Introduction

The greatest economic commitment made by most people involves the purchase of a home and the engagement of an experienced professional to assist in the evaluation of foundation support conditions can greatly help that investment.

Homes are not normally constructed in areas where the underlying support soils are perfect. In many instances, potential home site are selected for a variety of reasons, some of which included availability, costs, proximity to employment, access to transportation routes, and proximity to schools and other amenities. Unfortunately, economics will take precedence in the decision on where to construct homes. Site selection is very rarely governed by the soil conditions at the proposed building site.

It is generally recommended that a geotechnical engineer be consulted to assist in determining if any marginal soil conditions are present at the proposed building site. It is recognized that this level of engineering scrutiny is not typical for residential construction. However, when one considers that the greatest economic commitment made by most people involves the purchase of a home, the engagement of an experienced professional to assist in the evaluation of foundation support conditions is more than justified.

Residential construction often involves the placement of residential foundation systems, basement walls, retaining walls, concrete flatwork and landscaping features on or within challenging soil and bedrock conditions. These conditions can be generally classified by the following conditions.

Soil-Related Conditions

• The presence of swelling soils
• The presence of soils subjected to shrinkage upon drying (desiccation)
• The presence of collapsible soils and/or soils sensitive to the presence of moisture
• Inadequate bearing capacity for foundation system.
• Frost action

Construction-Related Conditions

• The presence of uncontrolled fill
• Non-uniform foundation support conditions
• Areas of poor surface drainage
• Subsurface groundwater due to infiltration and natural subsurface flows
• Leaking water service lines and underslab piping
• Unstable embankment/slope conditions
• Inadequate foundation design for anticipated loads

Lack of attention to the presence of these conditions often leads to substantial economic losses to both residential builders and homeowners. These losses typically take the form of supplemental foundation support (underpins), extensive re-grading of the ground surface, the installation of additional or new subsurface wall/footing drainage systems, concrete slab replacement, and miscellaneous interior repairs to walls, ceilings, doors and windows.

Swelling Soils

Certain types of soils, including highly plastic clays and weathered shale bedrock, are very susceptible to swelling or heaving when inundated with water. Figure 1 depicts the impact of swelling or heaving soils on a residential structure. The physiochemical make-up of these materials allow for this swelling phenomenon, especially when these materials exhibit low moisture contents (very dry and hard) at the time of construction and are then subjected to free water after construction of footings and floor slabs. Swell pressure in excess of 10,000 pounds/square foot and free swells approaching 7% of the wetted layer can occur. It therefore becomes very important to limit the construction of foundations and flatwork on very dry and hard clay soils that may be exposed to free water at a future date. The control of water becomes very important in addressing concerns with swelling soils.

Soil Shrinkage During Drying

Highly plastic clay soils will also exhibit an engineering characteristic called shrinkage when subjected to drying conditions. These types of clay soils will substantially reduce in volume as the clay dries out. This drying phenomenon is often referred to as "desiccation". The desiccation phenomenon can occur following extended periods of hot, dry weather and is often exacerbated or accelerated by the presence of large trees. Trees with root systems that extend both vertically and laterally can effectively reduce the moisture content of soils supporting a residential foundation system, resulting in a reduction in the volume of supporting soil. It therefore becomes important to maintain an on-going residential watering program to minimize the effects of desiccation.

Because of recent drought conditions throughout the United States, the zone of seasonal moisture variation (often referred to as the "active" soil zone) can extend to depths ranging from 15 feet to 20 feet. Obviously, foundation systems constructed for most residences rarely extend to these depths and, therefore, a shallow foundation system is often significantly impacted by fluctuations in soil moisture.

Collapsible and Moisture-Senvitive Soils

Certain types of soils, such as loess (silt) and severely weathered shale bedrock, may collapse or consolidate with the addition of water. Once again, the physical structure of these materials "breaks down" when exposed to free water, resulting in unacceptable vertical settlements. As with swelling soils, the control of water becomes very important in addressing concerns with collapsible and moisture-sensitive soils.

