704 - Steel Sheet Piling and Cofferdams
Steel sheet piling may permanent, temporary, or temporary left in place, meaning the sheeting was designed to support earth pressures during staged construction, and removal of the sheeting may be detrimental to permanent structures. Sheeting may also be part of a cofferdam, along with other structural components such as bracing, struts, walers, tie backs, and other related structural components act to create a water tight seal, as to construct substructure elements in the dry. Steel sheeting consists of new or used continuous interlock-type steel sections including connections and corner pieces. Steel sheet piling can resist loads via cantilever capacity, or in combination with bracing such as walers and struts.
Used steel sheet piling must be in good condition, and its use must be approved by the Engineer prior to driving. The most common steel sheet piling shapes are the PZ types. For example, for a PZ-27 section, the P stands for “pile” the Z is for the “type or shape”, and 27 represents the unit weight per ft2 of material. Figure 704-1 shows a typical PZ shape configuration:
Permanent steel sheet piling details will be included in the contract plans, and will be driven to the penetration elevation called for on the contract plans. The permanent steel sheet piling minimum nominal section modulus, material grade, and all other pertinent information will be shown on the plans.
Permanent steel sheet piling shall be furnished with suitable connections and corner pieces. The inside face is to be located along the footing outline. Cold rolled sheeting will be permitted.
The basis for payment is square foot, based on the full length from plan cutoff to plan penetration.
Temporary steel sheet piling details will not be included on the contract plans, other than general limits, locations, and cut off elevations shown in plan and section views typically as part of staged construction. The Contractor is responsible for designing, providing, installing, maintaining and removing temporary steel sheet piling, or cutting off temporary steel sheet piling left in place to the elevations as shown on the plans. Sheeting may be driven to the penetration as determined by the Contractor’s approved design unless otherwise noted by the design plans (i.e., the plans may specify a penetration depth regardless of the design).
The area measured for payment is based on the area of earth retention, and does not include the amount needed for the toe. Vertical dimensions shall be based on the difference in ground elevations at the sheeting line, or planned foundation excavation limits at the sheeting line, whichever is less. This line typically extends from ground line on the earth pressure side, to the bottom of the excavation on the staged construction side unless other limits are designated. If retaining earth on both sides of the same steel sheet piling during different construction stages, the quantity and basis of payment will be based on the stage requiring the least area of earth retention, not the sum of the areas for each stage.
Temporary steel sheet piling shall have a minimum nominal section modulus of 18.1 in3/ft of wall, and the Contractor’s plans shall show the required embedment depths, steel material grades, etc. The Contractor’s plans shall show connection and weld details, along with required bracing, walers, tie backs and other related structural components.
The basis of payment is square foot, based on all areas retained soil, including cut slopes from the excavation limits to existing ground.
A cofferdam is a substantially watertight enclosure consisting of steel sheet piling and structural bracing components which will permit substructure construction in waterways, or other areas where the construction must be isolated from the surroundings. Steel sheet piling is part of the overall structural system resisting earth and hydraulic pressures. Some cofferdams may require bracing, struts, walers, tie backs and dead men to maintain acceptable stresses on the members, while allowing no more than 2” of deflection at the top of the sheeting per 12DS704(A410).
Cofferdam construction typically also involves the construction of a tremie seal or subfooting using Grade T concrete per subsection 706.03.H.3 of the Standard Specifications for Construction. This allows substructure units to be constructed in the dry and without damage to the work.
The basis of payment is lump sum per unit, which includes designing, providing the required materials (steel sheet piling, bracing, struts, walers, tie backs, deadmen, etc.), installing, maintaining and removing the cofferdam if not to be left in place, or cutting off steel sheet piling, bracing, struts, walers, tie backs, deadmen, etc., if the cofferdam is to be left in place. Tremie concrete and geotextile filter bags for dewatering are paid for separately.
Hot rolled sections provide and excellent interlock seal, that works well against water seepage through the interlock. A ball and socket type interlock is common for hot rolled section, and this interlock is durable, and ideal for repeated use. See Figure 704-4 below for the ball and socket type interlock. Hot rolled sections are typically more expensive, and thus, not used as frequently by contractors.
