H20: Friend and Foe
Water can be a help or a hindrance during excavation and grading. Luckily there are ways to manage its behavior.
As a small boy on a sandy, chilly beach on the other side of the Atlantic, I discovered how independent and obstinate water could be. However many cities of sand and carefully engineered channels I dug, built, and barricaded, the water went where it wanted to go. One of the first gems of scientific wisdom I learned was that water seeks its own level. Overfilling a glass onto a clean tablecloth caused this parental comment. In many construction projects involving excavation, the resting place water seems to prefer is at the bottom of your trench to a depth just above your boots, or hiding in the ground under your machine. In some parts of this country, especially where new streets have been built over old streams, the ground can be downright jelloish or like a wet sponge just a few feet below yesterday's asphalt. Mats are often essential for the stability of loaders, excavators, trucks, and dozers on such terrain; sitting in a working excavator can feel like standing on a sidewalk in an earthquake. When the water table is just below the surface (and Florida is by no means the only place where that occurs), the ground is sure to be less stable and friendly to earthmoving machinery than sites where the top several feet of soil are dry and solid.
|Tracks handle wet, soft conditions well. You can have steel tracks over standard wheels to achieve a similar result.|
When water finds its preferred level, it usually stays there until an attractive new level is presented. It seldom goes away quickly by itself. There were several incidents last year - one of the most publicized was near Cincinnati, OH - where abandoned excavation sites filled with water that quickly became stagnant. There was a public outcry against the contractors who neglected to handle the situation correctly. One of the greatest dangers of waterholes such as those is that they are attractive to children (for throwing and skipping stones, wading in to find underwater treasures, and even trying to imitate Huckleberry Finn on discarded plywood). They are too often unfenced, easy-to-reach playgrounds for unsuspecting youngsters. Although the worst cases seem to have occurred when projects were halted or abandoned for financial reasons, with nobody accepting responsibility for any site condition, it is wise to check that the site where you have just finished your excavation or grading does not offer similar hazards.
Water can be the contractor's friend too when air pollution and excessive dust cause impaired vision in work areas. Water sprayed accurately from trucks helps control those problems, and its cost is generally at a special, lower construction rate, such as the water used for earthfill, drainfill, and rockfill. Measurement and payment will vary from state to state. Your first contact for the correct information could be your Natural Resources Conservation Service. Some contractors - those who work in these conditions constantly - prefer to use their own water trucks and drivers, but others like to use the services of local companies whose expertise is in dust control at construction sites.
Water Often Attacks Below the Surface
|Having the right-size pumps available at the right time has solved many emergency and temporary water problems.|
It always seems easier to combat the enemy you can see rather than the one you can't. Ask those contractors who regularly dig trenches. Even ground that looks dry and stable can cave in, with terrible results for people and property, and that is why regulations for shoring and shielding are so strict (and welcome). When the ground is wet, it seems to be more unstable, more inclined to fill any hole you care to excavate. (That is, after all, water's mission in life - to fill holes. What else is an ocean?) In many of the state regulations we saw, there was emphasis on the presence of a competent person where there was excavation and trenching. Such a person should understand how soils are classified and be able to judge the slope required for a trench or the most appropriate form of cave-in protection. Usually there is no protection required if the trench is less than 5 ft. deep or if the excavation is being made in stable rock. If shielding or shoring is mandated, it should be the right size. Does that sound obvious? Installing inadequate protection would be similar to securing the expensive equipment in your yard with a wet paper fence.
The competent person is also responsible for daily inspections of the site and inspection after a rainstorm. It is his/her responsibility to ensure that there is no accumulated water in excavations or trenches where there will be people working. The competent person will also be aware of the water table and natural drainage and know if there has been any change in the moisture content of the soil. Moisture, water, and runoff are threats below the surface that are too easy to ignore. Perhaps the most important regulation (in California and others states) concerning trenches and excavations is that nobody works in a trench without a lookout standing by. One of the observations made by researchers into the disposal of nuclear waste was that water tended to seep into some trenches used for burial of wastes at a greater rate than out of them. This is known as the "bathtub effect." It does not happen at every site, but it does emphasize the importance of knowing the soil conditions at every site where you excavate, trench, or grade.
Heavy rain is an obvious threat to trench work. Trenches should be inspected by a competent person after each rain before employees are permitted to go back into them. Many contractors say they take their workers out of the trenches during a rainstorm because it's just not known how quickly the walls will collapse. Controlling water on the surface might help prevent problems below. Benching, sloping, shielding, and shoring can help. What you use will depend on the type of soil. Benching, for example, cannot be used in most places if the soil is Type C. In a crude test with your finger, Type C soil would allow your finger to penetrate easily. It often indicates that the soil is wet or that it shifts easily when wet. Soil testing sounds like something expensive and even academic, but it isn't - it's something that your competent person should be able to do. You will come across such terms as cohesive, noncohesive, and granular. You will find out the characteristics (especially as they are affected by water) of clay, silty clay, sandy clay, angular gravel, silt, silt loam, gravel sand, and loamy sand.