Inadequate Foundation Bearing Conditions

At time of foundation construction, the soil located immediately below the proposed support footing can consist of material that exhibits low bearing capacity. A low bearing capacity can result in excessive foundation settlement as the residence is constructed. Low bearing capacity is typically caused by a combination of low in-place density and high moisture contents. These conditions can often be identified by a pre-construction geotechnical investigation or at time of footing construction. Unfortunately, qualified professionals are not often consulted to conduct investigations or examine bearing conditions at time of construction, and unacceptable bearing conditions are not properly identified and expedited as shown in Figure 2.

Frost Actions

The damage to unheated slabs on grade and unheated shallow foundation systems (such as shallow footings in a crawl space) caused by frost action in soil subgrade constitutes one of the most difficult problems associated with foundation engineering. Although it is often thought that frost heave is caused by the expansion of freezing water within the soil matrix, the actual engineering explanation of frost heave relies on the development of ice lenses. In a frost heave situation, ice layers or lenses actually form in a soil subgrade, causing an incremental increase in the overall volume of soil, resulting in frost heave as shown in Figure 3.

The development of these layers of ice, and the phenomenon referred to as frost heave, occurs in the following manner.

1. The frost line penetrates downward, freezing small volumes of water within the soil. This action actually "freeze dries" the soil.
2. As the soil dries because of the freezing temperatures, an upward capillary force or suction occurs, drawing water from deeper layers of soil.
3. This new water also freezes when it reaches the advancing freeze line, forming new ice crystals and lenses.
4. The layer of ice increases in thickness, much as ice forms on a pond, resulting in a thickening of the ice layer in a downward direction.
5. This thickening results in a heave as long as the ice layers is supplied with water and freezing temperatures exist in the soil.
6. As long as freezing temperatures occur in the soil, a ready supply of water is present to feed the advancing frost line, and the soil is comprised of a material with favorable characteristics for rapid capillary rise of water, the ice layer will grow in thickness, increasing the potential damage from frost heave.

Silty soils represent the soil type most prone to impact from frost action, followed by clay soils with a higher permeability than typical for clay. Coarse-grained soils, such as sand and gravel, are less prone to the effect of frost action. Therefore, if one can maintain the water table well below the ground surface and limit foundation support soils to granular material, the potential impact of frost action can be greatly reduced.

Uncontrolled Fill

Many residential developments require substantial earthwork operations, resulting in cuts and fill throughout the project. If substantial fills are present beneath the footprint of a proposed residence (see Figure 4a, next page), and if this fill was not properly engineered prior to construction and observed/tested for compliance with engineering recommendations during the construction process, substantial time-dependent settlements (approaching 2-3 inches) could occur, resulting in extensive structural damage to the residence.

Non-Uniform Foundation Support Conditions

During footing excavation for residential basement areas, that portion of the residence located in the deepest part of the excavation is typically supported by stiffer soils (or possibly bedrock), whereas the area of the residence located in the area of limited excavation (such as with walkout basements) is supported by significantly more compressible soil, see Figures 4b & 4c. When subjected to the same level of structural load, the stiffer soils will deform (settle) less than the softer soil, resulting in differential settlement from the front to the back of the residence. It becomes very important to place the foundation systems for any residence on material with similar engineering properties to preclude the development of differential settlements.

Area of Poor Surface Drainage

Finish grading around any residence has a direct impact on the future performance of the structure. If surface water is allowed to collect along foundation walls and infiltrate to the foundation level or collect beneath slabs-on-grade, a number of detrimental conditions can result, including swelling, collapse, or heave associated with frost. Figures 5a and 5c depict these detrimental effects. Therefore, it becomes imperative that the final surface grade around any residence be constructed to facilitate the rapid collection and discharge of surface water away from the structure, as demonstrated in Figures 5b and 5d.

