Patent Publication Number: US-2003233798-A1

Title: Post-tensioned, below-grade concrete foundation system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims priority to the following related applications, each of which is hereby incorporated in its entirety by reference as though fully set forth herein: U.S. provisional application No. 60/390,738 filed Jun. 21, 2002, entitled “Post-Tensioned Concrete Foundation System;” U.S. provisional application No. 60/410,532 filed Sep. 13, 2002, entitled “Post-Tensioned Below Grade Concrete Foundation System;” and U.S. provisional application No. 60/410,533 filed Sep. 13, 2002, entitled “Engineering Process for Post-Tensioned Below Grade Concrete Foundation System.” 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] Expansive soils are a significant problem for the construction industry. This is especially true in the area of residential home construction. Over time, expansive soils can increase in volume under a home causing the foundation to crack and compromise the structural integrity of the home. Further, soil expansion can cause concrete slab floors to buckle and crack, which may damage finished interior flooring, for example, tile or hardwood. Fixing the problems created in foundations and concrete slabs by expansive soils can be extremely expensive and difficult to implement. The problems associated with expansive soils adversely affect both the homeowner and the residential construction trade.  
       [0003] In order to circumvent the problems caused by expansive soils, homebuilders have employed various building techniques to mitigate potential damage from expanding soil underneath a home. One such solution, shown in FIG. 1 as a general representation of exemplary prior art, is to support the foundation on concrete piers or caissons. FIG. 1 depicts a portion of a house foundation  6  supported by a concrete caisson  4 . The house of FIG. 1 is modeled to include a basement  7  excavated below grade  9 . However, the foundation  6  could likewise be a foundation on grade  9  without a basement and similarly be supported on caissons  4 . The caisson  4  preferably rests upon bedrock  2  (or other suitable support) in order to provide a stable support for the foundation  6 . The foundation  6  may be in the form of a concrete perimeter beam or footer at the level of the excavation floor  10 . A concrete foundation wall  12  is formed on top of the foundation  6  and extends from the excavation floor  10  to above grade  9 .  
       [0004] A caisson  4  is constructed by drilling a shaft in the ground, for example, until bedrock  2  is reached. The shaft may be anywhere between 10 and 30 feet deep or more. The concrete caisson  4  is then formed to rest upon the bedrock by pouring a cement mixture in the shaft. Multiple concrete caissons are put in place at a single home site in order to support the foundation  6 . Again, the caisson  4  may support a foundation  6  of several configurations, for example, for a house with a basement  7 , or a house built on grade  9 .  
       [0005] One known system for withstanding the forces of expansive soils for houses built on grade is a post-tensioned concrete slab. Such post-tensioned concrete slabs act as both the foundation and the floor of the house. Steel tendons or cables are placed both longitudinally and laterally within the forms for the concrete slab. An additional cable may also be run around the perimeter of the foundation form. One end of each cable is anchored and the other end is stretched after the concrete slab has at least partially cured, placing the cable under tension and compressing the concrete slab, thereby increasing its tensile and bending strength under load. In some configurations, shallow, post-tensioned concrete beams are formed underneath the slab to provide further support. Because the post-tensioned concrete slab is formed directly on grade, while unitary integrity may be maintained, the slab, and therefore the entire house, may still move laterally on the soil surface as the soil expands and contracts. One way to counteract such movement is to support these slab-on-grade foundations by piers or caissons as previously described.  
       [0006] Understandably the cost and time involved in building a house increases significantly with the use of caissons. Cost increases are found in the expense of hiring a drilling rig plus the concrete forming the caissons. Further, additional time must be factored into the building process to allow for the drilling of the caisson shafts.  
       [0007] A construction design resistant to expansive soil effects generally used in areas where homes are built with basements is to substitute a structural floor  15 , as shown in FIG. 1, for a concrete slab on the excavated floor  10  of the basement. Caissons  4 , as previously described, may be incorporated in this design to provide additional support for the foundation  6 . Once the foundation  6  and foundation walls  12  are formed, hangers  8  are attached above the excavated floor  10  along the foundation walls  12 , and the desired structural system  15 , e.g., wood, steel, or concrete, is installed to span the space between the foundation walls  12  to support the basement floor  16 . In this design, a void or crawl space  14  remains between the excavated floor  10  at the bottom of the basement and the basement floor  16  suspended a few feet above. Because the structural basement floor  16  is spaced above the soil of the basement excavation, the soil can expand and contract without affecting the basement floor  16 .  
       [0008] One of the downsides to using a structural basement floor  15  to combat expansive soils is increased costs and time in the building process. Creating the void under the structural basement floor  15  necessitates a greater basement excavation depth. This means there is greater time and cost in the excavation portion of construction. Also, in order to provide a standard eight-foot ceiling in the basement, for example, the concrete basement foundation walls  12  need to be higher than normal, perhaps as high as 11 feet, to provide the void  14  under the structural basement floor  15 . Obviously, the additional concrete used in the foundation walls  12  result in a greater cost to the construction project. Another problem associated with this building technique is that the crawl space  14  under the structural basement floor  16  may promote an unhealthy environment within the home. The dank, dark void area  14  between the excavation floor  10  and the structural basement floor  16  is a conducive environment for vermin, spiders, and snakes. Further, it has been found that molds can flourish in this environment, which can create significant allergy and respiratory problems for the occupants of the home.  
       [0009] In view of the various drawbacks to the present construction designs and techniques for ameliorating the effects of expansive soils in residential construction, an alternative system for construction of homes with basements or other below-grade foundations would be desirable. The present invention addresses this issue.  
       SUMMARY OF INVENTION  
       [0010] The present invention seeks to combat the problem of expansive soils and avoid several disadvantages of the prior structures used to address expansive soil issues as discussed above. The system of the present invention provides a post-tensioned concrete floor slab formed below grade in the basement excavation of a home. Post-tensioning the floor slab provides compression of the floor slab in both planar directions (i.e., longitudinal and lateral directions). This compression significantly increases the tensile and bending or beam strength of the floor slab, providing greater resistance against the forces of expansion of the soil. The foundation walls, rather being supported on footings or caissons, as is the common practice in home construction, are supported directly by the perimeter edge of the top surface of the floor slab (hereinafter referred to merely as the perimeter edge). In this manner, the entire weight of the house sits upon and is distributed across the post-tensioned floor slab. Because the floor slab is post-tensioned, it can support the weight of the concrete walls in addition to the entire weight of the house. The post-tensioning of the concrete floor slab further distributes the weight of the house across the entire concrete floor slab. The combination of the post-tensioning of the floor slab and the increased downward force due to the weight of the house helps counteract the force of the expanding soil and significantly mitigates the typical effects of expanding soil on a foundation. With this design, the house will move as an integral unit. Because the house moves as an integral unit, the soil expansion must be sufficient to move the weight of the entire house. As a result, the house is more likely to remain unaffected by slight soil expansion forces that may have previously caused cracks to appear in the floor or in the foundation. Generally, the expansive soil force must be great enough to overcome the weight of the entire structure, which is distributed over the entire foundation floor. If the force of the expansive soil overcomes the weight of the entire structure, then the structure will likely react as a single unit, and the foundation and may shift slightly. This invention reduces the likelihood of structural damage normally caused by expansive soils, for example, cracking floors and walls.  
