Groundwater in borings for bldg - what now? 1

[Dennis59]

Hello all,
I couldn't think of a better title for this post - sorry.

Here's the scenario: You're a consulting engineer - hired to issue plans and specs for a project. A geotechnical investigation is undertaken and the geotech firm drills some borings. They find groundwater several feet ~above~ your planned bottom of building. (Assume that the building will be waterproofed and designed to resist buoyancy - that's not at issue for my question.)

What is your responsibility now in terms of providing information for the Contractor who will eventually win the bid and have to dewater and dig the hole?

Assume that the building you are designing is pretty much in the middle of nowhere - there are no issues with sensitive adjacent buildings and there are no significant issues with finding a place to get rid of the pumped-out groundwater.

Would you, as the consulting engineer, order some kind of additional tests at this point (during the preparation of plans and specs) to determine just how much water might be down there, and how difficult the dewatering might be? For example, might you order that a test well be installed and pumped in order to see what the water level does in some nearby boring that has ~not~ been backfilled with cuttings and/or grouted full? (It is my experience that soil borings are backfilled almost immediately, so the driller would have to be instructed in advance to install piezometers in at least a few water-containing bore holes.)

Or would you just include the geotech report in the plans and specs and let the potential contractors figure out what to do about dewatering?

It is my experience that Owners are reluctant to even do enough soil borings to figure out bearing capacity/settlement - much less install a test well. What is your experience?

If no test well is installed in advance, what happens when the construction starts and it turns out that there is so much water that 'conventional dewatering' is ineffectual? Would this typically be considered a "changed condition" (read "more $$") even if the groundwater level is exactly where the soil borings said it would be, and the subgrade is the same type of soil as the borings indicated? If so, at what point does it become a "changed condition"?

Do contractors (or dewatering subcontractors) ever go to a job site in advance of submitting a bid and do their own tests (on their own nickel)? I have heard tales of this, but have never actually seen it myself...

If there is a 'standard of care' with respect to this issue, I would like to know what it is so I can attempt to abide by it.

Thanks for reading, and for any response you care to offer.

 

[fattdad]

I guess I'm somewhere in the middle on this (leaning to option "b" however). I would properly characterize the geology (as required for the geotechnical engineering study) and include sieve/classificaiton testing for the soils below the water table.

I would then give a performance specification that directs the contractor to provide a minimum of 12 inch separation between the working subgrade and the water table and to use the appropriate dewatering system to realize this objective. Whether well points, deep wells, sumps/pumps or whatever - it's the contractor's responsibility to select the appropriate means and methods for the geology that you've characterized by the report. I'd also reference in your report that bidding contractors should obtain any/all supplemental informaiton that they may need (i.e., if not included in the report), prior to submitting their bids.

Let's say, you choose to be a nice guy and determine that the project requires 30 gpm to acheive dewatering. If it actually 50 gpm, the contractor will yell ($$$).

Other's may have other ideas.

f-d

¡papá gordo ain’t no madre flaca!

 

[VAD]

In your case a testpit would be best since this would allow you to determine if this is a true GWT or a perched one. This would be informative for the contractor as he can understand that all he may need is a sump at the corner of excavation etc. For certain jobs as you describe testpits are usually the best or should complement drilling. Pictures of testpitting etc placed in report is agood idea as well. Too often we just have holes drilled without the full understanding of what the project is all about. Thereafter it becomes a guessing game and of course the owner does not want to pay for additional work. There is aneed to discuss upfront but the practice is still "Geotec drill me some holes" or How much bwould it Cost to drill a couple of holes - Drill not testpit

 

[jike]

I would request the Geotech to provide statements in his report regarding dewatering. Is it perched water? Can it be dewatered by conventional means (sump pump) or does it require well points?

We routinely request that the Geotech provide a monitoring well and periodically go back to the site and measure the water depth. If he has not installed a well, you may have to request that be done.

