Need help confirming if this is a structural issue

Just got back from an inspection here in Parker, co. The basement has a raised subfloor with acres goes into the below crawl space. The front driveway and garage have severe erosion and cracking. At the back of the house where the walk it basement is, it’s at the bottom of a steep hill. From the outside I don’t see anything fishy but when I go into the crawl space I can’t tell of this foundation is ok it if it has erosion. Please see pics.! It doesn’t look normal to me. But with the raised subfloor maybe it is? 20210202_135203|375x500

That is void form. Placed during construction of the concrete foundation. It is designed to disintegrate from moisture after the concrete has been cured to create a void for soils to expand. But in Colorado we usually don’t have enough moisture to help it disintegrate properly.

The main issue is if it maintains its form there is no void for the soils to expand. I’ve seen cases where this has occurred and the foundation was cracking due to the soils. If there is concern someone can just go in there and remove it to create the void.

5 Likes

https://voidform.com/

2 Likes

That makes sense. I did tell my client I wasn’t sure it was an actual issue buy that I’d check and get back. The main thing that looks scary is that there is nice poured and shaped concrete and then you have this jaggedy crumbly looking area. Thanks for the help.

This is something to special attention to, especially on a steep hill. Narrate it into your report to protect your buyer and yourself.

3 Likes

That foundation is grade beam sitting on caissons. Typical for CO, especially when soil reports indicate highly expansive soil. That is also why the raised structural floor in the basement. Sometimes a high water table will have an engineer call for one as well.
There should have been 1/2" to 3/4" gravel bedding a perferated foundation drain or “drain tile” as some would call it level with or just above the SUREVOID (brand of void material) at the base of the wall/grade beam. The void is basically wax coated cardboard and will decay over time. It’s ugly but not an issue.
I did notice in the 3rd picture a crack in the wall with signs of water penetration. A rusted form tie and what appears to be rust on the void material.
Could be several reasons why, but might want to have it investigated further. What were the conditions under the paper barrier? What was directly opposite of the wall on the exterior where the crack is located? Do you have pics of the outside in that area?

2 Likes

There was thick plastic spread out over the whole crawl space. There did seem to be some gravel underneath of it. And a radon mitigation system. Directly outside was the base of the steep hill. It evened out to a almost level but a very slight positive grade still right where this picture is. All gravel. Looked generally ok.

That plastic should have been taped to the foundation and at any seam also. Especially if there was a radon mitigation system.

The void form essentially turns the wall above it into a grade beam. That is why the code requires a solid, continuous footing (with no void forms). To span an opening like the one shown in the pic an engineered design is required. Notice the crack in the foundation wall above the void.

This statement is incorrect for this location and construction. The foundation walls/grade beams are supported on drilled piers. Pretty common in COLORADO and in this part of PARKER COLORADO.

1 Like

The building code adopted by Parker Colorado calls for solid, continuous (no void form) footings in section R403.1. To install a drilled pier type foundation would require an engineered design.

Russell - I’m not trying to argue with you because that would be pointless for both of us and it doesn’t help the OP or address his original question. In my experience in the Denver Metro Area all plans for commercial or residential construction get reviewed by the AHJ plans examiner and get approval prior to construction.

We don’t typically follow IRC or IBC strictly on the foundation because a big factor in construction out here are the underlying soil types. They vary greatly across the Denver Basin, and up and down the front range due to our geology. Soils are a big enough factor here, that we have a State specific test performed here that is not part of any standard including ASTM standards. The other factor everywhere is the cost of construction.

A little history on the foundation construction in the Denver Area. This is very general but is based on age and location of the house and my understanding on the area. It is not exhaustive and I only use this so I have an idea of what I may expect to encounter prior to arrival onsite.
*Most houses constructed pre-mid 1990’s were constructed using continuous footing construction methods. Houses constructed in the later 1970’s through the 1980’s were constructed on continuous footings and in areas that have potential high expansive soils, some of those houses were torn apart due to the expansive soils. Mid 1990’s through early/mid 2000’s most houses were constructed using drilled pier foundations with grade beam/foundation walls and crawlspaces. The reasoning being the builders were a little more cautious, the areas being built up had potentially high expansive soils and the cost of earth moving was high. (Side note: This is especially true of the Westwoods area of Arvada and Highlands Ranch. Broomfield had a boom starting around 2004 where they were overexing instead of drilling piers.) Enter 2008 basically most construction for housing slowed to a crawl, and a lot of earthwork contracting companies downsized or went out of business. We had a period of about 4 lean years. 2012 due to demand for more housing a construction boom started and exploded around 2016 which we are currently in. Earthwork contractors were cheap due to a lot of factors so instead of building houses on drilled piers, it again became more profitable to overex entire neighborhoods 10+ feet to try to mitigate the soils. Plus builders started adding 10 year warranties on the structural components of the houses. And we are back to continuous footing construction.

This is very general because I know there are outliers where houses built in 2015 have drilled piers. And it is location dependent, I wouldn’t expect to see a house built in the Lowry or Green Valley Ranch area of Denver or anywhere within a couple miles of a river bed, no matter the age, built with drilled piers. It’s mostly sands and gravels, not expansive clays. Like wise most houses built in the Highlands Ranch Area as stated above are built on drilled piers.

Just trying to help educate if anyone else has additional information I am all ears to learn more.

9 Likes

Regardless of what any code book says, foundation designs around here are based on soil reports. In some cases voids are in fact used under spread footings if the soil reports indicate expansive soil and the dead load of the structure may not be greater than the hydrolic forces of said soil. For example:


As you can see, this spread footing foundation has void under the footing in various spots. This set up allows for the soil to expand horizontally away from where the footing is in direct contact with the soil and for vertical swelling in areas where the dead load is minimal. This home had a home to one side that sat on caissons/piers and grade beams while the home on the other side was spread footing without void at all. 3 lots spanning about 200 feet in total at the street. Same builder, similar floor plans and building footprint, but very different soil composition from one end to the other.

Tony is 100% correct with this statement. If you travel 40 to 60 north of where he is, what is under ground can vary from highly expansive clay with a high bentonite content, to clay mixed with gravel, to silt stone, to sand stone with vertically configured shelves and layers due to volcanic upheaval, to sand and gravel river bottom, to literally sand dunes covered in prairie grass and sage bushes (which some of these dunes consist of super fine blow sand with what seems to have no bottom. I have buried many o big rig in them so I speak from experience.) I have even been on job sites where underground creeks and streams were found when drilling for piers and excavating for basements. Talk about an instant swimming pool.
This afternoon I was in an area in NUNN, CO where the soil looked like this gravel filled dirt:

And just 2 miles away it looked like:


Full on mixture of everything. Sand, clay, rock, shale, and old fashioned dirt.
So as Tony pointed out down in the Denver metro area the soil types and conditions can vary quite a bit. Move north and east it varies even more. From Boulder to Brighton to Cheyenne and everything in between the only thing that is consistent is the inconsistency of the soil conditions, so builders have to adjust building designs as they go.

2 Likes

@jobryan, If you can, drive around some new subdivisions down there where different foundation styles are in the process of being built and a lot of what Tony and I said will make more sense. Especially if you can find some where drilling is going on. Look for a pile of what looks like boxes with markings similar to this:


This is what the void material looks like before it rots away…

3 Likes

Jeremy,

I found some more info that may be of help to you: Void Forms Concrete Foundation Applications

1 Like

All great info. Thanks for the help

2 Likes