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(May Update)
By Bill Merz
Landings Association Bridge Committee
London Bridge was recycled by reconstructing it in Lake Havasu City, Arizona. Our bridge will be recycled in a somewhat less glamorous fashion.
The working parts of the drawbridge mechanism, such as the gears,, will be delivered to Chatham County for use as spare parts for an identical drawbridge on Islands Expressway. The remaining metal parts of the bridge will be recycled as scrap metal. The asphalt pavement on the bridge approaches will be ground up and used in future asphalt paving mixes.
The bascule bridge abutments will be demolished by blasting. An expert blasting subcontractor will drill strategically placed holes to permit complete fracturing of the concrete in stages involving no more than five pound dynamite charges. Prior to the blasts, the abutments will be covered with steel cable mesh mats, and road and boat traffic will be stopped well clear of the blast area. This work must be done at low tide in November for environmental reasons. A barge-mounted crane will remove the structural steel and blasted concrete from the waterway.
The remaining concrete deck and piers will be demolished by hydraulic equipment and hauled to a crusher for processing as aggregate in future concrete mixes. Reinforcing steel will be separated from the concrete and recycled. Some of the larger concrete pieces may be used for an oyster reef.
The black temporary retaining walls adjacent to the existing road will be covered with fill material which will create a sloping embankment. A future article will discuss the restoration of the marshland in the bridge area, which is an important requirement of this contract. When the demolition and restoration phase of the contract is complete, there will be no evidence of the existing drawbridge or approaches.
(April Update)
By Bill Merz
Landings Association Bridge Committee
Most highway construction projects are bid on a unit price basis. The State provides the bidding contractors with a complete set of plans and specifications with a listing of the quantities required. The bidding contractor submits a price for each unit of work in place (e.g., pounds of reinforcing steel, or cubic yards of deck concrete) to arrive at a total price to build the project as designed. Normally, the contractor submitting the lowest total price is awarded the contract. Some quantities may vary as actual conditions differ from design assumptions (e.g., linear feet of bearing piles required to support a bridge), and the contract total price is adjusted accordingly. The contractor is required to complete the project within the time specified, but that time can be extended if quantities of work increase significantly due to field conditions or due to mistakes in the plans.
A somewhat newer method of contracting that was used for our bridge is called Design-Build. For this type of contract, the State provides a description of the project, and the bidding contractors may bid on a design of their choice that meets the criteria set forth. For example, the criteria for our bridge included the width and maximum slope of the roadway, the horizontal and vertical clearance over the Intracoastal Waterway, the need to maintain traffic during construction, and a few other similar requirements. The bidding contractors had to include the cost of all construction as well as the cost of project design, and of obtaining approvals for environmental, traffic safety, and utility protection provisions. The lump sum contract price had to cover all costs needed to complete the project within the allotted time. It is possible that different contractors could bid on designs that varied in the number, size, and type of beams; the number of piers; the type and size of piling; and in a variety of other details. All materials and construction methods must meet the requirements of Georgia Department of Transportation (GDOT) standards as well as standards adopted by the American Association of State Transportation Officials (AASHTO).
On July 19, 2009, United Contractors, LLC, submitted the low bid of $22,489,997. Five other bids were received, ranging from $22,869,375 to just over $30 million.
The Georgia Department of Transportation has established an Office of Innovative Program Delivery to select a limited number of projects suitable for the Design-Build method. To date, 10 Design-Build contracts have been awarded, of which our bridge is one of the larger in dollar value and complexity. The advantages to the State include the decreased time span from project conception to completion in that the design and construction are carried out somewhat simultaneously. There likely are cost savings in that the contractors will bid on a design that is most economically suited to their equipment and manpower capabilities.
Some of those advantages are also attractive to the bidding contractors. On the downside, contractors must spend more time and design costs to prepare a responsible bid on a project where they may not prevail. To be allowed to submit a bid, the contractor must demonstrate that he has the experience and financial resources required. This often results in a combined effort by a contractor and an engineering design firm.
As our bridge continues to progress toward a likely early opening, many people have asked if the contractor will receive a bonus payment for early completion. Unfortunately for our contractor, the only cost benefit for finishing early is the savings he will realize through reduced overhead and equipment rental. Perhaps he could sell lottery tickets with a prize for correctly predicting the opening date. Any takers?
