Handling Potential Groundwater on Project as Owner's Risk

[coloeng]

We have a pipeline that will be constructed that has an area that has potential groundwater that is primarily driven by irrigation in the area. It is difficult to determine how much groundwater there will be at any given time or point. We will be bidding the project in the next month and one of the potential bidders has asked if the owner would accept the risk for the potential groundwater. Does anyone have any ideas how to setup a method to value the measures that the contractor might use so that we could determine a cost associated with those measures?

 

[SlideRuleEra]

...potential groundwater that is primarily driven by irrigation in the area.

If the Contractor will provide a schedule with a firm commitment, ask the Owner if they will commit to not irrigating for the necessary time period.

IMHO, Both parties have to take some risk: the Contractor's firm schedule, the Owner's not irrigating at for the required time. If they cannot reach an agreement, there is no deal... the end.

 

[coloeng]

The irrigation water is from outside sources, i.e. adjacent farmers irrigating their fields, so the owner has no control over the irrigation water. Unfortunately, we can't ask the farmers to not irrigate.

 

[SlideRuleEra]

Understood. We handled high ground water on our industrial projects by designing with the assumption that the water table is at top of ground elevation... which is true several times a year. Is it too late to put that requirement into the design?

 

[coloeng]

Not to late to put it in the design or schedule of values, but how do we set the units so that the contractors can develop a bid value. Don't want to set it as lump-sum or we could spend $1 million. Looking for a way to measure how much groundwater the contractor has to handle and can set a value for it.

 

[SlideRuleEra]

Would a reasonable upper bound on dewatering volume equal the volume of irrigation water distributed recently? I assume the volume of irrigation water used is known, or can be accurately estimated.

 

[dik]

Irrigation may only be a part of the groundwater problem.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SlideRuleEra]

dik - The OP says "...potential groundwater that is primarily driven by irrigation...".

You are right, of course, but then the groundwater source is probably "infinite".

 

[coloeng]

So do I set a pumping rate, i.e. 3,000 gph times a dollar amount? Or something else?

 

[SlideRuleEra]

The pumping rate will depend on many factors, such as:
Area dewatered
Depth of dewatering
Elevation of water table is there was no irrigation
Soil properties
Length of time dewatering continues
Incoming water (irrigation, rainfall, etc.

The elapsed time (say, hours per day) that dewatering is operational seems to me to be the logical variable to monitor and pay for. The Contractor is going to have to remove whatever groundwater is there to do the job.

For our electric generating plants (located near "swamps") we have had to dewater several acres more or less continuously for up to 1.5 years (dewatering system down only a couple of hours per week for maintenance). Pumping rate is "high" at first then tapers to a lower mostly steady-state rate... but never stops.

There are other conditions where dewatering may have to operate for just a short time each day.

 

[SWComposites]

Just put an assumption in the bid that states "owner is responsible for dewatering the construction site sufficient to permit construction". Are there specialty dewatering companies?

 

[SlideRuleEra]

SWC - Yes, there are companies that specialize in dewatering, but they typically are interested in large scale, deep, long duration projects, at a defined location, like foundations.

A company qualified to install underground piping will usually make their own arrangements for dewatering since pipeline installation covers a moving, "long" work area. Dewatering equipment (usually well points for "shallow" depths, say, less than 20') will "leap frog" as the actual work area progresses along the line. Sometimes a Contractor can get by with just diaphragm (mud hog) pumps for local, shallow, short term dewatering and not have to install dewatering equipment.

Good suggestion, depending on details, perhaps it will be appropriate for the OP's project.

 

[coloeng]

Thanks SRE,SWC and dik for your responses.

This project is in southwest Colorado and the groundwater would not be present if not for the irrigation. We will be starting the project at the beginning of irrigation season, so the groundwater will be minor at the beginning of the project and increase as the project progresses. We will need to have the contractor do the actual dewatering and that is why we are trying to develop a unit cost that can be used. We don't want to stick it to the contractor, nor do we want the contractor to stick it to us. We have installed around 30 miles of pipeline in this area with very little groundwater issues, but this is in the low end of a valley where all the tailwater from the irrigation flows.

 

[SlideRuleEra]

How far below ground surface is the bottom of the pipeline?
Is that distance constant along length of the pipeline?

 

[swazimatt]

Maybe it is uncommon in your area, but as we have it almost for granted here we just have a lump sum item for dewatering excavations and it is then the contractor's responsibility to do it. Some contractors are smart and excavate long lengths of gravity trench witht a sump at the end allowing them to pump out the water while they are working on "higher ground". Others use well-points to lower the water table locally.
A common problem is that often your trench bottom becomes unworkable when wet and then you have a claim to over excavate and use geotextile and pea-gravel as pipe bedding

 

[coloeng]

SRE- The trench is 4.5' deep with an 8" pipe being installed. The depth is constant.

Swazimatt- We have a bid item for trench stabilization. Doing a lump sum item may be the only way to handle this.

 

[SlideRuleEra]

Agree with swazimatt, go with a lump sum. I don't have knowledge of soil conditions in CO, other that your description. Depth of 4.5' should not be a big problem to a qualified contractor. If you have bidders who are both qualified and interested is submitting a competitive bid, the price for dewatering should be reasonable (compared to total contract value).

 

[coloeng]

Thanks everyone, I appreciate the feedback.