Building a Bridge - Construction Part 3 - The Foundation - Where It Starts

I started on this series of posts, reached this point months ago, and found myself writing way too much about the minute details. I erased it all the other day, and started over. 

The bridge starts at the bottom. The bottom may be hundreds of feet below the surface, or right on top of rock. The design starts at this point, and expertise of geologists, and soil engineers, is necessary. At this point, the most important consideration is whether what's available can handle the load. 

The foundation of a bridge has to handle the dynamic load. The dynamic load is the dead weight, and the forces that will act on the final structure. The dead weight is determined by the culmination of all materials that will be placed, but that weight can't be calculated until all the required components are known. The final weight can't be determined until the required components are known, which can't be calculated until what will support the bridge is known, and all the forces that will act on the bridge are known. 

The final components are determined by theoretical design considerations, which require an understanding of what's available to support the bridge. That's first determined by knowing about what the bridge will rest on. Is their rock? If not, what type of soil is available? Is the available rock stable? Will the soil support the bridge with piling?

Bedrock can support a tremendous amount of weight, but the thickness of the rock, the amount of fissures, and the type determine the design. Geologists are consulted, and if necessary, samples are examined from drilled holes. After examination, the foundation may be secured by drilling dowels into the bedrock, or some rock may require removal to reach more stable rock underneath. 

Without rock, the soil determines the type of support. Some soil only requires driving piling to the point the friction against the soil is enough to support the structure. Other soils may require drilling down to a subsurface strata that can support the load, and filling the hole with reinforcing steel with poured in place concrete. That, or the instability of sand may require "water jetting" concrete piling with a hollow tube in the middle to a depth that will allow the soil friction to support the load. In some situations, the soil will support the bridge and the only thing to do is to spread the load with a spread footing. A spread footing can last the life of the bridge, or when certain conditions exist, a hundred year flood can lead to scouring, which erodes under the footing, and the bridge is compromised. 

Regardless of the method for the foundation, laying out the location can be an onerous task. Optical instruments have lessened the work, but arriving at control points may be as simple as walking with a trap on the end of pole, to spud barges, large tugs, and fighting a current to place sheet piling for a cofferdam around the future foundation, and the control points on the bottom of a body of water. The locations are critical, since all design loads are done on theoretical points on the final structure. Obstructions, locations errors, or broken piling can lead to construction stopping and new design parameters. Whether a single piling, or clusters of piling to handle larger structures, this is the point where the engineer representative calculates the bearing, if the piling is driven, and watches to ensure the contractor doesn't stop driving piling before it reaches bearing. 

Bearing is the amount of blows by a piling hammer. The weight of the hammer is calculated before driving, and may have been determined by driving test piling. The hammer may be a large single cylinder diesel engine that uses the explosion of the diesel to blow the piston upward, which causes an equal amount of force pushing downward on the pile, or a hydraulic ram, which accomplishes the same purpose. Bearing is achieved when the number of blows to drive a foot of the piling below the surface match known quantities for blows, to what is known as bearing, or "refusal". Refusal is when a piling is driven for a minute, and it doesn't go down any further, or the amount is less than an inch. This can be a disaster for a pile driver. If the engineer representative doesn't agree with the contractor, demands to keep driving, and the piling breaks, engineering design changes, extra costs, and possible shut-downs can lead to angry words, with resentment. I've seen this happen, and the project rep was wrong.

 Not achieving bearing requires adding to the piling, and driving again. On concrete piling, breaking the top of the concrete piling, adding an exact section with rebar, and concrete. On steel piling, welding a section is required. The welds are x-rayed for determining if the amount of porosity is within acceptable limits. Welds that fail are removed with arc-gouging, and/or grinding, then welded again.

Another aspect of piling is one I experienced on a project. The piling reached refusal about 7 feet above the final grade. The excess piling had to be removed. In that situation, the piling was 18 inches square pre-stressed concrete. The stressed cables prevented using a concrete saw, since releasing the tension can have disastrous results for the saw operator. We tried breaking the concrete with hand-held concrete breakers, since the pilings were above the water, and there was no access with larger breakers. That was taking to long. The final outcome was renting a machine that sheared the piles. It was expensive, but the time, with labor, and the cost of a piling crew shut down, was lower than the other method. 