The coming month will see the post-tensioning of the center spans as well as additional deck concrete placement. Next month, I'll describe the demolition phase of the contract.
(March Update)
By Bill Merz
Landings Association Bridge Committee
A very important aspect of any construction job is SAFETY. This is especially true when working around heavy equipment, near power lines, near traffic, and at great heights...all of which are conditions present on our bridge project.
All the power consumed by Skidaway Island is fed through the wires adjacent to the bridge. These lines carry a lot of power, and any contact with the crane booms operating in their vicinity could be disastrous.
Men are required to work at heights and around tripping hazards such as reinforcing steel. They are required to attach safety lanyards between their harnesses and secure cables whenever working above the ground.
Personal protective equipment including hard hats, safety vests, sturdy shoes, gloves, goggles, and life preservers are required when tasks require them.
Traffic on the Diamond Causeway is a constant concern, especially as trucks enter and exit the construction zone. Our readers are requested to observe the reduced speed limits at all times, and to use extra caution when construction vehicles are waiting to occupy the road.
An accident on a construction job has negative impact on morale as well as on costs. Insurance rates are adjusted according to a contractor's safety record. Any accident, especially a serious one, will cause a disruption to a project's time schedule. All contractors have safety programs dictating procedures to be followed to conform to, or exceed, federal OSHA (Occupational Safety and Health Administration) rules. United Contractor's onsite supervisors oversee safety on a day-to-day basis, but are backed up by monthly inspections by the company's safety manager. To date, there have been no lost time accidents on this project.
Work during the coming month will include further beam erection, including the drop-in beams over the Intracoastal Waterway. The project continues to be on or ahead of schedule.
(February Update)
By Bill Merz
Landings Association Bridge Committee
Enough about prestressing, post-tensioning, and precasting for awhile. Let's talk about some less glamorous, but necessary, parts of the bridge.
Concrete expands and contracts with changes of temperature, and allowances must be made for this phenomenon to avoid cracking as this movement occurs. Expansion joints in the bridge deck, sometimes called finger joints because they look like interlaced fingers, allow for up to three inches of movement between adjacent sections of the deck.
Neoprene bearing pads are placed under the ends of the beams to permit the beams to expand over the pier caps.
Rainwater is carried from the bridge deck to the ground under the bridge by a system of 12-inch plastic drain pipes connected to inlets, called scuppers, spaced along the roadway gutter lines.
A fender system will be constructed under the bridge to channel boat traffic on the Intracoastal Waterway. This will consist of piles supporting synthetic timber walls located 35 feet from each pier adjacent to the waterway. The distance between the fenders will provide a 180' wide boating channel.
Electric lights will be suspended from the bridge for navigation lighting.
Concrete retaining walls have been constructed along the north side of the roadway rather than sloping earth embankments, which would have encroached further into the marsh on that side of the road.
Work during the coming month will include erection of the final beams, giving the bridge its final shape. Additional sections of deck will be placed. The post-tensioning of the beams across the Intracoastal will be completed in March. The work continues to be on schedule for opening to traffic before November this year.
(January Update)
By Bill Merz
Landings Association Bridge Committee
On or about January 3, a convoy will set sail from Mabro Marine Service in Green Cove Springs Florida, near Jacksonville, for a 30-hour trip up the Intracoastal Waterway to the Skidaway Narrows.
This convoy will include a tug towing a crane-mounted barge, rafted to large empty barge, and two following push boats. The crane-mounted barge will stop at our bridge, while the empty barge will continue up the Savannah River to the Standard Concrete Products plant just downstream from the end of the Truman Parkway.
Here, five of the haunch girders will be loaded on the barge by an overhead crane that can straddle the barge as it lowers each beam onto the deck. That barge will make three trips back to our bridge where the ringer crane will erect the beams. Sufficient blocking will be required on the barge to keep the beams upright during the trip. The drop-in beams will be carried on the third barge trip in February.
