You may save a few dollars by using an assumed bearing value, but you take on the liability and when problem arises, and it all get dumped in the court, you become neglect. If you assume a low value, it costs the client, and if you assume a high value, you get movements.
The issue with foundations is usually to much movement. Often we expect movement, and then you have a choice, build, and allow for the movement, or not build. Some of the time, we can switch foundation types, and reduce movement to an acceptable level.
All foundations move. Movement is a response to changing load, and to often, changing moisture regimes. It is limiting these movements to an amount that is acceptable that is the trick.
Our building codes are out to lunch in some respects. It calls for foundation design utilising ULS, which should never control in soils, SLS will always govern. But the building code is written by a bunch of structural engineers, who do not understand soils. They force us soils types to adapt, to take what is right and bend it to fit a wrong code.
Wednesday, September 26, 2012
Friday, September 21, 2012
Engineered fill
Engineered fill is what we call the process of placing clay to support small buildings such as "traditional" houses. What is being built is far from traditional. And then there is the wet soil issues, where 0.6 metres of dry high plastic clay is placed and crusted over with a packer. The remainder of the fill is placed. Now what?
At the best, with traditional houses, we expect 2 inch of differential movement. Now a few construction problem/errors, tight basement finishes, and what do we have but houses who do not meet the owners expectations, but are within the expectations of the land developer and geotechnical engineers.
Such fun.
At the best, with traditional houses, we expect 2 inch of differential movement. Now a few construction problem/errors, tight basement finishes, and what do we have but houses who do not meet the owners expectations, but are within the expectations of the land developer and geotechnical engineers.
Such fun.
Tele-posts
Tele-posts, adjustable columns in the basement of most modern homes are to account for differential movement between different foundation types, and they are there for adjustment. They should be adjusted as required to keep the main floor straight. The basement walls must be constructed to allow for adjustment, and not to carry any vertical load to the basement slab. This will cause uplift of the floor, and partition cracking at doorways and other points of stress concentration.
Monday, September 10, 2012
Ice Jacking
Ice jacking is a phenomena that occurs when water is frozen, water is drawn to the freezing front through the soil, and freezes. There is also lateral frost jacking where water ingresses into a crack and freezes, producing large lateral forces. The amount of movement can become substantial when partial melting and cyclical freezing occurs.
This can occur in a number of situations. The typical one is in old stucco homes with no insulation and vapour barrier in the joist space over the basement. Water condenses and freezes, expands, and produces lateral force. The sun hit it, it melts, runs down, freezes and expands. You get the picture. Beam pockets over basement have a similar issue. Slabs outside have the same effect happening next to houses.
This week I saw a new occurrence of this, inside a unheated insulated garage. Cars drag in snow and it melts, and condensates on the grade beam. On nice days, this melts, runs down into the space between the grade beam and slab an freezes. This provides force, and in this case, after the slab moved a bit, it pushed the grade beam out of alignment.
The solution is not cheap. Heat, to prevent freezing, is one solution. But just insulation with a vapour barrier may be enough. The thermal gradient and water may be there, but these are separated to a point where the thermal capacity is not sufficient to produce frost in quantity to cause a problem. The vapour barrier must be attached to the concrete to prevent moisture ingress with a mastic or like.
This can occur in a number of situations. The typical one is in old stucco homes with no insulation and vapour barrier in the joist space over the basement. Water condenses and freezes, expands, and produces lateral force. The sun hit it, it melts, runs down, freezes and expands. You get the picture. Beam pockets over basement have a similar issue. Slabs outside have the same effect happening next to houses.
This week I saw a new occurrence of this, inside a unheated insulated garage. Cars drag in snow and it melts, and condensates on the grade beam. On nice days, this melts, runs down into the space between the grade beam and slab an freezes. This provides force, and in this case, after the slab moved a bit, it pushed the grade beam out of alignment.
The solution is not cheap. Heat, to prevent freezing, is one solution. But just insulation with a vapour barrier may be enough. The thermal gradient and water may be there, but these are separated to a point where the thermal capacity is not sufficient to produce frost in quantity to cause a problem. The vapour barrier must be attached to the concrete to prevent moisture ingress with a mastic or like.
Thursday, August 9, 2012
Overland Summer drainage
One of the local counties only permits overland summer drainage on road allowances. So what does one do with winter sump pump flows? Well apparently, there are not to be any within the county. So the county is trying to regulate nature? Good Luck county. Now what does one do? Build a glacier in the back yard, where it slopes to county land, so they can get a big melt in the spring, and comply with there overland summer drainage? Oh well, more government waste to tax money.
This is about like the Indiana State legislature in 19th defining the value of pi exactly, for "palatability." Or Pierre E idiot Trudeau ramming french on western Canada. It just ain't gonna work.
This is about like the Indiana State legislature in 19th defining the value of pi exactly, for "palatability." Or Pierre E idiot Trudeau ramming french on western Canada. It just ain't gonna work.
