Design Loads and Construction of Tremie Sealed Cofferdams - PDH Online Course for Engineers and Architects

Design Loads and Construction of Tremie Sealed Cofferdams

Course Outline

1. Learning objectives
2. Introduction
3. Design loads
4. Water elevation
5. Wave heights
6. Tidal and current loads
7. Mooring loads
8. Design stages
9. Pre-tremie loading
10. Dewatered loading
11. Construction

a. Access
b. Pile driving template
c. Setting template
d. Drive Piles
e. Excavate
f. Tremie
g. Dewater
h. Cleanup
i. Sealing leaks

12. Dismantle Cofferdam
13. Course summary

This course includes a multiple choice quiz at the end.

Learning Objective

The purpose of this course is to show that cofferdam construction is a complex process of design and construction through a series of stages. The designer must consider a number of forces than just the hydrostatic water loading. The student will have a better understanding of the design and construction process involved in building tremie sealed cofferdam in open water.

Course Introduction

This course explains how the various wind, current, waves and mooring forces are applied in addition to the hydrostatic head to tremie sealed cofferdams in open water for design purposes. The course then leads the student through the construction steps and the methods successfully used in the past. The examples of past problems are employed to illustrate the importance of careful planning and proper construction methods.

Tremie sealed cofferdams are used when construction must be preformed below the surrounding water level. Usually these cofferdams are in waterways such as lakes, rivers, and bays. In some cases, the free draining gravels will cause dewatering efforts to be less effective than a tremie sealed cofferdam. The tremie seal serves two purposes. First, it acts as a counterweight to prevent the cofferdam from floating out of the ground. Second, the tremie makes a solid foundation that will not heave or quicken from artesian water pressure.

The design of a cofferdam is a complex process that requires a detailed understanding of the various forces and construction methods that are used. The design must be compatible with the equipment and erection process. Intermediate stages of erection, internal permanent structures, and dismantling must all be considered in the design process.

Course Content

DESIGN LOADS

A typical tremie cofferdam will experience several loading conditions as it is being build and during the various construction stages. The significant forces are water pressure, buoyancy, soil active loads, water current, wave impact and mooring forces. In order to over come the displaced water buoyancy, the tremie seal thickness is about equal to the dewatered depth.

The first design parameter to select is the expected high water elevation. In a river it is a question of when and how long the cofferdam will remain dewatered. If the work can be completed during low summer flow, the cofferdam and tremie will be much shorter than if it has to be able to withstand winter and spring floods. Winter weather may also cause a concern for ice pressure and spring breakup. If the cofferdam is going to be dewatered for several months a selection of high water has to be made; a one, two, five or ten-year flood are common choices. This choice will depend on what damage can be done if the cofferdam is over topped by floodwater and for how long. In bays the highest expected tide will be the design water elevation.

Above the design high water elevation the cofferdam should have at least three feet of freeboard or higher than the maximum expected wave height. Wave forces will be significant factor in large bays and lakes where the fetch is several miles. Ship and boats can also generate large wake waves. The force generated by waves is asymmetrical and must be carried to the ground through the sheet piling in shear and bending. The waler system must be designed to transmit the wave forces to the sheet piles.

Tidal and river currents can generate significant asymmetrical forces and must be transmitted through the wale system to the sheet piles. The combination of high floodwater and fast current can result in scouring of the riverbed around the base of the cofferdam. Excessive scour can cause the cofferdam to become unstable, especially if the tremie seal is not yet in place. In loose sands and gravel scour can easily exceed ten feet deep in a matter of hours. If scour is a real possibility it may be necessary to armor the riverbed with riprap or mats to eliminate scour.

Mooring forces are derived form two separate actions. The first is the impact of the barge and tugboats as they moor to the cofferdam or the waves as they move the barges while moored. The other force is the wind pressure on the total sail area of the barge. Gale force wind is a common occurrence along most coasts and on large lakes. The combination of high wind and waves will cause major damage to the cofferdam and equipment if no preparation is made to accommodate those events.