After the basic part of the foundations are designed, such as pilings, manipulated rock, or spread footings, and the lowest foundation is finished, the next step can be as simple as leaving piling above the surface to the final elevation of the caps, or placing reinforced concrete with protruding dowels for the columns. The concrete is usually placed around clusters of piling, and the amount can be tremendous. Whether at ground level, or dozens of feet below the surface, the next step is building up, whether it's poured in place concrete, or pre-cast sections. That requires another post. 

Building a Bridge - Construction Part Two - Earthwork

On most projects, some type of earthwork is being performed during much of the project. Approaches may need embankment to reach the grade required for the abutments, or the subgrade may need the vegetation stripped, drainage concerns addressed, or something as exotic as a wick drain. It requires a foreman, or superintendent with a good knowledge of manipulating soil, and building a strong roadbed.

 If a wick drain is required, an excavator vibrates tubes into the ground to a target depth to allow water to be forced up when embankment is placed. A suitable drain strata is necessary to allow the water to escape, geotextile fabric may be placed before the embankment is placed and the embankment is placed in lifts over the area with wick drains. Eventually the heavy fill displaces the water. This allows the fill to not be compromised by excess water. On really swampy ground, the hydraulic fill may be placed above the final grade, the ground may sit for years, and the settling embankment eventually becomes stable. On one large project I watched, much of the saturated soil under the new embankment heaved outside the roadbed, and was eventually removed. The settlement was in feet. Although the embankment was placed decades ago, the roadbed is still sound, and thousands of vehicles travel the roadway daily. Even a direct hit from a hurricane didn't disturb the road.

The placing of embankment is more complex than just dumping dirt, and grading. Embankment is first tested for suitability. The testing involves determining the plasticity, optimum moisture, and thickness required to support the paving. Also, existing embankment is tested to determine if it can remain under the proposed fill. On some projects, grade profiles, with testing,  might indicate the embankment is all on site, and all that needs to be done is to place it in the correct position. This may require earthmovers, or excavators with dump trucks. Regardless of how the embankment is manipulated, fill requires placing the material in lifts, which usually is 12 inches or less. Eight inches is the usual rule of thumb, and exceeding the depth may cause a failure in compaction, scarifying the material, and re-compaction. On one project I was working, the material lifts weren't tested, the contractor continued their operation, and three feet later, when the inspector ran densities into the lower lifts, the material failed. The solution was pushing all the covering fill to the side, reworking all the material, and egg on the face of the contractor. At the time, I had the feeling the contractor was trying to pull a fast one, and was caught. I never knew, but the correction was obviously expensive.

All vegetation is required to be moved before any embankment is placed. The stripped material (usually 4 to 6 inches)may be hauled away, or stockpiled, for the final finish at the end of the project. The exposed subgrade is scarified to insure a good bond between the materials. Not performing this part of the requirement will lead to a layer of material that will never compact, delamination, or both. Delaminated embankment will slip and cause cracks, voids, or both. That, and if it's discovered, the new material will be removed at the expense of the contractor.

On some projects, the embankment is mixed with lime to consolidate and dry the material. For projects in urban areas, the lime is placed as a slurry. In open areas, the lime may be placed dry as a powder, or in a pellet form. Regardless of how it's placed, the usual percentage of lime is around 6% by volume, and requires large mixers (Like a large self-propelled garden tiller) water trucks, earthmoving equipment, and compactors. Mixing is to a depth two inches below the projected subgrade elevation. The material is checked for the lime content before the operation is complete. Failing to introduce the right amount of lime will lead to repeating the operation until successful. On a project, failing to place the correct amount is frowned upon. Continued failures lead to new supervisors. 

Placing lime with a slurry is probably the best method, since yield isn't affected by loss by wind, and removes the nuisance factor of lime dust. It can be delivered by transports in a slurry, or dry to be mixed in smaller trucks on site. A slurry pump is usually hooked to a fire hydrant, or a water pump for mixing on site. I doubt there are a few dozen of people in any state that have the experience to mix slurry on a project , and my near past experience shows transports of slurry being dumped on site. I've never figured the cost difference, but feel the difference isn't worth the problems.

