would be compensated by amaximum depth of scour 5% less than that estimated.

2.3 Changes in river regime during contract periodDuring the flood seasons in the summers of 1979 and 1980,the cross-section of the river changed dramatically, as canbe seen from Fig. 3. Early in 1979 during the low waterthere was only one deep channel spreading approximatelyfrom caisson 4 to caisson 6, although these caissons wouldat that time have been sited on sand islands.

The change in river regime resulted in there being manymore caissons in water of some depth; and, in fact, caiss-ons 3, 4, 5, 6, 8 and 9 were constructed within circular cellsof straight web piles filled with sand.

2.4 Site organisationThe project was divided into three camp sites, with eachcamp responsible for several caissons. All steel cuttingedges, lubricating pipe components, shutters, special weld-ments and similar equipment were fabricated at the mainfield fabrication yard.

A concrete batch plant was set up at each camp withaggregate storage areas and shops. At camp 1, the batchplant was near enough to boom out to the mixer with aconcrete bucket and furnish concrete for caisson 11. Con-crete was moved to caisson 10 with six cubic-yard ready-mix trucks. At camp 2, the batch plant was set up nearcaisson 7. At camp 3, the plant was adjacent to caisson 1and concrete was trucked to caisson 2.

For the sand-island caissons, two floating concreteplants were set up with aggregates stored on floatingbarges. All concrete batch plants used 0.68 m3 mixers.The rate of concrete placement at each caisson averaged20 m3/hr.

Coarse aggregate used for the caissons consisted of 50to 38 mm maximum size, crushed aggregate of stone,barged in from a local plant. Cement for the project wasshipped in bags from Korea. The design mix produced aconcrete exceeding 29.3 MPa at 7 days. Reinforcing steelvaried in diameter to 50 mm bars with a minimum yieldstrength of 414 MPa. The reinforcing steel also was fromKorea. Water for the concrete was obtained from wellssunk at each camp site. River water was used for the float-ing concrete plants.

320 IEE PROCEEDINGS, Vol. 131, Pt. C, No. 7, NOVEMBER 1984

caisson 1

west anchor tower

river- bed profile 1977(1978sirrjilar)

river-bed profile 1980(1979 similar)

east anchor tower

mean WL on 1st Dec.

LWL

mean WLon 1st Dec.

LWL

Fig. 3 River cross-sectionsWL = water level

LWL = lowest water level

horizontal scale of kilometres

0 1 2 3vertical scale of metres

10

2.5 Lubrication systemNormally, caissons do not have enough weight to over-come skin friction, and a jetting system is used to over-come skin friction. The contract specified, however, that nodisturbance of material below the design scour depthshould occur, because the stability of the caissons depend-

ed upon the integrity of the ground below such level. Alubrication system using bentonite slurry was thereforedeveloped (Fig. 4).

The leading edge of each caisson had a steel cuttingshoe 2.90 m high (Fig. 5). The first row of lubricationnozzles was placed just above the top of the shoe. Forty-

75mm dia. riser

pump suction—hose

I concrete trough(for mud collection)

lubricating mud

—(bentonite slurry)

75 mm thick

discharge nozzle

lubricating

ring 100 mm dia.

— cutting edge

<DT3O

i

Ei n0 0

>»'

1

E0 0

2.A

3

1'CM*~ 1

r

r

i

I

-I4-

A equalsegments

75 mm dia. riserpipe

100mm dia.nozzleheader

discharge nozzle 11at 851mm crs.for each segment

_each ring consistingof A segmentseach with individualriser

75 mm dia. riser pipe

Fig. 4 Caisson lubrication systema Diagrammatic view showing pipeworkb Section showing position of lubricating ringc Plan of typical lubricating ring


four nozzles, divided into quadrants of 11 nozzles each,were used at each level. Each quadrant of nozzles was fedby a separate riser.

three sections, a desilter, shaker, mud tank and two mudguns (Fig. 6). Additives were used to obtain a slurry withthe required specific gravity.

Fig. 5 Leading edge of caisson showing steel cutting shoeFig. 6 Bentonite slurry lubrication system

Breaking the lubrication system into quadrants provid-ed better control of the caisson, if it tended to lean orstopped its downward movement. Controlled jet lubrica-tion would start the caisson moving again and permitsome guidance of its movement.

The first eight levels of lubrication nozzles were placedat vertical intervals of 1.22 m and nozzles were staggeredat alternate levels. The close arrangement of the nozzles atthe base of each caisson was adopted to minimise thechance of channelling during circulation. Nozzles at thenext six levels were vertically spaced 2.44 m apart, and thefollowing 13 levels were spaced 4.88 m apart. The deepestcaisson had a total of 27 levels of lubrication nozzles,including 1188 nozzles and 108 risers.

The nozzles themselves were pipes, 25 mm in diameter,set horizontally with two vertical holes, 6.35 mm in diam-eter, and set back 134 mm from the face of the concrete.The concrete form surrounding each nozzle was a frustumof a cone, causing the lubrication stream to flow in avolute stream.

The location of nozzles was planned to clear the hori-zontal and vertical reinforcing steel used on both the outerand inner faces of the caissons.