       [0011] Several aspects to the invention can better be understood by reference to different practical embodiments. A first embodiment is a house with a basement excavation at a single level. A second embodiment is detailed with respect to a house built with a structural garage floor. A third embodiment is discussed with respect to a foundation built upon multiple excavation levels. A fourth embodiment concerns the integration of counterforts along the basement walls in order to resist the lateral forces of the soil below grade on the basement walls.  
       [0012] A first embodiment of a residential dwelling with a basement may includes of a post-tensioned concrete slab below grade and an exterior concrete basement wall located on a perimeter edge of the post-tensioned concrete slab. This and each of the other embodiments described herein fulfills one of the advantages of the invention to resist the stresses on the foundation caused by expansive soils. The foundation may be further include post-tensioned concrete beams running underneath the concrete slab. The post-tensioned concrete slabs and beams may be formed by including tendons in their structures that are stressed to compress the concrete. In some embodiments a tendon may extend within the both the concrete slab and the beam, the concrete slab and a concrete basement wall, or all three.  
       [0013] One method of constructing a such a foundation begins with the excavation of the soil at a site below grade level. A plurality of tendons may be placed upon the floor of the excavation. A cement mixture is then poured over the plurality of tendons on the level below grade. The cement is at least partially cured to form a concrete slab. The concrete slab is then post-tensioned by applying tensile forces to the plurality of tendons. Once the concrete slab is post-tensioned, the exterior concrete walls of the foundation are formed on a perimeter of the concrete slab.  
       [0014] In a second embodiment of the foundation mentioned above, a high load bearing floor (e.g., a garage with a structural floor) is supported by a plurality of beams spanning a first exterior concrete foundation wall and a second exterior concrete foundation wall. The beams may also, or alternatively, span an interior concrete wall and an exterior concrete foundation wall.  
       [0015] A third embodiment of a residential dwelling according to the present invention may be constructed with a multi-level foundation composed initially of a first concrete slab, an exterior concrete basement wall on at least a portion of a perimeter edge of the first concrete slab, and a means for post-tensioning the first concrete slab. The multi-level dwelling further has a second post-tensioned concrete slab adjacent to the exterior concrete basement wall. At least one of the levels is below grade. At least one tendon is used to structurally integrate the two levels. A first portion of the tendon extends within the second post-tensioned concrete slab, and a second portion of the tendon extends within the exterior concrete basement wall and may extend within the first concrete slab as well.  
       [0016] A fourth embodiment of the invention concerns the addition of counterforts to the foundation. The novel counterfort is formed by an extension of a post-tensioned concrete slab in a basement excavation beyond a wall of a foundation supported by the post-tensioned concrete slab. A concrete member generally normal to the extension of the post-tensioned concrete slab is supported by the extension of the post-tensioned slab.  
       [0017] Other features, utilities and advantages of various embodiments of the invention will be apparent from the following more particular description of embodiments of the invention as illustrated in the accompanying drawings and defined in the appended claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0018]FIG. 1 is an elevation view in cross-section of prior art construction utilizing caissons and suspended structural floors.  
     [0019]FIG. 2 is a plan view of a post-tensioned, below grade, foundation for a home according to a first embodiment of the present invention.  
     [0020]FIG. 3A is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 detailing the dead end of a tendon in position in the post-tensioned concrete slab.  
     [0021]FIG. 3B is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 detailing the live end of a tendon in position in the post-tensioned concrete slab.  
     [0022]FIG. 3C is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 depicting the placement of a tendon within the concrete slab.  
     [0023]FIG. 4A is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 detailing the concrete slab and beam supporting an exterior basement wall.  
     [0024]FIG. 4B is an elevation view in cross-section of a portion of the post-tensioned foundation of FIG. 2 detailing tendon placement in the concrete slab, beam, and exterior basement wall.  
     [0025]FIG. 5 is a plan view of a post-tensioned, below grade foundation supporting a structural floor garage according to a second embodiment of the invention.  
     [0026]FIG. 6A is an elevation view in cross-section of a detail of the structural garage floor of FIG. 5 attached to a basement wall.  
     [0027]FIG. 6B is an elevation view in cross-section of a portion of the structural garage floor of FIG. 5 supported by the basement wall and meeting with a driveway slab.  
     [0028]FIG. 7 is a plan view of a multi-level, below grade, post-tensioned foundation for a home according to a third embodiment of the present invention.  
     [0029]FIG. 8A is an elevation in cross-section of the multi-level interface of the post-tensioned foundation of FIG. 7 detailing the cantilever of the upper level of the foundation.  
     [0030]FIG. 8B is a partial elevation view in cross-section of the upper level slab, beam, and sill of the post-tensioned foundation of FIG. 2.  
     [0031]FIG. 9A is an elevation view in cross-section of a portion of the multi-level, post-tensioned foundation of FIG. 7 detailing a counterfort poured on top of a projection of a post-tensioned slab and beam.  
     [0032]FIG. 9B is a plan view of the counterfort of FIG. 9A.  
     [0033]FIG. 10 is a plan view of an arcuate window well counterfort placed on a projection of a post-tensioned slab.  
     [0034]FIG. 11A is a plan view of a window well counterfort housing an opening for a sump pump placed on a projection of a post-tensioned slab.  
     [0035]FIG. 11B is an elevation view in cross-section of the window well counterfort of FIG. 10A.  
     [0036]FIG. 12 is a fragmented elevation view in cross-section of a side wall of the post-tensioned foundation of FIG. 7 detailing an integrated wall and beam structure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0037] The present invention is directed to a novel foundation structure for a residential dwelling with a basement or wherein the foundation of the house or dwelling is otherwise built in an excavation below grade. While the examples of the invention are described herein for use in residential dwellings, it is contemplated that the invention could also be implemented in commercial or municipal buildings, or any other structure amenable to these inventive techniques. The inventive foundation disclosed herein is designed to be substantially resistant to the forces of expansive soil that may act upon the foundation. At its base, the foundation structure of the present invention, for example, as depicted herein in FIGS.  1 - 4 B, is a post-tensioned concrete floor slab formed upon the soil bottom of a basement excavation for the house. Exterior concrete foundation walls are built directly upon the perimeter of the post-tensioned concrete floor slab. In one embodiment, for example, as shown in FIG. 12, the foundation walls are structurally integrated with the post-tensioned floor slab by running portions of tensioning cables used to post-tension the floor slab both through the floor slab and upward into the foundation walls as well. In this manner the floor slab and exterior walls operate together as load supporting beams to support the load of the dwelling. Interior concrete basement walls or buttresses may likewise be formed on and structurally integrated with the basement floor slab with tensioning cables to further support interior loads of the home. Because the foundation is formed below grade, there is little problem of lateral movement of the foundation as found with post-tensioned slabs on grade.  