Transports are great, until an exuberant dozer operator yanks the axle from under the truck hauling the lime. The operation comes to a halt, someone will probably get lime burns trying to the fix the truck, and finger pointing starts for the charges. A good operation has a basically firm bed to start with, so the transports only need a little help to pull through the subgrade  to prevent tire slippage. Spreading the slurry at the right speed removes the need to make another pass in the same location. Dragging lime transports is a delicate operation, and damaging one can be expensive.

Mixing is required to consolidate the material. Over days, the mixers eventually turn the mixed soil to a fine grain, the dozer, or motor graders, shape it to grade, and the compactors make continuous passes to compact the embankment. After compaction is reached, water trucks keep the embankment wet for three days. 

Embankment rarely has the optimum moisture content. If it's too wet, it will track, pump and fail the compaction test. If it's too dry, it will not reach compaction. To remove moisture, only blade mixing with a road grader may be all that's needed. Blade mixing is grading up a windrow on the edge, blading the windrow back and forth, and hoping the material becomes dry enough to be compacted to specifications. After two days of frustration one project, I was forced to rent a mixer to break a heavy clay small enough to dry. My boss didn't like it, but realized it was the necessary solution.

 Too dry of material is windrowed, sprinkled with water from a water truck, and either blade mixed, or mixed with a mixer. When the moisture is correct, compacters pack the material, and the motor graders fine tune the grade. Good crews make the process look easy. Bad crew make a mess, when they get some sections too wet, and others too dry. The goal is to get the material equally hydrated, and a good water truck driver can distribute the right amount of water in one pass. 

Compacters for embankment usually have pads, or sheepfoot studs. When optimum moisture is reached, the pads eventually "walk" out of the material, while motor graders sweep enough material across the roadbed to keep the indentions filled. When complete, the pads don't sink, and only leave a surface mark on the material. At this point, it's imperative to get the grade to the correct elevations. Failing to do so, might lead to the necessity of scarifying the material again, re-grading, and compaction. Good clays get as hard as concrete when compacted at the correct moisture. Trying to just cut enough material to make grade can lead to the blade only skidding on the surface, a loud squalling, and a frustrated operator.

Embankment may be needed to form a subgrade for construction activities, or to fill to an elevation ready for pile driving. I worked on one project where the embankment supervisor crossed up up with the bridge supervisor. The embankment supervisor wanted to continue his operation into the work area of the bridge supervisor, which the bridge supervisor had "homesteaded". His laydown area was to his liking, and access was good from a nearby road. The embankment would ruin the arrangement, and the bridge supervisor would need to stop production for a day to rearrange the area. The embankment supervisor go his wish. The bridge supervisor was far from happy, which he showed with expletives, threats, and some stomping around. In the end, I felt the embankment supervisor was over his head. Allowing a few hundred feet of space from the edge of a bridge at grade offers the opportunity to adjust the final grade of the road to compensate for minor errors in the bridge elevations. How do I know? I saw it happen before, and manipulating dirt is much cheaper than trying to adjust grade with an asphalt bond breaker, or concrete. The same embankment foreman showed his ignorance many times during the project, and was eventually let go, when the project was finished. If I hadn't been the subcontractor, and was the supervisor for the prime contractor, I wouldn't have hired him. I had experience with him before, and had no confidence with his work.

With the embankment operation in progress, pile driving is more than necessary to complete. That requires another post. 

Building a Bridge - Construction Part 1 - Getting Started

After a contractor is notified they won a bid, a few things happen over the month, or longer, period before the ground is broken. First is the getting all the submittals for materials, subcontractors, EEO paperwork, insurance certificates, and any special designs, such as a falsework plans. I have no idea how many in a large company are delegated to this task, but know smaller companies may have only a few. During my career, it could be a daunting task, since I worked for a small company, and had other things to do, but it was part of my job description. Bidding was too, but it wasn't full time. There were way too many pieces of paper to be accumulated, and too much of it was bureaucratic make work. My supervisors/owners and office staff helped, and we scrambled to complete everything before the pre-construction meetings. Large bridges were beyond our bonding capacity. We did build some small bridges, but work on larger bridges was as a subcontractor for various. 