Sinking of the caissons did not require the use of all 27rings of lubrication jets, but after each caisson was foundedthe jets were used to void the annular space of bentoniteand replace it with cement/bentonite grout. No more thaneight of the lower-level rings of nozzles were used in theoperation, and the bottom four levels were used contin-uously.

As a further precautionary measure, 18 sounding wells,152 mm in diameter, were placed at intervals through thecentre of each caisson wall. Through these wells, a pipecould be lowered to jet around the interior of the cuttingedge to remove hard soil if the lubrication system failed toget the caisson moving.

As the bentonite lubrication system operated contin-uously during sinking, a trough of precast concrete wasplaced around each caisson at ground level to collect thebentonite slurry forced upward through the annular space.The bentonite was piped to a desilter and shaker screenand into a mud tank for recirculation.

Each caisson structure had an adjacent bentonite slurrylubrication system consisting of a large tank divided into

2.6 Sinking procedureA procedure was established for the construction of thecaissons. All ground for a radius of 30 m, except for thesix sand islands, was filled and levelled to elevation+ 10.7 m. Owing to the presence of a soft, clayey silt layerat the surface, each site was excavated to about elevation+ 4 m and filled with compacted granular material to ele-vation + 8.5 m.

At this elevation, timber mud sills measuring 200 mmx 300 mm were centred beneath the cutting-edge shoe,radiating from the centre of the caisson. The six steel-cutting-edge segments were placed on the timber mud sillsand secured for field butt welding. This formed the bottomof the caisson. On completion of the welding, the interiorand exterior areas of the caisson were back-filled with gra-nular material to elevation + 10.7 m.

The interior of the cutting edge was braced to withstandpressure from the back-fill material. The back-fillingsecured the steel cutting edge for concreting. Concrete wasthen placed in the cutting edge to a height of 1.5 m. Thereinforcing steel started above this initial concrete. Anadditional 1.2 m lift of concrete was placed, completing theconcrete in the steel cutting shoe.

At this point, the first rings of horizontal reinforcingsteel were placed, and the first level of lubrication nozzleswith quadrant headers and risers were set.

The interior face of the cutting edge was sloped to aheight of 7.6 m. Above the 2.9 m steel cutting edge, steelshutters for concreting were used. Two more lifts of con-crete were placed to complete concreting of the cuttingedge.

From this point up, circular steel shutters 2.5 m highwere used to place 2.44 m lifts of concrete. Nine segments,each of exterior and interior panels, made up a completeshutter.

Interior excavation began after the 7.6 m high cuttingedge was concreted. At this point, the timber mud sills hadbegun to crack and the caisson had started to sink. Exca-vation was done by crane grab until the caisson had settledapproximately 30 m. Excavation was accomplished byairlift below the 30 m depth.

Even though the bentonite slurry was constantly circu-lated to reduce skin friction, every precaution was taken toexcavate uniformly to ensure that the caissons did not tilt.


Concrete lifts of 2.44 m were added until the caissonhad reached a depth of about 46 m. Beyond this depth,three and even four lifts of concrete were added beforeexcavation was resumed, to sink the caisson an additional7.32 m or 9.76 m. Setting reinforcing steel, placing con-crete, setting the lubrication system, recirculating bentonitelubricant and airlifting continued until each caisson wasfounded.

The final step, after founding each caisson, was thepumping of grout into the annular space between thecaisson wall and surrounding soil. The grout consisted ofcement with small quantities of bentonite and retarderadded. The grout was pumped in measured amountsthrough the lubricating nozzles, beginning at the lowestlevel. As each measured amount was completed, pumpingwas started at the next level. This was repeated until theentire annular space was grouted.

After each caisson was founded and grouted, the inte-rior was excavated and back-filled, as detailed in Fig. 1.

The final lift of each caisson included the anchoragesystem to support the transmission tower. A reinforcedconcrete slab was placed across the top of each caisson. Totransmit horizontal shear from the four tower legs into thecaisson, it was necessary to post-tension the deck slabdiagonally.

2.7 Sand islandsSix of the 11 caissons were founded in the river. It was notpossible to use float-in caissons while recirculating thebentonite slurry lubricant. Without recovery and recircula-tion of the lubricant, the cost of bentonite would have beenexceedingly high. Therefore, cells 24.4 m in diameter weredesigned for these caissons (Fig. 7). A two-tiered template

Fig. 7 Cell under construction for caisson to be founded in river

supported on spud piles was erected to guide the driving ofthe sheet piles. Cutoff elevation of sheeting was + 10.7 m.Sheeting 30 to 37 m long was used. One problem here wasto protect the cells from scouring, and this is discussed inSection 2.8. Each entire cell was pitched with sheet pilesfor proper closure, before sheeting was driven to +10.7 mwith vibratory hammers. After completion of each cell, theinterior was back-filled with sand.

As steel-sheet piling was being pitched for caisson 6, avery strong wind gust hit the area and collapsed the struc-ture. Most of the sheet piling was removed, including thedriving frame. Some structural members were lost in thestrong current. As redriving the cell in the same locationwould have entailed the danger of encountering buriedsteel, the caisson was moved to avoid the submerged

IEE PROCEEDINGS, Vol. 131, Pt. C, No. 7, NOVEMBER 1984

debris. It was decided to move the cellular coffer dam24.4 m west, towards caisson 5, but set the centreline of thenumber 6 caisson 21.3 m west inside the cell. The cell waschecked for this eccentricity and determined to be safe.

