Basement Construction

[Steel2Steel]

A friend of mine is about to build a house. He has a basement that is fully buried on one side, about half of the side walls are buried, and the rear is completely exposed. I have a couple of questions.

1. What construction is better, CMU or Concrete? Obviously the Concrete is more expensive but as far as waterproofing goes what would be the better choice. He is near a lake so the water table is likely higher than on other sites.

2. Once the wall becomes completely exposed, say the rear wall, should you switch to a timber wall for economical reasons?

Thanks in advance!

 

[concretemasonry]

Both concrete and reinforced concrete can easily be built to resiat the lateral loads for a "normal" configuration basement wall.

It is not realistic to be able to expect a 100% waterproof basement if there is a high water table. Whether you use concrete or concrete masonry, you will still have a joint between the wall and the concrete floor slab that usually shrinks and pulls away from the wall.

For either material, because of the proximity of water, it is absolutely necessary to install interior or exterior drain tile (or both) at the level of the bottom of the footings. Enclose the tile with a "drainage sock", surround the pipe with granular soil and backfill with granular fill material. Cap with clay or an impermeable material and make sure you have positive drainage (away from the home). Control the roof drainage. This is not a big investment if it is planned and done during construction and not as an after-thought. The advantage of the interior drain tile is further insurance and better control of any moisture/pressure under the slab.

One construction problem to be concerned with is the excavation for utilities AND the access ramps for excavating equipment that can actually collect surface water and funnel it back to the foundation where it must be removed by a drain tile system.

dick

 

[msquared48]

For the rear wall seeing a full earth load, I wouldextend the top of the concrete to the underside of the PT sill plate below the floor joists. To stop it short would create a knuckle joint in the wall system leading to lateral deflection and instability.

If the level of the earth above the basement does not exceed four feet, then stopping the concrete with a wood pony wall afove is OK as the lateral pressure is not too great. Any higher with the soil, either use a non-yielding concrete retaining wall design (if you want to stop the top of the concrete short), or carry the concrete wall full height to the bottom of the wall plate below the floor joists.

Note, if the force is too great from the soil, you may need to hang the joists off the sill plate with top flange bearing joist hangers. And, if the joists are parallel to the wall, you will need to provide blocking for the first two or three joist bays four feet or so on center, depending on the soil load to the floor diaphragm.

Hope this helps.

Mike McCann
McCann Engineering

 

[SlideRuleEra]

As stated above, constructing a waterproof underground concrete structure is very difficult. IMHO, the most cost effective and easiest to use product is prefabricated, bentonite filled sheets of cardboard. These are simply fastened to the exterior of the concrete. Here is a link to a typical product:

http://www.cetco.com/bmg/Products/Waterproofing/Volclay_Panels.htm

http://www.SlideRuleEra.net

 

[twinnell]

msquared48,

I don't think he is saying he is going to start with concrete or cmu and then switch to timber vertically. I think he is saying that as you walk around the perimeter of the house, the grade slopes such the back of the house is a walkout basement and since it is fully exposed and not retaining any soil, would it be preffered to switch to timber rather then continue with cmu/concrete. Picture a square box, three sides cmu or concrete, one side wood.

 

[FinnB]

concretemasonry

I was interested in the part of your above post where you mentioned using drainage tiles and particuary using them internally to control moisture and uplift on the basement slab. Am I right in thinking that with internal tiles water is allowed to pass through the basement wall and is then collected in a floor drain. How effective is this system in relieving any water pressure that may build up behind the wall i.e. between say an impervious clay and the concrete wall. Do the tile companies advertise that internal drainage tiles wil significantly relieve water pressure.

Could someone post a web address for a drainage tile manufacturer. Thanks

 

[twinnell]

FinnB,

I think what concretemasonry is saying is at the exterior of the wall footing, place a foundation drain wrapped with sock and covered with granular material. At grade, cover the granular material with clay to keep rain water from seeping into the soil.

