WELL FOUNDATION

CONSTRUCTION OF HIGH LEVEL BRIDGE OVER SUTLEJ RIVER

PROJECT BREIFING

This bridge forms a part of a bye-pass project near Ropar The project is undertaken by B&R division of PWD. Total cost of the bridge over Sutlej is 63 crs. The contract is given to the company on the lump sum

basis.

Length 982 m

Lanes 4 lane

Spans 28

Length of span 36.5 m ( middle 23 spans)

24 m (end spans)

Formation level 273.5 m

High flood level 264.2 m

Alignment •Alignment of the bridge passes through :

New I.I.T. Road

Sutlej river

Nwansheher highway

•Alignment crosses river at an angle therefore it is a Skew alignment

Height of Bridge

Bridge is high level bridge i.e. formation level is above H.F.L.

Height if bridge is determined by examining the clearance of bridge over highway

As per IRC 5:1998 Min. clearance needed = 5 mRoad level =267Width of beam and slab =1.7 mReq. formation level= 273.7 mActual formation level=274.7 m

Extent of exploration

The exploration shall cover the entire length of the bridge and also extend at either side for a distance of about twice the depth below bed of the last main foundations i.e. if depth of foundation below bed is 24mts. then exploration will be done up to 48mts away from ends.

The depth of exploration should be at least 1 ½ times the minimum width of the foundation below the proposed foundation level in case of open foundations and deep well foundation

Where such investigation end in any unsuitable or questionable foundation material, the exploration shall be extended to a sufficient depth into firm and stable soil or rock but not less than four times the minimum depth of foundation below the earlier contemplated foundation level.

Scour depth

Depth of scour below the high flood level.Scour depth has important in determination

depth of foundation.DeterminationThe maximum depth of scour should be measured

with reference to existing structures near the proposed bridge site, if this is possible. Such soundings are best done during or immediately after a flood.

Mean scour depth (dsm )= 9.2 mMax. scour depth for piers = 2 dsm = 18.4 m

Depth of foundation

Well foundation are taken up to depth such that min. grip of foundation below max. scour depth is 1/3 rd of max. scour depth.

The max. Bearing pressure on the subsoil under the foundation resulting from any combination of the loads and forces except wind and seismic forces should not exceed the safe bearing capacity of the subsoil, after taking into account the effect of scour.

WELL FOUNDATION•Well foundation lies under the category of open caisson

•They are open at both ends

Well ComponentsCutting edge

Well curb

Well steining

Bottom plug

Sand fill

Top plug

Well cap

Cutting edge

•Cutting edge is the lowermost part of a well It is the part which cuts through the earth.•The mild steel cutting edge shall be made from structural steel sections.• The cutting edge shall weigh not less than 40 kg per metre length and be properly anchored into the well curb• sections are bent into shape by cold bended no heating is done.

The picture above shows a three 160mm MS

Sections curved and welded to form Cutting Edge.

Well curb

•Well curb is a structure with outer wall of curb being straight and inner at an appropriate angle which is fabricated over cutting edge.•The internal angle of curb can be kept between 30O to 37o .•It serves the purpose of transferring•Load from steining to bottom plug.•Thickness of well curb is 75mm more than that of steining to prevent tilts A fabricated well curb set atop wooden

planks

Well curb

Reinforcements in awell curb

Vertical bars -20 dia.

Triangular bars -20 dia.

Well steining

It is the longest part of well and it transfers load from well cap to the well curb at desired depth.

Track of length is taken by marking wells on 4 sides by gauges.

Steining is built in lifts of 2.15m and each lift is aligned with previous one.

As per IRC 78:2001. The amount of vertical reinforcement

provided should not be less than .2% of cross sectional area of steining.

2. At the inner face reinforcement must not be less than .06% of area.

3. Transverse reinforcement must not be less than .04% of volume/unit length of steining.

Reinforcements in steining

For wells 12 to 23Outer dia.= 5.35 m inner dia. = 3.4 mArea of steining= 13.394Min. reinforcement for steining (20 dia.

Bars)= .02*area/area of bar = 85.3 = 86 bars

Actual no. =62+26 =88 barsMin. reinforcements for inner wall(20

dia.)=.oo6*area/area of bar =25.59 = 26 bars

Actual no. =26 bars

Well steining

Determination of thickness of steining The min. thickness should not be less than 500mm and should be

according:

h=kd√l where : h- min. thickness in m

d=external dia. Of well

l=depth of well below well cap

k= a constant = 0.03

At the site: avg. depth of well = 25m

dia. = 5.2m

So Min. thickness required = .78 m

Well steining

But some adjustments are needed in thickness according to the strata as given by the table:

As the strata is sand with boulders final thickness is = .858 mOriginal thickness =.78 m (well no. 1-11 and 24

-29) = .88m (well no. 12-23)

Strata Adjustment

Very soft clay strata -10%

Hard clay strata +10%

Boulder strata or well resting on rock

+10%

Bottom plug- transfers the load of structure from well to the ground. It is cast under water after the final depth of well is reached. No reinforcement is provided. The mix should have min. cement content of 330Kg/m3

Sand filling : Sand filling shall commence after a period of 3 days of laying of bottom plug. It helps in increasing the weight of the foundation which provides more stability to the structure. It also counteract buoyant force of water.

