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Which Way Is It Moving?
Guidelines for Diagnosing Heave, Subsidence and Settlement
Ron Kelm, P.E. | Nicole Wylie, P.E. | Forensic Engineers Inc. | Houston TX | www.forensicengineersinc.com
Basically heave occurs because the moisture increases in an active soil. At the molecular level, a negative pressure potential (suction) in the soil particles attracts water molecules to a tight bond around the surface of the soil particles. Because water is incompressible, the clay particles are forced apart, causing soil movement. The graphics below (courtesy of Environmental Soil Stabilization, LCC) demonstrate this behavior.
Clay particles exhibit a net negative charge and pack tightly when dry
When water becomes available, it is attracted by the clay’s negative charges, and bonds tightly to the surface of the clay.
Because water is incompressible, it pushes the clay particles apart, causing an expansion or swelling of the clay.
It will occur in any direction the laws of physics will allow. At deeper penetrations, the soil + water particles will be restrained, reducing the potential for attraction of water particles and subsequent expansion. Closer to grade, depending on the amount of overburden from the soil and structures, the soil movement will expand upward, defined as “heave” and sometimes laterally, commonly known as “walking” when observed in flatwork or other foundations.
Most certainly. As water leaves an active soil, the gaps between soil particles close and shrinkage occurs, the vertical downward component of which is defined as “subsidence”. This is the usual reason for needing to lift an older foundation in the Houston area. Builder’s piers are commonly added to new slab-on-grade foundations in the Houston area to later prevent or minimize this effect from newly planted trees.
The most common reason is trees are planted or naturally occur, mature, and are then removed. Tree or other large vegetation roots are capable of removing water from active soils such that subsidence occurs. This takes years or decades as the tree matures and its roots propagate deeper and farther away from the trunk, creating a powerful suction. However when the trees are removed, the sudden lack of water uptake by the tree creates an imbalance, with the ongoing soil suction attracting available moisture from any direction. This moisture movement can happen over several months or several years depending on the quantities of cracks and root channels in the soil fabric. Rehydration continues until the soil reaches moisture equilibrium, determined by the amount of available moisture.
The below graphics illustrate the cycle from subsidence to heave that is commonly experienced in the Houston area:
At year 1, a small tree is planted (or naturally occurs) in clay soils. The soil around the tree has uniform moisture content; the soil is in equilibrium. The tree receives water and sunlight, and it thrives.
By year 30 (depending on the species and other factors), the tree is mature. The above graphic shows the typical cycle of drawing large quantities of water from the soil and releasing nearly all of it into the air by the process of transpiration. Depending on the surrounding vegetation and cover, a mature tree often requires more groundwater than the environment can supply and consequently the soil in the root zone becomes desiccated, creating a high suction potential in the structure of the clay. As the clay soil is desiccated it subsides, as shown in the graphic above. The dashed line is the original soil level.
The following year, the mature tree is felled so that a house can be constructed on the site. The high suction potential in the clay structure is unchanged. Before the foundation is placed, compacted fill is added to the subsided area to level the pad. Some of the upper tree roots may be removed prior to adding fill, though this is not always the case. Then the foundation is placed, often very soon after tree felling, before the soil has a chance to reach moisture equilibrium. The roots and rootlets (i.e., hair-like roots) in the soil shrivel up and die after the tree is felled, and in their place they leave tiny hair-like channels.
Capillary action is a natural occurrence via the rootlet channels, as moisture seeks to reach equilibrium and migrates from the wet soil to the dry desiccated soil, i.e., the soil with a high suction potential. With the tree gone, the transpiration has stopped, so that the moisture pulled in by capillary action is abundant in the soil and rehydrates it.
All new foundations provide a prime condition for water collection as their coverage stops the normal evaporation process that existed before construction. By the time construction is complete, the clay soil has already begun to rehydrate. As the rehydration occurs over a period of (typically) years, it causes the formerly desiccated and subsided soil to expand and heave. The fill, foundation and house are forced upward over the root zone, leading to differential movement of the foundation and cracking in the superstructure. The further from the edge of the foundation that the felled tree occurred, the longer this process usually takes, unless an interior water source, such as a plumbing leak, is present. Poor drainage, i.e., soils sloping towards the foundation, often causes the process to speed up, as more water is available for rehydration.