There are at least two stages that must be designed. The first stage is when the cofferdam is fully excavated but prior to placing the tremie seal concrete ballast. Usually the cofferdam is installed and the excavation is accomplished by crane and clam bucket. The sheet piles support the excavation face. If the soil is soft and the excavation is shallow enough the piles can extend below the bottom of the excavation and the sheet piles are acting as simple beam spanning from the lowest wale to the ground below the excavation. Often the excavation is too deep and/or the soil is too stiff to allow sheet pile penetration below the bottom of the excavation. Often the stiffness of the soil requires a dig and drive operation. In this case, the sheet piles are acting in cantilever, bending around the lowest wale. The water elevation in the cofferdam is kept at least equal to the surrounding water surface elevation. Often water is pumped into the cofferdam to insure no negative differential head develops during tidal changes. The active lateral soil pressure under water is about 15 pcf. At this first stage the cofferdam the least stable and is vulnerable to wave, current and mooring forces. I have had a major cofferdam lean to the side due to soil failure during the dig and drive sequence.

The second stage is after the tremie seal is poured and the cofferdam is dewatered. The mass of tremie concrete stabilizes the cofferdam, but the system must be able to resist the water pressure, current, wave and mooring forces. In most protected bays waves will generally be five feet or less from crest to trough. The design free board should be at least 1.5 times the expected wave amplitude. The pressure of the wave can be taken as the water density times the wave height. If the cofferdam is exposed to the open ocean or large and deep lakes a thorough analysis of the anticipated wave amplitude, wave length and pressures generated is needed. The current pressure can be figured as the current velocity squared times the density of the water divided by 2 times the gravity acceleration. Mooring forces are difficult to quantify, but I use at least 1,000 lb/lf along the wale closest to the water surface. In an open ocean environment where large barges are wave and wind driven an analysis of the potential impact forces will be required. From a practical standpoint sheet pile cofferdams are not usually built in open ocean waters. This is because of the extreme natural forces that can be commonly and suddenly experienced at sea.

Sometimes to facilitate the construction inside the cofferdam the lower struts and/or wales will be removed and the new internal structure will be used to support the sheet piles. If this can not be accomplished, it is important to arrange the struts to minimize the impact on the new structure. The struts are a major interference to the interior cofferdam construction and will significantly slow nearly all productivity as forms and rebar is placed through and around the struts. Blockouts may have to be formed around the struts so that they can be removed later. It is common to need divers to dismantle the lower wale systems and plug the blockouts. This is a slow and expensive process that requires extensive support equipment and support personnel. Because of the potential danger to the divers a careful and detailed plan needs to be drafted. This plan must also address all foreseeable events that could endanger the divers and have emergency procedures in place and communicated to all involved parties.

Pm = Mooring Force = 1,000 +/- ,lb/lf

Pc = Current Force = GwDwV^2/2Ge, in lb/lf

Pw = Wave Force = GwHDw, lb/lf

Pa = Soil Active Force = Ga(De+Dt)^2/2

Rpa = Soil Passive Reaction at the upstream toe

Rpb = Soil Passive Reaction at the downstream toe

Rpc = Soil Passive reaction to resist overturning

H = Wave Height in feet

V = Current Velocity if feet per second

Dw = Water depth to the ground

De = Depth of the excavation

Gw = Water density, 62.4 pcf for fresh water and 64 pcf for seawater.

Ga = Active soil pressure, usually about 15 pcf

Ge = Gravitational Constant = 32.2 ft^2/sec

Gp = Soil Passive Pressure, usually about 300 pcf

Several assumptions must be made at this stage of design. On the positive side the duration between completing the excavation and placing the tremie seal concrete is usually a matter of only a few days, so the exposure is minimal. Both the mooring force and the wave force are short-term dynamic impact forces so the passive resistance of the soil does not need to be reduced by submergence. The current force is a steady load, but it is usually small when compared to the mooring plus the wave force. If the Rpa is set equal to Pa, then the conservation of Moments and Horizontal forces is used to readily determine the passive forces Rpb and Rpc.

The second stage loading is after the tremie seal is poured and the cofferdam is dewatered.

Pt = water pressure to the top of the tremie seal = GwDt^2/2

Df = Effective Fixity below the top of the tremie concrete

It is also reasonable to assume a pinned support at the top of the tremie concrete at the sheet pile contact line.

