Problem Solving

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Introduction

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.

Soil-Related Issues

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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The pictures above show that the impact of swelling soils on a residential structure can be quite dramatic.  If subgrade soils swell beneath exterior footings or interior floor slabs, excessive structural distortion and cracking can occur.

Collapsible and Moisture-Sensitive 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 below.

 

 

 

 

 

 

 

 

 

A discrete prism of soil provides the ultimate structural support for a footing system.  If this soil is not acceptable, it must be removed and replaced with an engineered soil material that provides suitable load support with minimal vertical settlement.

Frost Action

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.

 

 

 

 

 

 

 

 

 

The development of frost lenses below foundations and floor slabs can cause an upward movement of support soils, leading to cracking and distortion.

Construction-Related Concerns

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.