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.