Retaining wall cracks 1

[a3a]

I was observing a neighbor's retaining wall. It is a stem wall on a strip footing (at least a foot deep). The wall ranges from 2.5 to about 4 feet high and roughly 40-45 feet. I want to say it was 8" thick at the top (possibly 6&quot. Not sure if the wall and footing are monolithic, but know that there is rebar in it.

I did not notice any type of crack control joints (and can't think of one retaining wall I've seen that had any type of joints in it). His wall is around 2 years old and it is cracking. The cracks are vertical and propagate from the footing to the top of the wall periodically throughout the length of the wall. He reasons that his other neigbor's drainage into the wall with no footing drain is the cause of the cracking. He also claims there was no cracks during the year before his neighbor built their house next to his, so the cracking have to be from the drainage into the wall and resting there with no drain.

While looking at it I don't see how a wall like that wouldn't crack even if it was not retaining soil since here are no joints in it. I've always heard and seen the dire need for joints in slabs, walks, and pavements. Is it common practice for retaining walls to omit joints?

2 other confusion points.

1. Is he right cliaming that the water is the culprit and that a drain would prevent the cracks. It is evident that water (mostly sprinklers since dry climate)is flowing beneith the wall into his yard in some sections. No server undermining of the footing, however.

2. The other is the verticle cracking. What could be the cause cracks verticle in nature if it is not shrinkage. I believe shear cracking propogates at 45 degrees.

This problem is between those two guys, but I'm curious what is really going on. Thanks. Sorry about the length.

 

[phillyfrank]

Retaining walls of any type are subject to water pressures, even if intermittent after rains, and must have drains. Also, ALL concrete cracks, unless an extraordinary amount of rebar and QC are applied. His neighbor may have exacerbated the problem with draining onto your neighbor's property, but the fundamental problem is the lack of drains. They can be drilled in after the fact, but a clean sand layer behind the wall should have been put in first to allow the water to easily reach the drains. Good luck, and sorry for the bad news.

 

[boo1]

I agree with PhillyFrank on draninage. This is one area that can be commonly overlooked, or at least underestimated, is the necessity to drain the backfill of rainwater and/or groundwater. Hydrostatic pressure can cause or induce retaining wall failure, or at least damage.

Drainage of water as a result of rainfall or other wet conditions is very important to the stability of a retaining wall. Without proper drainage the backfill can become saturated, which has the dual impact of increasing the pressure on the wall and lessening the resistance of the backfill material to sliding. Granular backfill material offers the benefits of good drainage, easy compaction, and increased sliding resistance.

Drainage systems usually utilize weepholes and drainage lines.

Weepholes actually penetrate the retaining wall and drain the area immediately behind the wall. Weepholes should have a minimum diameter so as to permit free drainage; for large walls, 4 inch weepholes are common. Adequate spacing between weepholes allows uniform drainage from behind the wall. Weepholes should always have some kind of filter material between the wall and the backfill to prevent fines migration, weephole clogging, and loss of backfill and caving.

Drainage lines are often perforated and wrapped in geotextile or buried in a granular filter bed, and serve to carry water to the weepholes from areas deeper within the backfill.

 

[boo1]

In most areas, "retaining wall designs taller than about 4 feet must be designed by or approved by a qualified, licensed professional engineer. Additionally, it is important to check with and adhere to local building codes prior to any construction, even when walls are shorter than four feet.
Designing any retaining wall requires a knowledge of lateral earth pressure, the pressure developed by the backfill. It is the force generated by the lateral earth pressure that constitutes a large part of the load that the wall must carry.
To determine the lateral earth pressure acting against the retaining wall, several soil parameters must be known in order for the qualified engineer to assess a particular wall design and its overall stability:
soil unit weight
angle of internal friction of the soil
cohesion and plasticity indices for cohesive soils (for instance, clays)
water table location.
Once the lateral earth pressures are known, the wall is checked for stability. This includes checks for wall overturning, base sliding, and soil bearing capacity failures."
The lack of drains increases the lateral pressure.

 

[a3a]

Thanks guys.

Well, I can understand that water is a big concern, and probably the biggest contributer to the cracks. I just thought there was more to it than only the added hydrostatic pressure and loss in soil stability(caused by the water) for the mentioned scale of the wall.

I took another look at their wall and it is 8" thick (no weep holes either). For a wall that ranges from 2-4 feet high, I just had a hard time seeing that the water would cause verticle cracks in such a seemingly stout wall--especially when the cracks are even present in the 2 foot high regions. I was thinking that there had to be some serious shrinkage stresses due to not having any expansion joins for a wall that long. So, I was also trying to determine if shrinkage stresses would cause verticle cracks. I will conclude, though, that the additional pressure is causing bending perpendicular to the verticle plane of the wall. Moreover, that the stress distribution along the length of the wall is not uniform (i.e. one area is collecting less water than 10 feet down so it doesn't experience the same additional streeses as the "wet" areas, hence the intermittent verticle cracks. Thus, the "perfectly" verticle cracks are no different than the bending stresses you may find on a long beam with multiple supports (in this case the beam is verticle and 8" deep). Would that be good reasoning?

Though I'm not a professional (and I kept my comments to myself), I would blame the wall's designer for not putting weep holes in it knowing that the wall was intended to retain soil around the other persons yard. The wall's owner was making a huge fuss over how it was the other neigbors responsibility to provide drainage so the owners wall doesn't crack. Is that really how it works? If some one puts up a retaining wall wthout weep holes, the other person has to go through the expense of providing for drainage not to harm the wall? I wouldn't even want to call these verticle cracks a wall "failure." I think he is mad just because it is not as pretty. My contention is that some cracking would be expected. Is that wrong?

I just found the whole thing interesting (since I'm not involved), but something I'd like to learn something from.

Thanks.

 

[jheidt2543]

If the wall is in the Northern part of the country, in the winter that water behind the wall turns to ice. The pressure of frozen ground behind the wall is a considerable force (more than the hydrostatic pressure). There are many a retaining wall of concrete, masonry and stone that get pushed over by the force of ice behind the wall. All the drainage comments of those above are on the money, even more so in areas suseptable to freezing.

 

[emmgjld]

The above discussion has been good regarding drainage and pressures against the wall HOWEVER, the original question has not been answered.
YES, the cracking in the retaining wall may very well be dur to concrete shrinkage. I have observed both small and large concrete retaining walls, decorative walls (no soil load) and most basement walls suffer from regular, nearly vertical cracking, with no appreciable displacement. Normal concrete must shrink and if provision is not made, cracking is bound to occur. Indeed, it should be expected.
Keep in mind that actual cracking may not appear for quite a while but, the stresses are present within the mass. The cracks may appear after minor settlement, extra lateral loading due to poor drainage or changes of the days/seasons (thermal)
Please note that poor or lack of drainage behind a wall may the the 'final straw' but, shrinkage stresses are probably already present in the concrete mass. The next issue is wheather excessive movement is occuring and are the cracks exhibiting appreciable displacement. Now the plot thickens.

 

[boo1]

EEMLGLD has a good point. A pour mix or inadaquate reinforcement (rebar) can lead to shrinkage cracks.
But a wall of 40 ft long x 2 ft to 4 ft high x 8 inches should be able to resist shrinkage stresses. As Frank and I indicated most retaining wall faults the culprit is drainage.