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BOTTOM PLUG IN WELL FOUNDATIONS Ref : IRC publication in ‘Indian Highways’ – May 2006 : Paper – “ A Ana nalysis of Tre T rem mie Techni chnique que in Concreting ting of Botto Bottom m Plug Pl ug of We Wells”
By
Dhruba Dhr ubajj yoti yoti Bhattach Bhattacharya arya Presented thi this Pape Paper in the the Workshop orkshop on Dee Deep Foun Foundati dations ons ( Well / P Piile ) for Bri Bridge dges and and Fl Flyove yovers – th Optimal Optimal Solution’ at Central Road Research Institute, Institute, New Delhi Delhi on 30 November 2006.
ABSTRACT The The flo flow w ch character of of the co concrete du during ing op operation ion of of well bottom plug lug ha has mainly inly be been discussed in this session. Concept on the following technologies, related with the well bottom plug concrete operation, has been clarified in this discussion. The The ‘Tre ‘Tremie Tec Technolog logy’, ap applied lied du during ing Bo Bottom Plu Plug op operation ion in well fou foundation ion, va varies ies wide widely from from the‘Conventional Trem Tremie Technol chnology’ ogy’ being appli applie ed in cast-i cast-in-si n-situ tu bored bored pi piles. In In general concept of conventi conventiona onal ‘Tremie Technol chnology’ ogy’,, appl applied iin n bored bored pil piles is is “pouring of concrete concreteunder water through tr trem emie pipe pipe from bottomto top of the dredge hole hole in a continuous continuous process’ ss’.. In I n this conventional conventional Tre T rem mie Technology, Technology, upward upward flow fl ow of concrete is very very im i mportant to avoid discon disconti tinu nuiity of concrete as wel well as to di discard the initi initial al contam contaminate nated d concrete. te. I n well bottom plug plug concreting, althoug although h ‘T ‘Tremie Technol echnology’ ogy’ is appli pplied, ed, due to large large diam diameter ter of the dredge hole, upward flow of concrete does not occur from bottom to top. Concrete, during well bottom plug operation, spills down over the old concrete. Normally well bottom sump is not considered in design. Normal sump with parabolic curvature is beneficial from the load distribution point of view. The focus point of this sump parabola formation is preferably to be on or above the cutting edge level. I n general neral for circul ci rcular ar well, well , diam di ameter ter is i s above above 2 metre and and li limited ted to 12 metre. Num Numbers bers of tremie pipe sets, required to be used for bottom plug concreting in circular wells depend on the influenced coverage area of each tremie pipe set. Similarly for “D” shaped or “Double D” shaped shaped wells, the numbers of tremie pipe pipe sets are decided cided based on the bottom bottom area area of the wells. Pri Prior to 1983, 1983, bottom plug plug concrete concrete under under wate waterr in in wel wells was was being execute xecuted d by ‘Sk ‘Skiip Box’ Box’ system only. only. Si Since 1983 to 19 1995, 95, both ‘Tremie Pipe Pi pe’ and and ‘Sk ‘Skiip Box’ Box’ methods were bei being adopte dopted d for for concreti concreting ng of wel well bottom bottom plug. plug. Af A fter ter 1995, 1995, adoption adoption of the ‘Tremie Pipe Pipe’ method was madecompul compulsory sory in i n well bottom plug plug concrete. concrete.
* By Dhr Dhruba ubajj yoti yoti Bhattacharya, Bhattacharya, Chie Chi ef General M anage anager, Cons C onsulting ulti ng Enginee Engineering Ser Services. E-Ma E-M ail :
[email protected]
In IRC:78-2000, clause 708.8.2 states for well bottom plug under water “ the concrete shall be placed gently by tremie boxes under still water condition ”. Based on the analysis, made in this session and the paper titled as “Analysis of tremie technique in concreting of bottom plug of wells”, published by Indian Road Congress in the journal ‘Indian Highways – May 2006’, the clause 708.8 of IRC:78-2000, requires further modification.