The surface of the tremie concrete will vary about two to three feet in elevation from the high points in the center to the low point s at the sheet piles. As the tremie is poured there will be some minor segregation and water entrainment. This causes the concrete to swell from batch volume about 5% when measured in the cofferdam. This is to be expected and is not a cause for concern.

Note that all the loads are shown as point (mooring), triangular (hydraulic head) or rectangular (wave and current). This is applied to simplify the calculation process. The mooring load is an impact load that is short duration. The kinetic energy of 1,000 ton barge moving at 1mph or about 1.5 feet per second generates about 1,000 ton x 2,000 lbs/ton x (1.5 fps)^2 / 32.2 ft/sec^2 = 140,000 ft –lb. The momentum is nearly 100,000 lb/sec, if this is absorbed by the fender compression and the flexing of the cofferdam totaling one-foot the force is 100,000 lbs. Sound and heat dissipates the remainder of the kinetic energy. A 100-foot long cofferdam waler nearest the water level will transmit 1,000 lb/ft through the cofferdam. The sheet piles then must transfer the load to the ground and/or tremie seal.

Often the sheet piles are cantilevered too much to absorb that load without yielding. For this reason heavy vertical cross bracing between the upper and lower struts is required to effectively transmit the mooring load to a point where the sheet piles can safely absorb the bending load. The author prefers a cable bracing system rather than rigid steel bracing because the cables will stretch and allows lateral cofferdam movement to help absorb mooring and wave impact forces.

In rivers, the high current and high water occur simultaneously with gale force wind and wave generated impact forces. In bays where tides generate the water level fluctuations, slack tide or no current accompanies high tide. In a large bay, such as San Francisco Bay, steady storm winds can generate large waves and raise high tide by several feet.

Determining the expected wave impact is a complex procedure and there are several methods of calculation. The configuration of the body of water, depth, length of fetch, wind speed, wind direction, duration of the wind, and gusting all play a significant role. The rectangular load diagram presented above is at best an approximation. By adding the wave height to the high water elevation, a single triangular load diagram can be used to calculate the dewatered waler and sheet pile stresses.

Usually in rivers, waves are not a significant consideration. However, some river will generate very swift currents, especially during flood stage. A 7 mph current or 10 fps will cause a 200 psf differential load on the cofferdam or over three feet of water head difference from one side to the other. Often this load controls the design of the waler system cross bracing and the sheet pile selection. The rectangular configuration shown is an approximation to facilitate the calculation process.

Most major cofferdams are indeterminate structures. The design and calculation process requires the use of deflection formulas to determine load and stress distribution. For this reason the load diagrams are kept as simple as possible. There is no point in refining load diagrams to complex configurations. The mooring, current, wave and wind loads are, at best, judgmental. Usually the worst case events such as 100-year floods and storms are not used to design cofferdams. The cost would be prohibitive. For this same reason, earthquakes are not usually considered in the temporary construction. Depending on exposure, risk and cost usually a 2-year to a 10-year events are used as the cofferdam design criteria.

CONSTRUCTION

The successful cofferdam construction of depends greatly on adhering to proper procedures and sequences. The designer and builder must understand that exacting tolerances can not be maintained, with deflections and misalignments are measured in inches or feet. Piles are easily deflected off line by rocks, obstructions, and changing soil conditions. Even improper installation methods can result in major damage.

From a practical standpoint, cofferdams are limited to 60-foot long sheet piles. Manufacturing, transporting, handling, threading and driving sheets longer than 60 feet creates major problems. We have built cofferdams with 40 feet of dewatered depth by excavating below the tip of 60-foot long sheet piles. This could be done only because the ground below the sheet piles was stiff enough to stand vertically under water long enough to place the tremie concrete. We have also chemically grouted sand lens to prevent underwater cave-ins. Usually tremie sealed cofferdams are limited to about 30-feet or less of dewatered depth, plus a 30-foot deep tremie seal.