As this offset of 21.3 m would change the catenariesbetween towers 5 and 6, the height of caisson 6 wasincreased by 4.9 m, to elevation + 17m, to maintain thedesigned vertical clearance between the bottom of the cate-nary and water level. The caisson was founded at elevation- 8 6 m.

From this point the procedure was the same as for land-based caissons, with the exception that materials weretransported by barges and handled by floating cranes.

Tower cranes 26 m high, with boom radius of 30.5 m,were used for construction of all six sand-island caissons.The cranes were supported on a piled foundation adjacentto each cell. These cranes proved very efficient. The cellsand extended decks also served as a storage and work areaduring erection of the towers. The cellular sheet piles wereremoved on completion of the towers and stringing ofconductors.

2.8 Scour protectionA major challenge was the protection of the sand-islandcaissons against scouring in velocities up to 4 m/s. Theprotective measures adopted consisted of plastic cementbags filled with sand. The bags were placed during low-water season. Before the construction of cells, two layers ofbags were placed in a radius of 46 m from the centre line ofeach caisson. Additional bags were placed around each cellafter the sheeting was driven. Bags were stacked in threelayers at the outer end and increased to four layers nearthe cells. The upper layer consisted of wired gabions filledwith sandbags and placed over the lower layers. Some200000 bags were required for scour protection at eachcaisson. The bags were inspected frequently, and recentinspections show that some bags were broken but the pro-tection they provide is still good.

2.9 Mud density predictionsIn 1979, when the bentonite mud method was examined,the aim was to balance the 'at-rest' pressure of the loosesand, partly to prevent disturbance of the sand and partlyto provide a margin of safety against complete collapse ofthe sand at depth. Based on experience with diaphragm-wall mud-trench stability a pressure reduction factor of 2was chosen to take account of the curvature of the sandwall. This led to the conclusion that the mud densitymaximum at founding level would have to be about1.20 t/m3. It was anticipated that heavy minerals, such asbarytes, would be added.

In March 1981, it was found that caisson 7 had reachedlevel —44.8 m, and caisson 11 had reached —42.7 m, yetthe mud density was measured to be only 1.03 t/m3. Thesupport of the sand at those stages relied on the fact thatthe ground-water level was at + 3.2 m for caisson 7 and at+ 6.1 m for caisson 11 to the side of the river channel,whereas the mud trench was at level +10 m. The criticalcondition was that of caisson 11, where the excess head ofmud over that of ground water was only 4 m. At the baseof the mud annulus, at that stage, the back-figured lateralpressure coefficient Ka of the loose sand had to be 0.11 orless, for there was no evidence of failure. Taking ft = 31°for the loose sand, Ka for an infinite plane would be 0.32so that the factor for curvature, together with anyunknown cementing between grains, was nearer 3 than 2.Using this observed enhanced stability, one could calculatethat a mud density of 1.15 t/m3 was the minimum require-

323

ment at levels of the cutting edge below — 61 m, when theflood water rose later that year to level + 10.7 m.

The danger thought to exist at that time was that anycollapse of the loose sand near the base might result inliquefaction and cause the caisson to sink to an unknowndepth, together with unknown consequences to the stateand stiffness of the sand below scour level.

2.10 Caisson 11 blowAt 3 a.m. on 15th June 1981, the cutting edge of caisson 11had reached level —78.3 m and the sand plug was 5.5 mthick. The water level inside the caisson was 1.8 to 2.1 mabove ground-water level, which by this time had risen tolevel +7.3 m. The bentonitic system was in circulationvia the first 4 rings from the base, and the airlift was exca-vating sand all as normal. The caisson was slowly sinkingas intended; and, according to Fig. 8, the implied friction

internal height of sand above cutting edge, m

\ 2 A 6

- 2 5

- 5 0

- 7 5

0'=31-32°

-TOOL

Fig. 8 Caisson 11 sinking: cutting edge level against internal height ofsand above cutting edge

After the movements had ceased, the ground level insidethe caisson was first found to be at level —50 m, namely22.9 m higher than before the base failed. The airliftequipment had been buried. The caisson had sunk only0.5 m. The bentonite annulus had disappeared.

In the following three weeks, during which the materialsinside the caisson settled back under the larger head ofwater, a survey indicated sand from —79 to — 66 m,20% sand and 80% mud from -66 to -60 m and mud to-52 m, while the water level was dropping below +17 m,see Fig. 9. The difference in vertical pressure between thesematerials inside the caisson and ground-water static press-ure outside the caisson was 23 m of water, so that this isindicative of the minimum excess pore pressure generatedat the base to cause the 'blow'.

The fact that 530 m3 of bentonitic mud entered thecaisson out of a total of 830 m3 supplied to form the mud

ground crack atbatcher plant

zone of majorsubsidence

\

\mud 215m?• sand 1

possible zoneof partialliquefaction

x l

•9.A5

water

mud345m3

sand

sand

rise in water level

GWL»73

-50 T --52

-60

-66

-73 -Li

-78.3 to /-78.8 /

"̂ gravel layer -89.3

Fig. 9 Survey of materials inside caisson 11 after base failure and blow

GWL = ground-water level

angle near the unloaded base was close to 30°, implying asand state with an in situ angle close to 26° under the fulloverburden pressure.