We must put a paragraph in here about the water that runs off construction sites, especially from excavated ground. There have been many instances of contractors being sued for causing pollution with the construction water that ran into lakes and streams. It is an aspect of excavation and construction that you cannot afford to ignore.
Pumps: Simple Solutions to Water Problems
|For regular wetland excavation, excavators can be given amphibious undercarriages. |
|This tracked excavator has a PowerTilt bucket to cope with the particular problems of ditch work|
The simplest way to keep water out of excavated holes is to pump it out. Remember that this water is seldom clean, and the pumps that work best are those designed to handle dirty water or water with contaminants (bits and pieces, twigs, stones, gravel, silt, and sand) in it. They are even called "contractors' pumps" by some manufacturers. You see them categorized as trash or diaphragm pumps; both types expect to handle solids in the water. Manufacturer names mentioned frequently by contractors were Gorman-Rupp, Honda, Flygt, Grindex, Chicago Pump, Bowie, Grundfos, and Tsurumi. In our conversations nationwide, comments made on pumping systems tended to be related to styles rather than brand names, with almost every interviewed person agreeing that there are many excellent pump manufacturers out there. Popular trash pumps will pass solids of 1 in. or so, and you can expect them to pump out around 100 gpm - more if you have a larger engine, such as the WT40X from Honda with an 11-hp overhead valve engine; that model will pump more than 500 gpm when there is a low head, as there often is in everyday excavation work. If you have to move huge amounts of water, 16-in. pumps from Crisafulli will discharge 10,000 gpm; they have successfully handled floods and could claim to have outpumped the Mississippi River on several occasions. Most were powered by tractors because the surging water was threatening farmland by the river, but options for electrical and diesel power have become popular too.
Shortcuts do not work with pumping systems. With most, there are three components that must match: the pump itself, the inlet hose, and the discharge hose. "A smaller-than-necessary inlet hose will seriously affect the system's performance," warns Kip Rohr, an experienced professional with Pump Systems Inc., in Dickinson, ND. The company sells, rents, and services several brands that would be appropriate to contractor work. "When users complain about the wrong pressure or too little volume, it is too often because they have tried to go around the recommended setup." Pumps are carefully engineered products; the prolific amount of performance curves published (and readily available from distributors) will show that there is probably a pump for every application. "If a contractor who has regular excavation and trenching projects has a 4- or 6-inch pump, he will find he can do almost any job," adds Rohr. He counsels that smaller sizes can be suitable for many applications, but the bread-and-butter models for companies involved in frequent excavation are the 4- and 6-in. sizes. An observation made by several users was that the solidly constructed discharge hose, rather than the lay-flat variety, has shown more efficiency and stability for construction-related applications.
A word that seems to occur frequently when talking of water problems is "bypass," and that is because many water-related problems are caused by broken or malfunctioning pipe. Much of the pipe in the United States is older than most of us, installed when our grandparents were young. It has served well but, like us, develops faults and frailties. When the sewer pipe on Main Street breaks - a sewer carries very dirty water - the repairs must be done, if possible, without disturbing the flow of the pipe. It sounds impossible, but it's what customers want. Repairs usually involve precise excavation; it can't be done efficiently if sewage is flowing over the excavator bucket. The contractor makes a temporary link where the pipe is broken so that the flow can continue while repairs are done. Pumps that do this are in constant demand, and they must work without interruption. Many contractors expressed high regard for the distributors and rental yards that supplied their pumping systems. Manufacturers insist on the highest service standards from their own staff and their distributors because most of the situations are genuine emergencies.
The Right Equipment at the Right Time
|Special long-reach booms can improve the versatility of excavators in projects where water is involved.|
|Tracked loaders have low ground pressure that enables them to work on soft terrain and avoid damage to sensitive ground.|
In Massachusetts, a 96-in. sewer and an 18-in. force main had to be relocated in a road construction project, and the interstate had to be kept open and construction access maintained to help build a new bridge pier. The main contractor used Godwin pumps, 26 of the 12-in. DPC300 Dri-Prime models, with almost 1.5 mi. of pipe. Eight of the same pumps helped authorities in Philadelphia control up to 35 million gpd to complete sewer work with no damage to historic and environmentally sensitive areas. Similar to many others, Godwin pumps often are rented for emergency situations; renting is a solution you should consider if your water problem is not permanent or unlikely to be frequent. Contractors who work in areas subject to regular but not frequent flooding also rent the pumps. Godwin offers a Regional Re-rental Program that gives distributors the opportunity to rent from Godwin at a discount from national rental rates. Kieger Enterprises in Hugo, MN, is a contractor working with the Federal Emergency Management Agency. When the Mississippi crested at a dangerous level, Kieger needed pumps immediately. "Because of the nature of their business, the Kieger people expect an answer within five minutes," notes Dave Lillquist, operations manager for Ziegler Rental, a Godwin distributor. "We responded to calls from seven states," adds Pat Iwan, general manager at Kieger Enterprises. "We had pumps on the road within an hour of receiving them, thanks to the quick turnaround at Ziegler and Godwin. It wasn't only the Mississippi that was giving trouble, but the St. Croix, Red, and Minnesota too."