It is anticipated that this operation of delivering and erecting the 10 haunch beams will take approximately 10 days. A fair amount of work will be required before the drop-in beams can be put in place. Erecting the drop-in beams will require closing the Intracoastal for five two-hour periods. The ringer crane is so named for the circular, or ring shaped, support that distributes the load onto the barge. The barge will be secured in position by two spud piles attached to the barge and driven into the waterway bed.
Meanwhile, work continues on the Truman Parkway, and is scheduled for completion in September 2013. Work during the coming month will also include some roadway work on the approach ramps and final preparation for the erection of the remaining beams on the Savannah side of the bridge.
(December Update)
By Bill Merz
Landings Association Bridge Committee
Last month, we talked about the post-tensioning operation required to make the five beams act as a unit over the three spans over and adjacent to the Intracoastal Waterway. The materials, equipment, and expertise required for this operation will be furnished by Dywidag Corporation. This firm is based in Germany, with branch offices around the world, and is an acknowledged expert in post-tensioning.
Positioning of the beams prior to post-tensioning will require the construction of temporary shoring towers. These towers will be supported on pilings driven by vibratory hammers to attain the required load capacity. The drop-in center beam and the end beams will be held in place by temporary steel frameworks, or strongbacks.
Once the five beams are in place and in the proper alignment, steel cables will be threaded through the three tubes, or ducts, that were cast in the beams. The ducts have an interior diameter of four inches, slightly larger than the diameter of the cable. The open space between the cable and the tube will be filled with grout when the tensioning operation is complete, to protect the cable from corrosion.
The cables are composed of 15 strands of high strength steel and are strategically placed to resist tension as the beams are loaded. These beams are also pre-stressed; that is, other cables were imbedded in the concrete at the time of casting.
Jacks will be placed at each end of the end beams to tension the cables, stretching them about four feet. The cables will be securely clamped to the end of the end beams before they are released and allowed to compress, or post-tension, the five beams together.
So the Germans help us with the post-tensioning, while the concrete is supplied by a Colombian firm, and the bridge is built by a South Carolina contractor...truly an international operation!
The coming month will see the installation of the shoring towers, erection of approach span beams, and placement of additional deck concrete. The center span beams will be placed in January.
(November Update)
By Bill Merz
Landings Association Bridge Committee
Because of the wide span required over the Intracoastal Waterway, our bridge will utilize a somewhat unusual method of construction. Instead of three beams for the three spans, our bridge will have five beams tied together by post-tensioning cables.
Haunch Beams will be placed on the piers adjacent to the Intracoastal Waterway. The gap between these haunch beams will be filled by a "drop-in beam" according to very detailed procedure.
The first step will be the construction of temporary supports followed by the erection of the haunch beams, each weighing 129 tons and delivered by barge. These will be hoisted into place by a 500-ton capacity barge-mounted crane (much larger than the yellow crane now onsite). Then the end beams will be placed on the temporary supports. Finally, the drop-in beams will fill the space between the haunch beams.
Each of these beams will have three 3.5-inch tubes to receive steel cables, or tendons, fed 600 feet from the back of one end beam to the other. Hydraulic jacks will stretch these tendons with 685,000 pounds of force, stretching them approximately 4 feet. Each end of the tendons will be clamped at the end of the end beams before the jacks are released. As the tendons try to return to their original length, they will squeeze the five beams together to cause them to act as a unit. This procedure is called Post-tensioning.
The sequence in which the tendons are post-tensioned in conjunction with the placement of deck concrete is spelled out on the plans and is more complex and detailed than I can cover in this article. More about post-tensioning next month.
Some of the work on the center spans is expected to begin in November. The temporary support towers will be constructed on steel pipe piling.
Meanwhile, erection of the remaining approach spans will continue, as will additional deck placement. The project remains on or ahead of schedule.
(October Update)
By Bill Merz
Landings Association Bridge Committee
Quality. Not only must the Skidaway Narrows replacement bridge be built to the right size and in the right location, but the materials and workmanship must meet quality standards established by the Georgia Department of Transportation.
The quality of the materials is determined by numerous and varied tests. The steel for the reinforcing bars is tested initially at the steel mill, where each batch of steel is tested and certified. After being formed into reinforcing bars, it is again tested for tensile strength, and those bars are marked for quality. Before concrete is placed around the bars, the location and spacing is checked by onsite inspectors. Likewise, other materials such as Portland cement, guard rails, asphalt for paving, and even paint for pavement marking are tested at the source of manufacture.