At 3.30 a.m. a cracking noise was heard at the batcherplant conveyor connection, located 27 m to the north-eastof the caisson. Investigation revealed a 75 mm wide crackin the ground and a sudden subsidence of the groundtowards the caisson. From 3.30 to 4.0 a.m. the ground levelaround the caisson continued to subside to a maximum ofabout 1.2 m, while the water level inside the caisson roseand overflowed the top of the caisson at level + 17 m. Ben-tonite flowed back up the riser pipes to exit level + 18 m.

cake, the mud trench and the mud annulus indicates thatthe loose micaceous sand around the lower portion of thecaisson must have partially liquefied [2], almost certainlydue to inwards collapse onto the caisson. (There was noevidence of artesian pressure in gravel layers on the site,nor would the flat topography lead one to expect suchpressure.) Once the rise in pore pressure spread to belowthe plug, the subsequent inflow of sand and water wouldhave affected a volume such as indicated in Fig. 9 so aseventually to carry the mud down past the 2.9 m of theshoe and up into the caisson. The gradual rise of thematerials over a period of 30 min is more typical of pore


pressure generation and spread than of a sudden 'blow'under artesian pressure of free water. Once the loss of mudoccurred, the sand gripped the caisson in shaft friction andarrested the gradual sinking process of the caisson. Thiswas fortunate, for the relative speed at which the caissonmight sink was unknown.

Back analysis of the ground state just prior to collapseindicates a lateral effective pressure coefficient of 0.076;namely, some 4 times smaller than the Rankine coefficient.In the case of compressible loose micaceous sand this indi-cates a remarkable effect of arching round a circle of12.3 m diameter, unless assisted by weak cementationunder the high overburden pressure. Experiments con-ducted at simulated pressures in a Rowe consolidation cellhave shown similar low coefficients of earth pressure atcollapse, and it is probable that the stiffness induced by thehigh overburden stress is the cause of the high archingfactor.

The question remained as to the state of the sand thenexisting below scour level, on which the lateral stability ofthe caisson would eventually rely. The site survey indicateda surface depression of some 400 m3. The mud annulusvolume is normally found to be about 0.4 x the liquidvolume of mud supplied, allowing for the cake formation.The volume of liquid mud in the caisson was530 m3, so the equivalent liquid annulus loss was esti-mated at 212 m3. This, together with the 369 m3 of sand,implied an excess of ground removed over that supplied bysubsidence of 181 m3. Estimating roughly from Fig. 9 thatthe sand came from a zone of 37 m diameter over a depthof 30 m, namely from a volume of 36245 m3, led to adecrease in sand density in situ by some 0.5%. This wouldimply an increase in compressibility of 11%. However, thecone penetrometer did not detect any clear differencebetween readings before and after the 'blow', so that loss invertical stress due to arching on to the caisson from undis-turbed ground appeared not to be significant. By compari-son, grouting the annulus prevented a loss of density ofabout 0.4% so that the effect of the blow appeared to beroughly equivalent to that of omitting the grouting.Caisson 11 was sited inland, and was taken to level —91m, one of the deepest, and had been designed against along-term possible deviation of the river channel. In theevent, it was decided that there was insufficient evidence tojustify the delay and cost of the construction of an alterna-tive replacement caisson.

The bentonite lubrication system was re-established byinjection through each ring of nozzles, commencing at thehighest level, and, on 15th July, the caisson once againstarted to move.

2.11 Analysis of sinking recordsThe analysis is based on the records for caissons 1, 2, 7, 10and 11. The construction period was such that the groundwater level rose as the caissons were being sunk, andfounding occurred at a time of peak ground water level,see Fig. 10.

The level of water inside the caissons was kept generally1.5 m or more above ground-water level. The actual levelvaried as shown in Fig. 11 between a minimum of 1 m anda maximum of 4 m; but, for analytical purposes, a figure of1.5 m has been used throughout.

Appendix 8.2 shows the calculation of friction force overthe outside of the cutting shoe, in which account is takenof the varying level of ground water. The main uncertaintylies in the product K tan 5. The pressure coefficient K isnot the 'at rest' value because shear acts over the interface.Back analysis of the records during the sinking of the Har-

dinge bridge piers [1] gives K tan 3 = 0.20. However, asexplained above, the sinking was accompanied by liquefac-

ground-water level,m(PWD)

2 A 6 8

- 2 5

± - 5 0j

U

- 7 5

-100L

Fig. 10 Rise of ground-water level as caissons were being sunkFounding occurred at peak ground-water level

tion of the sand around the base of the caisson. Hence, inthe present case, one would expect to find K tan 5 valueshigher than 0.20 when sinking slowly through relativelyundisturbed ground. Adopting tan <5 = 0.5 results theoreti-cally in K tan d = 0.33. During construction the contractorused a value of 0.25. Later data from the friction sleeve ofthe Dutch cone penetrometer gave a wide scatter with amean of 0.4, so that, on balance, the calculated value of0.33 is reasonably close to alternative estimates for thepurpose of this analysis.