1.
CONCEPT OF POURING CONCRETE IN WEL L BOTT OM PL UG. •
PRIVIOUS CONCEPT Prior to 1983 : Using skip box system 1983 – 1995 : Using both ‘skip box’ system & ‘Tremie pipe’ system. Since 1995 : ‘Tremie pipe’ system is compulsory.
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PRESENT CONCEPT Pouring concrete in Well Bottom Plug using ‘Tremie Pipe Technology’.
Before we go for discussion on the concept of pouring concrete in well bottom plug, i.e., concept of ‘Tremie Pipe Technology’, adopted in well bottom plug, a brief discussion is required on ‘Skip Box System’. – Why skip box system in well bottom plug concretewas madeobsolete? 1.1 Hazards of ‘Skip Box System’ in Well Bottom Plug : • Pouring of concrete from top, using “Skip Box System” creates ‘turbulence’ in water and ‘time gap’ between the stages of pour of concrete, which subsequently wash out cement of the green concrete from top layers. • Controlling proper and smooth flow of concrete under water to all edges over the all space of sump and the space inside the well curb upto the end of cutting edge is difficult. • Using “Skip Box System”, flow of concrete to the desired level and space occurs only by normal gravity flow of concrete owing to high slump, overcoming the buoyancy effect. Comments for overcoming the hazards created in skip box system : • Flow of concrete is preferred to be from bottom to top to minimize the wash of cement from green concrete top. • Unless the flow of concrete is controlled under pressure, integrity of concrete under water is difficult to ensure.
* By Dhrubajyoti Bhattacharya, Chief General M anager, Consulting Engineering Services. E-Mail :
[email protected]
1.2 In view of the above, ‘Tremie Pipe Technology’ is adopted for well bottom plug concreting to ensure : • The flow of concrete at sump bottom level and subsequently integrity of concrete in well bottom plug. Poured concrete gets additional pressure due to height of the tremie pipe in addition to the normal gravitational force at bottom level. • Minimization of wash of cement from green concrete top by adopting pour of concrete from bottom to top. 1.3 Prior to discussion of Tremie Pipe Technology’, adopted for well bottom plug concrete, let us first brief on – ‘What is the Conventional Tremie Technology ?’ ‘Conventional Tremie Technology’, as normally known to be adopted in Cast-in-Situ Bored Piles, is as below : • Adopted only in dredgehole normally for concreting under water. • Pour of concrete through tremie pipe shall be in a continuous process. • Pour of concrete shall be from bottom to top of the dredge hole resulting to up-flow of concrete in sequence. Observed effects in ‘Conventional Tremie Technology’ :
Due to “Up-Flow Character of the Concrete”, the first poured concrete with initial contamination shall come above the desired level. The stages of concrete shall flow up in sequence.
1.4 Concept of ‘Tremie Pipe Technology, adopted in Well Bottom Plug Concrete’ is different from the ‘Conventional Tremie Technology, adopted in Cast-in-Situ Bored Piles’ The ‘Tremie Pipe Technology, adopted in Well Bottom Plug Concrete is as under : ¾
Adopted in dredge hole (well) for concreting under water.
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Continuity of concrete is maintained by pouring concrete into the tremie hopper by concretepump or by pouring from transit mixer directly in a continuous manner.
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Pour of concrete occurs from bottom to top but concrete flow from bottom to top in sequence does not occur owing to large area of the well. Concrete spills down over the old concrete. The first poured concretedoes not comeup.
1.5 In well bottom plug concrete while concrete does not flow from bottom to top in sequence then – What is the “Flow Character of Concrete during concreting in well bottom plug”. Now let us discuss on the same.
* By Dhrubajyoti Bhattacharya, Chief General M anager, Consulting Engineering Services. E-Mail :
[email protected]
8M (APPROX)
TOP LEVEL FORMATION OF CONCRETE PLUG
TREMIE PIPE
STEINING DESIRED LEVEL OF PLUG TOP
SUMP
4 M (APPROX) CENTRAL ZONE ¾
After initial spreading of the concrete in total area at the well bottom and reaching above the tremie pipe bottom level, new concrete pushes up the old concrete covering the area of maximum 4 m dia.