Onsite testing in accomplished or supervised by two GADOT employees and three consultant inspectors. This testing includes sieve analysis of backfill material, strength testing of concrete, density testing of asphalt paving, and bearing capacity of supporting piles. Inspectors also check location and placement of all components of the bridge. The contractor also has a quality control program to avoid delays and to maintain his reputation for good construction.
During the coming month, we will witness the first of the deck concrete placement. The wet concrete is placed in front of a Bidwell Bridge Deck Finishing Machine.
A carriage supporting two rotating rollers moves transversely under the machine, while the machine advances longitudinally on rails. The machine and the rails are carefully adjusted to provide the required thickness and cross slope of the finished deck. The rollers smooth and compact the concrete.
We also will witness additional beam erection (construction term), building of diaphragms (another construction term), and form stripping (ditto).
(September Update)
By Bill Merz
Landings Association Bridge Committee
Many skills are required to build our bridge. Before construction starts, engineers prepare the plans, estimators assemble the bids, and finance people obtain the bonds and insurance necessary for the contract. Lawyers must approve the legal documents, which a company officer will sign.
Once the contract is awarded, surveyors establish the location and elevation of the piers and abutments. Operating engineers staff the equipment needed to excavate for foundations and to move and compact fill material for embankments delivered by truck drivers. Operating engineers also run the cranes. Pile drivers know how to align sheet piles to form cofferdams, which are pumped dry to permit construction of piers in the water areas. Pile drivers then use their skills to assure the bearing piles are driven plumb and to the depths required for the needed bearing capacity. Carpenters build and erect forms to contain concrete for the footings, pier caps, walls, and deck. Ironworkers place the reinforcing steel within the forms by assembling the right size and shape bars according to the plans. Concrete finishers know how to move the concrete from the concrete trucks to the structure, make sure it fills the forms without air pockets or voids, and then apply a finish for durability and appearance. Laborers are used for less skillful tasks such as carrying supplies, hand excavation, and general assistance to other crafts.
All of these crafts are supervised by a project superintendent who may have an engineering background, or who may have learned the business through years of construction experience.
While the contractor’s non-union crew may include individuals who possess the skills of several, or even all of the trades mentioned above, it often is necessary or practical to hire subcontractors to do specific items of work.
The contractor for the bridge replacement project, United Contractors, LLC, has elected to use about 14 subcontractors to perform such operations as guard rail and reinforcing steel installation, post-tensioning of beams, surveying, slip forming of parapets, paving, and blasting for demolition. (Slip forming and post tensioning will be discussed in future articles.) Some of the subcontracting is required by the State to provide opportunities for disadvantaged businesses, including minority contractors. Other subcontracting offers specialized equipment and efficiency that lower the contractor’s costs.
During the coming month, additional beams will be erected, and work on the center piers will continue. The project remains on or ahead of schedule.
(August Update)
By Bill Merz
Landings Association Bridge Committee
This month, we'll review how the beams for the bridge replacement project are made at the casting facility of Standard Concrete Products near the north end of the Truman Parkway. The casting beds are 430 feet long to hold three beams end-to-end. Reinforcing steel and prestressing wires (cable) are put in place along with end forms. The prestressing wires are stretched by hydraulic jacks as much as four feet over the 430 length. After the concrete has reached a minimum strength in about a day, the wires are cut at each end of each beam, allowing them to "squeeze" the concrete into compression. This prestressing of the concrete allows the beam to span a much greater distance than plain, reinforced concrete.
An overhead crane places the side forms. The concrete is delivered from an onsite concrete plant and placed in the form by the conveyor on an unusual vehicle. The reinforcing bars protruding from the top of the beams will be bent after erection to become part of the deck reinforcing.
Although we think of concrete as rigid and unbendable, these beams actually arch upward by two inches in the center when the prestressing cables are cut. The weight of the concrete deck will cause the beam to bend back to its original straight line form.
My previous article said the beams would be erected during day in early July unless permission was granted for travel at night. That permission was granted, and daytime traffic closures were not required.