The bearing loads on the inside inclined cutting-shoefaces have been estimated, on the basis that, as the shoesinks, the sand is displaced inwards and upwards as in thecase of passive sand resistance behind an inclined plate.Appendix 8.2 and Fig. 28 give the equations which makeuse of the earth pressure tables of Caquot and Kerisel [3].Account is taken also of the increase in vertical effectivestress in the sand plug due to downward seepage under theexcess head of water inside the caisson compared to thatoutside.

Using the line marked A in Fig. 8, relating level ofcutting edge to depth of sand plug, the value </>' = 41° givesthe bearing load shown on line 2a, Fig. 12; and likewise(f)' = 42° gives the load on line 2b. Similarly, values of<f>' = 31° and 32° fit line B, Fig. 8, to lines 2a, 2b in Fig.12; and likewise 0' = 26° to 28° for line C.

The bearing load range, lines 2a, 2b, Fig. 12, whenadded to the friction load line 1 gives the net effective


caisson load lines 3a, 3b. As the concrete lift is added,the weight of the caisson, for a given level of cutting edge,

level of water inside caisson above ground -water level m

. 2 U

-25

-50

-75

100

i\ i

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(t

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i

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<<»

Fig. 11 Level of water inside caisson above ground-water level as caiss-ons were being sunk

increases to line 3b. Excavation leads to an increase incutting-edge depth for constant weight, reaching line 3a.For a given level of cutting edge, the maximum weight, line3b, is that to overcome shearing resistance at theminimum depth of plug, and the higher angle <f>' = 42°

load.t5000 10000

-100

Fig. 12 Loads acting during caisson sinking(i) Cutting shoe friction using Fig. 27 and K tan <5 = 0.33(ii) Bearing load range using lines A and B and (j> values shown in Fig. 8(iii) Net weight of caisson

applies. Shearing ceases as the caisson comes to rest atminimum weight for a given elevation, consistent with thelower angle 4>' = 31°. This assumes no generation of excesspore water pressure during the slow rate of sinking.However, if movement is sudden from an abnormally lowdepth of plug, as when a hard-pan layer is met, and if thecaisson then sinks faster, the generation of excess porepressure would cause the plug depth to increase past lineB. The progress of caisson 7, for example, is shown withlines joining the principal levels of cutting edge and depthof plug, Fig. 8. At elevation — 40 m, a hard-pan layer ledto a plug only 0.3 m deep before sudden movement pro-duced a plug 6 m deep, beyond even limit line C. Towardsfounding level, the heights of concrete lift were decreasedand the depth of plug was maintained within a narrowrange of 1.8 to 3 m.

The range of $' values, 42° to 31°, which fit the aboveback analysis in bearing appear to straddle the range 39°to 34° estimated from the corrected SPT values. However,the sand in the plug had been unloaded by the excavationprocess by amounts up to about 1000 kN/m2. Laboratorytests show that, under these conditions, the value of (j)' offine sand in the loose state is 4° higher than when normallyconsolidated under the in situ stress of 1000 kN/m2, andwith sand in a medium dense state the difference is about6°. Hence the back-figured range of 42° to 31° becomes 36°to 27°, corresponding to the in situ conditions prior toexcavation, and this is significantly lower than that pre-dicted by the SPT data.

3 Tower erection and conductor stringing

Compared with conventional river crossings, the problemsof construction of the Jamuna River crossing were con-cerned primarily with scale and access. With normalsingle-span river crossings, tower erection and conductorstringing follow well established practices.

In this case, most of the tower locations were accessibleonly by boat, and normal conductor stringing techniqueswould have been impracticable due to the number of spansand lack of dry land on which to locate conventionalstringing equipment. Indeed, owing to changes in thecourse of the river no prior assumptions could have beenmade as to which tower sites might or might not have beensituated on dry land.

4 Tower construction

Owing to the restricted access available for storage andsorting at the base of the towers, the main aspect of towererection was logistics, Fig. 13. All composite members andtower panels were partially assembled in the main store-yard and shipped by barge, where necessary, to the towerposition, each item having been loaded in the correctsequence for erection. At one stage during construction,some seven towers were being erected simultaneously,necessitating the handling and sorting of some 3500 t ofsteel.

The 124 60 mm-diameter anchor bolts per tower werecast into the caisson walls to an accuracy of ± 1 mm byemploying an 'X' template constructed of welded beams.The base-plate arrangement was located over the project-ing bolts to the same accuracy, and fixed by a high-strength (48 000 kN/m2) nonshrink grout. To ensure no airpockets remained on the underside of the base plate, 10breather holes in each plate were provided.

The nuts of the anchor bolts were tightened to a torqueof 140 kg m effectively to prestress the bolts, thereby mini-


mising uplift movement of the base plate under loading.Locknuts were fitted before the base plates were sealed

freight and cross-river ferry traffic; the former can be assmall as a rowing boat and the latter of a size capable of

Fig. 13 Tower base, showing components sorted and stored in correctsequence for erection

Fig. 14 Tower under construction, showing crane used

with concrete and bituminous paint as a precautionagainst standing water.