Subsurface Groundwater

Water does occur below ground level and results from naturally occurring sources (such as perched water on top of bedrock or infiltrated rainwater) or from artificial water (such as infiltration of lawn sprinkler systems). Water that is moving through the subsurface regime can encounter the basement structure of a residence and may lead to several of the detrimental conditions listed above. Figure 6a illustrates a poorly designed drainage system for a residence. To that end, a well constructed and properly located (relative to depth) foundation/wall drainage system, as shown in Figure 6b, can control these potential negative impacts. If the footing drain is placed below the bearing level of the footing, groundwater can be intercepted prior to saturating the soils immediately beneath the footing, thereby reducing the potential for swell or settlement. Normally, the footing/foundation drain system is connected to a sump pump (or pumps) to facilitate rapid removal of intercepted water. On occasion, the drain system can be "daylighted" to a natural drainage course.
One source of "artificial" groundwater that has become increasingly prevalent as a cause of foundation problems is water released from leaking water service lines and/or leaking underslab sewer drainage pipes. At times, these lines and pipes can be damaged, separated, or otherwise impinged due to damage caused during construction, backfill settlement, inadequate joint gluing, frost action, and a variety of other causes. In some cases, water infiltrating from the surface can also be transported to the residence along the exterior of a pipe within the pipe trench backfill, with the system essentially acting as a conduit to bring water to the basement wall and wall footings.

If excessive and unsubstantiated water usage is noted when paying your water bill, a leaking water service line may be the culprit. If a foundation system has experienced substantial settlements, it is often advised to have your underslab sewer pipes examined with a in-pipe video camera to determine if any portions of the pipe is damaged and introducing water into the foundation bearing zone.

Unstable Embarkment Slopes

With the desire to live in a residence with an aesthetically pleasing backyard view, embankment-like fill slopes and extensive landscaping walls are often constructed. If these slopes and wall structures are not properly designed and constructed, rotational and block-type slope failures often occur at unanticipated future dates. The failures are often initiated during high rainfall events. Rainfall infiltration can reduce the ability of the slope to resist applied structural loads because of possible strength reductions in the fill soils caused by the saturation of the slope. This same infiltration water increases the total weight of the soil in the slope mass, thereby effectively increasing the driving forces "pushing" the slope in a downhill direction. Landscaping construction (such as short retaining walls and supplemental fill) also increases the applied load to the overall slope, further impacting the driving forces acting on the slope. The combination of reduced soil strength within the slope and these increases in driving forces often results in slope failures. In areas where creeks exist along the backside of residential lots, on-going erosion of toe material at the base of natural slope, in concert with the phenomenon listed above, may also contribute to a future slope failures by reducing the overall base/toe support for the upper slope material.

Introduction

Foundation problems are often identified when a homeowner observes or notices cracks (or other anomalies) in various places throughout the residence. If these types of conditions are observed, it is imperative that the homeowner make contact with a qualified professional engineer (either geotechnical or structural) to determine the probable cause of the cracking, the general severity and urgency of the problem, and the appropriate courses of action (if any).

The symptoms described below may be caused by heave or settlement. Settlement (Figure 7a) typically occurs most frequently along the exterior foundation system, whereas heave ( Figure 7b) is typically encountered within interior areas of a basement or crawl space. Of course, exceptions to both of these situations are possible, thereby requiring the involvement of a qualified engineering professional to assess the mechanics of the observed movement.