Cold rolled sections are more common due the fact they can be cut to any length or quantity, and are less expensive to produce than hot rolled sections. A hook and grip type interlock is common for cold rolled sections, which are known for allowing seepage through the interlock, and may become brittle over time due to the cold forming process. See Figure 704-5 below for the hook and grip type interlock. Pay close attention to interlocks getting jammed easily from soil, and water seepage in their final position.
All steel sheet piling designs are done in accordance with the AASHTO Standard Specification for Highway Bridges, 17th edition, the MDOT Bridge Design Manual, and the specifics as shown in the contract plans and specifications.
Permanent steel sheet piling details are included in the contract plans, and should have a familiar look and feel with respect to the MDOT plan format. Typically these designs are completed, or approved by Geotechnical Services prior to inclusion in the final plans. The required steel sheet piling sections, embedment depths, elevations, and material grades will be shown on the contract plans and specifications.
Temporary steel sheet piling, and cofferdams will be conceptually shown on the plans with respect to rough plan and section views, but this is for bidding purposes only, and not based on any detailed analysis and design. The Contractor is responsible for the design of the temporary steel sheeting and cofferdam installations based on the substructure size and staging requirements, and the soil borings, and other geotechnical information as shown in the contract plans and specifications.
At locations where the combined depth of retained water or soil is less than 6 feet, the Contractor is to submit the temporary steel sheeting piling, and cofferdam working drawings for approval per subsection 104.02 of the Standard Specifications for Construction.
At locations where the combined depth of retained water or soil is greater than 6 feet, the Contractor is to submit the temporary steel sheeting piling, and cofferdam working drawings and design calculations for approval per subsection 104.02 of the Standard Specifications for Construction.
The MDOT Geotechnical Services and Bridge Field Services areas will review the temporary steel sheet piling and cofferdam designs for conformance to the appropriate geotechnical and structural requirements. When reviewing the Contractor’s temporary steel sheet piling and cofferdam working drawings and calculations, the following must be shown:
- Steel sheet piling section modulus – must be at least 18.1 in3/ft of wall
- Design methodology for bracing, struts, walers, welded or bolted connections, and other structural components, i.e. AISC Allowable Stress, AISC LRFD, AASHTO LRFD, etc.
- Design methodology for tie backs, deadmen, ground anchors, or other externally applied anchor supports, i.e. FHWA Publication No. FHWA-IF-99-015 (Geotechnical Engineering Circular No. 4, Ground Anchors and Anchored System)
- Embedment depths, toe information, and depth of earth retained
- Material grade of steel for all structural members
- Other than the anticipated earth pressures and the 360 psf live load surcharge (when stage line is adjacent to traffic), consideration of any temporary loads such as construction equipment, anticipated material stockpiles, and unbalanced hydrostatic pressures
- Overall plan view or temporary steel sheet piling and cofferdam installations documenting each stage of construction, with profile and cross section views to ensure all load cases are accounted for, and the system can be built as intended
Working drawings for temporary steel sheet piling and cofferdams will typically not follow the MDOT plan format, as they are developed by the Contractor. Take the time to review and understand the overall staging scheme, and the sequence of construction for each stage. Once the Contractor’s design is approved, ensure it is built according to the approved design. Any variations, material or section substitutions, or changes in loading or site conditions will require further review by MDOT Geotechnical Services and Bridge Field Services.
If there are questions at any time during the temporary steel sheet piling and cofferdam installation, please contact Geotechnical Services or Bridge Field Services for assistance.
Unless prohibited otherwise on the plans, there are two methods for driving steel sheet piling, diesel impact (or drop hammers), and vibratory.
Diesel impact or drop hammers consist of a lead mounted short stroke heavy ram, with a drive cap to fit on to the steel sheet piling shape. Steel sheet piling is driven via impact force from the ram. See Figure 704-2 for a typical impact hammer set up.
Vibratory hammers consist of a suppressor unit used to transfer energy to the gearbox, and a clamp device to hold the steel sheet pile in place for driving. Vibratory hammers allow for easy driving in clays and soft soils, but may be prohibited in the vicinity of utilities, and other structures that may suffer damage due to the vibrations. See Figure 3 for a typical vibratory hammer setup.