The 150 t bottom panel of the tower was constructedwith the use of a mobile crane mounted on a bargemoored to the side of the caisson. The remainder of thetower was erected either by a conventional long derrick orby climbing crane Fig. 14. The climbing crane, which'grows' with the tower, had a rated lifting capacity of 2.15 tat 21 m working radius.

After each tower panel was erected, the tower waschecked for squareness and verticality. This was necessarydue to the comparative stiffness of the members and todetermine the cause of any lack-of-fit. As a result, the finalverticality achieved was well within the specified toleranceof 0.3% of tower height.

All bolts were locked by means of palnuts, as a precau-tion against loosening due to wind-excited vibration.

The rates of erection progress varied initially from 85days, using the derrick, and 74 days, using the climbingcrane, to 45 days at best for both methods, after the gangshad familiarised themselves with the processes. The climb-ing crane proved ultimately to be less useful in terms ofprogress than was originally expected.

All 11 towers were erected and complete for conductorstringing 20 weeks after commencement.

5 Conductor stringing

In all seasons of the year, the River Jamuna is a navigablewaterway used extensively by private and commercial

carrying several fully laden trucks and buses. When plan-ning the system of stringing to be employed, the riverauthority's requirement of minimum interference of rivertraffic in navigable channels had to be observed; both toavoid danger to personnel and traffic and, of course,damage to conductors and towers.

The River Jamuna, at the point of crossing is about12 km wide when in flood. At the lowest water level, theriver reduces to one navigable channel with several minorpassages. The river regime changes from year to year andno definite assumptions can be made as to which towerswill, in the dry season, be situated on islands. Planning forstringing the crossing had to take the above major con-siderations into account. It was, in any event, not possibleat the outset to plan for stringing at low water level, owingto the tight construction programme and the uncertaintyas to the problems that may have been encountered withcaisson sinking, which could have affected the tower erec-tion and stringing programme.

Several methods of stringing the crossing were exam-ined before the 'closed-loop system' was adopted. Thissystem involves a pilot wire being strung across the riverutilising, on a double-circuit tower, two of the 8 conductorpositions (for example, the two earth wires), and the endsjoined to form a closed loop. Subsequently, two drumlengths of conductor are introduced into the loop at eachriver bank, the loop is then circulated for a distance of oneconductor drum length, two more drum lengths are thenintroduced and the process continued until two completecrossing lengths are erected.

1EE PROCEEDINGS, Vol. 131, Pt. C, No. 7, NOVEMBER 1984 327

A schematic plan view of the system employed is shownin Fig. 15 and detailed in the subsequent 13 Figures towhich reference is made.

Stage 1 of the operation was to run out two pilot wires,using conventional techniques, through blocks suspendedfrom the two earth-conductor positions (la and \b in Fig.

—.turn sheave

conductor drum River Jamuna

guide pulleys t o w e f ) t o w e r 2 t o w e r 1 0 t ( A w r 1 ,

' I T § § >overland westtowers anchor

tower 65m 645m 1.22km 1.22km

13.635 km

650m

east anchor tower

c80m

Fig. 15 Plan of closed loop stringing system

tower 1

west bank east bank

turn sheave

Fig. 16 Closed loop—main elements

The main elements of the closed loop are shown in Fig.16. The suspension crossing towers are numbered 1-11from west to east, here illustrated in diagrammatic formwith only two conductor suspension points shown fromwhich running-out blocks of 1 m diameter are suspended.On each river bank is a turn sheave, 2 m in diameter,mounted horizontally on a trolley (Fig. 17). The guidepulleys, which are located between the turn sleeve andadjacent crossing towers, are for the purpose of deviatingthe loop horizontally through 22° and 37° on the west andeast banks, respectively. It will be seen later that about1600 m of land beyond both towers 1 and 11 is requiredfor the conductor loop; and, to simplify conductor hand-ling, deviations were necessary to avoid the anchor towers,the land section towers and other obstacles on the riverbanks.

18). This necessitated closing the river to traffic for shortperiods, due notice having been given to the waterauthority and major river traffic operators. The ends ofeach pilot wire were jointed on each bank, using compres-sion fittings, to form a loop around the turn sheaves. Thepilot-wire loop was then tensioned to obtain and maintainclearance above the river and enabled the river to beopened to river traffic. A tension in the loop of about 4 twas achieved by means of winches and pulleys located onboth river banks. The arrangement on the east bank is

1 13a

0

l a

12a

I2b

o1b

r i3b Ab

Fig. 17 Turn sheave on river bank, mounted horizontally

328

Fig. 18 Earth conductors, la and 1b, and line conductors


shown in Fig. 19; ground rollers are shown diagram-matically and were employed between the turn sheavesand towers 1 and 11, respectively, to ensure that the con-ductor, during running out, did not come into contact withthe ground or any other objects which might have causeddamage to the conductor surface.

Stage 2, shown in plan view in Fig. 20, entailed atta-ching wedge-type conductor pulling clamps to the pilotwires at points C on both banks and, by means of thecapstan winch at D on the east bank, the stringing tensionin the crossing was maintained. The turn sheave tension

was then reduced and the pilot wires cut at positions El5E2.