Many people have asked about the pile that is out of plumb. This pile hit an underground obstruction which caused it to lean, but it still has the carrying capacity to do the intended job.
The project continues to be on or ahead of schedule. The coming month will see additional beams being erected, and work continuing on the piers adjacent to the waterway.
(July Update)
By Bill Merz
Landings Association Bridge Committee
It’s going to start looking more like a bridge this month as the beams are erected on the first six piers at each end. The beams will be delivered by special trucks from the pre-casting plant at the north end of the Truman Parkway. One end of the beam will be supported on the tractor part of a tractor-trailer. The beam itself will form the trailer part as the far end of the beam will have a steerable dolly attached to it. Because the beams are 130 feet long, going around corners is made easier by having the ability to steer the rear axle. The truck driver has the control to steer the rear axle; however, it could be steered by a remote control in another vehicle. The beams weigh 80 tons each. Because of the weight, and the extreme length, the State requires each truck to have a permit and be escorted by two vehicles. Another restriction imposed by the State prohibits such loads from moving between sunset and sunrise. Therefore our beams will be delivered and erected during the day. (Unless a special waiver is obtained from the State).
The beams will be hoisted from the trucks to their final position on the pier caps by two cranes strategically placed to handle each end of a beam. Placement of the cranes for the beams on the span closest to The Landings is limited by the embankment already in place. This will require the beam truck to block a travel lane during unloading. The five beams for that span will take two days to offload and erect, and traffic will be restricted to one lane between 9 a.m. and 4 p.m. during that time. If the waiver is obtained, the lane closures will be at night. As of this writing, the lane restrictions will be in place on July 11 and 12, weather permitting. The Landings Association will distribute Email Bulletins in advance, to allow residents to make plans to avoid using the causeway at those times.
The 55 beams for the other 11 spans will be delivered off the main road and should not cause a traffic problem. The beams for the center three spans are entirely different and will be described in a later article. Work is continuing on the foundations for the center piers, and we will see concrete placed in the circular columns during this month.
Remember to plan your trips on and off the island to avoid the expected traffic tie-ups on those two days in early July.
(June Update)
By Bill Merz
Landings Association Bridge Committee
What word is used in conjunction with golf swings, income tax submissions, verbs, private school years, women’s undergarments, and concrete construction? The answer, of course, is “forms”. Forms are required to contain concrete in its liquid state until it hardens into the desired shape. They may be constructed of wood, plastic, metal, fiberglass, and, in some cases, dirt. Several firms dominate the market for reusable metal forms, an $800 million annual business. A fiber-based form for circular columns contributes another $30 million. I doubt if any figures are available for the quantity of plywood and framing lumber that is consumed by forms constructed on the job, but it would be a very large amount.
Below are the different forming requirements for our bridge:
a. Moving forms which slide with the machine distributing the concrete (How this is done will be covered in a later article.)
b. Under deck forms made of corrugated metal which will remain in place for the life of the bridge
c. Beam forms used for this shape; pre-stressed concrete beams
d. Reusable metal forms
e. Semicircular metal forms for the round columns on piers 6 through 11
f. Reusable metal forms for the footings on piers 6 through 11
g. Metal forms used by precast concrete manufacturer
h. Not shown: wood forms constructed on the site for the end abutments
While the cost of concrete and reinforcing steel is approximately the same for different contractors, the efficient and innovative use of forms often gives one a competitive edge in the bidding process.
Horizontal forms must be strong enough to withstand the dead weight of plastic concrete, and vertical side forms must be braced to resist the hydrostatic horizontal forces. Even slight movement of the forms during concrete placement could create an unacceptable product. Forms are oiled to keep concrete from sticking, much like Pam in a frying pan. Reinforcing steel often is supported by the forms, but is also kept at the prescribed distance from the finished surfaces by various kinds of spacers.
United Contractors expects to complete pile driving this month and will continue work on forming and pouring the pier caps. They do not expect any traffic delays other than those caused by rubber-necking drivers from you-know-where.
Information about golfing forms and the other forms mentioned at the start of this article will have to be found elsewhere.
(May Update)
By Bill Merz
Landings Association Bridge Committee
Telling a concrete contractor that all concrete mixes are the same is like telling a gourmet chef that his meals are just like everyone else's.