     [0038] Additional structural features can be combined with the basement post-tensioned floor slab foundation structure to either create more versatile home designs, further reinforce the strength of the foundation, or both. Because of the strength of the post-tensioned floor slab design, the dwelling can support substantial loads that might otherwise not be possible with regular home construction techniques. For example, in one embodiment as shown in FIGS. 5, 6A, and  6 B herein, a structural garage floor is provided wherein the garage may be built over the basement. The significant weight of a car in the garage is transferred to and supported by the foundation. Additionally, because of this added weight providing a downward force on the foundation, the foundation is more resistant to the upward forces of underlying expansive soils.  
     [0039] Another possible structural variation provides for multi-level foundations. For example, rather than a structural garage as described above, a garage floor built on grade may be structurally integrated with the post-tensioned basement foundation as shown in FIG. 7. The garage floor slab may itself be post-tensioned to create an integral unit. Then the garage floor slab may further be integrated with the house foundation by training tensioning cables within both the garage floor slab and the basement floor slab and extending through a system of buttresses and counterforts. The garage floor slab may be viewed as cantilevered off of the adjacent foundation wall. With this design, the house foundation and the garage floor slab will respond to soil expansion as a unitary structure, thereby increasing the expansive soil force necessary to affect the structure, and also reducing the stresses on the superstructures of the house and garage, particularly at the intersection of each. Using the structural concept exemplified by the cantilevered garage, a foundation may be designed integrate post-tensioned basement floor slabs (and foundation walls) formed at multiple excavation levels into an integral structural unit. By transferring the load of the basement walls and the superstructure of the house to the post-tensioned floor slab, the same resistance to expansive soil problems is achieved.  
     [0040] With this design, the house will move as an integral unit. Because the house moves as an integral unit, the soil expansion must be sufficient to move the weight of the entire house. As a result, the house is more likely to remain unaffected by slight soil expansion forces that may have previously caused cracks to appear in the floor or in the foundation. Generally, the expansive soil force must be great enough to overcome the weight of the entire structure, which is distributed over the entire foundation floor. If the force of the expansive soil overcomes the weight of the entire structure, then the structure will likely react as a single unit, and the foundation and may shift slightly. This invention reduces the likelihood of structural damage normally caused by expansive soils, for example, cracking floors and walls.  
     [0041]FIGS. 2, 3A,  3 B,  3 C,  4 A and  4 B depict a first embodiment of a below-grade, post-tensioned foundation according to the present invention. The post-tensioned foundation  17  of FIG. 2 is shown in plan view and details the locations of various structural components of the foundation  17 . The post-tensioned, concrete, basement floor slab  20  rests upon soil  19  at the bottom of the basement excavation as shown in greater detail in FIG. 3C. Concrete beams  18  (indicated by the thick dashed line in FIG. 2), which may be integral with the basement floor slab (e.g., formed monolithically), extend beneath the post-tensioned basement floor slab  20 . The width and depth of the concrete beams  18  is determined according to engineering requirements based upon a number of factors, for example, the expansiveness of the soil and the load to be supported by the foundation. The concrete beams  18  are located around the perimeter edge  56  of the post-tensioned basement floor slab  20  and are also placed in positions across both the length and the width of the post-tensioned basement floor slab  20 . The concrete beams  18  provide added structural support to the post-tensioned basement floor slab  20  (if needed) as will be described in greater detail later herein. The width of the concrete beams  18  may be expanded in certain locations to act as footers  21  to better support point loads bearing down from the superstructure of the house, for example, posts supporting a load bearing beam. In other embodiments, the post-tensioned, concrete basement floor slab  20  may be engineered to a sufficient thickness with appropriate tendon  22  placement and other reinforcement that the addition of beams  18  to the design is unnecessary.  
     [0042] Tensioning cables or tendons  22  (indicated by the thinner dashed lines of FIG. 2) are placed within the post-tensioned, concrete basement floor slab  20  running across both the length and width of the post-tensioned floor slab  20 . In this manner an orthogonal web of tendons  22  is formed within the post-tensioned floor slab  20 . In alternative embodiments the tendons  22  may run in only one direction or at varying angels according to engineering specifications. In a location where a concrete beam  18  extends beneath the post-tensioned floor slab  20 , a tendon  22  may undulate such that portions of the tendon  22  are within the area defined by the beam  18  and portions of the tendon  22  are within an area defined by the post-tensioned floor slab  20 . Alternatively, or additionally, depending upon the engineering requirements, separate tendons  22  may each run solely within the beam  18  and solely within the area defined by the post-tensioned floor slab  20 , as shown in FIG. 3C. The tendons  22  may be placed upon chairs  52  to vary the height of the tendons  22  within the post-tensioned, concrete floor slab  20  and the beams  18 . Locations and heights of the various chairs  52  are specified according to engineering requirements.  
     [0043] The ends of each tendon  22  differ as depicted in FIG. 2 and in greater detail in FIGS. 3A and 3B. A first end  28  of a tendon  22  is a dead end anchorage  26 . This first end  28  is embedded in the concrete post-tensioned floor slab  20  or concrete beam  18  at a first side or end  38  of the post-tensioned floor slab  20  or beam  18  and does not move once the concrete  40  cures. The second end  32  of the tendon  22  is similarly embedded within and near the second side or end  48  of the concrete post-tensioned floor slab  20  or the concrete beam  18 . The second end  32  of the tendon  22  is an active end wherein the tendon  22  protrudes from the concrete post-tensioned floor slab  20  or beam  18 , which allows a force to be applied to the tendon  22  and place it under tension during the building process and permanently thereafter.  
     [0044] Also depicted in FIG. 2., the exterior foundation walls  54  (shown as solid lines) of the basement are formed directly on top of at least a portion of the top surface perimeter  56  of the concrete floor slab  20 . FIG. 4A shows a cross section of the intersection of a foundation wall  54  and the post-tensioned floor slab  20  in greater detail. Reinforcement members  58 , for example, rebar or other mild steel reinforcement, may be placed within the concrete  40  to strengthen the foundation walls  54  and tie the foundation walls  54  to the post-tensioned floor slab  20 . An angled reinforcement member  59  may also be placed to vertically support the position of a tendon  60  in the foundation wall  54 . A sill  61  may be mounted to the top of the foundation walls  54  by anchor bolts  57  embedded within the foundation wall  54 . The sill  61  provides the base for framing the superstructure of the house or other structure to be built upon the foundation  17 .  