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Flow-up force will gradually reduce from centre to the periphery and as a result a heap of concretelike ‘cone’ will be created centrally around the tremie pipe.
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The said cone collapses gradually while concrete height increases or tremie pipe is jerked / lifted.
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Subsequently, in case well diameter is more than 4 metre, concrete spreads over the outer ring area, i.e., beyond 4 m dia zone. Thus old concrete does not remain in seriatim from top.
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The old concrete in this outer ring area, beyond 4 m dia central zone, does not flow up significantly, as the upward pressure, generated centrally, does not influence this zone considerably.
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So, the old concrete is not being flowed up in seriatim; new concrete is ever accumulated over the old concrete spilling down from the central heap/cone of concrete.
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To ensure flow of concrete to cutting edge and to all end surfaces, high slump and workability of concrete is very important.
1.6 As the “Flow Character of Concrete” in Conventional Tremie Pipe Technology, adopted in Cast-in-Situ Bored Piles is different from the Tremie Pipe Technology, adopted in Well Bottom Plug ’, the following concepts are required to be clarified in view of right execution at site.
* By Dhrubajyoti Bhattacharya, Chief General M anager, Consulting Engineering Services. E-Mail :
[email protected]
1.6.1 Addition of Extra Cement in concrete : I n Conventional Tremie Pipe In Tremie Pipe Technology, adopted Technology, adopted in Cast-in-Situ in Well Bottom Plug Bored Piles • In conventional “Tremie Pipe • In well bottom plug concrete, new Technology”, as applied, in general, in concrete is accumulated on old cast-in-situ bored piles, no extra cement concrete. Thus top surface of each is added. Refer Clause 6.3.3 of IS:2911 layer of concrete gets washed with (Part I / Sec 2). water and partially becomes weak in It is because the first poured concrete, strength. So, integrity of concrete is which gets contaminated or washed not totally ensured. To overcome this with water, flows up to top, beyond problemand to strengthen the joints of desired level. concrete layers, to the extent possible, 10% extra cement is added to the concrete.
1.6.2 Time limit of concrete pouring : I n Conventional Tremie Pipe In Tremie Pipe Technology, adopted Technology, adopted in Cast-in-Situ in Well Bottom Plug Bored Piles • In all cast-in-situ bored piles, even of • In well bottom plug, concrete does large diameter (limited to 4 m) piles, not flow up in seriatim. New up-flow of old concrete is totally concrete accumulates on old followed in seriatim as desired in concrete. conventional tremie pipe technology. • ‘Final setting time’ factor of • So the total concrete operation in concrete will not therefore be such pile is required to be completed applicable for such bottom plug within final setting time of concrete concrete in wells. (final setting time of concrete is generally increased, based on • So, no restriction of time for total requirement, by using suitable concrete operation during well admixture), as movement of initial bottom plug is imposed poured concrete ends at the time of completion of the total concrete operation.
1.6.3 Final level of concrete : I n Conventional Tremie Pipe In Tremie Pipe Technology, adopted Technology, adopted in Cast-in-Situ in Well Bottom Plug Bored Piles • In cast-in-situ bored piles, initially • In well bottom plug, as the initial poured concrete gets contaminated with concrete does not flows up and
* By Dhrubajyoti Bhattacharya, Chief General M anager, Consulting Engineering Services. E-Mail :
[email protected]
bottom sump mud and week due to remains at the bottom of well, no wash of cement from concrete while it additional concrete is required to be first comes out from tremie bottom. raised beyond the desired level. Due to up-flow of concrete this initially poured concrete comes on top of the • At the final stage of well bottom plug dredge hole above the desired level. At concrete, it is generally observed that later stage this extra concrete is the central heap / cone of concrete removed. remains above the desired level. The height of the central concrete cone may be minimized by jerking of the tremie pipe at the last stage of pouring concrete. • Full minimization of the cone may not be possible and the central heap of concrete may remain in position, provided the periphery concrete reaches the desired level.