The meat and potatoes of concrete are Portland Cement and aggregates (stone and sand) along with water. There are many recipes telling how to combine these for different purposes. And then there are the spices, or additives, that add the subtleties. Additives make the concrete stronger, set faster, or slower, resist freeze/thaw stresses, become more water resistant, and vary other qualities.
Portland Cement is the ingredient that makes a mixture of sand, stone, and water turn into a structural element. It is made from special limestone which has been heated to very high temperatures. When combined with water, it undergoes a chemical reaction causing it to harden into the material that binds the aggregate into a solid mass. Concrete does not "dry" during the setting process; in fact the best concrete is cured by not allowing the water to escape by evaporation. Fly ash is added to the mix in place of some cement. Fly ash is obtained from burning coal at power plants and provides some beneficial qualities to the concrete.
The curing process continues for a long time. Most strength tests are taken at 28 days, but the material continues to gain strength at a declining rate for months, even years, after that.
Much of the concrete for our bridge is provided by Standard Concrete Products in the form of precast piling and pre-stressed beams. The remainder of the concrete for the footings, columns, and deck will be provided by LaFarge from a temporary batch plant located at the Truman Parkway site on Waters Avenue, shown in the accompanying picture. Although it looks a little rough, this is a computer-controlled plant with accurate scales to assure the correct proportions of ingredients. Adjustments in the amount of water added are made to allow for varying moisture content of the aggregates. This plant will discharge the appropriate weights of fine aggregate (sand), course aggregate (stone), cement, fly ash, additives, and water into concrete trucks with rotating barrels containing fins that thoroughly mix the ingredients. The concrete is discharged from the truck by reversing the direction of rotation of the drum to cause the mixing fins to lift the mix onto a chute. From there, it will be transported to its final destination in a concrete bucket suspended from a crane, or by a concrete pump which pushes the mix through pipes.
The above is "Concrete for Kindergarten". Those of you who are interested can attend several courses on the subject at Georgia Tech, or join more than 50,000 individuals who attend the World of Concrete convention each year in Las Vegas. Just don't say that all concrete mixes are the same.
Pile driving will continue during the month, and concrete will be placed in the columns of the piers on each side of the Intracoastal. Forms will be constructed for the transverse pier caps on the top of the tall piles.
Although there are no anticipated major traffic disruptions expected, some short closures will be needed for material delivery and, possibly, maintenance on the existing roadway. The Landings Association has again met with the contractor find ways to keep traffic interruption to a minimum.
(April Update)
By Bill Merz
Landings Association Bridge Committee
Our bridge is being built with concrete and steel, but very little steel will be visible when it is finished. Steel is a very important part of the bridge, for without reinforcing steel, concrete is good for building only things like birdbaths. This bridge requires more than 1,000 tons of reinforcing steel. Concrete can withstand heavy loads in compression, or forces pushing it together, but is weak in resisting tension, or forces pulling it apart. Steel, on the other hand, is strong in tension, and when placed in the concrete, takes over that part of the job. Why not use all steel? It's more expensive and not nearly as resistant to corrosion and fire. So, combining the right amount of reinforcing steel with the concrete results in the best, most economical structure.
Reinforcing steel bars (rebars) are placed in a form and surrounded with wet concrete. They become an integral part of the structure when the concrete hardens. To make an even stronger structure, some of the reinforcing is composed of high-strength cables that are stretched and held taut in the forms before the concrete is placed, and then released when the concrete has hardened. As the steel cables tend to return to their original length, they compress the concrete, giving it even greater strength. This procedure is called prestressing, and will be used in the prestressed piling and prestressed beams. For those of you who want to understand fully this process, I recommend a course at Georgia Tech. For everyone else, just accept the fact that it works.
Most prestressing takes place at special manufacturing plants equipped to make precast units that are then hauled to the construction site for installation. Sometimes, however, the prestressing is done on the jobsite after precast units are erected and is called post-tensioning. The beams on the center spans of our bridge will be cast with tubes through which prestressing cables can be installed on the site. I'll try to describe this process more fully later.