Access to the cofferdam site is by trestle or barge. Several circumstances will determine which is the better access. A trestle offers the easiest and most stable access, but deep water and great distance from the shore may cause barge access to be more economical. Ship channels may also prevent the use of an access trestle. If rough seas and high wind are common, barge access will be limited causing excessive delay while waiting for calm weather. In such cases, a trestle may prove to be more economical in the end.

The first construction step after the access is in place is to position the wale system. The wales can be assembled on a barge and floated into position. Guide piles and support frames are installed to hold the wale system in place. The barge can often be partially flooded and towed from under the suspended whale frame. The wale frame is then lowered to elevation using cranes or hydraulic jacks. The wales are then used as a guide to thread and drive the sheet piling.

Usually there are at least two layers of wales. The top and bottom layers will act as stabilizing template to control the sheet piles. In any marine environment, there will be some waves, current, and wind. Without a supporting template to guide the sheet piles it is almost impossible to maintain the vertical and horizontal alignment necessary to close the cofferdam and prevent the interlocks from splitting open. If the sheet piles are not kept plumb the interlocks will split apart in tension or the closing pair can bind up due to compressive friction and refuse to be driven.

Vibratory pile driving hammers have largely replaced impact type driving hammers. The vibratory hammer is faster, quieter, and is less likely to cause damage to the sheet piles. Drilling holes for the piling is the preferred methods of installation when the soil contains cobbles or is too hard to allow pile driving.

The first step to cofferdam installation is making a driving template. Usually the waler system is used as a driving template. Someone must help thread the interlocks.

The template wales should be marked with the proper location of every sheet pile pair interlock that touches the wale. To allow for deflection and some misalignment that will occur, it is common to build the template 4” to 6” wider than the designed size. One way to accomplish this is to band 2 or 3x12 wood planks on the outside of the walers. Special care should be taken to insure the first pair is set plumb and in the proper location, since it will act as guide for the rest of the sheet piles. One real advantage of the vibratory pile hammer is the hydraulic pile grip is used to pick the sheet pile pair from stockpile and thread the interlocks. When the sheet pile pair is properly threaded and aligned, it should be driven to the top of the template wale. A C-clamp can be used to keep the free interlock from fanning out. Sheet piles will tend to tilt along the wale because of the unsymmetrical interlock friction during the initial driving, so the top should be restrained from walking along the wale.

If there is a prevailing wind or water current, start setting the sheet piles on the center of the upwind or up-current side. Complete this up wind side installation of sheets including the corner pile by alternating from left to right when adding sheet pairs. Make sure that the corner piles are truly plump in both directions. It is a lot easier to correct misalignments as the sheets are being threaded than discovering a problem when the final closure is attempted.

Final closure should never be made at a corner. The reason for this is the corner works in both directions. If either sheet wall line is out of plumb, the sheet interlock will probably split open. The other reason to be careful in initial alignment is that this will largely define the direction the piles will take as they continue to penetrate the ground. If the interlock is started off tight and out of line, it will likely split apart as it is being driven. This will damage the pile and may require a very expensive and time consuming repair procedures.

When the sheet piles are fully in place and driven to the top of the upper template, the template wales can be lowered, if needed. The pairs of sheet piles should be advanced in about five foot increments. Drive alternate pairs so that the interlock friction stays symmetrical for every pair. This will help maintain pile alignment. Constantly check the sheets for plumbness and alignment. If the sheets start to walk out of plumb or alignment, extract the sheet pair and advance the pair on each side of the problem sheets. Sometimes by working the problem sheet pair up and down a few times, the pile will realign and driving can continue. This ability to extract and drive the sheets with a vibratory hammer is a huge advantage over impact hammers, which usually only can drive the pile efficiently. If the misalignment can not be corrected and is serious enough to require additional action, the only practical solution may be to excavate to the toe of the sheets and remove the obstruction. It may even be necessary to install temporary walers at unplanned elevations. This is another reason to have the design on a computer, so you can react quickly to address the problems as they arise.

Cofferdams are rarely installed as easily as they are planned and designed. You must expect and anticipate problems that will require redesign and innovative solutions. However, it is rewarding to solve the demanding construction and knowing it will help successfully complete the project.