Symptoms

The symptoms of potential foundation problems generally consist of the following observations

Exterior Cracking and Separations

•  Diagonal Cracking at Corners of Doors and Windows
•  Cracks in the Exposed Grade Beam Foundation Wall or Basement Wall
•  Cracks in Exterior Masonry/Brick Walls and Mortar Joints
•  Vertical and Lateral Cracks or Gaps in Attached Concrete Slabs (Sidewalks, Patios, or Driveways)
•  Gaps between Exterior Trim and Vertical Walls
•  Gaps between Chimney and Exterior Walls

Interior Cracking

• Cracks at Corners of Door and Window Openings
• Cracks in Ceilings and Hallways
• Cracks in Concrete Floor Slabs (Basement) or Floor Areas with Stone Tile

Gaps between Walls and Opening

•  Gaps between Baseboard Trim and Floor Coverings
•  Gaps between Fireplace and Interior Walls
•  Gaps between Soffit/Ceiling and Cabinets

Operation of Doors and Window

•  Difficulty Opening and Closing Doors
•  Difficulty Opening and Closing Windows
•  Cabinet or Cupboard Doors Not Fitting Properly

Miscellaneous Impacts

•  Sloping Floors and Countertops
•  Damaged or Impinged Sub-Slab Piping (Water and Sewer)
•  Damage to Water Lines or Gas Lines Entering Through the Foundation Walls

On occasion, the observed cracking and damage is cosmetic in nature, resulting in no impact to the structural integrity of the foundation or framing of the residence. However, a qualified foundation repair contractor, geotechnical engineer, or structural engineer should be consulted to determine if the problem is indeed cosmetic or of a more serious nature.

Introduction

Introduction

The maintenance of residential foundations is often neglected because the behavior of soil when subjected to drying and/or wetting is not clearly understood by the typical homeowner. Foundation maintenance becomes extremely important during periods of dry weather when the potential for shrinkage of foundation support soils is the greatest. It is imperative that an on-going watering routine be established (see Figure 8) and maintained throughout the year to develop a near-constant moisture condition adjacent to and beneath the foundation of a residence.

The use of a "soaker" hose is oftensuggested, with the hose being located approximately 24 inches to 36 inches away from the residence. It is very important to understand that a watering program must be on going and not just performed when cracks are already present in the soil backfill along foundations or adjacent to basement walls. Understanding that the greatest drying often occurs from July through September, a watering program must be started well in advance of these months. Obviously, the watering program should be adjusted to reflect actual rainfall received on a year-by-year basis.

Additional Maintenance Issues

Other items that must become part of any foundation maintenance program include the following.

• Development of Good Surface Water Drainage
• Proper Foundation Drainage at Basement Level
• Control of Vegetation/Trees
• Regular Observation and Examination of Foundation

Surface Water Drainage

It is extremely important that the surface water drains away the foundation of the residence. If water is allowed to collect against the foundation, and a gap has developed between the soil and the foundation wall, water will have free access to the soil providing support to the residence. Depending on the moisture conditions of the soil, excessive settlement or upward heaving could occur. The lateral forces on the basement wall may also increase, causing wall deflection and damage. Proper grading of the ground surface must be maintained.
Gutters should be clear of all obstructions to allow for rapid discharge of roof runoff water and all downspouts should be located and directed in such a way so that that all water is discharged at least 3 feet to 5 feet away from the residence. Figure 9 depicts a proper approach to the control of water discharge from the roof gutter system.

Basement Wall Drainage

If necessary, a foundation wall drainage system should be established or maintained to ensure that excessive water does not "build up" against foundation walls. The use of a foundation wall drain that gravity drains (daylights) to some type of drainage feature or to a structure-internal sump pump is very important to minimize the impact of infiltration water on basement walls and footings. Excessive water pressure against basement walls can cause inward deflection, or "bowing", of the basement walls and excessive water at the footing level can cause settlement or heaving as described previously.

Control of Vegetation

Highly plastic clay soils will exhibit an engineering characteristic called shrinkage when subjected to drying conditions. These types of clay soils will substantially reduce in volume as the clay dries out, thereby requiring the use of a watering program as described earlier.

This drying phenomenon is often referred to as "desiccation". The desiccation phenomenon can occur following extended periods of hot, dry weather and is often exacerbated or accelerated by the presence of large trees.