1.6.4 Interruption in continuity of concrete : I n Conventional Tremie Pipe In Tremie Pipe Technology, adopted Technology, adopted in Cast-in-Situ in Well Bottom Plug Bored Piles • For well bottom plug, continuation of • For well bottom plug, continuation of concrete in a single operation is must concrete in a single operation is must at least till the pressure equilibrium at least till the pressure equilibrium condition is achieved at the tremie condition is achieved at the tremie pipe bottom level. pipe bottom level. • Further concrete is also preferably to • Further concrete is also preferably to be done in continuity. Due to be done in continuity. Due to discontinuity of concrete for more discontinuity of concrete for more than concrete final setting time or than concrete final setting time or delay in operation, the tremie pipe delay in operation, the tremie pipe may get choked with concrete as well may get choked with concrete. as the concrete in dredge hole may get set, resulting to difficulties in further up-flow of concrete. • In any case, the tremie pipe must be • In any case, the tremie pipe must be well embedded into the concrete, all well embedded into the concrete, all the time, till end of the operation and the time, till end of the operation and entry of water in to the tremie pipe entry of water in to the tremie pipe must not occur. must not occur.
* By Dhrubajyoti Bhattacharya, Chief General M anager, Consulting Engineering Services. E-Mail :
[email protected]
2.
VI TAL OBSERVATI ONS PRIOR TO COMMENCE WEL L BOTTOM PLUG CONCRETE •
Water inside the well shall be in stand still condition. There must not be any flow of water from bottom of the well due to artesian condition or whatsoever.
• The well shall be brought to an absolute stable condition, at least for last 48 hours before bottom plug operation is executed. • The water inside the well shall be kept undisturbed for minimum last 24 hours to allow all sedimentations and to get the water table inside the well stable. 3.
LIMITATION OF DEPTH OF WELL BOTTOM SUMP
During Sinking operation : • During sinking operation of the well, the central depth of the sump should preferably be limited to half of the internal diameter of the well. • In any case, due to stiff clay or any other reason, the central depth of the sump must not exceed 3 m during sinking operation, as recommended vide Clause 2.3 of Appendix - 4 of IRC:78-2000. On termination of the well at founding level : • Final central sump, on termination of the well at founding level, below cutting edge should preferably be around 1/6 of outer diameter of the well, limited to ¼ of outer diameter of the well. 3.1 Limi tation of depth of well bottom sump as considered in design :
• Bottom sump below cutting edge is not absolute necessity from design point of view, as in general, designs are based on consideration of flat bottom surface at cutting edge level, where the reaction pressures developed are considered to be axial and vertical. • Consideration of parabolic arch formation between curb faces, owing to the concrete plug with sump, may economize the design, as more bottom area is available resulting to less pressure at the bottom surface. But due to uncertainty of the sump formation and unpredicted soil strata variation, normally sump is not considered in the design, except some specific cases. • As well as, if concrete bulb formed in the bottom sump is very deep, the reaction pressure at the bottom concrete surface may be non-uniform, which may result to more initial settlement than what is obtained with a flat bottom surface. To overcome the non-uniform pressure effect on bottom plug concrete, while it is in stiff sump, it is preferable to fill the bottom sump bulb with coarse sand or concrete of requisite strength to get the suitable ( ‘X’
FOR X > D/4 : Value of ‘a’ will be ‘< ‘X’
While X = D/4
Whil e X = D/3 a = 3/16 x D, a = 9/16 x X [a < X] Focus remains below cutting edge level. Stiff Sump
¼
a= xD a=X Focus remains on cutting edge level. Reasonable Sump While X = D/6 a = 3/8 x D a = 2.25 x X [a > X] Focus remains above cutting edge level. Reasonable Sump
Whil e X = D/2 a = 1/8 x D, a = 0.25 x X [a < X] Focus remains quite below cutting edge level. Very Stiff Sump
* By Dhrubajyoti Bhattacharya, Chief General M anager, Consulting Engineering Services. E-Mail :
[email protected]
3.3 Conclusion for well bottom sump : Central sump of the well during termination should preferably be limited to ¼ O.D. of the well.