     [0045] In addition to exterior foundation walls  54  of the basement, interior foundation walls  55 , as shown in FIG. 2, may similarly be formed upon the post-tensioned, concrete floor slab  20 . Such interior basement walls  55  may perform several functions, for example, support a point load from the superstructure above, act as a beam to support the superstructure above, support the ends of beams spanning from the exterior foundation walls  54 , and acting as a buttress to exterior foundation walls  54  to counteract the lateral forces of the soil backfilled against the exterior foundation walls  54 . Rather than full interior walls  55 , interior concrete buttresses  57  may be formed on the post-tensioned floor slab  20  to oppose lateral soil forces on the exterior foundation walls  54 . Alternatively, counterforts  170  external to the exterior foundation walls  54  may be provided to oppose the lateral soil forces on the exterior foundation walls  54 . As shown in FIG. 2, the counterforts  164  may be constructed, in one embodiment, by forming vertical concrete members  165  on extensions  170  of the post-tensioned floor slab  20 . The counterforts  164  may be structurally integrated into the post-tensioned floor slab  20  by extending a tendon  22  from the post-tensioned floor slab  20  into the extension  170  and the vertical concrete member  165 . Various novel embodiments of counterforts  164  will be described later herein with respect to FIGS.  9 A- 11 B.  
     [0046] Application of one aspect of the invention may be described generally with respect to a home constructed with a basement wherein the basement is excavated at a single level as shown in plan view in FIG. 2. In order to prepare a post-tensioned, concrete foundation system  17  below grade, at either single or multiple levels, according the present invention, the basement or other below grade levels must first be excavated at the home site. In some instances, an advantage of the present inventive foundation system  17  is that the excavation will generally be several feet less than the depth necessary to build an elevated structural floor. This is because the extra foundation height required for a structural floor is not necessary. The excavation will, however, need to be a few feet wider than the actual length and width of the foundation  17  on two adjacent sides of the excavation to allow access to the active ends of the tendons  22  to tension the concrete  40 . Once the home site has been excavated, it may be desirable to additionally trench the perimeter area over which the foundation floor slab  20  is to be poured in order to pour structural concrete beams  18  under the perimeter  56  of the basement floor slab  20  as shown in FIG. 2. It may be further desirable to trench areas across the length and width of the floor slab  20  in order to pour beams  18  to add additional support to the floor slab  20 . The beams  18  can be poured either monolithically with the floor slab  20  or separately as described below. The use of beams  18  may not be necessary as previously indicated depending upon the engineering design of the structure. In some instances, especially with zero property line build-outs in many residential neighborhoods today, it may be advantageous to excavate two or more home sites in a single pit, thereby overlapping the additional width needed to tension the tendons during the concrete cure.  
     [0047] Once the trenches for beams  18  have been excavated, tendons  22  for tensioning the beams may be placed. One method for post-tensioning concrete known in the art that may be used is the addition of a mono-strand tendon  22  with a dead-end anchorage  26  on a first end  28  and a stressing anchorage  30  on a second active end  32  to the beam  18 . The monostrand tendon  22  may consist of a seven wire, steel strand  24  coated with a corrosion-inhibiting grease. The seven wire, steel strand  24  may be encased in an extruded plastic protective sheathing  34 . An iron or steel plate  36  is attached to the first end  28  of the monostrand  24  normal to the length of the tendon  22  to provide the dead-end anchorage as shown in FIG. 3A. The dead-end anchor  26  will generally be placed a few inches from a first end  38  of the beam  18  and supported such that it is completely embedded in the concrete  40  when the concrete  40  is poured. On a second end  32  of the tendon  22 , the stressing anchorage  30  is supplied as shown in FIG. 3B. The stressing anchorage  30  may be an iron or steel casting  42  that grips the mono-strand  24 . The casting  42  is placed immediately behind a plastic grommet  44  through which the tendon  22  extends. The grommet  44  is placed against the concrete form  46  at the second end  48  of the beam  18  to create a pocket  50  around the tendon  22  protruding therefrom and preventing concrete  40 , when poured, from contacting the exposed steel mono-strand  24  of the tendon  22 .  
     [0048] The tendons  22  are placed along the length of the beam  18  by draping points of the tendon  22  over one or more support chairs  52  as shown in FIG. 3C. The support chairs  52  may be of varying heights and are placed according to engineering specifications to alter the vertical position of the tendon  22  through the concrete beam  18  in order to achieve the desired structural strengthening effect. Portions of the tendons  22  may be raised above the top of the beam  18  to extend within the area of the post-tensioned floor slab  20  to structurally integrate the two. Reinforcement members  58 , e.g., mild steel rebar, may also be placed throughout the trenches in order to provide additional reinforcement for the beams  18  as desired. The beams  18  may be poured either monolithically or separately from the post-tensioned floor slab  20 . If a beam  18  is poured separately from the pouring of the post-tensioned floor slab  20 , additional reinforcement members  58  may be placed protruding above the beam  18  in order to tie the beam  18  into the post-tensioned floor slab  20  when the concrete for the post-tensioned floor slab  20  is poured.  
     [0049] Once tendons  22  have been placed for the beams  18 , additional tendons are then placed for tensioning the floor slab  20  itself. Tendons  22  may be placed laterally and longitudinally across the area of the floor slab  20  according to desired engineering specifications. As used with the beams  18 , each of the tendons  22  has a dead-end anchorage  26  on a first end  28  and a stressing anchorage  30  on an active second end  32 . Preferably, when multiple tendons  22  are used, each of the dead-end anchorages  26  of adjacent, parallel tendons  22  are placed along the same perimeter edge  56  of the foundation structure  17 . The tendons  22  are again draped along support chairs  52  of varying heights, with the height and spacing of the chairs  52  determined according to engineering specifications for the post-tensioned floor slab  20 . Again, reinforcement members  58  may be placed throughout the area of the floor slab  20 , for example, to reinforce corners, to reinforce cut-out areas within the floor slabs  20  (e.g., for placement of a sump-pump), and for tying foundation walls  54  into the post-tensioned floor slab  20 . Once the tendons  22  and reinforcement members  58  are in place, a cement mixture can then be poured to form the concrete  40  of the post-tensioned floor slab  20 . As previously indicated, the post-tensioned floor slab  20  may be poured monolithically with the beams  18  directly on floor of the basement excavation. When leveling and smoothing the post-tensioned floor slab  20  during the pour, it may be desirable to leave the perimeter edges  56  of the post-tensioned floor slab  20  rough to lessen slippage between the floor slab  20  and the foundation walls  54  when the foundation walls  54  are later poured.  