We had one cofferdam where the crew let the sheets get out of plumb and the closure sheet pair could not be driven to the required penetration. They elected to cut the jammed sheet pair flush with the top of the adjacent sheets so nobody would notice. Unfortunately, the tip of the jammed sheet pair was above the tremie seal concrete but about ten feet into the clay bay bottom. When the dewatering was nearly complete the clay plug blew out and the cofferdam filled with water so fast that two men got wet to their waist before they could ride the crane hook out. The water rose about twenty feet in just a few seconds. It also took another month to seal the blow out and complete the cofferdam dewatering. Even a split interlock is expensive and time consuming to repair. A one-inch wide split a foot or so long will spew more water than a fire hose if it is forty feet below the water level.

In another incident, we had a major cofferdam tip to one side about 15 degrees. The cofferdam was 140 feet long, 60 feet long with 60-foot long sheet piles. The cofferdam weighed over 500 tons. The Geologist assured us the weak rock at the sheet pile tips would support the weight while the tremie excavation proceeded to further depth. It took a month to right the cofferdam using barge-mounted cranes and hydraulic jacks. The cost of the mistake cost us over $1,000,000 to fix and we still had to install the support piles to carry the weight of the cofferdam. The cost of the support piles was only $25,000. Obviously, the risk taken to save a few dollars was not a good one. Cutting corners when building major cofferdams is only begging for disaster.

With the sheets carefully driven and the wale in position, often the sheets are welded or bolted to the top wale to provide cofferdam stability during excavation operations. A crane and a clam bucket usually perform the excavation, although in some instances a backhoe can be effective.

If the soils are stiff, the ground can be “Swiss cheesed” with a crane mounted drill. This allows the bucket teeth to grip and cut through the soil rather than just scraping along the surface.

Always excavate along the sheet piles first, keeping a low hump in the middle. This allows the clam bucket to rest against the sheets and stay upright so it can stuff the bucket. If a depression is created in the middle of the excavation, the bucket will roll on its side and be unable to excavate the wedge of soil adjacent to the sheet piles. When the excavation is nearly complete, slide a steel beam spud between the wales and the sheet pile alcoves. Almost always soil will cling in the alcoves. This plug of soil can easily blow out during dewatering, causing great expense and delay. The cause of a major leak that prevents dewatering can be very difficult to even locate, often requiring divers to probe for the leak. If there is more than one plug of soil to blow, you may gain repeated experience by finding such leaks as they sequentially blow out with each attempt to dewater the cofferdam.

Tremie concreting is done in a manner so as to minimize the flowing concrete contact with the water. The method is to induce the fresh concrete under the previously placed concrete and pillow it up and out. Never allow the concrete to fall through the water, if that happens, the cement will wash out and you will have a pile of gravel with a weak cement paste icing on top. This icing or surface latents will happen to a limited extent no matter how carefully the concrete is placed. The concrete can be pumped by first filling the pump hose with concrete and weak wiring a watertight cap over the end of the hose. The hose is lowered to the bottom of the excavation and the concrete continuously pumped raising the hose only when the backpressure slows the pumping production or the concrete has risen to the desired grade. More than one pump can be employed. Pumping points should be at about 25 feet on center at the most. Tremie tubes made from 12” steel pipe can be used. These tubes act the same way as the pump hose. First the tube is lowered on the bottom and a rubber ball is pushed into the tube, forming a seal. Then concrete fills the tube forcing the ball all the way to the bottom. When the tube is completely full of concrete, it is eased up from the bottom until the concrete starts to flow. The tube is kept almost full at all times be adjusting the tube up or down to compensate for the flow rate of concrete. The tube is extracted and restarted in a new location only after the concrete pillow has reached the desired elevation.

The tremie placement is a continuous operation until completed, going 24 hours a day without interruption. Tremie pours usually involve large volumes of concrete, often several thousand cubic yards of concrete. Usually one or more concrete plant and backup are dedicated solely to the tremie pour until it is complete. One of the worst things that can be done is stopping the tremie before it is completed. Any cold joint formed will be a thick, inclined and very weak plane, which may easily fail from the weight of the structure it is designed to support.

