Expert: Daniel Humphrey - 1/12/2012

Question
QUESTION: Life to me is simple, you plan for the worse and hope for the best. In bad soil with clay content how deep do you bury the footings? If my house has problems moving up and down then the pier and beams need stability, right? Ive heard some people dig under the house and pour concrete then put the blocks back under the beams, if that is true how deep do you dig to pour concrete 1 foot, 18 inches? Any advise that you can offer will be greatly appreciated.

ANSWER: Dear Danny,

Digging down and pouring a continuous footer and concrete stem wall all around the house is the best solution, but the most expensive in the short term. Banks like this feature, homeowners like it, and future buyers like it. The footer spreads the weight, and the stem wall adds rigidity and mass so that the whole wall must lift as a unit. If the wall does lift, the change is not as noticeable because the rest of the house goes with it. To support the mid span of the floor, another footer and beam are poured. This floor support is sometimes shimmed with paired wedges so that it can be adjusted if the floor should lift or sink in the middle.

Since you don't have frost to worry about, the depth of the footer doesn't matter much unless the soil has big differences close to the surface. For example, if there is hard pan within three or four feet, then you want to dig down that far and use it as a foundation. If you have a high water table, then you generally want to stay above it at least a foot.

One alternative to a continuous footer is a system of piers with belled footers. The footer is bell-shaped to resist up and down movement, and is generally buried deep enough (6 or 8 feet) to provide some hold-down. I have had good success in clay soils with this method as well. Sometimes, the pier supports are adjustable also. When the weight of the house bears down on the piers, some adjustments are often necessary until the system reaches equilibrium.

Simply adding more concrete beneath existing supports will help only slightly unless the pier footers are buried very deep.

Images and specifications of continuous footer/stemwalls and belled piers are easy to find with an internet search.

I hope this helps.

Daniel

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QUESTION: Thank you very much for your answer. I need to give you better informatio so that you can narrow down your answer to my problem.1. my house is about 1400sq.ft.2.I have blocks 2 high completely around the house[about18 inches high] they rest on a concrete footing thats about 1 ft. deep. 3.Last july when I had the house leveled the cracks disapeared, It was about 1/2 inch low in the back part of the house. 4. the house came with 2 beams and I had another beam installed in the center of the house for support.5. the dirt that is about 5 ft. away from house holds water when it rains[its saturated]. 6. the dirt under the house is bone dry and hard to dig.The man who leveled my house came today to check and found the house droped about 1/2 inch to 3/4 inch along the back beam[1 of the original beams] when he leveled the house he added blocks to the beams I have a 18x18x4 base block with clinder blocks on top every 6 ft.The house dropped almost overnight after 2 rains and a cold snap. I would greatly appreciate your help thank you   Danny

ANSWER: You probably are safe in assuming the sinking is due to the clay getting wet and being displaced by the weight of the building. When this happens, the sinking is permanent because the clay has been squished out.

You should dig a test hole out where the ground gets wet and determine whether there is a dry layer somewhere deeper. If you do find that the ground is dry a couple of feet below the surface, then that is where the footer should be.

To prevent clay displacement, you must either keep the clay dry or make the footer large enough to spread the weight of the building so that it can't displace the clay. Most solid, semi-dry clay can support only one ton per square foot. This is sufficient bearing capacity for most houses, if the foundation is a perimeter footer/stemwall. Figure that the average, occupied, slightly built wood-frame house weighs about 200 pounds per square foot, then your house (concrete, cinder blocks, and all) might weigh about 140 tons. You would need a total bearing surface of 140 square feet on top of the clay, if it is semi-dry. If the clay is saturated, then the bearing capacity could be almost nothing.

We also have expansive clays, which swell up when they get wet! You sometimes see a problem with floors bulging after a crawl space is flooded. The perimeter in this case is not sinking, but the previously dry interior is puffing up.

Adding more footer area and keeping the water draining away is the most practical solution for an existing house.

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QUESTION: Dear Daniel, Thank you for taking time to answer my questions, with your help I will get the job done. The man that leveled my house came out sat. and checked the house and found that the back beam had dropped from 1/2 to 3/4 of a inch. the 3/4 drop was close to the center of the house.Its only been 5 months ago that it was level. I dug a hole in a wet spot in the yard about 18 inches deep and it was moist at 18 in. We have had 3 inches of rain ths week. I talked to a man that built this house and he said that the footers for the cinder block were 1 ft. deep and they should be deeper and the land is very wet. I estimated the footings under the house and came up 87.17 sqare ft.,that includes the blocks around the house. How many footing will i need to put under the house to do a good job? Im told that I will have to cut holes in the floor of the house and dig down to dry dirt pour concrete then put blocks on concrete. The back beam is the problem, thats why im asking how many concrete pads do i dig? thank you, danny

Answer
Danny,

Put the spread footers where the support is needed most. If the footer beneath the back beam is sinking in the middle, you should put more support there.

Concrete block stemwalls don't really span anything, unlike reinforced concrete stemwalls. Cinder blocks require a broad footer and really good soil with lots of bearing capacity, which you don't have.

I would put most of the effort into the perimeter, rather than the middle. The house is constructed so that the majority of the weight is on the perimeter. Interior floors just need some support to keep from bouncing.

Some things you can consider doing:

1. Increase the footer area along the perimeter walls. You can dig down below the existing perimeter footer and add spread footers that support the existing system. Try to add at least 60 more square feet of bearing surface near the perimeter.

2. Replace the cinder blocks with a real concrete stemwall with lots of rebar.

3. Beef up the back wall so that it has more shear strength. This means adding a layer of sturdy plywood to both the outside and inside,gluing them to the studs. Plywood shear walls require the plywood to be applied with 8 foot length and 4 foot height, and none of the seams can line up.

If you have any other ideas, feel free to pass them by me.

Daniel