Steel Sheet Pile CofferdamConstruction Essentials

1. Overhaul andInspection of Steel Sheet Piles on the Site

Lassensteel sheet piles should be repaired and inspected after arriving at theconstruction site. Remove impurities in the latch (such as welding slags andwaste filler, etc.) and repair the defect part. Correct twisted and"bent" latches identified during inspection. Steel sheet pilesexceeding the deviation should not be used. Carry out complete inspection ofmissing parts, remaining traces, uncleanliness, scale and curliness of the pilebody and adopt relevant measures to ensure normal use.

2. Extension ofLassen Steel Sheet Pile

Extendthe qualified Lassen steel sheet piles with only one weld. First, build theassembly platform and bevel the welding part. To reduce deformation, locate thesteel sheet pile using pressing plate before welding, flatten it using clip,keep the web of two steel sheet piles in the same plane and fix them on thebase. Polish and then weld the steel sheet pile. After steel sheet pile isextended, pull an about 2m long steel sheet pile through the latch on bothsides and inspect whether the latches are on the same straight line and whetherthere are welding slags and other obstacles in the latch.

The   steel sheet pile is extended to smoothly insert and remove the Lassen steelsheet pile during construction and increase its antiseepage performance. The latchof each steel sheet pile should be lubricated with oil mixture and theproportion ratio should be butter: dry saw dust=5:3. The cofferdam and internalsupport should be machined on the site, such as I steel splicing and cornerbrace fabrication, and the fabrication process and quality must meet thespecification requirements.

3. Erection ofSteel Sheet Pil

Thesteel sheet pile must be erected according to the principle of "verticalerection and timely adjustment". The steel sheet pile should be hoistedusing truck crane and erected using static pressure pile driver. The locationand perpendicularity of the first lateral pile is the key to control theposition of steel sheet pile cofferdam and steel sheet piling in the laterperiod and must be strictly controlled during construction. When erecting thesteel sheet pile, observe using theodolite in two mutually vertical directionsto ensure the perpendicularity. Then, erect steel sheet piles cyclically on oneside based on the first one. The steel sheet pile should be erected in theupstream and closed at a corner in the downstream. Special corner piles should beerected at corners; pallets should be set at the latch in the piling directionand measures should be adopted against displacement of steel sheet pipe toensure construction quality.

4. Closure ofSteel Sheet Pile

Steelsheet pile closure technology is the key for steel sheet pile cofferdamconstruction and the closure result determines the cofferdam constructionquality. First, set a closure opening in the middle of the cofferdam on thelast erecting side, with 3- 5 steel sheet piles. To control the perpendicularityof steel sheet pile at the closure opening, the gradient should be strictlycontrolled when there are over 10 steel sheet piles on both sides of theclosure opening. In order that the closed steel sheet pile engages withadjacent piles well, the height of piles on both sides of the closed pileshould be different and the height difference should be controlled to100- 200mm. In this case, the steel sheet piles can be erected in the latch ofthe closed steel sheet pile from high to low. When erecting the closed pile,the closed pile should freely drop into the latch of the adjacent pile untilreaching the preset elevation. Evaluation criteria for closure quality: Theclosed steel sheet piling time is basically consistent with the general steelsheet piling time. In the piling process, the latch of the closed steel sheetpile should be perfectly connected to that of the adjacent pile withoutbreakage; after the closed steel sheet pile is erected, the gradient of twoadjacent piles should be limited to 0.2% and consistent with that before closure.

Solve the leakage problem of steel sheetpile cofferdam

1. Identify thecause of leakage

Leakageof steel sheet pile cofferdam is often caused for the following reasons:

1)The steel sheet pile is damaged and twisted and the latch is deformed in thetransportation and handling process; 2) The pile body is incomplete andseriously deformed; 3) The joint of the latch of the steel sheet pile isseriously deformed; 4) The hoisting hole used for hoisting the steel sheet pileis not welded before use; 5) The steel sheet pile is not extended well, withwelding deformation; 6) The steel sheet pile is not closed well; 7) The bottomconcrete and the steel sheet pile are not bound tightly; 8) The bottom concretestrength does not meet the design requirements.  

Consultacceptance certificate for incoming materials, acceptance record onpre- hoisting welding quality, construction log, closure quality inspectionrecord and bottom concrete strength report to identify the primary cause ofcofferdam leakage. That is, the steel sheet pile has been used repeatedly,there is a large gap between the latches and the latches are not engagedtightly.

2. Formulatesolutions

Specificto the leakage problem caused by great deformation of the latch joint and looseengagement of latch, we adopt many measures.

SolutionI: Wrap the steel sheet pile with waterproof color strip cloth and keep itclose to the steel sheet pile under water pressure to resist water.

SolutionII: After all steel sheet piles are erected and when pumping water to build thecofferdam, based on practical experience, pump water and mix dry fine sand andsaw dust with wet sand below the joint simultaneously, and absorb fine sandinto the joint under water pressure to stop leakage.

SolutionIII: Fill the joint with cotton after installation.

Conclusion

Inconclusion, to ensure smooth steel sheet pile cofferdam construction and avoidleakage, pay attention to the followings:

1. Reasonably design the structuralform of steel sheet pile cofferdam to ensure construction quality andstructural safety.

2. Select new steel sheet pileswith well-engaged latch to minimize the possibility of leakage.  

3. Before using old steel sheetpiles, inspect whether there are defects. If any, they should not be adopted.

4. When extending Lassen steelsheet piles, ensure the welding quality meets the specification requirements;inspect whether the latches are on the same straight line before use.

5. The steel sheet pile must beerected according to the principle of "vertical erection and timelyadjustment".

6. The steel sheet pile closureresult determines the cofferdam construction quality, and the gradient of steelsheet pile should be strictly controlled.

7. Waterproof color strip cloththat is commonly used in daily life, fine dry sand or saw dust mixed with wetsand and other measures can be adopted to effectively stop leakage of steelsheet pile cofferdam.