One task that was very important after winning a bid was creating a spreadsheet with all the bid items. Included were the anticipated labor costs, material costs, subcontractor costs, overhead items, profit, TIBS (taxes, insurance, BS/miscellaneous) and time. There were dozens of bid items included, and the information was needed to load the project into the accounting program. I know there are canned software applications out there for construction project management, but we never really had the need for the expenditure. We all came from the old school where we originally worked with columnar tablets, and the accounting was done the same. Years of building spreadsheets developed a method to take care of all we needed, and the canned accounting software was the only one that could handle the bid items when we first bought it. Over time, the software was upgraded, and we stuck with it. Manipulating spreadsheets became second nature. 

With TxDot work, and I imagine most other states, and municipalities, the placing of barricades is the first task. This may only be a handful of temporary advanced warning signs, but may include temporary striping, barriers, and message boards. Without these things, and SW3P items (storm water prevention) the project is not allowed to start. Storm water runoff items can a few feet of silt fence, or a elaborate configuration of silt fence, soil erosion logs, and rock filter dams. 

After the barricading is completed, field survey crews station the project, place temporary controls, and are usually available during the project for surveying tasks. Their efforts require a good understanding of the plans, and the instruments they use. While some of the surveying is completed by GPS, accurate measurements require a total station. 

Total stations aren't affected by the weather, shade, or the failure to allow the GPS instrument the time required to be as accurate as possible. Some, called robot guns, can be controlled by the controls on the rod, which can make accurate measurements easily performed by one person. It removes the job of one person, but they're tens of thousands to purchase. During my career, it was one of those items I knew I would never see. It was far beyond what a small company could justify for what little use it would have. Mine required someone to hold the rod, and I was happy to have it.

Once a total station is set up, known coordinates are shot to determine the current location of the total station, the height of the total station above a point, or the ground, is entered, the station can be used to either record, or find locations. Elaborate software allows placing offsets, the points in a curve, and does so with accuracies within a fraction of an inch. Coordinates can be downloaded into the instrument, and when set up correctly with a good rodman, be more accurate than any measuring tape over long distances. Wood stakes, with nails, are used to set temporary controls. These are surrounded with lathes covered in barricade tape. Lathes, with information written with a sharpie, are used to show stations. These are holy objects on a construction site, and disturbing any is frowned upon; especially temporary benchmarks. 

Paint may show stationing, or known underground utilities. With stationing, the paint is a reference for a location. The stationing is a good reference for determining the big picture of the project, and for a quantity reference in the future. The stationing on the ground may show a discrepancy in the plans, or help with determining access to parts of the project. Regardless of how accurate plans can be, plans don't indicate ground condition, or the fact the local rancher has a little drive he uses daily. With ground conditions, the planned storing of materials in a boggy area is usually a bad idea. With the rancher, he probably owns half the county and hob nobs with state officials a few times each year. Both are problems that need to be handled before the crews arrive, and a good superintendent takes care of both contingencies way before that happens.

Paint for utility marking is color coded. White is for the contractor, orange for communication cables, red is for electrical, green is for sewer, blue is for water, yellow is for gas, and none can be assumed to be marked correctly. Some markings may match the plans, but others may indicate an underground utility missed by the initial survey, and cause problems. One of my worst situations was water line running right through the center of a large proposed junction box. We had paving to break, a large hole to excavate, and the project was shut down. My calls to the engineer I knew were wasted on voicemail messages, but I was lucky when a supervisor of the water utility happened to arrive on the site. With a few minutes conversation, and cutting off the water main, we broke the paving, excavated for access, and his crew relocated the main in a few hours. 

With enough surveying completed, and required items handled, the next part of the project is embankment, piling, or both. That requires another post.

Building a Bridge - Engineering Part 3

 Altogether, the posts on building a bridge will be numerous posts. I'm doing this to keep from a large volume, and hopefully, keep those interested from being bored. 

The next phase in engineering the building a bridge involves the drawing of plans, final funding, and the specifications for construction. How much goes on in the background is beyond my experience, but I have a feeling many projects are shot down before construction due to politics, and lack of funding. 

The plans are drawn by draftsmen using CAD programs. In the past, there drafting was done with pencils, and the drafting tools used for the craft. In my experience, AutoCad is the leading software package. It's expensive, especially when many of the plug-ins are purchased. Those proficient in the use of the software have my admiration. I base my opinion on what little experience I've had with simple two dimensional CAD programs. AutoCad is much more robust, more complicated, and can be used for some amazing, scaled three dimensional drawings. Considering the complexities, many of the drawings I've seen for construction projects are probably simple drawings for the more experienced draftsman.