A number of people have asked what the new bridge will look like compared with the existing bridge. The new roadway will be 50 feet higher than the existing at the center of the Intracoastal Waterway, as shown on the sketch above. The slope of the approach road will be a 5% grade, or less than the 6% grade of the Westcross bridge over Green Island Road.
Pile driving will continue during the next month, with each pile requiring about several hundred blows and taking about a half hour to complete. The columns on the center piers will be constructed with cast-in-place concrete, and the approach fills will be substantially completed. The large red crane will be disassembled and trucked to the other side of the waterway to complete the pile driving by the end of the month.
No significant traffic interruptions are anticipated.
(March Update)
By Bill Merz
Landings Association Bridge Committee
And now for some more complicated bridge construction. I hope I don't lose too many readers here.
Where underlying soil cannot support the weight of a structure, that structure often is supported on pilings. The piles are driven into the ground until the friction with the surrounding soil is strong enough to prevent the pilings from sinking deeper under the weight of the structure. That depth required is determined initially by test borings and pile tests to determine the soil characteristics, and finally by the number of blows of the pile hammer required to drive the pile its final inches.
Our bridge is using pre-stressed concrete piles. Several of the piers will utilize 24-inch square piles driven out of sight, while others will use 36-inch square piles, the tops of which will reach to the underside of the pier caps.
There will be 154 piles, totaling 8,500 feet in length. They will be manufactured by Standard Concrete Products near the north end of the Truman Parkway, and hauled to the site by truck.
The piles are driven with a diesel pile hammer. The hammer for the heavier piles weighs 17,700 pounds, and is raised about 13 feet after each blow by exploding diesel fuel, much the same as a piston in a diesel engine. The hammer then drops by gravity to impact the pile again.
Why doesn't the concrete crack when hit so hard? These piles are constructed with very high strength concrete that is pre-stressed with steel tendons (more about pre-stressing as applied to the piles and the bridge beams in a later article). The top of the pile is cushioned with a layer of plywood, which allows the energy of the hammer to be used to drive the pile into the ground without damaging the concrete.
The longest 36-inch piles are up to 85 feet long and weigh 95,000 pounds. There will be six of these piles on each of the four piers leading from the embankment approaches on both sides. As noted above, the top 30-to-50 feet of these will be exposed to view from the pier cap to ground level.
The 24-inch piles supporting the three piers in each side of the waterway will be driven about 35 feet deep and will be covered with a concrete cap at approximately water level. Two 48-inch columns will be constructed from this cap to the bottom of the pier cap.
There may be some traffic disruption as these piles are delivered to the site, but they will be of short duration as trucks are maneuvered into the work area.
The work to be done in the coming month includes completing the approach embankments, continuing pile driving, and placing the concrete pile caps adjacent to the waterway.
The contractor informs us that the project is on schedule.
(February Update)
By Bill Merz
Landings Association Bridge Committee
Concrete, steel, asphalt, and dirt are the main items needed for our new bridge. Today we'll talk about dirt (fill, borrow, embankment) and save the other materials for later. It will take approximately 7,500 truckloads of dirt to build the ramps on either side of the bridge. About 95% of those loads will be placed before the bridge can be opened, and the remainder after the existing road is abandoned. Before the new fill could be placed, unsuitable materials, including vegetation and muck, have been removed from the site.
As the ramps are constructed next to the existing roadway, a temporary retaining wall will be required. This wall is composed of wire mesh backed with black plastic fabric and is held in place by straps buried in the fill. The lower sections of this wall can be seen now. When the fills are completed, the wall will be as high as 20 feet. After the new roadway is opened to traffic and the existing road is abandoned, this temporary wall will be buried under sloping embankments. Because the existing soil under the roadway will move and compact as the weight of the fill is placed on it, the fill must be allowed to settle for possibly as much as one year before the final pavement can be placed on it. We will see no construction work on the approaches for that time, but Mother Nature and gravity will be hard at work.
Meanwhile, work continues on driving the concrete piles for the bridge piers, a topic that will be covered in more detail in the next article. There is no anticipated traffic interruption for the next month. The tie-up last month was caused by the state inspecting the existing bridge without giving advance notice, and was not caused by or related to the new construction.
Information about the general contractor, United Contractors, LLC, can be found at www.UIG.net. | 
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