     [0050] After the walls  54  have cured for a sufficient time, for example, several days, the tendons  22  in the post-tensioned floor slab  20  and beams  18  may be tensioned by any methods known in the art. Although one method for tensioning the floor slab  20  and beams  18  is described herein, other methods may be used with the same result. In an exemplary method, each exposed steel mono-strand  24  is first inspected and marked to indicate its pre-tensioning position. The grommet  44  is removed and a conical, two- or three-piece wedge (not shown) is placed around the exposed steel mono-strand  44  in the recess in the concrete formed by the grommet  44 . A hydraulic jack (not shown) is attached to the steel mono-strand  24  and tensions the tendon  22  until it stretches the desired engineered distance. The wedge around the steel mono-strand  24  is placed such that it prevents the tendon  22  from retracting back into the floor slab  20 . The corrosion-resistant grease and protective sheathing  34  around the steel mono-strand  24  act to break the bond between the concrete  40  and the tendon  22  and reduce friction on the steel mono-strand  24  during the tensioning process. The wedge maintains the tension on the tendon  22  and helps transfer the tension to the surrounding concrete  40 . After sufficient time, for example, several days, the exposed steel mono-strands  24  of the tendons  22  are again inspected to ensure that there has been no slippage of the tendon  22 . If everything is in order, the steel mono-strand  24  is cut to not extend beyond the edge of the concrete floor slab  20  and the recess formed by the grommet  44  is grouted with a corrosion-resistant mortar to seal the end of the tendon  22 . Because the tendons  22  are permanently stressed, a constant compressive force acts upon the concrete floor slab  20  and beams  18  of the foundation  17 . This compression counteracts the tensile forces created by loads upon the concrete floor slab  20 , for example, cars, furniture, and the weight of the structure of the house itself, and increases the bending or beam strength of the floor slab  20  and beams  18 . By post-tensioning the concrete floor slab  20 , the load carrying capacity of the floor slab  20  is significantly increased.  
     [0051] At a suitable time after the tendons  22  in the post-tensioned floor slab  20  and beams  18  have been tensioned, the foundation walls  54  may be formed and poured as shown in FIG. 4A. Forms (not shown) for the exterior foundation walls  54  of the basement are placed over the perimeter edge  56  of the post-tensioned floor slab  20  so that the resultant foundation walls are supported on the perimeter of the foundation. Any reinforcement members  58  or tensioning cables  60  for strengthening the walls  54  are then placed appropriately within the forms. A cement mixture is then poured within the forms to create the concrete foundation walls  54 . The concrete foundation walls  54  are formed directly onto the rough perimeter edge  56  of the post-tensioned floor slab  20 . As previously indicated, the rough interface between the perimeter edge  56  and the foundation walls  54  helps reduce any slippage of the foundation walls  54  on the post-tensioned floor slab  20 . Although formed concrete foundation walls  54  are primarily described herein, other types of foundation walls, for example, concrete block, brick, rock, and wood frame, may all be formed on the below grade post-tensioned floor slab  20  of the present invention. Thus, the foundation walls  54  are supported by the post-tensioned floor slab  20 , rather than footings or caissons, and help to exert opposing force to counteract expansive soil forces.  
     [0052] Alternately, as shown in FIG. 4B in a design with a long basement wall, portions of the tendon  22  running in the plane of an exterior foundation wall  54  may undulate between the beam  54 , the post-tensioned floor slab  20 , and the foundation wall  54 . The tendon  22  may terminate at each end within the foundation wall  54 , rather than in the beam  18  or post-tensioned floor slab  20 . In this manner, the beam  18 , post-tensioned floor slab  20 , and foundation wall  152  can be tensioned and compressed together, whereby the foundation wall  54  now acts as part of the load supporting beam  18  and further integrates the structure to react unitarily in opposition to expansive soil forces. Actual tendon placement may depend upon various factors, e.g., the load to be carried, the design tolerances required, the expansiveness of underlying soil, and the geometry of the house design. In a foundation design with a thick post-tensioned floor slab and without beams as described earlier, the tendon may undulate and alternatively run within a length of the basement slab and within a length of the basement wall. By similarly integrating all of the foundation walls, beams, and slabs under compression through the use of tensioning tendons, a monocoque structure is created to resist the forces of expansive soil and help cause the house to act as a unit. With the unification of the foundation wall  54  and the post-tensioned floor slab  20 , the function of the beam  18  is distributed upward into the foundation wall  54  above the post-tensioned floor slab  20 , thereby increasing the load carrying capacity of the foundation  17 , lessening the excavation depth required for the beam  18 , or both, and helping resist the effect of expansive soil.  
     [0053] By constructing a post-tensioned concrete floor slab  20  in an excavation as a basement floor and forming the concrete foundation walls  54  of the basement directly on the post-tensioned floor slab  20 , the detrimental effects of expansive soils on the house foundation may be mitigated. The use of this construction structure and technique also offers several advantages over prior art construction structures and techniques designed to circumvent expansive soil problems. First, as noted, the post-tensioned floor slab  20  has greater resistance to soil expansion because the weight of the entire house, including the concrete basement walls  54 , rests upon the post-tensioned floor slab  20  rather than merely upon beams  18 . Second, because the post-tensioned floor slab  20  is poured directly on soil there is no space underneath the basement floor. Therefore, there is greatly reduced and likely no void space between the post-tensioned floor slab  20  and the soil at the base of the excavation and thus reduced opportunity for mold growth or vermin infestation as in a raised structural floor. Third, pouring a concrete post-tensioned floor slab  20  may be less time-consuming and labor intensive than building a structural floor. Fourth, less concrete may be used in forming the foundation walls  54 , as the foundation walls  54  of the present design may be several feet shorter in height than the walls required for structural floor construction. Fifth, there is potentially less excavation involved because the excavated level for pouring a post-tensioned floor slab  20  of the present invention is several feet above the level required for building a home with a structural floor.  
     [0054] The post-tensioned, below-grade foundation  17  of the present invention is designed to be an integral unit in order to mitigate the effects of expansive soil. Construction of an integral foundation  17  through post-tensioning of the basement floor slab  20 , forming the foundation walls  54  directly upon the post-tensioned floor slab  20 , and structurally tying the foundation walls  54  and the post-tensioned floor slab  20  together helps create a unitary structure. If the foundation  17  moves at all due to soil expansion, the foundation  17  will move as a unitary body, thus reducing the likelihood of cracking and buckling of the foundation  17  or causing stress to the superstructure of the house.  