On the interior of the footing, also install a foundation drain wrapped with sock and covered with granular material. No clay is needed here.

 

[concretemasonry]

With interior drsin tiles, I was referring to the practice of installing a drain tile/sock/ rock system along the interior side of the foundation footing. This is commonly done when the exterior is not accessible, when the floor plan is not condusive to an exterior system OR to collect moisture from under the slab by drain tile under the slab or through a layer of permeable fill under the slab. - The water would be collected the drain tile and fed into a sump/sump pump (commonly) or drained to daylight(which is rare). In both systems, the water is controlled, collected and disposed of before any leaks can be caused.

During new construction with a CMU basement the cores of the block bearing on the footing may be pierced (or special block are used) and plastic tubing is installed to direct any possible core leakage into the drain tile area.

The purpose is the same as the exterior drain system - that is to collect water and releive pressure that can cause structural problems or causes leaks through problematic details such as the slab/wall joint. The "floating" slab poured on the footing will always shrink and pull away from the wall.

I have seen interior, exterior and sometimes both on new construction. I know of one builder that vowed never to have a basement problem that always used both interior and exterior systems, usually tied together on ever home he built. He felt it was much cheaper during new construction than having a problem later. I never heard of any of his more than 10,000 homes having a problem.

 

[FinnB]

concretemasonry

I have used land drains externally on top of basement footing and also perforated drainage pipes under the slab to reduce water pressure and to control leaks. Unfortunately in Dublin, Ireland where I work the local authorities will not allow ground water to be pupmed into local authority drains so unless soakaways are viable there is no way of disposing of the pumped water.

I haven't herad the term drain tile before so I am having difficulty picturing what it looks like.

 

[FinnB]

Twinnel

Could placing clay over the external drain lead to water being trapped between the basement wall and the clay backfill. What if the drain blocked and hydrostatic pressures built up behind the wall and under the slab and the "floating slab" really did float due to water uplift.

 

[twinnell]

I beleive the clay is at grade level. It is placed above the granular fill. Foundation drains can get clogged. Then you have a failed system. You try to keep them from getting clogged by wrapping them with fabric and then encassing them in gravel and backfilling with sand. The few inches of clay at grade just limits the amount of rain water that percs into the ground at the foundation wall.

 

[concretemasonry]

Finn -

We have the same regulations in much of the U.S. There is a requirement that the drained ground water cannot be pumped into a drain line leading to a sanitary waste water treatment facility without special approval.

Some countries allow it to be pumped into a gray water collection system.

In the U.S., the proper disposal if a municipal line is to available, is to pump it outside a proper distance so it can be absorbed by the soil or run off since it is really surface water unless you have an unusual situation.

The term "drain tile" is just an American term that has developed over the years. Originally, short sections of perforated ceramic tile (100mm+) were used without tight joist to collect water around building footings, to intercept shallow ground water or to darin low moist fields. Now, a 4" PVC pipe is common with holes along the length at 4:00 o'clock and 8:00 o'clock. It is surrounded by a well graded, cleam granulat fill material. Frequently, a fabic is used around the PVC.

 

[FinnB]

Thanks guys

I am interested in how basements are drained to avoid buildup of water pressure. The opinion in the relevant code over here is that you cannot predict how water levels may rise or how water can buildup between impervious soils and the basements walls and slab. To design for the full life of the building the recommendation is to take a water table level of 1.0m below ground level. This makes for deep floor slabs or the use of rock anchors. Do you have similar requirements.

concretemasonry

The problem I often encounter is that clays don't absorb the runoff and give poor soakage. This problem becomes a headache if the site is in the city centre were only soakaways are possible to drain water back into the ground. Don't know if you use the term soakaway but what I means is basically a large hole in the ground filled with stone, the stone being wrapped in a free draining geotextile. I like the idea in clays of using free draining stone behind the retaining wall with a pumped perforated drain at the base to relieve water pressure but where I am practicing this doesn't seem to be an allowable solution any more considering the full design life of the building.