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Old 08-29-2011, 11:26 AM
 
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We're in the process of buying a home that has a heaving concrete garage floor. The impact is that water off the car will drain back towards the house.

Is heaving common in Colorado due to the expansive clay soil? Should we be concerned about the foundation's integrity or risk of further heaving? Would we expect to see similiar behavior in the basement at some point? Any idea how much it costs to replace a concrete slab?
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Old 08-29-2011, 11:50 AM
 
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Expansive clay soils ... bentonite ... are commonly found in the area.

If you are seeing obvious damage in one area of a structure, anticipate that you will have problems with the rest of the structure; if not now, at sometime in the future.

In either case, you can expect to spend a fair amount of money to mitigate the problem. At a minimum, you need a soils engineer/structural engineer evaluation for the house under consideration.

Before making any final purchase decision, you need to have all the information available to make an informed purchase decision.

I have seen houses sold in the area that would cost more than the cost of the house to repair the existing damage and mitigate the site/foundation so that the problem(s) would not reocurr. There's ways to divert the water flows around a structure so that the soil doesn't expand ... but it's only to the extent of the best engineering available at the time and the given water flows.

You are still then at risk of changed water flows in the future. For example, I had a house in a subdivision where the builder specified that the sites were not to be regraded. The sites were graded in such a way as to divert the area water run-off to land between the developed sites, but this required that a sizable portion of everybody's back yards were essentially an unusable hillside. Subseqent owners moved in and re-graded the yards to give them the utility of the whole site they owned, unaware that they were creating problems for the folks downhill from them. Many houses were affected by the seasonal run-off flows, especially in the years when stronger rainstorms passed through. The ability to forecast that this would happen is limited, although a good site/soils engineer can predict the possibility; proving the source of damage to your house structure a 1/2 mile away is costly and problematic. I've also seen situations where excavation repairs to a failed sewer line from a house changed the sub-surface water flows for adjacent properties and caused serious damage to those other houses when the soiil got wet in season.

If the real estate agents involved have not fully disclosed the extent of bentonite soils in the neighborhood or the specific problems with the property you have under consideration, you need to light fires under these people. The motivation to get an earned commission at a closing can outweigh the need for full disclosure among many of these agents in the area, but the problems with a structure after a closing are all yours to deal with. I've seen a lot of times where a lawsuit after the fact and upon discovery of new structural damage is the only recourse you have; you can bank on it being expensive and with no guarantee that you will prevail in your claim. Even a home owner's warranty policy may be no insurance, or inadequate coverage to repair and mitigate this type of problem in the area.

For my money, given visible obvious signs of bentonite soil damage in progress ... I'd be looking for another property unless the sellers were willing to undertake full resolution of the issue before the closing and before I spent any money at all. Outside of the structural concerns, it's a real problem when doorways get out of kilter, kitchen cabinets get to where the doors don't close, or interior floors start buckling ... and I've seen that and many other effects from the foundation stresses and failures due to bentonite soils. In some neighborhoods, you'll see proper drains around the houses, proper site prep, and even then ... sump pumps in the basements to remove the water that flows too close to the house sub-surface.

Additionally, you need to be very careful about your grounds maintenance and watering. Setting up a sprinkler system where it irrigates the expansive soils adjacent to the structure can be a serious issue ... and I've seen a lot of architects from out of the area who don't have experience with bentonite design structures that won't take any seasonal sub-surface moisture build places that were set up to fail at the first wet season.
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Old 08-29-2011, 12:47 PM
 
Location: Bend Or.
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"For my money, given visible obvious signs of bentonite soil damage in progress ... I'd be looking for another property"

Solid advice.
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Old 08-29-2011, 04:25 PM
 
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So expansive clay soils heaving concrete garage floors is not common in Colorado? We didn't see other signs of structural damange, but I am concerned about potential future headaches.

If anyone could message me a good structural/soil engineer in the Denver area I'd really appreciate it.
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Old 08-29-2011, 05:36 PM
Status: "October is the eighth month" (set 19 days ago)
 
Location: Just south of Denver since 1989
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try jacking up the floor and have a new one poured.
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Old 08-29-2011, 06:01 PM
 
Location: San Ramon, Ca
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you can do a grout injection or simply remove and replace the slab. about $5,000 either way.
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Old 08-29-2011, 06:04 PM
 
Location: San Ramon, Ca
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that type of heaving happens. most garage floors are a separate pour to allow for movement. have you looked into the development having a history of soil issues? check with the neighbors.
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Old 08-29-2011, 06:55 PM
 
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Originally Posted by markfromCA View Post
that type of heaving happens. most garage floors are a separate pour to allow for movement. have you looked into the development having a history of soil issues? check with the neighbors.
Why do I get the impression that you may be unfamiliar with the nature of the bentonite soil problem/expansive soils issues here in Colorado? This is a relatively local regional problem due to the fact that we're in a desert environment, so the soil dries out seasonally. Unless there's a source of moisture, then the soil is stable. But once a sub-surface flow of water comes through, then the soil expands with devastating force. Heaving slabs here is highly indicative of a very common problem in this area, and if a slab is heaving, it is not "normal".