Referring to Fig. 21 and Stage 3 of the operation, theturn sheave end of the plot wire from Ej was attached tothe capstan winch at A, the corresponding loose E2 beingconnected to the first drum of conductor and the conduc-tor hauled slowly over ground rollers around the turnsheave in an anticlockwise direction until the full drumlength of conductor has been run out. A similar operationwas carried out simultaneously on the west bank. The sig-nificance of the distance between the turn sheave and posi-

tower 11east bank

jointingpositions ground rollers

C 7 1 &

turn sheave dynamometertrolley mounted \ capstan

winch

anchor

looptensioningequipment

Fig. 19 Stringing stage 1: winch and pulley system on east bank

tower 11

guidepulley

east bank

Ei—X-

capstanwinch D

capstanwinch A

conductorclamp

E 2

- K -

I -v-

Fig. 20 Stage 2; plan viewconductordrum

east bank

pilot wire/ACSR joint

conductorclamp

turn sheave

ACSR

conductor drum850m

Fig. 21 Stage 3


tions El5 E2 can now be appreciated as being half thenominal conductor drum lengths of 1700 m. Jointing posi-tions had been prepared at Ei and E2 and the ACSR

resistance of the various running-out blocks was consider-ably less. It will be appreciated that one of the merits ofthis system is that the power required to circulate the loop

pilot wire

east bank

ACSR to pilotwire joint

, - - / - •

ACSR

C \conductorclamp

capstanwinch D

u — /&>E, y * \ caps tan winch A

jointingposition

Fig. 22 Stage 4 conductor drum

Fig. 23 Stage 5

guidepulley

capstanwinchD

east bank

pilot wire

capstanwinch A

ACSR

guidepulley

east bank

capstanwinch D

capstanwinch A

turn sheave

J—-,.-.-—Fig. 24 Repetition of stages 3, 4 and 5 unit conductor was introduced into the complete earth conductor loop

jointed here to the pilot wires in the crossing. Fig. 22 illus-trates this stage (Stage 4) of the operation. The turnsheaves on both banks were then tensioned to 4 t and thewedge-type pulling clamps released.

With the introduction into the loop of conductor drumlengths on the east and west banks, the complete crossingloop was then circulated in a clockwise direction by meansof the capstan winch A, situated on the east bank andacting on the pilot wire, until the complete drum lengths ofconductor had passed positions E2 on the east and westbanks, respectively; this is shown in Fig. 23. The contrac-tors had originally considered that the loop could havebeen circulated by means of manpower, but, in the event, itwas decided that better control would be obtained withless possibility of damage to the conductor if this wasachieved by means of a winch. The winch A was rated at7 t, although it had been calculated and subsequentlyshown that the tension to overcome inertia and the rolling

is relatively small, having only to overcome the friction inthe running-out blocks and turn sheaves.

Stages 3, 4 and 5 were then repeated (Fig. 24) until theconductor was introduced into the complete earth conduc-tor loop, with only the equivalent of two conductor drumlengths of pilot wire remaining around the turn sheaves.Joints between conductor drum lengths were normal com-pression midspan joints. To avoid unacceptable permanentdeformation of the joints as they were passed around theturn sheave and through the running-out pulleys on thetowers, a joint protector, Fig. 25, was developed by thecontractors and fully tested before stringing operationsbegan. The joint protector comprised a rigid steel casetotally enveloping the joint and proved entirely satisfac-tory in performing the function it was designed for. Theouter layers of the smooth-body conductor, as used forboth the earth and line conductors, are susceptible todamage and birdcaging if mishandled and passed over


Fig. 25 Protector for joints between conductor drum lengths

pulleys of small radius. It was decided, in conjunction withthe conductor manufacturer, that the proposed diameter of2 m for the turn sheaves would be adequate as would1.0 m for the diameter of the running-out pulleys hungfrom the conductor crossarms. These dimensions alsoproved to be satisfactory.

Reverting to the next stage of stringing, illustrated inFig. 26, the turn sheave tension was increased from 4 t toapproximately 6.5 t, the 'everyday tension', and the con-ductor in the crossing tensioned off at C, using the conduc-tor clamps and the capstan winch D on the east bank.

Again, to avoid damage to the conductor, it was foundnecessary to employ more than one clamp at each posi-tion. The conductor was separated from the pilot wires atEx and E2, released from the guide pulleys, swung acrossto the anchor tower, passed through pulleys suspendedfrom the conductor tension attachment point andanchored by means of conductor clamps to the winch atH. Tensioning winch H allowed clamps at C to be releasedand allowed the conductor in the crossing and anchorspans to take up the correct 'initial stringing condition'profile. Sag boards attached to towers in 3 spans were usedto obtain and check that the calculated conductor sag hadbeen achieved. Allowance was made for conductor creepby means of the 'temperature shift method', 15°C for theearth conductor and 25°C for the larger phase conductor.It is interesting to note that the phase conductor 'final' sagat 25° is 65.6 m and, with a design temperatue range of

east bank

15°C to 80°C, the conductor sag varies about this 'every-day' value by only —0.7 m and +4.0 m, respectively. Fol-lowing sagging, the conductor was tensioned off ontotension sets attached to the anchor towers and the conduc-tor in the crossing spans, transferred from the running-outblocks to the saddle clamps. On completion of this, theconductor sags in the crossing spans were again checked.

