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Article # 0034
MOVEMENT OF SHALLOW CONCRETE
SLAB-ON-GRADE FOUNDATIONS
By Terry Dyson, PE
Shallow concrete slab-on-grade foundations are generally utilized for residential construction, light commercial construction, warehouse floors, driveways and parking lots. Slab-on-grade foundations generally have thickened perimeters, floor slabs of 4 to 6 inch thickness and may or may not have shallow integral interior beams. Reinforcement of the concrete slab-on-grade foundations can be conventional steel reinforcing bar, post tensioned cabling or no reinforcement at all.
Definitions:
“Dictionary of Architectural & Construction” Third Edition by Cyrill M. Harris
Foundation:
Any part of a structure that serves to transmit the load to the earth or rock.
Structure:
A combination of units constructed and so interconnected, in an organized way, as to provide rigidity between its elements.
For this discussion roadways and parking lots will be considered as shallow concrete slab-on-grade foundations.
Foundation Movement:
Foundation movement can either be vertical or lateral. Foundations can move laterally due to thermal expansion and contraction of the foundation and due to major ground movement such as earthquakes. For this discussion, we will concentrate on vertical foundation movement – heave (uplift) and settlement of shallow foundations.
Causes of Movement of Concrete Slab-On-Grade Foundations:
The causes of settlement of concrete slab-on-grade foundations are varied and generally due to the following.
1. Earthquakes.
2. Hurricanes, Storms & Floods.
3. Inappropriate Fill.
4. Inadequately Compacted Fill and Subgrade.
5. Changes in Soil Moisture.
6. Poorly constructed foundations, which is not included as part of this discussion.
Earthquakes:
When the ground moves, everything moves which is the problem with earthquakes. Earthquakes have minor impact in Texas.
The most advantageous method to avoid earthquake impacts to foundations is not to construct in earthquake prone zones.
Hurricanes, Storms & Floods:
Hurricanes, storms and floods generally create sudden impact to foundations and structures. The wind action such as from hurricanes and tornadoes can have a physical impact to the foundations but generally less than to the supported structure.
Hurricanes and floods can undermine a foundation by erosion of the soil resulting in movement and potential failure of the foundation. Storm water can also weaken supporting soils on slopes with resultant landslides and failure of the foundations supported by these soils.
Damage from floods and hurricanes can be mitigated by not constructing in such zones or on slopes that are prone to failure.
Inappropriate Fill:
To create a level building pad, the fastest and cheapest method is to cut down the high side of the site and move this cut material to fill the low side of the site, thus the term cut and fill.
The problem that arises in many cases is that the site is not first grubbed which is the removal of the vegetation and humus layer before the cut and fill is undertaken. The vegetation and humus layer then become incorporated into the fill material. The incorporation of the vegetation and the humus layer into the fill material (soil) results in a non-homogenous fill that does not have the same bearing capacity of the underlying and exposed soils. Even if the fill is adequately compacted, this non-homogenous soil will shrink as the vegetation and humus material decomposes with resultant foundation settlement. The importing and use of inappropriate fill can also create this condition.
Mitigation of potential foundation movement from inappropriate fill starts at the beginning of the preparation of the building pad with adequate grubbing of the site before the cut and fill are undertaken and with the use of imported appropriate fill materials.
Inadequately Compacted Fill & Subgrade:
When fill is placed and the subgrade is disturbed from the construction activities, a technique of compaction is to run a tractor or bulldozer across the subgrade and fill which is generally not an acceptable method of compaction.
Too often, the speed of construction takes precedent and the depth of fill is inadequately controlled which results in an increased depth of the fill which can not be adequately compacted. There are also times when the fill is just placed and spread without compaction or minimal compaction that only compacts the surface layer of the fill.
Inadequately compacted fill and subgrade will settle through time resulting in settlement of the foundation.
Changes in Soil Moisture:
Even if the fill and subgrade are prepared correctly, there are still potential problems with changes in soil moisture.
Soils will experience a change in volume with a sufficient change in moisture content. This change in volume is related to the change in soil moisture and the expansive properties of the soil. Change of soil moisture is either desiccation of the soil (loss of soil moisture) or increase of soil moisture.
Loss of sufficient soil moisture will result in shrinkage of the soil. The amount of shrinkage is dependent on the amount of soil moisture loss and dependent on the expansive properties of the soil.
Sufficient increase of soil moisture will result in expansion of the soil. The amount of expansion is dependent on the amount of moisture added to the soil and dependent on the expansive properties of the soil. Soils can become sufficiently saturated that the soils decreases in load bearing capacity. This is demonstrated by a soil that will support foot traffic, but when sufficiently saturated, the soil looses bearing capacity and will no longer support foot traffic. Consistent wetting of the soils can also result in loss of fines and loss of bearing capacity of the soil.
Settlement of Shallow Concrete Slab-On-Grade Foundations:
Even though inappropriate fill and inadequately compacted fill can impact the settlement of concrete slab-on-grade foundations, the greatest cause of settlement is due to desiccation of moisture from the supporting soil by cyclic changes in the weather and from trees and vegetation.
