Block foundation failure 2

[Hammerslinger]

thread507-73922

I've been reading about block basement foundation failures and it seems the consensus is that concrete is better.

I have to disagree. Mostly the failure masonry occurs because of undersized block, lack of vertical and horizontal reinfocing along with improper backfilling.

I speak from experience having built masonry basements. Using 10 inch block with a #5 40ksi bar in a grouted core every 4-5 feet along with horizontal truss type galvanized steel every other course. I installed the vertical steel after the wall was built. Backfill with gravel up to the frost line. In Wisconsin that is usually 4 ft. below grade. Finish the backfill with soil, which acts as a frost insulator.

I remember building a home with my method of block wall construction while a neighboring builder was building with concrete. A few weeks after his house was framed and backfilled I went over and he showed me an ugly vertical crack from top to bottom in the concrete basement foundation.

I have to say I snickered. I could see that the span of that wall, even for reinforced concrete was too long.

As for my block foundations it's been 20 years for some and none have failed even with the heavy clay soil we have in Wisconsin.

I do see many foundation failures with block wall construction and it's usually due to no vertical reinforcement and improper backfilling.

My father-in-law had to have his block basement repaired with structural steel beams at the cost of $27,000. Before that one company installed the anchor system which failed. Most likely it failed due to soil and concrete slug shear.

My basement is built with 10 inch block. It is dry and there is not a crack in it.

Semi-retired builder--carpenter/mason
Structural engineering Tech.
Former building inspector

 

[oldestguy]

Sounds nice. Unfortunately many a block wall is done without any reinforcing. Some distort or worse, while many get by and no problems. Pat yourself on the back, good job.

 

[Hammerslinger]

I imagine one day in the future the powers that be will outlaw the use of block for basement construction.

 

[oldestguy]

I have to differ with the thought block walls will be outlawed. In Wisconsin the building codes are pretty much written by carpenters, masons, etc. on state committees. I have much experience studying and installing perforated pipe drains for wet soil conditions, including the main subject of my Master's Degree thesis. All of this info, etc tells me that the backfill to these drains should be concrete sand gradation to prevent plugging, etc. Well the code requires coarse aggregate as used for concrete, coarse gravel!! I met with the state committee that writes that code. Well the code requires coarse aggregate for concrete, coarse gravel!! I met with the state committee, made up mostly of plumbers.
They were nice enough to listen to me, but no change was done. Reason: "We have been using gravel for years and don't know of any problems". Problem is they build the systems, but are not called back to fix them later.

So any committee handling use of block likely will not change, since they don;t get back to fix what they built.

 

[Forensic74]

Over my years, I have seen a handful of dramatic block basement wall failures. ie, half of the back yard ends up in the basement. Unreinforced CMU walls are common and allowed by the IRC; however, when I see them fail, is because they maxed out the allowable backfill height and didn't pay attention to wall drainage. IMHO, while IRC requires wall drainage, it does a poor job conveying it.

Do realize that "heavy clay" seems to work better with block walls. The failures I see are typically a sandy loam type material.....A backfill with poor cohesiveness that can readily place a lot of out of plane load on the wall.

Personally, while I dont think block walls should be outlawed, they should revise the tables in the IRC to assume the worst soil type and have a clearer requirement for foundation drainage.

 

[BridgeSmith]

Any given block or concrete wall has a particular capacity depending on how it is constructed and reinforced. Where higher strength is called for, generally concrete is preferred, since the continuity is better and it is easier to add reinforcing where it is most effective - near the faces. Where a block wall is adequate, it is simpler and generally cheaper to construct.

Either type will work fine if properly designed for the loading it will experience; both will have problems otherwise. The problem with the "neighboring builder" wasn't the material; it was inadequate reinforcing due to an improper design (or more likely assumptions substituted for a proper geotechnical/structural design).

 

[MotorCity]

In most cases, a wall can be cmu or concrete if properly designed and constructed. I think that a cmu wall is inherently more susceptible to failures than a concrete wall simply because each head joint and bed joint is a potential "built in" failure surface. I agree that cmu sometimes gets a bad rap when compared to concrete.

 

[oldestguy]

Somewhere here we need a discussion as to proper backfilling of walls, not only dealing with soil types as well as backfilling procedures and what can be done to minimize pressures. It is a subject that seems to be plagued with ideas that are either of no value or even bad practice, including many listed above.. For what it is worth I did an extensive study of filling against a wall some 30 feet high using sand. It was possible to see increases of pressure on the wall at intervals up the wall when compactors were working near the wall as high as 18 feet above a pressure measurement. Also in order to have compactors not affect pressure they had to be over 3 feet from the wall. Generally that uncompacted zone does not settle noticeably later due to the silo effect.

 

[Hammerslinger]

Wouldn't the angle of repose of the backfill material provide for a fairly accurate calculation for wall pressure?