With most bridge projects plans I've dealt with, the centerline proposed for the construction is the main control. Regardless of service roads, or surrounding structures, the centerline is the best baseline, since it eliminates any crossover stationing. I ,once, bid on parts of a project designed by a consulting firm, which had a very confusing conglomeration of stationing. Since the project involved many sections of highways with existing stationing, including the stationing was probably their solution to the complex problem of converting all stationing to one baseline. Their job was probably easier, but to the contractor, examining the plans was tedious. 

With a baseline, there is a starting point, which on a new project will be station 0+00. With tie-ins, or renovations of an existing road, the stationing for the project may start at some station as 256+58.5 which would be part, or a continuation, of a project point that had 256 one hundred feet stations, plus 58 feet 6  inches, which is 6/12 or .50 feet. I have worked with metric, when TxDot dabbled in metrics, and found the English units too entrenched to stay at metrics. Metrics are easier to use for calculations, but many manufacturers have English dimensioning, and these dimensions don't quite match the metric dimensions. Soft metrics can be used, but in reality, manufacturers aren't going to spend millions to convert, and in engineering, the calculations don't quite match. Since materials strengths, and dimensions require accurate calculations, errors in converting data can lead to problems. 

In stationing, and the centerline is used for the baseline, offsets are used to indicate future, or existing sections. With a 60 feet wide road, the edges would be defined by left and right offsets. At station 100+00, the far right edge would be station 100+00 - 30 feet right (100 feet from the starting point, and 30 feet perpendicular from that point). The left edge 100+00 - 30 feet left. It's an easy method to determine locations, and when it comes down to existing utilities, it's important to the owners to understand where, and how far to move any obstructing utilities. Utilities are usually moved before projects are let, since the operation can take a long time. Contractors charge for downtime, and justifiably so. Equipment, labor, and time add costs, which a contractor must learn how to handle. Failing to fall procedures outlined in the specifications, which are part of the contract, can lead to monetary losses by the contractor, and a bad working relationship between the engineers and contractor.

The plans are not just drawings of the future bridge. With TxDot, and most Texas municipal drawings that usually follow TxDot standards, included are a multitude of pages with standard drawings, paving designs, drainage structures, elevations views, existing utilities, typical design sheets, phasing, traffic control, environmental protection requirements, quantities, typical sections, and any special design features. There are more, but to really understand the requirements, you would have to examine the TxDot website that details the necessary requirements. Last, but not least, are the specifications for the project, which reference what is commonly called the "Bible". Txdot occasionally updates their standard specification book, which has all the standard construction, and material requirements. An important reference is always available for approved material suppliers. These suppliers provide materials that were tested by TxDot, and in some situations, a contractor can have a problem by bidding using a material not supplied by an approved supplier. The cost difference may be substantial, and lead to financial woes for the contractor.

Usually, there's a pre-bid meeting. The bidders may just meet at the office of the owner, an onsite meeting may be all that's required, or both may occur at the pre-bid. It's at this point questions are asked, answers are produced, or the engineer will answer the questions in writing before the bid date. If an addendum is required, it will be issued before the bid date. Acknowledging the receipt of addendums is usually part of the bid documents, and failing to acknowledge the addendum will lead to a bid being thrown out.

When all is compiled, checked, and approved, the project is released for bids. This leads to the contractor to examine the plans, and specifications, figure out the costs associated to perform the work, add a profit, fill out all the blanks correctly, notify the bonding company, and cross their fingers.

Bidding is once a month for TxDot projects, and at council meetings for cities. The bids are checked for following the rules of bidding, and any numerical irregularities. Whether the bid is local, or let in a multiple bid letting, some allow electronic bidding. It's at this point the contractor finds out if they won the bid, and can now start worrying about what they left out. Some are so far out of balance, the contractor is consulted, and allowed to remove their bid. This can come at a price, if the project requires a bid bond. The contractor forfeits the bond, and the bonding company frowns upon such things. 