I would make a point of discussing this at a pre-bid meeting with all bidders present. You might even have the Geotech there also to answer the bidders questions regarding dewatering.

 

[DRC1]

A couple of random thoughts:
A.)Unless the anicipated depth of the water is shallow, yu will want to remove it by eithe gravity or pumping. Watertight strucures are rarely water tight for long and seepage will beme an ongoing problem. Therefore you will want to have some idea of the flow conditions for your work.

B.) The better quality data you give the bidders, the less contingency they will carry. Although there are cheaper tests, such as a slug test, I like the pump test as it gives you an idea of the permiability of the whole area. Permiability can vary significantly on some sites.

C.) Some what contrary to B, We do a lot of dewatering on sites that athe soil conditions are fairly typical to the area and we pretty much know how to deal with them, so one question to ask is if this area is similar to most soils in the area or is it somewhat unusual? That would play a factor in the amount of testing I would do.

D.)To answer your other question, unless it is a signifcant project with a lot of bid time, the bidders will probably not do borings. They are generaly busy bidding the package they have been given and trust as part of the bid the site has been properly characterized. Considerable case law supports this position.

E.) You may want to have an early preconstruction conference and dicuss the dewatering with the bidders and see how cmfortable they feel with the data they have and make ajudgement on additional testing from that.

 

[fattdad]

I respectfully disagree in using test pits to identify whether the water table is perched or not. If you have a satuated sand atop a saturated silt and excavate through the sand and into the silt, you will observe water inflowing into the excavation from the sand layer, seemingly concluding that the water in the sand is perched atop the silt. This may or may not be the case. The best way to conclude whether it is a perched water condition is to install piezometers. I say this because I've seen this error played out many times - folks reaching the wrong conclusion from a simple observation.

I do agree that a pumping test or a slug test (albiet to a smaller scale) will help characterize the hydraulic conductivity and further quantify the dewatering requirements for the project. That said, if the geology is fairly uniform and there is local precident on the properties, some sieve tests and correlation between hydraulic conductivity and D20 (or D10) may suffice. If you want this correlation, go to my web page and click on the link to hydrology:

http://www.fatt-dad.com

f-d

¡papá gordo ain’t no madre flaca!

 

[VAD]

Fattdad:

Based on your example if your conclusion is a perched water table then you are incorrect in my books. Kindly remember that each of us have our own approaches. I therefore conclude that the observation from testpits are for the trained eye and does not necessarily mean that is all one does i.e watch for water. If piezometers are not installed correctly with respect to the various strata then spurious results can be achieved. How many times do we seal off strata that may not be water bearing in our installation. Common practice is often to slot the entire length of standpipe and place it in the hole. This is often so prevalent that if contractors were to worry about some of the results then they would not want to bid on the job. There is alot more on this subject. However, we all have different experiences and I stick to my concepts.

 

[fattdad]

No problems from my side, just a difference of opinion. I completly agree that an improper well screen is equally a problem. I also agree on the value of the trained eye, which is needed throughout field exploration. I did not mean to imply that anybody posting on this thread does not have one, I just mean to state that test pits can (have) led to incorrect conclusions from my side of the perch (no pun intended).

Enjoy your weekend!

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

There is a basic question I didn't see clearly spelled out by the discussors.

How do you furnish the soils data to the contractors? Do you include it with the bid dopcuments? or do you tell them it is available and you leave it up to them to decide on its merit?

Obviously the more the contractor has to guess, the higher the bid. However, later if there should be some problem and you are in court trying to figure out if there are changed conditions or other argument, you might not be in too good a position if the data was bound with the specs.

I prefer to make it available, and allow them to do all the test pits or borings they want, provided they doln't mess up the support originally there.

