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Reinforcing Three reinforcing options exist for the design of slabs-on-grade. These are bonded reinforcing (conventional reinforcing), post-tensioning and hybrid systems combining post-tensioning and bonded reinforcing. Any of these systems can produce acceptable performance if properly design and constructed. Design Methods There are three generally accepted design methods available for the design of slab-on-grade foundations on expansive clay. These are:
The Geotechnical Report The design engineer should obtain a geotechnical report prior to designing a foundation on expansive clay. The geotechnical report should contain:
When implementing one of the recognized foundation design methodologies (PTI, BRAB or General Fill In many developments, uncontrolled fill is placed over large areas, commonly resulting from the excavation of roadbeds and general grading for site drainage. This may occur several years before construction of buildings is started. The foundation engineer must ensure that uncontrolled fill has not been placed on the lot, or that the uncontrolled fill has been accounted for in the design. This could be done be removing and replacing the uncontrolled fill, or perhaps ensuring that the grade beams bear on undisturbed soil. Select Structural Fill Select structural fill can be used to reduce the amount of potential vertical movement. If select fill is used, care must be taken to ensure the fill is properly compacted and meets the requirements for select fill. This normally requires the services of a geotechnical testing firm. Consider testing the physical properties of the fill (such as PI and LL) after the fill is placed to ensure compliance with specifications. mpermeable Perimeter Cap Movement of water into and out of the soil under the slab is the primary cause of foundation movement. When the slab is built on select fill, the problem is exacerbated because the select fill frequently extends outside of the building perimeter and is relatively permeable compared to the native soils. This can result in the select fill acting as a bathtub, exposing the underlying expansive clay to large amounts of water (Figure 1). 
Figure 1: Possible Bathtub Effect of Fill. The solution to this problem is to install an impermeable cap of fat clay around the perimeter of the building. Ensure that this cap provides proper drainage away from the foundation (Figure 2). 
Figure 2: Solution to Bathtub Effect of Fill. Beam Bearing Good practice indicates that the beams should bear on undisturbed native soils or compacted structural fill. Foundations that are cast on improperly compacted fill will experience differential settlement resulting from consolidation of the fill. Corners All three design methods treat the slab as a one-way system. However, at the foundation corners, a biaxial state of bending occurs. In slabs with widely spaced beams, the point of maximum moment at the corner may not cross a beam. Additional beams or a diagonal beam running to the corner to the first beam intersection should be considered in these areas. Ground Penetration The perimeter grade beam also serves as a vertical moisture barrier. The deeper the grade beam penetrates into the soil, the more effective it will be in stabilizing the soil moisture. At least a foot of penetration is generally recommended. Drainage Proper drainage away from the foundation is important to maintain constant soil moisture. The minimum slope that is generally accepted is 5% within 10 feet of the building perimeter. This drainage must be established using impermeable fill. The engineer should verify that local drainage is being taken care of by the civil or landscape architects. If this is not the case, proper drainage must be ensured on the structural drawings. Tree Removal Trees significantly alter the soil moisture balance of the soil, reducing the equilibrium soil moisture in their vicinity. If a foundation is constructed over an area where a tree was recently removed, the soil will gain moisture over time and heave after the foundation is constructed. This effect is increased if the tree is removed during a dry period and construction is started soon after the tree is removed. Landscaping Beds Improperly constructed planting beds can result in saturated soil around the perimeter of the building, even when the soil surface nominally has positive drainage away fro the building (Figure 3). This problem is exacerbates if the building is constructed on select fill extending under the planting beds. IN this case, improperly constructed planting beds can act to inject water directly into the select fill (Figure 1). 
Figure 3: Effect of Improper Landscaping Trees Trees planted near a foundation can upset the soil moisture balance due to the water demand of mature trees, especially during drought cycles. While it may take a number of years before the tree gets large enough to cause structural damage, this will eventually occur if the tree is close enough to the slab. In general, the distance from the tree to the foundation must be at least half the height of the tree, but the required distance varies with tree species. The engineer should consider discussing landscaping expectations with the owner and/or architect. If landscaping requirements dictates that trees must be planted near the foundation, the engineer can recommend over-designing the foundation to account for the effect of trees. Plumbing Trenches Plumbing trenches should be backfilled with compacted select fill in order to prevent entry of moisture under the slab through void space in the trench backfill material. Trenches should never be backfilled with sand of granular materials. Consider requiring the use of a fat clay plug at the building perimeter. Slab Reinforcing Overall structural performance of stiffened slabs is generally independent of the performance of the thin slab in the areas between beams. This portion of the foundation slab is generally intended only to acts as a separator between the building and the soil below. However, if thermal or shrinkage cracking is noted in these areas, many owners will perceive the foundation is in a failed state. This is particularly important if the owner anticipates the use of tile or stone finishes. Therefore, performance expectations with respect to slab cracking should be discussed with the owner and architect prior to design. In the past, many engineers have relied on the minimum temperature and shrinkage steel requirements from The engineer should instead refer to |