Dry weather phases will desiccate the soil around the perimeter of the foundation and the desiccation of the soil will extend under the foundation. The corners of the foundation will show shrinkage of the soil sooner due to having two edges of the foundation exposed to the dry weather phases. With the shrinkage of the soil, the corner of the foundation settles.
Tress and vegetation around the perimeter of the foundation will also desiccate the supporting soil under the foundation. Trees will grow roots under the foundation to obtain moisture from the soil with resultant shrinkage of the soil and settlement of the shallow concrete slab-on-grade foundation.
Heave of Shallow Concrete Slab-On-Grade Foundations:
Addition of sufficient moisture to expansive soils under foundations can result in expansion of the soil with resultant heave of the foundation. This moisture can come from leaking domestic water piping, from leaking sanitary sewer piping and from ponding water along the foundation perimeter.
If the supporting soils become sufficiently saturated, the supporting soils loose their bearing capacity and the foundation will settle.
Foundation Movement Indicators:
Residential and light commercial structures constructed on shallow concrete slab-on-grade foundations will show foundation movement generally as:
1. Cracks in the exterior façade.
2. Cracks in the interior walls and ceilings.
3. Cracks in the foundation.
4. Doors and windows that will not correctly open and close.
Roadways and parking lots will show movement generally as:
1. Cracks in the concrete pavement.
2. Vertical movement between adjacent sections of pavement across joints and across cracks.
3. Settled pavement may also result in areas of ponding water.
Many warehouses are constructed with leave-outs in the floor slab around the perimeter of the building to allow the interior slabs to be constructed and then the exterior walls to be constructed. The backfilling of the soil along the exterior wall can be impacted by improper fill and inadequate compaction resulting in settlement of the leave-out floor slab and opening of the joint between the leave out floor and the interior floor.
Summation:
Foundation levelness is a function of time, subgrade characteristics and soil moisture content. Additionally, the foundation levelness can be influenced by the original design, construction of the foundation, and subsequent movements in the subgrade soil due to the geotechnical properties of the soil, perimeter grading and surface drainage conditions, irrigation or lack of irrigation, the compaction or lack of compaction of any fill materials and the subgrade, plumbing leaks and remedial work to the foundation.
The levelness of slab-on-grade foundations can also be impacted by the increase of and loss of moisture from expansive soil supporting the foundation. Loss of soil moisture in the expansive soil can result in the shrinkage of the soil, loss of support to the foundation and resultant settlement of the foundation. Increase of soil moisture in expansive soil can result in the swelling of the soil, which can result in heave (uplift) of the foundation. Soil moisture content is influenced by changes in climatic conditions during the changing seasons, irrigation or lack of irrigation, by landscaping, by trees and by plumbing leaks.
Prevention and Correction of Movement of Shallow Foundations:
Movement of shallow concrete slab-on-grade foundations can be mitigated by:
1. Installation and use of an irrigation system through all four seasons.
2. Installation of root barriers at the location of trees.
3. Tapering of soil away from the foundation.
4. Installation of piers and/or footings.
5. The use of appropriate fill and adequate compaction.
Article # 0034 TEST QUESTIONS:
TEST QUESTIONS:
1. Foundations are?
a. Any part of a structure that serves to transmit the load to the earth or rock.
b. Constructed of concrete.
c. Constructed of wood.
d. All of the above.
2. Foundations move?
a. Vertical.
b. Lateral.
c. Neither a nor b.
d. Both a and b.
3. Which is not a cause of movement of shallow concrete slab-on-grade foundations?
a. Earthquakes
b. Inappropriate fill.
c. Changes in soil moisture.
d. Adequately compacted subgrade and fill.
4. Changes in soil moisture can result in?
a. Shrinkage of the soil.
b. Expansion of the soil.
c. No change.
d. All of the above.
5. What can cause shrinkage of soils under shallow foundations?
a. Dry weather phases.
b. Trees.
c. Vegetation.
d. All of the above.
6. What can cause expansion of soils under shallow foundations?
a. Leaks in domestic water piping.
b. Ponding water along the perimeter of the foundation.
c. Trees.
d. Both a and b.
7. What are the foundation movement indicators in residential and light commercial construction?
a. Cracks in the exterior facades
b. Cracks in interior walls and ceilings.
c. Doors and windows that will not open and close correctly.
d. All of the above.
8. What are the foundation movement indicators in roadways and parking lots?
a. Cracks in the concrete pavement.
b. Ponding water.
c. Neither a nor b.
d. Both a and b.
9. Foundation levelness is a function of:
a. Subgrade characteristics.
b. Soil moisture.
c. Neither a nor b.
d. Both a and b.
10. What preventative and corrective actions can be taken to address movement of shallow foundations?
a. Installation of piers and/or footings.
b. Installation of root barriers.
c. Installation and use of an irrigation system.
d. All of the above.
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