A soil test could reveal those results. The problem is most home builders don't have or know what to do with that information. Rarely have I ever seen a foundation for a house designed by an engineer.

And now many who repair failed foundations guess at the structural member used to brace the sections of the wall.

 

[oldestguy]

Hammer thrower: Angle of repose is for loosely dumped granular material with no cohesive content. Compact it and its friction angle (not angle of repose because it may stand without support at times)can be much higher. Take a fat clay, it can have zero angle of repose at high water content.. What I am getting at is the wide range of soils (gravel to fat clay) and the friction angel as well as cohesion (the "glue" that holds particles together) can be highly different as to pressure imposed on walls. How about soils that take on water and swell? Then along comes another factor, such as water from rain or melting snow, along with buoyancy when saturated or loss of cohesion, etc. Then comes drainage differences and possible plugging of them. One of the worst situations is the wall acting like a dam and holding saturated soil, a very heavy liquid. Jetting of backfill can cause this also. Thus for the non-engineer practitioner some general rules are the best we can hope for, no computations needed. That's where building codes come in, if at all. As you know even those "rules" don't always get followed, especially by do-it-yourself home owners. Unfortunately they usually go for block walls, built by them. Ask them to install re-bars or reinforcing in the horizontal mortar beds and you get a blank stare.. Even worse vertical re-bars in the cavities. How do you get anyone to haul in special backfill material when there is plenty of soil on-site that came from the excavation? Right now there is an experienced builder starting his own house in the next block. Due to everyone very busy here his job had been delayed. He has foundations in and one main wall up in below 32 degree weather. Ground is frozen and weather is not cooperating. He is concerned the footings will heave up some before he can enclose things. Partial wall backfill had him add bracing on inside today in case that backfill pushes towards the non-filled side. Hopefully it works. .Ain't this subject a fun one? Full of many factors to consider.

 

[Hammerslinger]

Action fueled with knowledge is a great thing.

 

[oldestguy]

OG with one more, since frost depth was mentioned by poster. Here in Wisconsin we have a variety of soils both gradation and particle sizes, as well as frost heave potential. The code for frost depth is only very rough. Frost heaving potential depends mainly on the silt percentage (roughly). Water content also is very important, since when freezing, water gives off heat, negating to some extent freezing front going deeper. Clean sands don't heave. Silty soils usually do heave, if water is present. So least depth of freezing here just north of Madison in water saturated soil never reaches 4 feet. But silty soil here still heaves plenty. In LaCrosse, in the clean sand there freezing front easily goes over 8 feet deep even before Christmas. But, little to no silt, so there is no heave. So how deep the frost goes depends mainly on water content.

So as to walls and heaving causing high wall pressure, clean sands are great, but silty soils not so good. Whether block or concrete in Madison with silty sands, etc. and high moisture content, residential basement walls do push in usually with a long crack horizontally at mid height. Interesting to see those walls with insulation are worse off due to no heat lost from inside negating the advance of freezing.

 

[XR250]

Many basement wall failures I witness are due to a lack of bracing of the wall at the floor system - either by too few anchor bolts or the joists being parallel to the wall.
Concrete walls seem to hold up better to these dearths. I think the IRC is deficient in regards to basement wall bracing as well. I believe it states they need to be braced at 8 ft. intervals?

 

[oldestguy]

We (I) have worked this subject pretty much out of the main topic of wall types. However, the subject of "frost depth" might need a little more space here. In the case of how deep to set footings so that they don't get heaved due to frost heave, remembering that frost heave is due to three things. You need water, usually plenty, you need below freezing temp here, and you need soil that will develop frost lenses that grow and lift. Knowing that water gives off heat of fusion at a given number of calories per unit of volume, the more water per unit of volume, the higher the water content, the slower the rate of frost front moves down in soil. Thus, from a practical standpoint, tying in area cold weather history, snow cover,etc. as well as soil type (frost heaving susceptible or not) and common water content (fine grain soil holds a lot more than coarse grained), experience shows that setting footings below a given depth, places them below commonly experienced freezing depth in frost susceptible soil. Freeing temps going below that depth due to low moisture content,as in clean sand, do not cause frost heaving. Thus soil grains with a percentage passing the 200 sieve exceeding 5 % do heave, more as thus number increases, to a point. Getting into the finer grained soil in the clay ranges, the permeability is so low that frost lenses cannot grow. That's why fat clay areas may have some frost heaving, but under similar circumstances, those more in the silt range do heave the most. Theoretically in clean sandy soils with no frost heaving there is no reason to worry about shallow depth footings, but codes can't get that detailed. Unuff said!!

The comes water mains. They have to go below the depth of freezing. So In LaCrosse, WI, they gotta go very deep.

 

[charliealphabravo]

The Canadian codes have this table. I was a bit surprised to see how many "No Limit" entries there were for minimum foundation depth but it makes sense with OG's explanation.

