Building levelling system

Details Building levelling system Building levelling system

2002-06-11

2004-07-27

Yip, Winnie (Department: 3635)

Static structures (e.g., buildings)

Specified terranean relationship

With drain or vent exterior to foundation perimeter

C052S169140, C052S302300, C052S741100, C405S036000, C405S054000, C405S129500, C405S229000

Reexamination Certificate

active

06766620

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a building levelling system for a building having a foundation supported on expansive soil and more particularly to a building levelling system for preventing uneven settling of a building supported on expansive soils.
BACKGROUND
Many homes are troubled with settlement which is due to changes in the moisture content of the soil. The majority of the land area of North America includes a type of soil referred to herein as expansive soil which swells when it is wetted and shrinks as it is dried.
Soils with the potential to shrink and swell are found throughout the United States, Canada and in almost all parts of the world. Soils with this shrink and swell potential create difficult performance problems for buildings constructed on these soils because as the soil water content increases, the soil swells and heaves upward and as the soil water content decreases, the soil shrinks and the ground surface recedes and pulls away from the foundation walls.
Expansive soils are also known as swelling soils, heaving soils, volume change soils and shrinkable soils. By whatever name these soils are clay soils. Sometimes the clay has been compressed by great weight at some time in its geologic past and is called a shale, which can also be expansive. Nearly all clay soils swell when they get wetter and shrink when they get drier. Although there are many types of clay minerals, three that are most commonly encountered are those known as kaolinite, illite and smectite.
When building on expansive soil it is desirable to keep the water content of the soil consistent, however this can be difficult in some climates, for example semi-arid climates. Many dwellings have been built with concrete foundations resting on concrete footings. A weeping tile system has been installed which, in past construction, drained rainwater away into the sanitary sewer system and, more recently, drains water into a sump pit from which it is pumped outdoors away from the house.
In this way, water seepage is tidily disposed of without any flooding of basements. However, this system tends to dry out the soil to elevations even below the footings due to the capillary action of the soil. Settlement takes place as the soil dries beneath the footings.
In colder climates, for example Canada and the northern United States, further drying of the soil is caused by the phenomenon known as stack effect. During winter months the temperature difference between the inside of a building and outdoors can be 100 to 110 degrees Fahrenheit. The greater this temperature difference, the greater is the difference between the densities of inside and outdoor air.
Under these conditions, there is a small pressure difference between the inside of a heated building and the outdoors, this difference being greatest at the lowest elevation of the building. This difference causes outdoor air to leak into the lower regions of this building through doors, windows and other openings in the structure while heated air escapes at upper elevations through windows and other openings such as a roof space trap door.
This is stack effect and can be observed in a two story house equipped with leaky horizontal sliding windows. In severe weather, the windows on the main floor will be clear while the second story windows are partially or fully fogged up with moisture and ice. In this case, dry outside air enters the building through the main floor windows while moist warm air exits through the second story windows. This warm air condenses moisture onto the glass as it cools and leaks out to the outdoors. Homes equipped with fossil fuel fired heating systems (natural gas, propane or fuel oil) have chimneys which add to this stack effect as they exhaust combustion products from the heating equipment.
The basement of the structure is the lowest elevation where stack effect is at its greatest. During the heating season, cold, dry outdoor air passes down the exterior of the basement walls through cracks in the soil and enters the building through the weeping tile system. In the upper regions of the frozen soil, ice crystals in the soil sublime into the air that is passing downward causing shrinkage of the soil. At lower levels, moisture is also picked up by this outdoor air so that the above shrinkage occurs even to the footings and below due to the capillary action of the clay thereby resulting in settlement. A gap between the soil and outer sides of the concrete walls of the basement foundation are known to occur during prolonged severe weather due to this drying action. As little as a ¼ inch may not seem like much but if it exists all the way around the building it is a significant opening. An average size house may have a perimeter of 158 feet for example, which multiplied by the ¼ inch gap computes to 3.3 square feet of opening.
So far, conditions which tend to dry the soil uniformly have been described. There are conditions which tend to dry the soil differentially.
During the last 40 to 50 years, the automobile has become firmly entrenched in our lifestyle so that many of our homes are complete with attached garages that house two and, in some cases, three automobiles. Many such garages have front walls that project 4 feet to 6 feet beyond the front wall of the house and may include a roof which extends over the front entrance pad. This roof is equipped with eaves troughs which conduct rainwater away to a remote location thereby providing a sheltered, dry front entrance for the dwelling.
This arrangement causes a wide range of water absorption by the soil. Before such a house was built, the water content of the soil was likely uniform so that shortly after its completion, differential shrinkage and settlement were underway. Many such houses are one to two inches lower at the corner of the house adjoining the garage. Along the length of the common wall between the house and garage the expansive soil is also denied rainwater which is drained away by the eaves troughs and down pipes to locations remote from the house.
Trees are a significant part of landscaping and planning should locate them far enough from dwellings to minimize their affect on the moisture content of the soil at the dwelling. A 22 foot caliper tree for example may require 45 gallons of water per day. With a root system extending twice as far as the branches maximum separation from dwellings is desirable.
When damage to the foundation results, in many cases, owners opt for the lesser cost of repair which involves leaving the foundation and concrete floor in the settled state. The contractor lifts the house with jacks in the basement using beams to heave the joists upward to a level state. Shims are then installed to support the wood joists.
A considerably more expensive arrangement involves getting under the footings, raising them to level and installing friction piles. Friction piles however are known to sink. The only trouble free installation is piles to refusal, for example to bedrock or hardpan. This type of repair will more than likely require replacement of the basement floor. As the soil under the footings dries out so does the soil under the basement floor slab. If footings only are raised, the periphery of the floor slab will also be raised with resultant cracking of the slab. This is so because typical construction does not include a structural floor.
Repairs done that are less than totally supporting the building on piles to refusal plus a structural basement floor quite likely will not prevent further settlement or differential movement as the moisture content continues to be depleted. Furthermore, the above noted repairs involve considerable cost and disruption to the owners and occupants of the building.
SUMMARY
According to one aspect of the present invention there is provided a building levelling system for a building having a basement with a floor and footings supported on expansive soil, the building levelling system comprising:
a water management system arranged to maintain water in the expansive soil at a level

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