 

[fattdad]

I'm with oldestguy. I believe that the raw data be provided within the contract documents - i.e., boring logs, location plan and laboratory testing data. The more complete the raw data set - i.e., sieves, water table data, perm. testing, Atterberg limits, natural moisture contents, etc. the better the description of the site from the owner to the contractor. Engineering conclusions drawn within the report should not be made a part of the contract documents, however. No doubt that these conclusions are reflected by the design of the structural components, the earthwork slopes, the use of drainage materials for the permanent construction, the pavement section, etc.

I look at dewatering as a means to the end and something that is in the complete control of the contractor. I don't see it as the owner's responsibility in that it does not contribute to the specified design (it's a constructability issue). That said, the engineering for the dewatering system is a challenging and important task.

I have no problems providing my geotechnical engineering study to bidding contractors - just that they use it for their own information and not be considered an integil part of the contract.

f-d

¡papá gordo ain’t no madre flaca!

 

[DarthSoilsGuy]

Almost every project I have worked on from the Geot. side, our report and borings have made it into the specifications right before earthwork. One service we would offer is to review contract documents to catch conflicts between the soils report and plans/specs. We would stay away from language to provide means/methods direction for localized dewatering. We would discuss french drains if permanent dewatering was required.

i think it's best for the owner and design team if all information is made available to the contractor. not sharing all the info, is an easy excuse for the contractor in my book.

in my experience people on the design team and the contractor have shown little evidence of paying attention to the soils report until after a hole is dug, even if the report was in the specs. Although, the design team members are better at faking it. We would joke about changing "3000 psf" to "three thousand pounds per square foot" to make people read the report a little more.

Is french draining the site around the building into the stormwater system and then again for a foundation drain (as a backup system and for water table rebound) not an option? Maybe I'm just biased against waterproofing, as I'm sure some people out there have been burned by improperly installed french drains.

Don't take the condition out that bidders are welcome to do their own pre-bid exploration ......

Do let your Geot. know that you want to put it in the specs (give them the opportunity to edit) if that is not the standard where you are. there are some things a Geot will say to an owner that are suggestions that should not be perceived as engineering design recommendations if the report was never meant for spec insertion.

 

[civilperson]

A test well with draw down characteristics should be installed and the results given to the bidders.

 

[oldestguy]

One more item.

There are some places where it is a good idea to place boring records on the plans. However, state a weasel clause there to say "no guarantee" , but it might not hold up in court. These are pipe line, roadway, sewers, etc. Of course never withhold anything, and make even the separate non-contract engineering geotech report available if they wish it.

I have found that placing borings directly on a site for a lift station, a caisson, etc. is not a good idea. If they hit a tight layer over a porous water bearing layer, that contractor has a more troublesome job than if no borings were done at all. So, then the weasel clause is needed, due to the off-site info.

 

[fattdad]

Darthsoilsguy: Including the geotechnical engineering report in the specification binder is different than including it as an integril component of the contract documents. It can be included as an addendum or as supplemental information without being in the specs. I don't have the direct language from ASFE, but they would support this exclusion as well as most professional liability companies. At the risk of repeating myself, I do include the actual data within the contract documents (i.e., boring logs, location plans and laboratory test data).

I do agree that the geotechnical engineer should be offerred the opportunity to review the plans and specifications to make sure that the report's recommendations are properly integrated into the design.

civilperson: If the design of the project neccesitates (sp) the site-specific determination of the hydraulic conductivity (this may be just such a job), I would completely agree. There is no substitute to a pumping test to get the aquifer characteristics. That said, in many instances such as this owners may not fully recognize the need to run the test for the required length of time. While slug test may be of some relavent value, it will not mobilize the areal extent of the aquifer as a constant discharge drawdown test will.

oldestguy: For the case that you just put the boring locations and the logs onto the plans these represent direct data points at a specific location ONLY. Now the issue (to me) is how frequently are you spacing the borings. I like to include borings at about 200 ft centers within the building footprint. I agree with the use of a cavet and really see the need when requested to produce geologic profiles as these show correlation of geology as well as engineering characteristics from one location to another. There's some risk to that, eh?

