Dewatering in Casper, Wyoming

You cannot place concrete in a lake or lay pipe in soup. Water in an excavation softens the bottom, floods the forms, floats the pipe and turns a one-day pour into a pump-out nobody budgeted. Dewatering lowers the water to below the work, holds it there until the work is backfilled and then lets it come back up without damaging what was built.

The job sounds like running a pump. It is closer to managing a small hydrologic system. We estimate how much water the ground will give up, pick a method that keeps up with it, route the discharge somewhere allowed and watch the nearby ground for settlement. Too little pumping and the trench fills. Too much, in the wrong soil, pulls fines out of the ground and settles the pavement next door.

We approach dewatering as part of the excavation plan from the first day, not as a rescue when the hole fills. Water that shows up on the schedule is a cost. Water that shows up in the trench is a delay.

Pick the method to fit the ground

Sumps and pumps are the simplest method. We dig a low spot in the excavation, drop in a pump with a screened intake and move the water out through a hose. That works for shallow trenches, clean seepage and clay soils that release water slowly. It fails in loose sand and gravel, where water pours in faster than a sump can handle.

For granular soils and deeper cuts, we use well points or deep wells. Well points are small pipes jetted into the ground around the excavation, connected to a header and a vacuum pump, and they lower the water table before the excavator arrives. Deep wells use submersible pumps in larger holes to pull down the water over a wider area. We size the system from the soil boring data and the pumping rate, then monitor the water level in observation wells and adjust. Every system gets a standby pump, because the day one fails is the day it rains.

Why the river sets the water table

Casper is located on the North Platte River, and the ground near the river carries groundwater that rises and falls with the season and with flows upstream. Upstream, Alcova Dam impounds a reservoir on the same river, and river levels shift with the season and with how that water is managed. A trench near the water that is dry in March may weep in June.

That variability is why we ask for the geotechnical report and for any local knowledge on groundwater before pricing. Borings show the water level on the day they were drilled, not the day we dig. Sewer, storm and foundation work near the river corridor, including the Platte River Commons area southwest of downtown, should assume that water will show up somewhere, and carry an allowance for it.

Where the water can go

Pumped groundwater is not free to dump. Discharge usually needs permission and often a permit, particularly if it carries sediment or could reach a stream. Clean water can go to a vegetated area, a storm system or a sediment bag. Muddy water must settle first, in a tank, a basin or a filter bag. We design the discharge route before we turn on a pump, not after a neighbor calls.

Contaminated groundwater is its own category. The Mystery Bridge and U.S. Highway 20 Superfund site in Evansville involves groundwater contamination believed to come from former industrial operations, and the former refinery property at Platte River Commons is still recovering after a remedy agreement in 2002. On or near sites with that kind of history, we sample before discharge, follow the environmental consultant's plan and may need to haul the water to an approved facility. A good record keeps the owner out of trouble.

Settlement, cold and the neighbors

Lowering the water table can settle adjacent ground, especially in soft or organic soils. We watch for it. Survey points go on nearby pavement, utilities and buildings before the pumps start, and we stop if movement appears. Filters on well points and screens on sumps keep fines in the ground.

Winter changes the setup. Discharge hoses freeze, pumps ice and a pump station can lock up overnight. We insulate or heat lines, drain them at shutdown and keep fuel and generators serviced. Noise and fuel are part of the job as well. Pumps run around the clock on long dewatering jobs, and neighbors notice. Quiet-rated units, fuel containment and a clear contact for complaints keep the peace.

What dewatering costs

Cost depends on flow, duration and method. A sump on a one-day trench is a small line item. A well point system on a long sewer run with granular soils can be a major part of the budget, with mobilization, setup, daily monitoring, fuel and removal. The discharge treatment adds cost if the water needs settling or testing. Commercial developers weigh that tradeoff on every site with a deep footing, and a mid-rise office building with an elevator pit or basement is the usual case.

We recommend pricing the work as a base system plus unit rates for extra wells or extra days. That way, an owner is not paying for a worst case that never occurs and is not surprised if the ground is wetter than the borings suggested. Where a project has both deep excavation and a high water table, early review of over-excavation and recompaction options may save money, because replacing wet soils can cost less than pumping them dry for weeks.

Wet holes are dangerous holes

Water makes a trench wall weaker. Saturated soil slides, and a trench that was stable when dry can fail when it is wet. We reevaluate the shoring plan whenever groundwater appears. Dewatering also keeps workers out of standing water, which carries its own electrical and slip hazards. Pumps and cords are rated for wet use, and any electrical equipment near water is protected by ground-fault devices.

Open excavations with water are fenced and signed after hours. Engine exhaust from pumps requires ventilation if used near confined spaces. The competent person inspects the excavation each day, after each rain and after any change in the dewatering rate. Related excavation work and the sanitary sewer installation that often needs dewatering follow the same inspection routine.

Tell us what the borings show

Send the geotechnical report and the excavation depths. We will recommend a method, a standby plan and a discharge route before the first cut.

Questions Owners Ask

How do I know if I will need dewatering?

The soil borings and the groundwater readings show it, but borings are a snapshot. If the excavation bottom is within a few feet of the recorded water level, expect to deal with water. Locations near the river, in low ground or after a wet spring carry higher risk. We recommend an allowance rather than hoping for dry ground.

Can I pump water onto the street or into a storm drain?

Only with permission and only if the water is clean. Sediment-laden or contaminated water cannot go to the street or the storm system. We route the discharge through filters or settling and follow the permit. If the water is suspect, it is tested first.

How long does dewatering run?

It runs until the work is backfilled or the structure is heavy enough to resist uplift. On a short trench that may be a day. On a deep foundation, it can run for weeks. We keep it going through curing and backfill rather than shutting off early.

What happens if the pumps fail overnight?

A standby pump and a fuel plan are part of every system. On long jobs we check the system on a schedule, including weekends. An alarm or a remote monitor is worth it on critical excavations, since a flooded foundation can ruin forms, rebar and a day of concrete.

Does dewatering damage nearby buildings?

It can, if done carelessly in soft soils. We install settlement points, filter the intakes and monitor water levels around the site. If we see movement, we change the rate or the method.

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