The concrete mix design is very important. The mix design must produce a free flowing and slow setting concrete. The concrete usually contains about 7 sacks of low heat of hydration cement, rounded aggregates, high sand content and water added to achieve a six to eight inch slump. Concrete set retarding, water reducing and anti wash agents are sometimes added to the mix design. The concrete mix design is a critical element to building a successful cofferdam. It is wise to consult with an expert in tremie concrete construction before committing to the work.

We have encountered engineers and owners who think they know all about tremie placement methods. When we ceded to their method demands, it always led to major problems. One Federal agency insisted we adhere to a cofferdam construction manual that was twenty years out of date. The author of the manual, an internationally acclaimed Engineer, finally told them that that the manual was obsolete and referred to his latest work. Another time, an engineer insisted on a tremie concrete placement method that resulted in soil seams and weak cold joints. This forced us to drill and high pressure grout the tremie seal concrete. This cost the project hundreds of thousands of dollars in unnecessary lost time and expense. Refuse to comply with improper suggestions.

If an owner insists on poor procedures, document to the engineer that you believe the methods are wrong before the work is started and you will file a claim for all remedial costs and loss of time. It is usually wise to have a recognized expert review your cofferdam construction plans and methods well in advance of the work and submit the review before the owner has a chance to interfere.

When the concrete has cured enough to gain enough strength to withstand the dewatering forces (about two or three days), dewatering can begin. Two major problems can arise at this time. The first is the pH of the water in the cofferdam is going to be at least 11, very basic and often too high to permit pumping back into the bay or river without treatment. The water will be murky, containing colloidal size cement particles that will not readily settle out. The other problem is the cofferdam will leak more and more as the water is drawn down, usually generating several thousands of gallons per minute of leakage. Once the differential head of water between the outside and inside becomes great enough to push the interlocks tighter, the leakage will be significantly reduced. But initially, massive volumes of water must be quickly removed from the cofferdam until it has a chance to seal itself up. The required pumping rate is measured in several thousand of gallons per minute in order to be effective. The draw down within the cofferdam must be fast enough to detect visually, at least 1” per minute initially or the pumps will be over whelmed by the leakage through the interlocks and the draw down will cease and additional pumps will be needed.

In times past it was no problem to pump directly into the surrounding waterway. Today with strict water quality regulations in place, disposal to the pumped water can be a huge problem. Recently in Canada several weeks were lost because there was no way to dispose of the pumped water fast enough. Only by adding flocculation, buffering chemicals and circulating the water through filters could the water quality be improved enough to allow pumping into the river. Huge holding ponds must be found or created or pumping directly back into the waterway is required. The time and cost to treat the cofferdam water can cause major costs and delays. Be sure a workable and approved plan is in place before the cofferdam must be dewatered.

After the cofferdam is dewatered, the clean up process can begin. The surface will be rough and undulating. There will be layers of mud, debris, and dead fish that must be cleaned up. Once the clean up is done, the top of the tremie concrete will have about six inches of laitance. The laitance is a weak layer of nearly pure cement that has been washed to the surface of the concrete by the dynamics of the concrete tremie placement. This is one reason to have a cement rich concrete mix design. Some of the cement will be washed from the concrete and some segregation will naturally occur.

At this point, a safety precaution is inserted. No gas-powered machinery should ever be allowed inside a cofferdam. The danger for explosion and carbon monoxide poisoning is too great. Even the use of diesel powered equipment in the cofferdam should be kept to an absolute minimum. Whenever it is possible, engines outside the cofferdam should power all machinery. These actions will both reduce congestion in the cofferdam and provide for safer working conditions.

The laitance must be removed from at least the areas of contact with the subsequent structure foundation. The laitance can be removed by jackhammer; a small rubber tired backhoe mounted hoe ram, or a very high-pressure water jet. The laitance is removed until the coarse aggregate is exposed. This insures that a structurally competent bond and bearing will be achieved between the tremie mass concrete and the reinforced foundation structure.