Friday, August 3, 2012
Frost Heave
Frost heave is a process during freezing that causes ice lenses to be formed. Water is drawn to the freezing front. This process required three conditions; a slowly advancing freezing front, a soil that is permeable enough, yet has capillary rise, and a water table within the capillary rise. It is even worse it freezing occurs below the water table. This is all extra to the simple phase change volume increase.
Simple phase change can cause volume changes of 4 percent of the freezing depth; however, 2 percent is more typical. Frost heaves on the other hand can much large, often 150 mms to 300 mms on rural roads, and similar earth structures. Note that the frost penetration is much less in ice lens as the energy loss required to freeze water is greater. Normal soil has no laden heat, and its thermal capacity is only 10 to 15 percent of water, but the soil density is 1.5 to 2.2 time that of water, so a unit volume of soil has typically 15 to 20 percent thermal capacity plus that of contained water. For this reason, frost penetration is greater in dry soil, and typically greater in sand than clay due to the water retention properties of clays.
Silt sized material, with fine sand and small amount of clays is about the worst material for frost heave. This is usually delta sorted material or river settlements. These frequently occur in low land where water is also found. Road fills also reduce the likelihood of frost penetration, which is one thing that can help. Insulation under the pavement structure can also help.
Frost heaves, in the City's, where no frost heave has previously occurred are often indicators of water leaks, as these provide a source of water. When opening these to expore the cause in clay tills, frequently, a water bearing crevasse infilling sand seam in east Edmonton / Sherwood Park area.
Simple phase change can cause volume changes of 4 percent of the freezing depth; however, 2 percent is more typical. Frost heaves on the other hand can much large, often 150 mms to 300 mms on rural roads, and similar earth structures. Note that the frost penetration is much less in ice lens as the energy loss required to freeze water is greater. Normal soil has no laden heat, and its thermal capacity is only 10 to 15 percent of water, but the soil density is 1.5 to 2.2 time that of water, so a unit volume of soil has typically 15 to 20 percent thermal capacity plus that of contained water. For this reason, frost penetration is greater in dry soil, and typically greater in sand than clay due to the water retention properties of clays.
Silt sized material, with fine sand and small amount of clays is about the worst material for frost heave. This is usually delta sorted material or river settlements. These frequently occur in low land where water is also found. Road fills also reduce the likelihood of frost penetration, which is one thing that can help. Insulation under the pavement structure can also help.
Frost heaves, in the City's, where no frost heave has previously occurred are often indicators of water leaks, as these provide a source of water. When opening these to expore the cause in clay tills, frequently, a water bearing crevasse infilling sand seam in east Edmonton / Sherwood Park area.
Thursday, August 2, 2012
Positive Drainage
Drainage away from your house and buildings is critical, and may require maintenance. Backfill against basements, grade beams and frost walls settle, removing drainage. Water will only run down a slope. Yup. A fellow engineer even forgot that. Oh well. The whole building drains to the elevator shaft, and the sump is just below basement floor level over one metre above the elevator shaft elevation.
The ABC building code requires a 150 millimetre vertical separation between any untreated wood and soil. This is essential to prevent rot, mould, from moisture wicking into the wood. In addition the first 1.5 metre away should drop another 150 millimetres. In new construction, we recommend 300 mms in 3 metres to allow for some settlement. Two percent is recommended beyond.
Now what do we do if there is not sufficient grade for drainage? Well, we weight for wet basements and insurance claims. Exterior sumps can be installed, usually to some depth below the weeping tile, and it may have weeping tile attached to pick up any surface ponding. These exterior sumps are high maintenance, as in this country, these pumps need to pulled out in the fall, and reinstalled in the spring, to control freezing conditions. The alternative is heat tape, insulation, and dealing with water in the winter, unless the sump goes dry. Now what about spring, when we have daily cycles of melting and thawing.
The simple solution is to allow enough grade for drainage.
Housing has become a commodity in some areas, rather than a life time investment. Many people have become detached from the care and maintenance of there houses.
The ABC building code requires a 150 millimetre vertical separation between any untreated wood and soil. This is essential to prevent rot, mould, from moisture wicking into the wood. In addition the first 1.5 metre away should drop another 150 millimetres. In new construction, we recommend 300 mms in 3 metres to allow for some settlement. Two percent is recommended beyond.
Now what do we do if there is not sufficient grade for drainage? Well, we weight for wet basements and insurance claims. Exterior sumps can be installed, usually to some depth below the weeping tile, and it may have weeping tile attached to pick up any surface ponding. These exterior sumps are high maintenance, as in this country, these pumps need to pulled out in the fall, and reinstalled in the spring, to control freezing conditions. The alternative is heat tape, insulation, and dealing with water in the winter, unless the sump goes dry. Now what about spring, when we have daily cycles of melting and thawing.
The simple solution is to allow enough grade for drainage.
Housing has become a commodity in some areas, rather than a life time investment. Many people have become detached from the care and maintenance of there houses.
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