     [0055] In homes constructed with a basement and an attached garage, integration of the entire foundation can be difficult. Given the significant weight of automobiles that are normally parked in an attached garage, the floor of the garage is generally a concrete slab poured on grade. The garage floor slab could be formed as a post-tensioned slab on grade to resist the expansive soil forces. However the grade for pouring a garage slab will generally be significantly higher than the base of the excavation on which the basement floor slab sits, as it is necessary for the garage to be at ground level to provide access for cars to and from the street. While the walls for an attached garage are generally integrated into the framed superstructure of the house, the slab for the garage may be poured independent of the rest of the foundation of the house. Therefore, in response to expanding soil conditions, the garage slab may move independent of the house foundation, which may result in structural problems in the structural frame at the juncture of the garage and the house in addition to any problems with the foundation.  
     [0056] As shown in FIGS. 5, 6A, and  6 B, one solution to the problem of integrating the garage  66  and the house is to construct a high load bearing, structural garage floor  64  over the post-tensioned basement foundation  68  of the present invention. The basement foundation  68  in FIG. 5 may be the foundation  17  shown in and described with respect to FIG. 2. FIG. 5 depicts only the foundation walls, and does not depict the tendons- 22  post-tensioning the concrete floor slab  20  and the beams  18  supporting the post-tensioned floor slab  20  as shown in FIG. 2. In this embodiment, the basement actually extends underneath the structural garage floor  64 . In this manner, the weight of the garage and any vehicles within it will be supported by the post-tensioned basement floor slab  70  and any beams underneath, if any. In this embodiment the substantial excess weight attributable to the garage  66  and automobiles will enhance the counteractive force of the foundation  68  against the pressure of expanding soils. The front and outside superstructure walls (not shown) of the garage are supported by the exterior basement walls ( 76  and  78 , respectively, in FIG. 5), which in turn sit upon the post-tensioned concrete slab basement floor  70 . In the design shown in FIG. 5, the front portion  82  of the interior garage wall superstructure (not shown) is also supported by a portion of the exterior basement wall  80  formed upon the post-tensioned floor slab  70 . The remainder of the structural garage floor  64  is supported by an interior basement wall  84  and an internal buttress  100 , each formed upon the post-tensioned floor slab  70 .  
     [0057] As shown in FIG. 6A, the structural garage floor  64  is supported by horizontal steel beams  86  that are attached at one end to the either the jutting exterior wall  80  or the interior basement wall  84  and at the opposite end to the exterior basement wall  78  by vertically oriented steel angle plates  88 . These steel angle plates  88  are bolted into each of the concrete jutting exterior wall  80 , the concrete interior basement wall  84 , and the concrete exterior basement wall  78  with expansion bolts  90  and bolted to the steel beams  86  with thru-bolts  92 . As shown in FIG. 6B, the front edge  94  of the structural garage floor  64  is supported partially by the front basement wall  76  itself and partially by horizontally oriented steel angle plates  96  that are also bolted to the front basement wall  76  with expansion bolts  90 .  
     [0058] The rear  98  of the structural garage floor  64  may be partially supported by an internal buttress  100  (as shown in FIG. 5) extending into the basement from the outside basement wall  78 . Steel angle plates  96  may likewise be used along the buttress  100  to support the rear  98  of the structural garage floor  64  in a similar manner as the steel angle plates  96  on the front basement wall  76  support the front edge  94  of the structural garage floor  64 . The internal buttress  100  is poured on top of the post-tensioned basement floor slab  70 . The interior buttress  100  need not extend the entire width of the garage  66  but may extend only a portion of the width of the garage  66 . The remaining span between the buttress  100  and the interior basement wall  84  may be spanned by another steel beam  102 . The steel beam  102  may rest in pockets  104  formed in the tops of the interior wall  84  and the buttress  100  as shown in FIG. 5.  
     [0059] Once the steel beams  86  are anchored in place, metal pans or decking  106 , as shown in FIG. 6A and 6B, may be placed on top of the beams  86  and overlap the front wall  76  and rear buttress  100 . Reinforcement members  108  may then be placed above the metal decking  106 . A cement mixture may then be poured over the metal decking  106  and reinforcement members  108  to form a concrete garage floor slab  62  for the structural garage floor  64 . If desired, tendons could likewise be used to reinforce the concrete garage floor slab  62 . Upon tensioning such tendons, the garage floor slab  62  would likewise be a post-tensioned structure. The garage floor slab  62  may be poured to extend over the top of the front basement wall  76  in order to meet a concrete slab for the driveway, which is poured on grade.  
     [0060] The internal buttress  100  supporting the rear  98  of the garage  66  serves an additional function of providing support to the outside basement wall  78  to resist the horizontal forces of the soil backfilled against the foundation walls. Also, by using the interior buttress  100  in cooperation with the steel beam  102  at the rear of the garage  66 , the size requirement for the steel beam  102 , especially one in a load bearing position, is substantially reduced. Further, by using the combination of the steel beam  102  and internal buttress  100  at the rear  98  of the garage  66 , rather than extending the buttress  100  as an internal concrete wall across the width of the structural garage floor  64 , access is allowed to the space under the garage  66 , creating a room of usable space with the post-tensioned basement slab  70  for a floor and the structural garage floor  64  for a ceiling.  
     [0061] An alternative configuration combining caissons and a post-tensioned structural floor is also contemplated. In this embodiment, caissons may be placed in the basement excavation according to design to support a structural floor. Rather than hanging steel beams from the foundation walls as in the garage discussed with respect to FIGS. 5, 6A, and  6 B, the steel beams may be supported by the caissons. A structural concrete floor slab as described with respect to FIGS. 6A and 6B may then be supported on the caissons and steel beams. The structural concrete floor slab may include tendons for post-tensioning. Once the structural floor slab is post-tensioned, the foundation walls may be formed on the post-tensioned floor slab as described with respect to FIG. 2. In this embodiment, there may be a void space beneath the structural slab floor and the soil at the base of the basement excavation.  
     [0062] In a further embodiment of the invention, the integration of multiple foundational slabs constructed at multiple excavation levels or at grade may be achieved, for example, as shown in FIGS. 7, 8A, and  8 B. In the plan view of the foundation  110  depicted in FIG. 7, a high load bearing garage floor slab  112  is constructed on grade, higher than the basement slab  114 , but is formed as an integral part of the foundation  110  according to the present invention. Through the disclosed design, the entire foundation  110 , including the garage floor slab  112 , reacts in a unitary manner to the effects of soil expansion. While a post-tensioned garage floor slab  112  on grade is described herein as an exemplary embodiment of a multi-level foundation according to the present invention, those skilled in the art will recognize that structures and methodologies used to integrate a post-tensioned slab on grade are likewise applicable to the integration of a separate foundation levels each below grade.  