The direct costs of clean up, laitance removal will require at least four person-hours per square yard, and the cost of the cofferdam associated equipment. Usually it takes at between a week and a month to fully prepare the tremie surface for the subsequent construction. The surface will vary in elevation approximately three feet or more. High spots usually must be chipped down. Starter forms must be custom cut and fit to the tremie surface at an expense of 2 to 4 square feet per person-hour. Often a leveling slab is placed to facilitate the construction to the subsequent structure.

While the clean up and laitance removal is progressing, the cofferdam will continue to leak and require substantial pumping. The leakage water will be contaminated by the mud and debris in the cofferdam until all remedial work and clean up is completed. All water removed from the cofferdam during this stage probably will have to be processed before returning the water to the river, lake, or bay. Sometimes barge-mounted filters are needed. At other times, a settling pond on the shore can be utilized. Pollution control measure requirements can be very extensive and costly.

As soon as the dewatering of the cofferdam is started, efforts to stop and control leaks should begin. The slower the leakage the quicker the cofferdam will be dewatered and the least amount of water will have to be handled and processed. Controlling leakage is the one major reason to adhere to strict and proper cofferdam installation methods. A properly installed cofferdam will allow quick and easy dewatering, while a badly constructed cofferdam may require weeks of work just to get to the point where it is even possible to dewater the cofferdam.

There are several ways to stop and slow the leaks. No cofferdam is totally free of leakage, but over time, the cofferdam will continue to seal up and decrease the leakage. Rust of the sheet pile steel and water borne particles will fill small gaps. After several weeks, a large cofferdam may need only a small 2” pump to keep the cofferdam dewatered. Wooden wedges can be driven into the larger seams. Fly ash can be poured on the outside over small leaks. Visqueen can be lowered on the outside to form a patch. A mixture of horse manure, sand, and sawdust is often very effective when dumped on the outside above the leak. The sand will add weight so the mixture will rapidly sink, the horse manure will add just enough stickiness to hold the mixture together, and the sawdust and sand will be sucked into the crack. The sawdust will absorb water, expand slightly to further seal the crack, and wedge itself in tighter.

Initially, an around the clock watch should be maintained to insure the pumping system does not fail. The cofferdam can easily fill overnight if the pumps fail or clog up. If that happens, then the whole process of dewatering must be started from scratch. Much of the previous leak control work will be undone and avoidable time and money will be lost. Even with a full pump watch, there should be redundancy of power source and standby pumps already in place that just need to be switched on. Sometimes automatic float switches and emergency relay gear can be used.

Removal of the cofferdam can be a single event or in stages. The single event is when all the construction below the water is completed, the cofferdam is flooded to relieve hydraulic pressure and the sheet piling are extracted and wales are dismantled. Sometimes the designer requires the sheet piling to be cut off at the ground line to enhance lateral stability of the structural foundation. That requirement will increase the cost of the cofferdam by the extra work of divers and lost salvage value of the piling.

The cofferdam will sometimes be removed in stages. The new structure is completed to just below the lower wale and backfilled between the sheets and the new structure. The lower wale and strut system is then removed. The new structure is then built to the next layer of wales and the process is repeated.

The single event allows the sheets and wale system to be removed with the hydraulic head removed, so the dismantling process is only a matter of retrieving the pieces. When the cofferdam is dismantled in stages the wale system will be under high pressure and the sheet piles will squeeze inward several inches as the gravel backfill absorbs the transferred hydraulic pressure and the piles deflect. The pressure is best transferred slowly so impact is minimized and the process is always fully controlled. The struts can be slowly cut near a wale with a torch until the remaining steel yields and the sheet piles move in. The preferred method is to heat the strut near one end with a rose bud torch until red-hot and the pressure collapses hot portion of the strut. The process can be carefully controlled and stopped at any time by cooling the steel. The bolts attaching the wale corners can be torched or driven out with an air hammer.

Every cofferdam is unique and requires thorough analysis. The designer must take into account a large number of parameters. The design must be compatible with the weather conditions, waves, currents, construction equipment, construction methods, internal permanent structures, and ground conditions. Comparable cost studies should be analyzed to determine if the cofferdam method is favored over other techniques, such as precast or caisson construction. Often the cofferdam designer must work closely with the project design engineer to arrive at a mutually satisfactory procedure.

Course Summary

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