Water, water everywhere and not a drop to drink

Staying dry when groundwater infiltrates the jobsite

By Tom Minihan

Such is the cry of many contractors when groundwater halts a construction job or even threatens bankruptcy of the project. Whether expected or unexpected, groundwater on a construction project can become the single largest cash drain of the project, regardless of whether the project is a pipeline or a building. The secret weapons in the contractor’s groundwater-fighting arsenal are respect, information and contingency planning.

As an estimator, project engineer, project manager, superintendent or foreman, you must know and respect the enemy. Groundwater is not the same as the sparkling glass of water you guzzled down on the jobsite or the hot, soothing water in your morning shower. Underground water takes on a much different personality and in many cases multiple personalities. Lack of respect for it — it’s only water, how hard can it be to handle — can shut down the project and has put many contractors out of business. Water contained in the soil acts more like molasses than the water in a lake or river. The soils through which the water moves — the clays, silts, sands, gravels, coquina, cobbles and boulders — determine how much water threatens the excavation, how it will approach the site, how it will enter the excavation and what it will carry with it .

 

There are several general terms relating to groundwater personalities that must be understood:

Perched Water sits on top of clay layers, or hard pans of compacted mineral deposits and can have dry soil underneath it.

Uplift Pressure is the nature of groundwater to seek its original water table level, which generates an upward pressure. The pressure can destabilize the soil, create boils and cause settlement and subsidence.

Permeability is the ability of the soil to pass water in feet per minute or feet per year. In clays water travels in feet per year whereas in gravels water can travel in feet per minute. Some soils like silt will retain water and act like jello.

A recharge source is a reservoir of water that can release water to the jobsite area once dewatering has begun. The Recharge Source is usually obvious such as a lake, stream or river but can be underground in gravel or fractured bedrock deposits. The distance the recharge source is from the jobsite is very important but even sources several thousand feet away can be a hazard. Remember also that recharge sources can be contaminated. Once you start dewatering your site the groundwater in the recharge source can move toward your location and bring the contaminants with it.

The jobsite albatross
Groundwater management falls into two major categories — handling the groundwater before it enters the excavation or handling it after it enters the excavation. The decision to do one, the other or both is the most important decision the Contractor will make for his project and requires diligent investigation of the jobsite soil conditions. Soil borings are invaluable as a risk management tool in today’s economy and should be done regardless of whether the Project Owner has provided a soils report or not. Unfortunately, Owners don’t commission soils investigation reports to gather information on how to construct the job, only to determine how to design the structure. Therefore, the idiosyncrasies of the groundwater environment are not given priority.

The contractor needs to know where the lenses of clay and hardpan are and what is sitting on top of them.   Where are layers of shell or gravel or coarse sand in relationship to the groundwater? Where is the bedrock, the coquina or the prehistoric ocean bottom? And are there cobbles or boulders present? Unexpectedly finding boulders the size of a Volkswagen and weighing several tons can ruin your day particularly if you”re dragging a trench box or driving sheeting.

One valuable technique in gathering soils information that the owner will probably not commission is taking continuous split-spoon soil samples. That is logging the soil types from the top of the ground to at least 15 feet below the deepest excavation continuously without missing an inch. Continuous sampling, at least in one boring, will identify soil types and soil profiles like silt layers on top of clay or seams of gravel above the bottom of the excavation that could devastate a job. Typically the driller doing owner-commissioned soil borings may not record hitting cobbles or boulders. If it doesn’t come up in his auger it’s not his problem. Contractor-commissioned soil boring puts information-gathering control in the hands of the contractor. Five thousand dollars up front could save millions when it’s too late in the project to turn back due to groundwater problems and added dewatering costs. Going in with a dewatering system after the problem is uncovered is much more costly than installing a system before the site is changed and may be impossible.

Digging test holes with an excavator is a common practice to gather soil information and gives valuable information on how the dirt digs. However, it is important to recognize that this technique is limited to reaching to the top of the water table or just below it and won’t expose the soil structures and their groundwater characteristics below the excavation.

After biting the bullet on boring and test dig expenses the Contractor can make a more confident decision on whether to handle groundwater before it enters the excavation or after. In this article we won’t discuss handling the groundwater after it enters the excavation, only before.

Smooth Sailing
Groundwater management offers a myriad of techniques. Depending on the soils information gathered above some sites can employ slurry walls or slurry trenches to greatly reduce the flow of groundwater into the site. The slurry is composed of bentonite clay mixed with the soil in a perimeter trench. Steel sheeting can be a water management tool also but is expensive. Active dewatering includes the use of deep wells with submersible pumps on high volume and deep excavation sites, vacuum wellpoints and vacuum wick points with geotextile fabric for shallower cuts to 18 feet or less and eductor wells up to 100 feet deep in tight soils or in contaminated soils.   Trenched under-drain pipe commonly called “sock” can be used for shallow cuts and horizontal wells installed by directional drilling works well for tunnels, highways or railroad tracks or for sites where above ground encumbrances are undesirable.
When choosing an active dewatering technique for a project, the first thing to remember is that one size doesn’t fit all. The site soil conditions may dictate that you use different techniques in different parts of the site. Wellpoints are a common dewatering tool but conventional slotted screen well points aren’t always the best selection.

Nowadays sizing the header manifold piping and using the new Wick Points with geotextile filter screening can produce a more efficient and economical dewatering system. This method can also produce less turbid discharge water without the need for “sand packing,” which makes the Environmental Engineer happy. It also reduces or eliminates subsidence and settlement problems. Excavations with perimeters of up to 1,400 ft using properly sized Wick Point systems can be handled with less than 10 horsepower single-phase electric pumping systems. This can greatly reduce dewatering costs over the life of the project. Using inert materials like polyethylene tubing for underground risers and connection piping can offer the option of converting the temporary construction dewatering into a permanent system for the project owner. Simply abandoning the economical Wick Points in place reduces removal expense without creating an environmental hazard.

New green pumping systems are in development, including systems powered by solar panels and other alternative energy sources. Dewatering systems involving fracturing — splitting silt and clay lenses vertically with continuous sand columns— is also being developed to intercept groundwater in low permeability soils for positive groundwater drainage while maintaining excavation slope integrity.

Piping systems are also advancing. Vari-use piping, with swivel ball and socket connections, is available to provide the Contractor one pipe system that can be used for water discharge, vacuum well points and wick points, eductor well systems, bypass and sumping systems and deep well systems. Vari-use pipe allows the contractor to change from one type of dewatering system to another immediately without changing piping systems. Made of polyethylene the pipe doesn’t require any tools to assemble or disassemble and is resilient to abuse and typical jobsite hazards. It can be repaired economically and modified to fit the job requirements.

Dewatering system installation techniques are advancing rapidly. With tracked excavator attachments less labor is required to install wells, wellpoints and Wick Points particularly in clayey or “boney” soils laden with cobbles and boulders or old construction debris. Up to 40-foot long deep wells and eductor wells can be installed with the same excavator the Contractor would use to dig an eighteen-foot deep pipe trench or excavation. The new installation apparatus also reduces hazards to workers thus reducing strains and sprains making the work environment much safer.

The bottom line is, respect Mother Nature and work with her, be humble and informed and don’t take groundwater for granted. If you do, you’ll lose. Choose wisely and get a good night’s sleep.

Tom Minihan is vice president with Griffin Dewatering Midwest LLC based in Hammond, Ind.