     [0063] As detailed in FIGS. 7, 8A, and  8 B, a post-tensioned garage floor slab  112  is formed on grade with tensioning cables ( 116 ,  118 ) running longitudinally and laterally through the slab. The longitudinal tendons  116  running from the front to the rear of the garage floor slab  112  may be extended through the top of the front wall  120  of the basement  122 . As shown in FIG. 5B, the post-tensioned garage floor slab  112  may be poured monolithically with a concrete beam  124  supporting the perimeter of the garage floor slab  112  and a concrete sill  128  rising above the top of the slab  112  and extending along each of the lateral sides ( 130 ,  132 ) of the garage  126  from the front  125  of the garage  126  to the rear  127  of the garage  126 . A first tendon  134 , as shown in FIG. 8A, may be run through the beam  124  along each of the lateral sides ( 130 ,  132 ) of the garage  126  from the front  125  of the garage  126  to the rear  127 . As shown in FIG. 8A, the bottom projection of the beam  124  for the interior side  132  of the garage  126  may be sloped downward toward the rear  127  of the garage  126  to eventually meet with the front edge  136  of the post-tensioned basement floor slab  114 .  
     [0064] On top of the post-tensioned basement floor slab  114  may be an interior buttress  138 , shown in FIGS. 7 and 8A, formed on top of the basement floor slab  114  normal to the front wall  120  of the basement  122  and extending from the front wall  120  of the basement  122  into the interior of the basement  122 . The buttress  138  may also function as an interior wall of the basement  122 . The slope of the beam  124  from the garage  126  approaching the basement floor slab  114  may be about  30 %, and could be more or less depending upon the application. The slope of the first tendon  134  between the garage beam  124  at grade  140  and the excavation level  142  of the post-tensioned basement floor slab  114  should be gradual rather than sharp in order to provide the strongest integration between the basement foundation  122  and the garage floor slab  112 . The first tendon  134  in the garage beam  124  follows the downward slope of the beam  124  and may be set within the buttress  138  above and parallel to the post-tensioned basement floor slab  114 . Alternatively, the a portion of the first tendon  134  may also extend into the basement floor slab  114 , further structurally integrating the garage  126  and the basement foundation  122 .  
     [0065] In a further embodiment, a concrete beam  139  may run underneath the basement floor slab  114  under the area of the buttress  138  and may also extend to the rear  158  of the basement foundation  122 . The first tendon  134  may thereby be run below the basement floor slab  114  within the beam  139 . Alternately, the first tendon  134  may undulate between the basement floor slab  114  and the beam  139  as called for by the engineering design. Further, the first tendon  134  may undulate between the basement floor slab  114 , the beam  139 , and the internal buttress  138 . If, the internal buttress  138  were instead an interior concrete wall, a portion of the first tendon  134  could likewise extend within such interior wall as well. This concept is further described with respect to FIG. 12 herein. In general, using present tendon technology, a tendon in transition from one level of foundation to another should not be bent at smaller than a five-foot radius.  
     [0066] A second tendon  144  may run through the concrete sill  128  on the interior side  132  of the garage  126  as shown in FIGS. 8A and 8B. This second tendon  144  may extend through the sill  128  from the front  125  of the garage  126  to the rear  127  of the garage  126  and through the top of the interior buttress  138  formed on the basement floor slab  114 . The second tendon  144  therefore remains generally level and does not angle downwardly like the first tendon  134  in the beam  124 .  
     [0067] In an alternative embodiment, the garage  126  may be positioned further rearward with respect to the house, for example, as positioned in FIG. 5. In this embodiment, the garage slab  112  would again be formed on grade and there would not be a basement area underneath the garage slab  112  as there is under the structural garage floor  64  of FIG. 5. Recognizing the short length of the garage slab  112  extending in front of the house (again, contemplating the garage slab  112  as formed on grade in the area of the garage  126  of FIG. 5), there may not be enough distance to extend the first tendon  134  through a gradually sloping beam  139  to ultimately extend the first tendon  134  into the basement floor slab  114 . An alternative solution presented by this embodiment is to use the interior wall  84  of FIG. 5 (which in this embodiment would actually be an exterior wall of the foundation) to transition the first tendon  134  from the garage slab  112  at grade to the level of the basement floor slab  114  at the bottom of the basement excavation. The second tendon  144  could extend through the sill  128  from the front  125  of the garage  126  to the rear  127  of the garage  126  and through the top of the interior wall  84 .  
     [0068] Returning to the embodiment of FIG. 7, as shown in greater detail in FIG. 12, a third tendon  150  may be placed in the foundation along the exterior side  130  of the garage  126  to tie the garage  126  into the basement foundation  122  as depicted in FIG. 12. The third tendon  150  may be run through the beam  124  below the garage slab  112 , which extends downward toward the rear  127  of the garage  126  at a gradual slope to meet with the post-tensioned basement floor slab  114 . In this instance, rather than a buttress, the third tendon  150  is run within the length of the outside basement wall  152  to the rear  158  of the basement. In one embodiment (not shown), the third tendon  150  may remain within the outside basement wall  152  parallel to the surface of the basement floor slab  114 . Alternatively, the third tendon  150  may be run within either basement floor slab  114  or the beam portion  154  under the perimeter of the basement floor slab  114 , parallel to the surface  156  of the basement floor slab  114 . Alternately, portions of the third tendon  150  may undulate between the beam  154 , the basement floor slab  114 , and the exterior basement wall  152 . When the third tendon  150  approaches the rear  158  of the basement foundation  122 , as shown in FIG. 12, the third tendon  150  may be angled upward, emerging out of the basement slab  114  to terminate within the outside basement wall  152  at the rear  158  of the foundation  122 .  
     [0069] A fourth tendon  160  may be run through the concrete sill  124  of the exterior side  130  of the garage  126  at or just below the surface of the garage floor slab  112 , extending from the front  125  of the garage  126 , to the rear  127  of the garage  126 , and beyond through the top of the exterior basement wall  152  to the rear  158  of the foundation  122 . The fourth tendon  160  may remain generally level with the top of the garage floor slab  112  for its entire length. The sloping beam  124 , the garage floor slab  112 , the interior buttress  138  on the post-tensioned basement floor slab  114 , and the front basement wall  120  and the exterior basement wall  152  may all be poured monolithically once all the tendons ( 116 ,  118 ,  134 , 144 ,  150 ,  160 ) are in place. Alternatively, these same elements may also be poured in stages, depending upon the practicalities of the design.  
