Slab on Grade Foundation Stabilization: Structural vs. Non-Structural
SLAB ON GRADE FOUNDATION SYSTEM
Stabilizing a slab on grade foundation system requires a design approach that mirrors its original design principle. Since shallow bearing foundation systems are designed as single units, they must be repaired as single units. At a minimum, the remedial structural design should uniformly stabilize the entire foundation system so it can resume resisting the soil, dead and live loads imposed upon it. A reliable, durable, and time tested method for uniformly stabilizing slab-on-grade foundation systems includes underpinning, where slab on grade foundation interior and exterior grade beams bear upon and span underpins (drilled and steel reinforced concrete piers, or DRCP’s). This is commonly known as “Full Piering”. DRCP’s convert the slab on grade foundation system from a shallow bearing structural element to one that bears on deep, yet stable soils. DTCP’s allow the engineer to specify and limit the vertical movement of the foundation system over its life, improve slab on grade foundation performance, and minimize foundation superstructure brittle material cracking.
“Partial Piering”, as it is commonly known, limits the number and placement location of underpins beneath a slab on grade foundation system. Partial piering restrains vertical movement near the part of the foundation system where the underpins are placed, and leaves the remaining portion of the foundation devoid of underpins to move unpredictably with the seasonal changes in soil moisture content. Over time, stress builds up between the restrained and unrestrained areas of the foundation system creating a hinge, or weakened area where the concrete can fracture and structurally fail.
The initial cost to underpin slab on grade foundation systems using DRCP’s can appear disproportionally high when compared to other non-engineered structural repair methods such as “partial piering”. Where soils are highly expansive, or when soils shear strengths are unusually low, the owner may decide that slab stabilization using DRCP’s is simply too costly. Cheaper structural repair alternatives to DRCP’s may have a lower initial cost and may appear to initially stabilize the foundation system, but over time, their life cycle costs often exceed typical DRCP life cycle costs. And since “partial piering” fails to uniformly stabilize a slab on grade foundation system over its entire expanse, this repair method is less reliable than DRCP’s and can cause further damage to an already weakened slab on grade foundation system.
Option 1: DRCP’s.
When properly engineered, DRCP’s are reliable, yet costly. For residential foundation systems, these underpins are strategically placed beneath the expanse of the foundation system, usually to depths exceeding eight feet to provide maximum support. Unlike the typical (and inexpensive) multiple segmented concrete pile components stacked atop one another like unstable kinder blocks, DRCP’s are poured in place, reinforced with vertical steel, and placed to bear on deep, yet stable soils, free from seasonal moisture variations. More importantly, and unlike segmented piles (which are often used in “Partial Piering”), DRCP’s provide superlative resistance to soil friction that tends to push the underpin up (float) or push the underpins from side-to-side (lateral). As such, DRCP’s provide a stable bearing surface for the damaged foundation system by limiting its ability to move vertically or horizontally. For a residence of like kind, quality, age and size, 30-40 reinforced concrete underpins drilled to depths exceeding ten feet and placed strategically under the interior and exterior perimeter beams of the foundation system may be expected. The depth, spacing, and number of DRCP’s are determined jointly by competent and experienced Geotechnical and Structural engineers.
Option 2: Non-Structural Repairs to Minimize Foundation Movement and Improve Performance.
Slab on grade foundation systems incorporate design criteria relating to climate, soil, and structure. Reinforced concrete foundation performance can be impacted by post-construction activities unrelated to its core design criteria. If rainfall is allowed to pond or collect adjacent to a structure built on expansive soil, the structure may be subjected to unscheduled distress caused by swelling bearing soils due to increased soil moisture content. Lot surfaces must be graded to drain away from the structure in accord with the International Residential Code R401.3. In accord with 304.100(a)(2) and section 7.3 of the ASCE Guidelines, the following summarizes the recommended non-structural remedial measures:
Roof Rain Gutter System (ASCE, section 7.34). Uncontrolled roof rainfall runoff can erode the ground surface along the foundation perimeter and provide a source of excessive and non-uniform water input to the foundation perimeter beam bearing soils. Variances in bearing soil moisture content distribution along the foundation perimeter can result in unscheduled foundation system vertical displacement and rotational movement. Rain gutters and downspouts should be placed along the entire house perimeter eave lines where the sloping roofline discharges rainfall runoff. The gutters will capture and convey roof rainfall runoff. The runoff is then discharged via downspouts into a ground surface swale, or into a subsurface solid pipe drain system. This type of gutter system will help to eliminate ground surface erosion, and prevent excess water accumulations near the foundation system.
Drainage Improvements (ASCE, section 7.35).
Surface Grading along the Foundation perimeter. For adjacent ground areas, a minimum slope of 5% (6” fall per 10’) away from the foundation should be provided for the first five feet all the way around. Swales shall have longitudinal slopes of at least 2% (6” per 25 ‘) if practical, and 1% (3” per 25’) minimum. Eroded surfaces should be replaced with vegetated surfaces. Gaps between concrete surfaces along the foundation system perimeter that allow surface water to infiltrate into the foundation bearing soils should be eliminated. Concrete surfaces that may allow water to flow towards the foundation system perimeter should be modified to direct water away from the foundation perimeter. Erosion Control. Ground cover should be placed in areas where ground surface erosion currently exists.
Surface Water Drainage Option A. The ground surface should be graded to slope to one or more subsurface solid drainpipe inlets. Cleanouts should be provided at 50 feet intervals for proper maintenance. Roof rainfall gutter downspouts may be connected to the subsurface solid pipe system provided the pipe has sufficient capacity to prevent a backwater condition. The pipe should have a minimum slope of 0.5 percent to the surface outfall. In any case, the ground surface slope along the foundation perimeter must comply with local code requirements.
Subsurface Water Drainage Option B. Subsurface water drains are appropriate to control surface water runoff. They may consist of a perforated pipe placed in an aggregate filled trench along with an optional filter fabric to prevent pipe stoppages. The pipe should have a minimum slope of 0.5 percent to the surface outfall. Cleanouts should be provided at 50 feet intervals for maintenance. In any case, the ground surface slope along the foundation perimeter must comply with local code requirements. Gutter downspouts should not be connected to a perforated pipe system.
Monitor foundation performance after completing all non-structural repair measures to assure their satisfactory implementation.