Intermediate plug :The function of the plug is to keep the sand filling sandwiched & undisturbed. The concrete used for intermediate plugging is also M25.

Top plug :After filling sand up to the required level a plug of concrete is provided over it as shown on the drawing. It at least serves as a shuttering for laying well cap.

Well cap- It is needed to transfer the loads and moments from the pier to the well or wells below. It is designed as two way slab.

Well parameters

Parameters Data

1.      Total no. of wells to be sunk59 n0.

2.      Grade of concrete used for well curb and steinning: M 30

3.      Grade of concrete used for bottom, intermediate and top plugging

M 25

4.      Type of Cement usedOPC 43-G

5.      Grade of steel used for reinforcement: FE 500

7.      Scouring levels from previous years data: 9 m

9.      Thickness of the cutting edge: 16 mm

Well parameters

Parameters Data

  Depth of the wells below ground: Well no. 1 to 11:24.0 m

Well no. 12 to 23: 26.5 m

Well no. 24 to 29: 24.0 m

Height of well curb: well no. 1 to 11 and 24,25: 1.410 m

Well no. 12 to 25: 1.530

Thickness of the well steinning: Well no. 1 to 11: 780 mm

Well no. 12 to 23: 880 mm

Well no. 24 to 29: 780 mm

Construction of well

Setting of well curb

Sinking of well

Bottom plugging

Sand filling

Closing of top by well cap

Setting of well curb

The centre of the well is marked accurately with help of theodolite.

Cutting edge is placed on the wooden sleepers such that centre of cutting edge coincide with the marked centre.

Reinforcements and formwork for curb is fabricated on the cutting edge and concreting is done.

After curing of well curb is complete the wooden slippers below the curb are removed by making soil below them diluted by water and is allowed to sink under its own weight.

In this picture a well curb is resting on the wooden slippers

Sinking of well/Construction of steinning

1. Fabrication of formwork

and reinforcements

2. Concreting

3. Dredging of earth to

support sinking

Sinking of WellFabrication of Formwork

The formwork for casting are supported on the channels bolted with steining on both sides.

As per design lap is provided after every fourth reinforcement at same section.

Concreting is done by pumping the concrete to the mould by concrete pump.The concrete is placed in 400mm thick layers along with continues vibrations.On avg. about 23m3 of concrete is needed for casting of one lift.

Fabrication Concreting

Sinking of Well

Dredging In order to sink the well,

continuous dredging is done. Soil is excavated from the dredge hole inside the well.

The dredging operation is performed evenly along the hole circular hole

Construction of well

Bottom plugging : it is done after the well has reached the founding level. Concreting is done inside water. For this tremie pipe is used. A tremie is long funnel made of mild steel . A flap door is provided at its bottom.

Well cap:After water filling, a truss system consisting of three trusses supported in well steining and cribs welded with it in criss-cross manner are used for top plugging. This system is covered with strong plywood and sealed with concrete slurry so that nothing can enter inside the well. Hence preparing it for the casting of well cap.

Methods to promote sinking

Kentledge -Total of 125 concrete cubes of 1m3 in size are made for kentledge loading which is approximately equal to 25t × 125.

Chiseling- When the boulders are very firmly cemented then chiselling is done. In this soil is hit hard by sharp end chisel.

Dewatering- dewatering of dredge hole is done to reduce buyout force on well thus promoting sinking.

Flushing with jet of water on the outside face of well reducing skin friction

Tilt and shift

Tilt and shift are deviation of well from its proposed position and orientation.

Max. tilt allowed is 1/80Max. shift allowed is 150mmMeasures to avoid tilt and shiftThe outer surface of steining and curb should be kept as

smooth as possible.The radius of well curb should be kept more than the

steining.The dredging should be done on all sides of well uniformly.CausesEccentric dredgingCan be caused due to boulder under one side

Method of checking tilt and shift

Tilt and shift are calculated with the help of a level and total station.

MethodDifference between height of points (1&

3)= R.L.(1)-R.L.(3)Tilt = [ R.L.(1)-R.L.(3)]/diameter Shift at bottom from top position

= tilt/ lengthNote : this process is repeated on sides (2 &

4) and in similar manner tilt is obtained and then combined tilt and shift is reported.

Shift of well at top is measured by total station. 

Correcting tilt and shift

Regulation of GrabbingEccentric LoadingStrutting The wellWater jetting or Digging Pit outside the

higher side of wellPulling the well or caisson Excavation on the higher side outside the

well

Problems

Sand blowing- This problem occurs when well bottom rests on sandy strata and dewatering is done below water table. In this sand from ground gushes into the well and there is sudden sinking of well. This sudden sinking is very dangerous and is very difficult to rectify.

Bridge scour -is the removal of sediment from bed such as sand and rocks from around bridge, abutments or piers. Scour, caused by swiftly moving water, can scoop out scour holes, compromising the integrity of a structure.

Sand heaving-When a well passes through soft strata over a considerable depth, the upward resistance acting on the outside surface of the well is less than the weight of the well. In that situation the well sinks down and quite often a heap is formed inside the dredge hole.

Bridge scour Sand heaving

Problems

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