The engineers might find they substantially underpriced the project, and wonder why. That, and if their track record with the contractor usually ends up with change orders for all their mistakes, they may wonder what their exposure will be, and how much it will cost. It's now their job to go through all the paperwork of the bid, and the contractor  will eventually be given a contract to sign. After signing, the contractor has to start submitting material sources, subcontractors, schedules, and have all prepared before the pre-construction meeting. When the start construction date is set, time is started on the project. Whether its a few months, or a few days, the contractor submitted schedule is used as a club, when the project falls behind.

Now it's time to start the project, and a new post.

Building a Bridge - Engineering Part 2

 I've worked on, or built, a few bridges during my career. Most were beam and girder, with one being a large cable stay bridge. When looking at the design, the fact they were built is a testament to wonders of modern engineering. The fact they stand for so long shows the design wasn't a foolish whim of a demented person, and after working on some older structures, the fine tuning of the design of newer bridges indicates some designs are now obsolete; even though the less desirable designs are still in use and safe. 

Not all bridges make it to the construction phase. We had a local bridge designed to the point a three dimensional model was made to scale. A historical society, and government officials, became involved with preserving the existing swing bridge, and the new bridge was cancelled. The old bridge was refurbished at a tremendous cost, was still an old, untenable, high maintenance bridge, and regardless of how ridiculous the effort was, remains as something that taxpayers shouldn't have to pay to maintain. 

The final design of a bridge requires knowledge of materials, the distances to be spanned, and the cost. Steel is one of the best materials, but requires coatings, which can be expensive over time. There is a steel that corrodes on the outside, but will not have deep corrosion. It's substantially more expensive than regular structural steel, but I have seen it used on overpasses. It's uglier than homemade soap, when the rust "bleeds" onto the concrete caps, and columns, supporting the girders made from the steel. Still, steel is the best material for the type of design, and the long spans required for flyovers, and other long spans in urban freeway congestion. 

Steel can be designed to span long distances, where concrete can't without overhead supports. What girders are used depends on the span distance, whether there is a typical wider flange beam that can handle the load, or specially fabricated girders with camber to offset deflection of the weight of the deck. On larger spans, the design can be a multitude of different steel configurations, including trusses, arches, and box beams. Regardless of the bridge design, steel will be used somewhere in the construction.

Concrete usually requires less maintenance, but concrete has limitations. The biggest limitation is how brittle it is. If a span has nothing but concrete, it will stay together for awhile, but the expansion and contraction will eventually cause large sections to crack, and fail. That's where reinforcing steel is necessary, and planned joints needed for temperature changes in the concrete. 

The "rebar" holds the concrete together, and distributes the load evenly. The more rebar, the more strength. This allows some of a bridge decks to be as thin as 8 inches, and still handle the weight of a full loaded semi at highway speeds. Concrete can be used for long span girders, but the only application I've seen is box girders with substantial dowels, or stay cables. What makes this design convenient is that huge sections can be precast, hauled to the site, and placed by cranes. Still, the concrete has to be reinforced, and maintenance requires constant monitoring of the concrete for corrosion of the reinforcement. Concrete always has cracks, and water will migrate to, and along, the reinforcement steel. Over time, the rust will expand, concrete will spall, and eventually a major section will fail.

For shorter spans, concrete girders can be used. These are prestressed before the concrete is poured with steel cable tendons. Camber is also cast in the girders. The camber required for the deck deflection is predetermined by years of testing, and even with the best of designs, the deflection may not be as wanted. I've driven over bridges that have a "lope" between spans, where the girders didn't deflect as designed. On one bridge, the inspector was constantly harassed about "his" bridge, which had this problem. Subsequent examination showed all elevations before the deck pour were accurate. The girders just didn't deflect as designed. 

With the design finally determined, the next phase is plans, specifications, final anticipated costs, and the approval of the highest powers involved. This can be very time consuming, but if many standard designs are involved, and the project is relatively small, the time can be very short. That requires another post.

