704 - Steel Sheet Piling and Cofferdams
GENERAL
Cofferdams
A cofferdam is a substantially watertight enclosure which will permit substructure construction above a tremie seal or subfooting, in the dry and without damage to the work. Cofferdams must conform to all conditions included in the Department of Environmental Quality (DEQ) or other project permits. The Contractor must obtain permission to deviate from the plans or approved permits.
Cofferdams are usually constructed around a template made of large timbers, H piling, or welded tube piling which serves as bracing when the enclosed area is dewatered. Care should be taken in driving the first sheet pile making sure it is as plumb as possible. Out-of-plumb sheeting leads to many problems, particularly at the corners. Water leakage through sheeting joints is common, but can usually be stopped by dropping fine cinders into the water outside the cofferdam where the leakage is occurring. The cinders will plug the joints and stop, or at least materially reduce, the water flow into the cofferdam.
MATERIALS
Sheet Piling
Steel sheet piling may be:
- permanent.
- temporary.
- part of a cofferdam.
Sheet piling must be of the continuous interlocked type, either new or used and in good condition.
Adequate toe and/or bracing must always be provided to support the piling and provide safe working conditions.
Check plans and specifications for the required minimum section modulus. Measure the individual pile length to determine driven lengths for payment considerations where sheet piling will or might be left in a permanent location.
Cold-rolled sheeting will be permitted for all applications.
CONSTRUCTION
Permanent Sheeting
Permanent sheeting will be driven to the penetration called for on the plans and the area measured for payment will include the full length from a plan cutoff to plan penetration. Permanent steel sheet piling section modulus shall be as specified on the plans and furnished with suitable connecting and corner pieces. Cold rolled sheeting will be permitted.
Temporary Sheeting
Temporary sheeting may be driven to the penetration determined by the Contractor unless otherwise noted by the design plans. The area measured for payment extends from ground line to the bottom of the excavation unless other limits are designated. It does not normally include the toe.
Temporary steel sheet piling shall have a minimum nominal section modulus of 18.1 in3/ft (973,000 mm3 per meter) of wall.
Inspection of Cofferdams
There are several general items to keep in mind about cofferdams:
The Contractor is responsible for its design and construction.
It should be constructed so that it does not hinder navigation or the river flow.
Its design should be adequate to provide a safe working area.
Where it is required to protect adjacent property or roadways, it will be "soil-engineered," and reviewed to ensure adequacy and stability with proper wales and bracing.
When requested by the Engineer, the Contractor is required to submit sketches of the proposed cofferdam for review.
The Contractor must secure an approval for his method of construction from DEQ in water crossing situations. A well point system may be used on land to control ground water. It cannot be used in waterways because it causes siltation.
If the Contractor's operation is suspect of water pollution or siltation, the Engineer will ask for a DEQ inspection as provided by Section 107 of the Standard Specifications for Construction.
Water pumped from cofferdams during the dewatering operation must be pumped through a geotextile filter bag before entering the watercourse.
Dewatering shall not begin until the tremie seal (if using) has obtained the minimum strength according to 706.03.H.3.
Piling used in a cofferdam is included in the pay item for Cofferdams, lump sum.
Cofferdams, left in place, include designing, furnishing, installing, maintaining and cut- off of the sheet piling at the elevation shown in the plans.
MEASUREMENT AND PAYMENT
-Reserved-