Sorry to be so long-winded. It's the curse of being a fast typer - ha.

f-d

¡papá gordo ain’t no madre flaca!

 

[DarthSoilsGuy]

FD,
you know... when people would throw around the words "contract documents" it use to make me cringe (passive-aggressive way of telling people to do something not because it's in the plans/specs but b/c it's their job and they can get their @$$ sued). I used to think these words should be banned from engineering language. I fully support your assertion that it isn't a catchall for "everything given at pre-bid conference". unfortunately, i've seen it used for the latter so many times that i'm becoming part of the problem. rant for a different post....

Cheers,
DSG

 

[fattdad]

Yeah, I'd really like it if folks would just use common sense and be nice. Maybe ASFE and their case studies have hardened me. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[DRC1]

Generally the courts have supported the use of owner supplied geotechnical information as a basis for the contractor's price despite even the best crafted exculpatory language. The theory being this information is made available to the bidder and is later found not to realistically reflect actual conditions,the courts will find that he owner will bear the cost of the change for two reasons: 1.) If the site had been accruatly characterized, the costs associated with the change would have most likely been caried in the bid and the owner would have had to pay these costs. 2.)As the owner of the site, the owner recieves the benifits of the improvement, so he should pay the cost of the unforseen work.

The problem is that many owners feel that if there is anforseen subsurface condition, the geotech has not done something nessesary and has some how failed the owner. While this may be true in some circumstances, Often it is not. Geotechnical conditions varry and can be unpredictable. The geotech must attempt to preform a sufficent investigation to reasonably anticipate changed conditions, but it can not for see the future or view the subsurface. If they could they would be in much more lucrative fields.

Having said that you may want to go to the owner with some options with pros & cons as to how to handle this and let him make the final descion on what to tell the bidders.

 

[BigH]

Mmmm, interesting discussion so far. In my view:
(1) The factual information should be provided in the contract documents. This, I think, is a given. The factual information includes the borehole logs, results of laboratory testing, information on groundwater conditions encountered. Interpretation for design should not be provided. Many times, then, two separate reports are provided - a clear and concise "Factual" report and an "Interpretive" report.
(2) If the groundwater conditions are such, in the view of the geotechnical engineer that it would affect the design and construction, then he should be suggesting that further study be undertaken to ascertain the groundwater regime and anticipated behaviour. In this case, a complimentary investigation consisting of a pumping test would be considered. If this is done, the factual report of the test results should be given to the contractors bidding the project. Remember that the main contractor will almost always be using a specialist dewatering expert/firm to look into the situation. I am aware of deep dewatering for the at ground portion of the Vancouver Skytrain but no special "prior to construction" testing was carried out for the benefit of the contractor - or even the designer. It was noted that the excavation for the below grade area was in a cohesive material which overlay a layer of sand under artesian conditions - the design took care of the dewatering problems in this case.
(3) I note that the groundwater was found only "several" feet above the base of the building. It is highly likely, even in moderately permeable sands that open excavations and pumping from sumps would be suitable. If the goundwater level was found "high" above the base of the building, this might be a different story.
(4) In general, weasel clauses do not seem to carry much weight anymore - and, in one instance that I am aware, engineering and common sense didn't either. But, as has been stated earlier, provide all the information of a factual nature you have, be up front with it all - it is really up the contractor, though, given his chosen work methods, etc. to interpret the data as to how if fits in with his work methodologies.

 

[fattdad]

I'm agreeing with BigH.

One more item to consider: I worked on a project that was to include waterproof, below-grade construction that would have required temporary dewatering. Please recognize that the use of the temporary dewatering will affect the state of stresses in the adjacent land (increased effective stress), which in turn may cause consolidation/compression of the neighboring soils. To what extent this becomes a problem is for the geotechnical engineer to determine. It must be considered however. . . .

f-d

¡papá gordo ain’t no madre flaca!