Building a Bridge - Engineering Part 1

 I've worked with many engineers during my career. Like all professions, engineers have years of school under their belt, and before they can officially stamp a set of plans, they have to become licensed. Licensing requires years of working under those that already secured their license, and tests specific to the branch of engineering they pursue. Not all engineers pass the test, and some spend their careers performing the tasks required to design, or build, but never have the legal authority to place a stamp signifying they take full responsibility for the design. Regardless of licensing, I worked with a brilliant engineer without a license that not only designed offshore semi-submersible platforms, he sailed with them on their maiden voyage to insure the "bugs" were shaken out of the platform, and the function was as required. He was an exception, and even some with licenses were far over their heads, and to make things worse, oblivious of the fact their efforts were not only sometimes foolish, they were dangerous as well.

Engineers that design bridges are usually civil engineers. In my circumstances, most of those I worked with were employed by the state of Texas. The hierarchy of responsibility has a small board in charge of the entire state, statewide engineers, with staff in Austin, districts with a chief engineer, and subordinate engineers on staff to support the workload. Some are more than qualified, and some are (to me) functional idiots. All usually are hired as graduate engineers, and some are contractually bonded to five years of service to the state for financial help while going to college. 

With a bridge, and the basic needs understood, rough design drawings start the process. The amount of lanes, the maximum span needed, and approaches are drawn to start the process from conception to completion. This determines the final weight, how the environment will affect the structure, and since it requires access, how existing roads will be modified for use. It's a start, but there's a long way to go.

In the past, airplanes were used to take aerial photographs of certain areas. Photos were used to build a collage of photos, which were used to create larger photos that could be shrunk to a size for ease of use. Surveyed marks were placed on paving for matching photos. You probably have seen these. They are large crosses, and circles, painted on paving painted white. Today GPS is used. It's very handy, but has some inherent problems. One being that an overhead view doesn't scale correctly to actual ground measurements. Changes in elevations, and the curvature of the Earth over a large distance, add up to differing quantities during actual construction. That, and locations shown on plans don't match-up in the field. Regardless of what is used, accurate plans require field surveys to have an accurate scale for measurement. 

During the initial design survey, samples of the existing soil are taken, and bore holes are made to acquire cores for analysis of the substrate. The cores are analyzed by a soils engineer, who specializes in determining the bearing of the existing soil, and what is underneath suitable for bearing the proposed structure. Sometimes, there is available data for some of the area, but bore holes are necessary (when possible) where bearing structures will be placed. Bedrock is the best for bearing, but even bedrock needs analyzing to determine the strength. When there is no bedrock, the soil needs analyzing to determine the length, and type of piling for construction. 

In my experience, which was mostly locally, the substrate is a mixture of clay and sand. The soil is analyzed to determine what bearing a driven, or drilled pile, will provide with friction. With the right diameter, number, and length, the piles can carry a tremendous amount of weight, and provide a firm base for a bridge designed to move. While movement can be small, it's necessary to handle the dynamic load of what the bridge will be used for. On some bridges I worked on , the movement was very noticeable, and while inside a cable stay bridge, I had a few moments of vertigo as heavy traffic passed overhead. 

With the information for the subgrade, the depth of a channel -if any- is now used for determining any construction to take place under the existing water, or the necessary span to satisfy the requirements. Determining the depth may be as easy as using a grade rod, or soundings from a small boat, to a marine survey using sonar mapping. I've never seen an attempt for soil bores in a channel, but have seen soil bores as near to the channel as the drilling rig allowed. Usually, the channel is designed to be free of obstructions to allow water traffic, or to maximize the drainage. If underwater construction is required, the design of a cofferdam is a necessary step. Cofferdams can be anything from dirt piles to allow pumping to the bottom, to elaborate sheet pile configurations to contain the water at deep depths. Whether they still use caissons is out of my knowledge, but they were used successfully in the past.

I have to add that all the efforts to determine underground conditions are not always successful. Years ago, I bid, but didn't win, the emergency repairs for a small bridge. One of the interior spans fell a few feet after a one hundred year flood. Long story short, piling driven for the pile cap reached refusal before the depth required. All involved theorized the sand was much stiffer at the interior bent, so when refusal (I'll explain "refusal" later) was reached, the driving was stopped, and the pile was cut off to grade. After a high volume of water eroded the bottom to a depth below the bottom of the piling, the piling settled, and the pile cap went down. Underwater examination at the now exposed bottom found a sandstone. The piling, which compacted the few feet of sand above the sandstone, successfully supported the bridge for decades, but when the and washed away during the flood, there was nothing under the piling for a few feet. I don't know how the successful contractor performed their repair, but my bid included a drilling rig to drill into the sandstone, and cased piling.  