The right equipment for handling problems caused by water might include an excavator. The Cobra Cutterhead, engineered and manufactured by pump company Crisafulli, can have an excavator mounting system that makes it a powerful solution for coping with water problems where the water might be full of weeds and silt. This system has been successful for smallish projects, such as golf course pond, and for much bigger challenges. In Hawaii, at the Oahu National Wildlife Refuge, there were severe problems with silt in the drinking-water lagoons. It could build up to a depth of 30 ft., and because of the disturbance caused by straightforward excavation, a method was required that would be as effective but with much less disturbance of the water supply. The Cobra Cutterhead on an excavator cut away the mud bottom and discharged it through flexible pipeline, proving to be an acceptable and affordable solution. Wetland Equipment Company, based in Thibodaux, LA, was involved in that Hawaiian project and has produced many amphibious undercarriages for similar work with excavators. The low-ground-pressure, 100% amphibious excavator carrier works with excavators from Kobelco, Volvo, Case, Caterpillar, Daewoo, Hitachi, John Deere, Hyundai, Komatsu, Link-Belt, and New Holland. Wetland Equipment also makes an amphibious backhoe carrier. "We have our machines working in more than 30 countries," states Jim Atkinson, general manager. "So far, however, we have not found anything that will compare with our Louisiana coastal marsh, and we work our machines in, out, and across that every day. We are tooling up to provide 100% amphibious excavators and supply buggies for a project along the coast here that may take 15 or 16 years to complete."
If water threatening your excavation or grading is more mud than water, tracks can be more efficient than wheels. Some contractors have tracks that they substitute for their standard wheels on such machines as skid-steer loaders. In northern states, having tracks for use on muddy or even icy ground has enabled contractors to extend their working season into the late fall and winter. Tracks give lower ground pressure and better traction. Bobcat offers tracks for some of its machines, and ASV (a Cat affiliate) specializes in the design and production of tracked loaders for soft ground conditions.
|Crisafulli has designed a cutterhead/pump system that mounts on an excavator for dredging silts that are full of weeds or other plants.|
Prompt delivery and easy setup apply to shoring protection too. One of the reasons that aluminum hydraulic shoring seems to be more popular now than timber shoring is that it is lightweight - often light enough to be installed by one worker - and the installer does not have to be in an unprotected trench to install the protection. Aluminum shoring is installed from the top down, with the top hydraulic cylinder recommended to be no more than 18 in. below the top of your excavation. There are other regulations regarding the bottom of the cylinder and the number of shores to be used. Manufacturers of shoring know these rules; they might be your best source of information and advice.
Shielding is different from shoring. Shoring supports the face of the trench; shielding protects workers by putting them in a safe box. To prevent movement of the box, the space between it and the wall of the excavation should be backfilled. "Protecting workers is the main goal of shielding," asserts Diane Beckerman of Comcore Utility Products in Falls Church, VA. "Shields are not intended to support the trench walls you have excavated but to guard the people inside against cave-ins." There are many types and brands of shoring and shielding available. Comcore aims to provide equal strength with less weight in its fiberglass composite products and has designed a "Crossing Service shield" that comprises an array of vertical and horizontal shoring panels to accommodate distribution lines and crossing services, at random heights and angles, as well as those other obstructions that always seem to exist in the trench you need. Another helpful point for those who are excavating to install pipe comes from Gordie Schmitt, who has years of experience with Montana Dakota Utilities. "Make sure that the open end of the pipe is out of any water. If water can't get into the pipe or what the pipe contains, it won't cause future problems."
Water does not warn you when it is about to break into your trench or excavated site. It arrives quietly, often in a seeping, creeping mode, and holds its ground until you force it away. If you know the soils in which you are working - are they Type A, B, or C? - you can be prepared for problems. For Type C soils, water invasion is much more likely. You can be prepared, too, by ensuring that your workers know about the hazards of water at their work sites. The training required is neither lengthy nor complicated. An excavator operator need not be a soils engineer, and an installer of utilities in trenches need not be a professor of hydraulics, but everybody should know what can happen, what to do if something dangerous does happen, and how to make the job site safe for themselves and their colleagues. It's a training program that is simple; you yourself can probably be the teacher. As with all simple programs, it is easy to neglect or assume that everybody knows all the answers without your checking.
We must have water. Most construction work requires water at some stage, for some technique of the project, for jobs as diverse as mixing concrete or cleaning tools and equipment. When it is creeping, seeping, or gushing where we don't want it, it becomes a problem. Good preparation will avoid damage and delay from water for most excavation and grading work. If one piece of advice was much more popular than any other from the contractors with whom we spoke, it was know the soils where you are going to work before you start the work.
Author's Bio: Paul Hull is a frequent contributor to Forester Media publications.