The preceding few paragraphs have described stringingthe first closed loop, which resulted in the earth conductorbeing landed on positions la and lb of the crossing towers(Fig. 18). For stringing the line conductors, the procedureswere repeated by forming loops for line conductor posi-tions 2a, 2b, 3a, 3b; and, finally, 4a, 4b.

As noted in Part 1, owing to the make up of the con-ductor (25% and 43% steel in the phase and earth conduc-tors, respectively), the design 'everyday' tension and spanlengths, it was considered essential to control conductorvibration during stringing operations, whenever the con-ductor was at rest for any length of time and until theconductors were clamped-in and the specified Stockbridgedampers fitted. To assist construction staff to easily iden-tify the two physically similar, but technically different,types of damper, the heavier dampers were painted redbefore leaving the factory.

The aircraft warning spheres were fitted to the earthconductors at the same time as the dampers, using a con-ventional conductor trolley. As noted in Part 1, these werefitted at approximately 80 m intervals on each earth con-ductor, but staggered so as to meet the International CivilAviation Organisation's recommendation that markers bedisplayed not more than 40 m apart.

Preparation for and stringing of the crossing, a total of4 loops comprising 2 earth conductors and 6 phase con-ductors, took 6 weeks, and, except for the stringing of thepilot wires, the passage of river traffic was not significantlyaffected. The closed-loop system of stringing employed isbelieved to be the first use of this method over such a suc-cession of long spans, and proved to be entirely successful.

6 Acknowledgments

The principal participants in the work were as follows:Joint consulting engineers were Merz and McLellan andRendel Palmer & Tritton. The specialist advisor in soilmechanics and caisson sinking was Prof. R.W. Rowe. The

capstan winch

dynamometer

capstanwinch guide0 pulley

terminaltower-landsection

anchor

tower 11


Fig. 26 Stage 6

331

work was contracted by a consortium of the followingKorean companies: Korea Development Corporation,Kolon Electric Machinery Co. Ltd. and Sansung Co. Ltd.The technical advisers to the contractors were RaymondTechnical Facilities of New York.

7 References

1 GALES, SIR R.: 'The Hardinge Bridge over the Lower Ganges atSara', Proc. Inst. Civ. Engrs., 1917-18, 18, p. 205

2 BJERRUM, L., KRINGSTAD, S., and KUMMENEJE, O.: 'The shearstrength of a fine sand'. Proc. 5th Int. Conf. Soil Mech. and Found. Eng.,1961, 1, pp. 29-37

3 CAQUOT and KERISEL: 'Tables for the calculation of passive press-ure, active pressure and bearing capacity of foundations' (Gauthier-Villars, Paris, 1948)

8 Appendixes

8.1 CostsThe final cost of the works

General itemsSite clearanceSubsoil investigationsFoundationsTowers and overhead lineSpare line materialsProvisional sumsDayworks

was as follows:

uss2 233 8002 789 200

365 20049420 50012042 500

547 20085 00025 600

US $ 67 509 000

Fig. 27 Mohr's circle of stress

bentonite

28.27% of the above was paid in local Taka currency andthe remainder in US dollars.

8.2 Friction force and end bearing on shoe

8.2.1 Friction force on shoe:Ground level GL = +Z = + 10.68 mGround-water level GWL = +yCutting edge level = — xTotal density of sand = ys = 1.9 t/m3

Submerged density = y' = 0.9 t/m3

Height of cutting shoe h = 2.9 mVertical effective stress at midheight of shoe =

- y)ysx 0.3048 t/m

Friction area A = 2nRh x 0.30482 m2

Lateral pressure coefficient KDuring shearing action, from(Fig. 27):

G'V — o'h = 2T tan <5

= 2oh tan2 5

Mohr's circle of stress

o'v 1 + 2 tan2 5

For tan 3 = 0.5, k tan 5 = %Friction force = o'v Ak tan 5

8.2.2 End bearing on shoeRe Fig. 28D = depth of sand plugAhw = excess head of water inside caisson relative toground-water levelFor D < Do = 1.5 mp = jy"D2 sec2 (3 b/unit circumferenceMean circumference = (2R — D tan PJn

332

Fig. 28 End bearing stresses on shoe

radius R

sin (fi + - D

sinsin

+ 5)61(2i? - Do

2 + S)b2(2R - Do

Vertical load L = n/2y"D2 sec2

tan 0JFor D > Do

L = n/2y"(D2 - (D - Do)2) sec2

tan px) + n/2y"(D - Do)2 sec2 0

tan /?! - (D - Do) tan p2)Where bu b2 are the passive coefficients from earth press-ure tables for given values of /?ls /?2 and selective values of(f> and 5. Owing to a higher permeability of the naturalsand below the caisson in the horizontal compared to thevertical direction, the excess head hw is dissipated mainlyover the depth of the plug.Effective vertical stress at depth x below surface of plug =

y =y + ^

Where y" is the equivalent sand density under seepage.

1EE PROCEEDINGS, Vol. 131, Pt. C, No. 7, NOVEMBER 1984

jamuna river 230 kv crossing, bangladesh. part 2: construction

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