     [0070] When the tendons are stretched and placed under tension, the garage floor slab  112  is then integrated with the post-tensioned basement floor slab  114  and the rest of the foundation  110  as shown in FIG. 7. The post-tensioned garage floor slab  112  may be viewed as being effectively cantilevered from the basement foundation  110 . In this manner, the post-tensioned garage floor slab  112  and the post-tensioned basement floor slab  114  are structurally tied together and the basement foundation  110  and the garage  126  become a single, integral unit. Again, it should be apparent that these inventive techniques described with respect to constructing a garage may be used to structurally integrate different sections of foundation formed at different levels at or below grade to construct a multilevel home. Thus, an integrated, multi-level, post-tensioned basement slab foundation may be constructed, which in turn supports the foundation walls and the superstructure of the house.  
     [0071] In a further embodiment of the invention, as shown in FIGS. 9A and 9B, the post-tensioned basement floor slab  162  is designed to provide counterforts  164  to help resist the horizontal soil forces on the basement walls  166  inward. While interior buttresses as described above work well in opposing the horizontal forces of soil backfill against basement walls, such buttresses may not be preferred as they intrude upon usable space in the basement. In order to strengthen the basement walls  166 , counterforts  164  on the exterior side of the basement walls  166  may be used. A new design for counterforts  164  is provided by the present invention. In this aspect of the invention, and extension  170  of the post-tensioned basement floor slab  162  projects beyond the perimeter  163  of the foundation in the desired location of a counterfort  164 . Reinforcement members  168  may be placed to protrude from the post-tensioned basement slab extension  170  and tie into additional reinforcement members  168  protruding from the basement wall  166  as shown in FIG. 9A. The counterfort  164  may then be formed on top of the post-tensioned slab extension  170  in a first embodiment as a vertical concrete member  165  normal to the adjacent basement wall  166 . However, the counterfort  164  may be formed in another orientation other than vertical. The counterfort  164  may be poured monolithically with or separately from the basement wall  166 .  
     [0072] By integrating the counterfort  164  with the post-tensioned floor slab  162 , greater resistance to fracturing of the counterfort  164  due to expanding soil as well as better counterfort anchoring may be provided. The weight of the soil on the post-tensioned slab extension  170  creates a downward force on the counterfort  164  opposing the horizontal force moment of the soil against the basement wall  166  because of the integral structure formed by the post-tensioned concrete slab extension  170  as the base of the counterfort  164 . The post-tensioned concrete slab extension  170  may also be formed integrally with a beam (not shown in FIGS. 9A and 9B) supporting the post-tensioned concrete slab  162 , which beam may be similarly be constructed to extend beyond the perimeter  163  of the foundation, similar to the post-tensioned slab extension  170  described above, and the counterfort  164  may then be constructed on top of the post-tensioned beam.  
     [0073] In an alternative embodiment, a counterfort  172  may be formed to further act as a window well  174  around a basement wall window opening  176  as shown in FIG. 10. In this embodiment, an extension  178  of the post-tensioned floor slab  162  may extend beyond the perimeter of the post-tensioned floor slab  162 , which may have a width slightly larger than the width of the window opening  176 . A window well  174  may be designed in any desired shape, e.g., rectangular, arcuate, or a combination, to best allow access to the window opening  176  or provide the required strength. An appropriately-shaped window well  174  may be constructed by forming a concrete counterfort  172  on top of the post-tensioned slab extension  178 , to which the counterfort  172  may be integrated by the use of reinforcement members (not shown in FIG. 10) protruding from the post-tensioned basement slab extension  178 . The counterfort  172  may be poured either separately from or monolithically with the basement wall  166  surrounding the window opening  176 . The post-tensioned slab extension  178  would be previously formed integrally with the post-tensioned floor slab  162 .  
     [0074] Alternatively, the counterfort  172  could be of pre-cast construction and placed upon the post-tensioned basement slab extension- 178 . In this arrangement, both the basement slab extension  178  and the basement wall  166  on either side of the window opening  176  may be provided with steel plates (not shown) embedded in the concrete. Opposing steel plates (not shown) may be formed within the pre-cast concrete counterfort  172  to abut the steel plates in the basement slab extension  178  and the basement wall  166  when the pre-cast counterfort  172  is set in place. The pre-cast counterfort  172  may then be affixed to a basement slab extension  178  and the basement wall  166  by welding or bolting each of the opposing steel plates together. Other suitable connection structures may likewise be utilized. By using a pre-cast counterfort  172 , time and money can be saved in the field by dispensing with the need to build an appropriate form and casting the counterfort  172  in place. Economies of scale can be achieved through the mass production of such pre-cast counterforts  172  in a factory environment. However, by connecting the pre-cast counterforts  172  to a post-tensioned slab extension  178 , the beam and bending strength of the post-tensioned floor slab  166  is imparted to the counterfort  172 , providing improved strength in construction over the prior art.  
     [0075] In a further variation on the window well counterfort design, an opening  180  for a sump pump, for example, for use as part of a drainage system for the foundation, may be provided in the post-tensioned basement slab extension  178  as shown in FIGS. 11A and 11B. By combining the basement slab extension  178  with a window well-type counterfort  184 , a sump pump may be placed outside the house within the opening  180  in the basement slab extension  178 , while access to the sump pump is provided by the well  182  within the counterfort  184 .  
     [0076] The post-tensioned concrete slab constructed below grade and supporting the entire mass of a residential dwelling, including foundation walls and the superstructure of the dwelling, has been presented herein as a novel construction alternative to resist the detrimental effects of expansive soils. Such post-tensioned basement foundations are quite versatile. The strength of such foundations allows for the support of heavy loads, for example, a garage, which further helps counteract the expansive soil forces. Post-tensioned foundations according to the present invention may further be composed of multiple levels that are all structurally integrated through the structures and techniques disclosed herein. Further, extensions of the post-tensioned slab concept allow for construction of additional reinforcing features, for example, integrated counterforts, that add to the stability and resistance of the foundation to the forces of expansive soils.  
     [0077] Further, by structurally integrating the foundation walls with the post-tensioned concrete slab formed below grade, and in some embodiments by additionally integrating multiple foundation levels, the entire structure may react to expansive soil forces in a more unitary manner. Because the structure moves as an integral unit, the soil expansion must be sufficient to move the weight of the entire house. As a result, the structure is more likely to remain unaffected by soil expansion forces that may have previously caused cracks to appear in the floor or in the foundation. If the force of the expansive soil overcomes the weight of the entire structure, then the structure will likely react as a single unit, and the foundation and may merely shift slightly rather than causing structural damage.  
     [0078] Although various embodiments of this invention have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention. It is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative only of particular embodiments and not limiting. Changes in detail or structure may be made without departing from the basic elements of the invention as defined in the following claims.