After the soil rehydrates, the soil suction drops and the capillary action ends. The moisture content in the previously desiccated soils will tend to go above moisture equilibrium as compared with the surrounding soils that are uncovered. However, another source of water, such as poor drainage or a pool or plumbing leak, will bring even more water to the pad area, making the soil too wet and causing a myriad of other problems for the foundation.
Subsidence is the reverse of heave. The below graphics illustrate how subsidence commonly occurs in the Houston area:
At year 1 the foundation is in place and a young tree is planted near the foundation. The soil is at a uniform moisture condition and the tree will grow. If there are gutter downspouts that exit at grade and flowerbeds installed around the house that are regularly watered, as is typical in new construction, the tree will receive extra water from the foundation side, and it will thrive.
As the tree grows, it desiccates the surrounding soils, including those soils at the edge and somewhat inward of the foundation. If poor drainage or an interior sewer leak is present, the tree will direct its roots towards and possibly under the foundation, causing the desiccation to reach even further under the foundation. As the soils subside, they take the shallowly supported foundation (i.e., a slab-on-grade or a foundation with piers founded in the moisture active zone) down with it, leading to differential movement of the foundation and distress in the superstructure. This is depicted in the above graphic. The mature tree, at ±30 years, depending on the species, has desiccated the soil, causing significant subsidence and the right edge of the foundation has subsided as well, damaging the superstructure.
The graphic below is an example of cyclical subsidence (the thick red line) of a foundation founded in clay soil due to the growth of a tree over a period of 9 years. The graphic also shows that after the offending tree is felled, the soil rehydrates over a period of several years, but it never reaches its original level. There is a portion of the downward movement labeled “Final distortion” that cannot be reversed except by adding soil.
Plot showing cyclical and permanent subsidence over time and partial heaving after tree felling. (taken from “Tree Root Damage to Buildings” by P.G. Biddle, 1998)
Note that Dr. Biddle’s labeling of “Dynamic settlement” in the above graphic should not be confused with our definition of settlement. His “Dynamic settlement” is actually a final vertical distortion of the soil that is not recoverable. Also note that the label “Recovery” is the rehydration or heave that occurs when a large tree in active clay is removed.
Settlement occurs when the vertical loads from above are in excess of the bearing capacity of the soil strata directly below the foundation, causing the foundation and superstructure to move downward. There are three common types of settlement: immediate, slope instability and long-term.
Immediate settlement, as the name implies occurs immediately upon loading due to an elastic consolidation and distortion of granular or clay soil particles. Adding a second story to a foundation designed for one-story loads may cause immediate settlement if the new loads exceed the bearing capacity of the clay soil strata beneath the foundation. The bearing capacity, directly related to shear strength, can be easily measured using a pocket penetrometer. Measurement of a clay soil’s shear strength is the best indicator to determine whether or not the movement type is settlement. Minimum values for the ultimate shear strength for several types of structures are given in the table below:
Failure Shear Strength of Clays at Grade Beam Depth*
* Based on 12″ wide grade beams, typical loading conditions, typical geometry of dimensional wood framing, standard bearing capacity theory for strip footings 12” below grade, without safety factors
Please note the more accepted method of determining undrained shear strength is a laboratory test called the unconfined compression test. However a simple field test using a pocket penetrometer gives acceptable results in the Houston area’s weak, homogeneous clays. The unconfined test should be used though in cases of non-uniform sediments, such as slickensided clays. From the above table, it can be seen that even for a typical three-story brick veneer structure, settlement will not be suspect unless the pocket penetrometer readings at the grade beam bearing levels are less than 0.5 tons/square-foot.
Slope instability settlement occurs when a foundation is built upon sloped land and does not penetrate deep enough. When the slope fails due to earthquake, landslide, faults, floods, or other natural occurrences, the foundation loses part of its stability and can move down and away with the soil.
Long-term consolidation settlement is caused by gradual expulsion of pore water from voids between saturated clay soil particles.
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