The soil engineer passes the information to other engineers, including the paving engineer. The paving engineer is responsible for designing a paving to handle the dynamic load of future traffic. Many things are considered, including materials for the paving, the bearing of the existing soil, and the requirement for any fill to bring the subgrade to the elevation needed for the final paving structure. 

The design of the bridge is now well on the way, but there are multiple tasks required to finally get the project to the bidding process. I'll write about that next.

Building a Bridge - Introduction and Getting Started

 I'm starting this series of posts because the years I spent in construction were a valuable education, and from personal experience, many are interested in the construction of a bridge. It's a fascinating, tedious process, with many involved. The process is multi-faceted, with many involved never even seeing the finished bridge; much less driving over the structure.

The process starts with the design. The design is never contemplated, unless there is a specific need. That's where engineers become involved, and regardless of opinion, they have a huge responsibility to get it right. Errors are not only expensive, they can be dangerous. 

It all starts with a survey. Property surveys are first. If the right-of-way is not secured, property owners are notified of the future construction, and the negotiations begin. The surveyor determines all corners, who is affected, officials contact the property owners, and an equitable value is determined for the land needed. This can be a daunting task, since many are happy with the metes and bounds of their land, have concerns about how the bridge will affect their lives, and may start a process of using the courts to stop the project. The time required may be years, and numerous legal challenges needing addressing. If all is taken care of, and the land is now the property of those wanting the bridge built, the next process is to make a detailed survey to determine elevations, obstructions, any utilities, and surrounding areas. 

A detailed survey is a time consuming process. Utility owners are contacted for locations of their utilities, adjoining property is examined for problems that may develop, such as height problems, noise, or vibration issues during the construction. Vibration? Yes. Some industrial facilities have vibration devices that prevent damage, or releases. If they are close to where heavy equipment, or pile driving, is in the future, the effects have to be considered. Shutting down an operation that makes millions each day is not something to be taken lightly. 

The start of the survey is determining known property bounds, marking corners, and then making a detailed survey of the land. Elevations need to be known, any obstructions noted, clearances known, access known, and finally, what's underground. 

Generally, the elevations are determined by creating a grid with a determined length for stationing, and offsets to baselines. The controlling baseline is usually the centerline of the proposed structure, which may already have stationing from previous projects. The stationing in the U.S. is on one hundred foot increments, and feet are broken into tenths of feet. There are no inches, since they are not only cumbersome to deal with, using a decimal system makes calculations much easier. In the construction phase, if inches are required, the decimal equivalents can be calculated by multiplying, or dividing by twelve.  Those in bridge construction become accustomed to the calculations, and do so without a calculator for even inches. Fractions are seldom used, since such precision isn't usually required. 

What's used for surveying? In the past, it was with transits and chains. A transit is a telescope, with a compass, and the telescope can be tilted 180 degrees for determining angles, or back sights. The compass is so well made, a magnifying glass is needed for the smaller increments of the azimuth angles. A chain is a strip of metal, with babbit at every foot. The babbit is punched, when the chain is stretched to a determined tension, at a certain temperature for the material of the chain. The chain is stretched horizontally from one known point, which is under the transit when beginning. The point is either a witness monument, steel rod, or wood stake with a nail driven at the correct point. Since the chain might be pulled at an angle, the transit operator sights the target, notes the degrees to the target, and calculations are performed with trigonometry to determine the actual horizontal distance. The height of the target held by the rodman is known, and the height of the transit telescope (HI) is known. The calculations use these corrections for accurate results. Since the actual elevation under the transit is a known elevation, calculations determine the elevation of the point sighted. It's time consuming, tedious, and not performed much any longer. New surveys use GPS. total stations, and computer software. This eliminates much of the time required, and errors in handwritten calculations. That, and many instruments don't require a second person to hold a rod. The savings can be tremendous over time, and the process can be much safer than wandering through the brush with a machete to clear a line of sight. Old timers have horror tales about snakes, alligators and other critters that they encountered during a survey.

After the survey is completed, the data is used in a CAD (computer aided drafting) program to start the drawings required for the project. It's a good start, but there's a lot of engineering yet to be performed. That I'll save for another post.