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Normally it is desirable that the focus point of the parabola (sump bottom formation line) remains on or above the cutting edge level.
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4.
CONTROL OF TREM IE PIPE DURING PLUG OPERATION
4.1
How many numbers Tremie Pipe Sets are required to be provided during well bottom plug concreting ? For circular wells upto 8 m dia
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For circular wells beyond 8 m dia and limited to 10 m dia :
For circular wells beyond 10 m dia and limited to 12 m dia :
Single Tremie Pipe Set at centre.
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Preferably 3 Tremie Pipe Sets as below :
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Preferably 4 Tremie Pipe Sets at 4 quadrants.
4.2 For “D” Shaped well ; double “D” Shaped and “Rectangular” well :
* By Dhrubajyoti Bhattacharya, Chief General M anager, Consulting Engineering Services. E-Mail :
[email protected]
•
•
Numbers of Tremie Pipe sets are to be decided based on the coverage area of each tremie pipe set as maximum8 metre dia circle. For double “D” shaped wells, coverage area of tremie pipe sets should be considered for the two compartments separately due to obstruction of the middle stem.
4.3 CONTROL OF TREMI E PIPE DURI NG PL UG OPERATION.
4.3.1 Diameter of tremie pipe during bottom plug operation. Diameter of tremie pipe during bottom plug operation shall preferably be decided based on the size of coarse aggregates used in concrete.
While 20 mm & down coarse aggregates are used : Tremie pipe dia – minimum 150 mm, maximum300 mmmay be used.
While 40 mm & down coarse aggregates are used : Tremie pipe dia – minimum 250 mm, maximum350 mmmay be used.
Higher diameter tremie pipes are better to be avoided as due to non matching of concrete feeding intensity into the hopper with the tremie diameter, discontinuity in concrete inside the tremie pipe may occur owing to expected air trapping inside the pipe. This air trapping not only hamper the concrete strength, it may cause entry of water inside the pipe, which results to non-functioning of tremie pipe technology.
4.3.2 Initial placement of tremie pipe and limitation of gap from sump bottom. •
•
•
Tremie pipe shall be placed into the well keeping a gap from the dredge hole bottom, so that concrete can smoothly flow down from tremie pipe through that gap. As such the said gap preferably should be minimum the diameter of the pipe and maximum twice the diameter of the pipe. In any case this gap must not be more than 600 mm. The tremie pipe should always be placed vertically, as far as possible.
4.3.3 Embedment of tremie pipe in concrete during stage lifting. •
•
Stage lifting of the tremie pipe shall be done ensuring sufficient embedment of the tremie pipe bottom into the concrete, to prevent entry of water into the tremie pipe. Tremie pipe with hopper, while jerked to loosen the friction of concrete with the pipe surface and to clear the jammed concrete in the pipe, the pipe bottom should not come * By Dhrubajyoti Bhattacharya, Chief General M anager, Consulting Engineering Services. E-Mail :
[email protected]
out of the concrete. Sufficient embedment of the tremie bottom into the concrete should be well ensured to prevent entry of water into the pipe. •
5.
The tremie pipe with hopper shall neither be lifted above concrete nor shifted in other location by dragging. THE CL AUSE MODIFICATION
708.8
OF
IRC:78-2000,
REQUI RES
FURTHER
Based on the discussions made, the clause 708.8 of IRC:78-2000, requires further modification.
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* By Dhrubajyoti Bhattacharya, Chief General M anager, Consulting Engineering Services. E-Mail :
[email protected]