Unlike riparian areas of the USA with a high water table all year 'round, the Front Range area of Colorado dries out. Then there's a winter snow layer which melts in the spring, feeding the sub-surface streams, and then the following expansion of the soils.

Yes, garage (and basement) floors typically are poured to be "free floating" from the footers and foundation in this area.

But heaving indicates a sub-surface expansion of the bentonite soil due to a sub-surface moisture source.

The problem was described as "heaving", not settling ... so mud jacking or similar repairs which address issues of lost soils under the slab are not appropriate fixes. They will not stabilize an expansive soil condition.

One of the key differences here with this situation is illustrated by the different materials used for foundation waterproofing in riparian areas of the USA compared to Colorado. Several of the more common exterior waterproofing coatings that will durably work in those other areas are dependent upon being immersed in water for most of their service life. Here in Colorado, after a few years of wet/dry cycling, they lose their waterproofing ability. Some contractors (and specifiers) know the difference; but again we have folk who are professionals who use big famous name products from other areas which the distributors are happy to sell without understanding the functional limitations of their application and the likely failure in the near term.

Outside of my 40 years of real estate experience along the Front Range of Colorado, I've had extensive contact with these problems as an industrial coatings distributor/applicator dealing with in-ground containments as well as concrete slabs on-grade. I've literally seen multi-million dollar industrial buildings designed and built by folk with no knowledge of the extent of bentonite soil issues in this region and watched as they were stuck with floors that couldn't hold an epoxy deck or were continuously cracking and heaving with serious structural defects when the seasonal sub-surface streams ran water by the foundations or slabs. I've personally gone into hundreds of residential garage flooring sales calls to evaluate coating and concrete issues and done moisture testing to verify that the slab-on-grade was seeing in excess of 5-8-10-even 12 lbs per 1,000 sq ft of moisture coming through ... and that was in an unheated garage, not an interior space which has yet stronger driving forces to bring moisture through the slab.

I've been in commercial shopping centers, retail malls, and similar developments as a consultant for concrete floors and industrial/retail epoxy coatings, or epoxy terrazzo. The first step when you observe a failed coating is to start measuring the moisture issues ... and I sure in heck will decline to install a epoxy deck coat system (unless it's breathable) in any situation where there's moisture vapor transmission through the concrete in excess of the rated capacity of the epoxy floor.

Folks, that moisture is coming from somewhere. It's in the sub-surface below the slab, and there's an inadequate or non-existent moisture vapor prevention construction detail for that site. There's a lot of shady builders in this region who understand the implications of "seasonal" water flows, and they only do their site compaction tests and engineering reports in the dry season to get approval for the next phase of the construction ... knowing that when the wet season hits, all bets are off as to the stability of the site. After the fact, they can point to having done their due diligence for the site prep ... but they know that they've gamed the system.

Similarly, I've seen hundreds of residences along the Front Range where folk finished off a basement with a flooring system that was damaged by water vapor transmission ... soggy carpets (with mold issues), popping VCT, peeling epoxy or paint floors, and laminate or wood floors that are warping. If they're lucky, there's no bentonite soil beneath to aggravate the problems with structural issues ... but then again, I've found the heaving slabs and damage under the flooring system. That's the time to call in a soils/structural engineer and get the rest of the story. I'm only able to indentify the problem, I'm not able or licensed to quantify it or correct it. Sometimes, houses have to be lifted off of their foundations, the whole foundation removed and excavated out and have a new foundation installed with proper french drains and drainage areas to keep the sub-surface area acceptably dry so the foundation can be stabilized.

As I pointed out above, these mitigation efforts can be as costly as the price of the house, if not more. I repeat ... if you are seeing visible bentonite soil issues with a house now, you'll save yourself a lot of time/effort/energy by moving on to a house without the issues or where they have been properly mitigated when the foundation was placed. There are neighborhoods were every house has some issues, or just a few houses in the subdivision, and some neighborhoods where only one house has a problem. There's no rhyme or reason to why this happens, you just need to be an informed buyer and inspect a house carefully before making a purchase ... if unable to inspect it yourself, then hire a professional to check for these concerns.

Last edited by sunsprit; 08-29-2011 at 08:18 PM..
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Old 08-29-2011, 07:06 PM
 
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I guess it would help if we defined some sort of "what's normal" movement from what is truly a problem amount of up/down movement.
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Old 08-29-2011, 07:27 PM
 
3 posts, read 16,759 times
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Quote:
Originally Posted by Mike from back east View Post
I guess it would help if we defined some sort of "what's normal" movement from what is truly a problem amount of up/down movement.
One concrete slab in the garage is raised by approximately 1-2" and another slab has a crack down the middle. I also noticed very slight cracks in the wood siding, and in one spot inside. Is the cracks normal wear/tear? The house is less than 10 years old.
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