Edit:

Do realize that "heavy clay" seems to work better with block walls.

I'm not sure I follow this. I think whatever you gain from soil cohesion you lose with the clay being much heavier that the kinds of soils the prescriptive codes were written for. I have never understood how clay is allowed as a backfill material when prescriptive codes only speak to the equivalent fluid pressure of granular and silty soils. And then there is the matter of active clays.

 

[Hammerslinger]

So with all this discussion does anyone know what the force in pounds per square foot acting on a basement wall with a 7 foot backfill of Sandy clayey soil?

 

[oldestguy]

OG here. Force on wall depends on what you do with the backfill and type. We usually refer to the backfill properties as if it was a fluid, calling it an equivalent fluid. Soil commonly has a unit weight somewhere between 90 and 140 pounds/c.f. Under some circumstances the pressure equivalent fluid can reach those numbers, such as due to compaction of that backfill. There also is the effect of the wall moving some to this pressure,giving the lowest pressure, called active pressure. However,loosely placed, or dumped the active pressureis in the range of 25 to 35 pounds per cubic foot equivalent fluid density. A common number for designers is one with a little compaction during placement and the wall doesn't move at numbers about half of the soil's density, meaning around 50 pounds per cubic foot equivalent fluid density for design, called "at rest". This is a common number used for most backfill. If the backfill is clean sand, the number might be a little lower. If it has a lot of clay, it can be somewhat higher. If one is using compactors, that will increase these numbers to as high as the actual density of the material. Keeping compactors at least 3 feet away from the wall is a good rule to follow to avoid this increase. The above is a simplifying description, since among the factors moisture in the soil affects this and some soil actually expands when wetted. Keep it drained and that is the best situation.

So for a wall backfill 7 feet high on the outside at 50 #/cf, equivalent fluid density, the horizontal push, calculates to 1,225 pounds per foot along the wall. Fortunately most walls have factors that reduce the bending effect by such factors as weight from the structure above. Imagine the wall is a stack of children blocks. If you put a load on a column of those blocks, you have some difficulty pushing over the stack. That's part of the reason unreinforced block basement walls manage to stay up.

 

[oldestguy]

This post has brought out both good and not so good things about basement walls built from concrete blocks. One thing missing now is how do you design these walls and add reinforcing that will do the most good? In addition to actually designing the basic wall as to block size, reinforcing and where to place it there must be some attention to loading. That loading is not just the backfill material and HOW TO BACKFILL,and horizontal stresses imposed, but the structural loads from above. Those walls with floor joists sitting on them plus walls and loads from above, such as second floor and roof.. Then comes also the walls with no floor joists but only a wall from the house. While Hammer-thrower gets by with reinforcing placed by guess and luck, it would be much better if there would be a system which follows some design logic as one would do with a reinforced concrete wall. For instance make a horizontal cut view in the block wall showing the re-bar(s), the block pattern, and any grout filling. Then show the stresses exerted on all... Ya big deal.. How? Does any grout carry compression? is it complete or with voids? Do the rods wonder back and forth from compression side to tension side, and how effective at those locations?

A bunch of questions, but can they be resolved. This sort of puts the block alternative at a disadvantage compared to concrete before we even try to design it. However maybe some one here can put fort a list of steps for design that can simplify things. I know experienced persons can pretty much build these with reasonably good results, but possibly over built never really kniowing how safe..

 

[Hammerslinger]

Oldest guy. I didn't just get by with a guess or luck. I happen to be an engineering school graduate and did a bit of investigation and design before building.

Building a block wall isn't rocket science. It takes a little know how and a little common sense. What mostly needs to be known to novice builders is that concrete and masonry are both weak when it comes to tension. Strong on compression and that is what most think about when it comes to building with concrete, stone, and masonry.

Vertical steel placed within the cores of a block wall will provide at least a minimum of tensile strength pick up. And prevent the buckling and separation seen in so many block wall failures.

I don't claim to know everything about the subject but have the experience and education to render a satisfactory example of the construction practices.

 

[oldestguy]

OG here. OK, fine so there was no actual formal design procedure for the walls of the subject post. My question for others is perhaps setting up a design procedure for reinforced block walls. What does the other home owner or contractor do now for how many vertical bars and their sizes and spacing? Sure we can set up some "standards" that work for most cases, but for me, I do not recommend "cook-booking" something that is claimed to be engineered when my livelihood can be totally ruined by a failure resulting by this "method". These days of law suits coming for all sorts of things really needs attention. This is especially true for an engineer involved with a project that has a catastrophe not caused at all by him, yet the attorneys bring in everyone hopefully getting the most judgement for his client. This again is just one reason for designing a reinforced block wall versus cookbooking it. I'd summarize by saying the safe way is engineer it, whether block or concrete.