Smith Custom Builders- Articles- Clay Soil Results in a Foundation of Woes

  As the soil expands or contracts. a well-designed slab will withstand the pressures created by the soil movement, and substantially maintain its initial geometry. The structure above will remain intact and will be spared the defects commonly associated with foundation movement.
  Stiffening the slab is effectively accomplished by the placement of stiffening ribs, known as grade beams (because they are formed by trenching the grade). The result is that a waffle-like grid is created under the slab that considerably enhances the slab performance.
REINFORCEMENT
  Reinforcing steel is placed within the concrete slab and grade beams to develop strength and to counteract the natural tendency of concrete to shrink and crack. Reinforcement is typically of two types: conventional steel and post-tensioned cables.
   Conventional steel reinforcement consists of deformed bars that bond with the concrete as it hardens.
Post-tensioned cables are strands of high-strength steel that are housed in plastic sheathing. The strands are tensioned and anchored at the slab edges after the concrete has hardened, leaving the slab and grade beams in a compression mode. So Iong as the concrete is in this mode, no cracking is possible in it.
  Understandably, the size of concrete elements and the amount of reinforcement provided in them are determined by the anticipated pressures. Slab and foundation systems supported on soils with high clay contents will require heavier foundations than those constructed on lower clay soils.
DRILLED PIERS
  Drilled piers. usually under-reamed (belled) at the bottom. are sometimes used as part of the foundation system and offer several advantages.
  They transfer the loads from the structure above to deeper soil strata that may be stronger than the surface soils and that are less exposed to the moisture fluctuations that cause them to heave or contract.
  The second advantage is that piers prevent slab and foundation settlement if the near-surface soils do contract.
  Another advantage is that piers may act as deep anchors that prevent the foundation system from moving upward if surface soils heave.


Clay Soil Results in a Foundation of Woes in Houston Construction Houston Business Journal- Week of April 26 - May 2, 2002
BY: DOUGLAS SMITH Smith Custom Builders Most commercial buildings and homes in the greater Houston area have foundations that are supported on soil (rather than rock, for example). Consequently, the characteristics and behavior of the soils have a significant impact on the performance of the foundation system. Soil is a compressible material; it "gives" when load is applied on it. Settlement of foundations occurs when the load is excessive for the supporting soil. It is not unusual in the design of multi-story com- mercial buildings to plan around potential settlements of several inches. Most houses,. on the other hand, are lightly-Ioaded structures which apply relatively small loads. These loads will result in minor settlement in the foundations. So why do many constructions in the Houston area have so many foundation problems? The answer is the type of soil on which they are built. A large portion of the soil in Houston has a high content of clay, which is usually very sensitive to moisture fluctuations. Introduction of water into clay causes it to expand and heave upward, subjecting foundations to pressures from underneath. Conversel,. if water is with- drawn. these clay soils will contract, sometimes creating voids that leave portions of the foundations unsupported. The successful design of foundation systems for any construction in the Houston area starts by analyzing and understanding the types and behavior of soil. Founda- tion type and features at a particular lot are chosen on the basis of soil characteristics. The higher the clay content, the stronger and stiffer the foundations have to be. SLAB-ON-GRADE The most commonly-constructed foundation systems include a concrete slab-on-grade. Successful design of all these foundations is predicated on detailing a slab stiff enough to counteract the effects of soil movement.	As the soil expands or contracts. a well-designed slab will withstand the pressures created by the soil movement, and substantially maintain its initial geometry. The structure above will remain intact and will be spared the defects commonly associated with foundation movement. Stiffening the slab is effectively accomplished by the placement of stiffening ribs, known as grade beams (because they are formed by trenching the grade). The result is that a waffle-like grid is created under the slab that considerably enhances the slab performance. REINFORCEMENT Reinforcing steel is placed within the concrete slab and grade beams to develop strength and to counteract the natural tendency of concrete to shrink and crack. Reinforcement is typically of two types: conventional steel and post-tensioned cables. Conventional steel reinforcement consists of deformed bars that bond with the concrete as it hardens. Post-tensioned cables are strands of high-strength steel that are housed in plastic sheathing. The strands are tensioned and anchored at the slab edges after the concrete has hardened, leaving the slab and grade beams in a compression mode. So Iong as the concrete is in this mode, no cracking is possible in it. Understandably, the size of concrete elements and the amount of reinforcement provided in them are determined by the anticipated pressures. Slab and foundation systems supported on soils with high clay contents will require heavier foundations than those constructed on lower clay soils. DRILLED PIERS Drilled piers. usually under-reamed (belled) at the bottom. are sometimes used as part of the foundation system and offer several advantages. They transfer the loads from the structure above to deeper soil strata that may be stronger than the surface soils and that are less exposed to the moisture fluctuations that cause them to heave or contract. The second advantage is that piers prevent slab and foundation settlement if the near-surface soils do contract. Another advantage is that piers may act as deep anchors that prevent the foundation system from moving upward if surface soils heave.	A variation on the conventional slab-on-grade is a structural slab that is separated from the grade by the use of void boxes. These boxes are typically made of wax-impregnated cardboard on which the concrete is poured When the concrete hardens, the slab will span between the grade beams and will not be supported directly on the soil. The soil may heave without exerting any pressures on the underside of the slab. PIER-AND-BEAM Some foundation systems do not involve a concrete slab. The floor is framed with wood stringers and joists (or trusses) that are raised above the ground, typically three to four feet. The wood framing is supported on continuous grade beams and pony walls, or on pilasters, usually of concrete masonry block (concrete masonry units or CMUs), supported either on drilled piers or spread footings. Again, the choice of footings depends on the type of soil. This system is known as a crawl-space foundation system. and is commonly referred to as a pier-and-beam system. Many older houses, and a large number of new houses in the Heights are,. are constructed with this system. Other foundation systems, such as driven piles, may be used, but their use is infrequent and generally restricted to certain localities. Such piles are very common in Galveston, where flood plain require- ments dictate that the finish floor is lifted considerably above the ground .

DOUG SMITH is owner of Smith Custom Builders, which recently was named winner of the Greater Houston Builders Association's Prism Awards for custom homes under $500,000 and remodel under $250,000. Engineer M.F. QADDUMI, owner of The Interfield Group, contributed to this commentary.