As depicted in Figure 10, trees with root systems that extend both vertically and laterally can effectively reduce the moisture content of soils supporting a residential foundation system, resulting in a reduction in the volume of foundation support soil. If large trees are present in close proximity to a residence, it becomes doubly important to maintain anon-going residential watering program to minimize the effects of desiccation.

Regular Inspections and Observations

During times of excessive rainfall or extreme drought conditions, a prudent homeowner will inspect the area around the home's foundation, and the foundation itself, to note any signs or indications of movement that may indicate a potential problem. If a problem is noted, a qualified engineer or construction professional should be contacted to determine the significance of these observations. When is comes to dealing with potential foundation problems, "An ounce of prevention is worth a pound of cure".

Introduction

To mitigate the problems often experienced by residential foundations, a variety of techniques and systems are available for consideration by the homeowner. As discussed in the section entitled Foundation Maintenance, a number of techniques are available to control issues such as surface/subsurface water drainage, desiccation/shrinkage of foundation soils, and the impact of vegetation.

Drainage Issues

In general, negative impacts on subgrade soil caused by drainage issues (as shown in Figure 11), such as swelling of foundation or subslab soils, can be easily addressed by using the following approaches.

1. Ensuring proper slope of the ground surface away from foundation areas
2. Install a properly designed and installed perimeter drain
3. Provide downspout extension and proper guttering
4. Install sump pumps/foundation drains to control subsurface water

Figure 12 illustrates a proper approach to residential water control.

For areas subject to extended periods of hot, dry weather, the use of a "soaker" hose or the institution of a regular lawn watering program will minimize the effects of desiccation (drying) of foundation soils, thereby minimizing potential vertical movements.

Dessication

Problems with desiccation caused by vegetation/large trees can be also addressed through an active and aggressive watering program.

Additionally, especially for new construction, trees and large shrubs should not be located in close proximity to the residence (as shown in Figure 13). Rather, trees should be located away from the residence as depicted in Figure 14 to minimize the impact of root systems of the performance of foundation soils.

Problems associated with an inward bowing/deflection of a basement wall, as possibly caused by swelling soils or excessive hydrostatic (water) forces, can best be addressed by using the following approach.

1. Completely Excavate and Remove Existing Wall Backfill
2. Straighten Wall by Removing Structural Load from Wall with Temporary Jacks
3. Install Drainage System -Wall Drain With Sump Pump
4. Replace Backfill with Clean Granular Material

Another approach to stabilizing walls that have experienced inward lateral deflections is through the installation of helical tiebacks as depicted in Figures 15a and 15b, below.

Heaving of Basement Floor Slabs

Problems associated with heaving of basement floor slabs can be addressed by using the following general approach.

1. Remove the Affected Portions of the Floor Slab
2. Underpin any Column Supports
3. Remove and Replace Floor Slab Subgrade with Low Swell Material
4. Install Underslab Drainage System
5. Provide Isolation Joints Around Column Pads
6. Install Collapsible Void / Expansion Allowance at Base of All Stud Walls Supported on the Floor Slab

Settlement of floor slabs or exterior concrete slabs can be addressed by using supplemental support components comprised of helical piles (see Figures 16 and 17 below).

Concerns with inadequate foundation support conditions or potentially excessive settlements (for new or existing structures) can be easily addressed with supplemental foundations systems comprised of helical piles or resistance piers.

For the construction of new residences, a helical pile system similar to the system depicted in Figure 18, can provide an economical and dependable foundation system for a variety of marginal soil conditions.

Unacceptable Settlements

For existing foundation systems that have experienced unacceptable settlements, several approaches could be used to mitigate the problem. Helical piles (Figure 19) or push piers (Figure 20) can be installed to provide the necessary supplemental support to reinstate the structural integrity of the foundation system. In some cases, "lifting" of the foundation system can be undertaken to restore the foundation, and the superstructure being supported, to its original position. Mudjacking, which is a low pressure grouting technique used to fill voids with a cementitious grout, is often used in conjunction with supplemental underpins. The sketches presented below provide a general depiction of these two techniques.