The following information is provided for an understanding of the different deep foundation support systems, their installations and limitations.

piers (DRILLED SHAFT PIERS)

(Recommended for new construction in all types of soils)

CONCRETE PIER INSTALLATIONS FOR RESIDENTIAL STRUCTURES:

A concrete pier is constructed by drilling a hole in the ground. Concrete is poured into the hole with reinforcing steel (rebar) added to give the pier strength in shear and in tension. The hole can be as big in diameter and as deep as necessary. A concrete pier can provide a sufficient safety factor to prevent future settling or heaving of the pier. A Safety Factor at the time of installation is usually designed to be from 1.2 (it will hold 120% of the weight of the building) to 2.0 (it will hold 200%, or twice, the weight of the building).

PILES (PILING)

(Not recommended for new construction in expansive soils)

CONCRETE PRESSED PILE INSTALLATIONS FOR RESIDENTIAL STRUCTURES:

Concrete pressed piles are typically installed by using the weight of the house as the resistive force against which hydraulic jacks push (press) the piles into the ground. They are pushed to "refusal". This means that the pile will be forced into the ground until it begins to lift the house, at which time they can no longer be pushed further into the ground. The Safety Factor at the time of installation is 1.0

The depth to which they are pushed is dependent upon the resistive force of the soil (skin friction and end bearing) at the time of installation, and the resistive weight of the house. If the depth to which the piles are pushed is insufficient to account for changes in the soil’s capacity to support the weight of the house, they will "fail."

Pressed piles are typically from 6" to 8" in diameter. The larger the diameter the pile, the shallower the resulting installation.

STEEL PILE INSTALLATIONS FOR RESIDENTIAL STRUCTURES:

Steel pressed piles are typically installed by using the weight of the house as the resistive force against which hydraulic jacks push (press) the piles into the ground. They are pushed to "refusal". This means that the pile will be forced into the ground until it begins to lift the house, at which time they can no longer be pushed further into the ground.

The depth to which they are pushed is dependent upon the resistive force of the soil (skin friction and end bearing) at the time of installation, and the resistive weight of the house. If the depth to which the piles are pushed is insufficient to account for changes in the soil’s capacity to support the weight of the house, they will "fail."

Steel pressed piles are typically from 2-7/8" to 3-1/2" in diameter. Some steel piles are open ended and some have solid ends or caps. The larger the diameter the pile, the shallower the resulting installation.

Steel pressed piles are typically driven to depths below the soil’s active zone since the skin friction and end bearing resistances are less than those of concrete pressed piles. The Safety Factor at the time of installation is 1.0

HELICAL PILE INSTALLATIONS FOR RESIDENTIAL STRUCTURES:

Helical Piles were introduced in the Dallas/Fort Worth Metroplex in the 1990’s, by A. B. Chance of Atlanta, Georgia. A helical anchor/pile is a segmented deep foundation system with helical bearing plates welded to a central steel shaft. Load is transferred from the shaft to the soil through these bearing plates. Central steel shafts are available in square shaft 1-1/4" to 2-1/4", or round shaft from 2-7/8" to 4-1/2". The helix sizes can range from 6" to 14" in diameter.

The helical pile is screwed into the ground using mechanical means with shear pins in a rotary mechanism to achieve the desired torque. The Safety Factor at the time of installation is indeterminate, as the depth to which they are screwed is dependent upon the resistive force of the soil at the time of installation. If the depth to which the piles are screwed is insufficient to account for changes in the soil’s capacity to support the weight of the house, or if it terminates within the active zone, they will "fail" as they will move with the active soil.

Since the piles are screwed into the ground, they loosen the soil as they are screwed into the ground, thus their primary ultimate capacity is in end bearing.

PRESSURE GROUTING

Mudjacking

Pressure grout injection, commonly known as mudjacking, is the introduction of cement sand slurry under the slab. This method uses the pressure introduced to raise the slab. The uplifting force can be varied by varying the pressure under which the slurry is injected. A more recent introduction to the pressure injection market is the introduction of a urethane product into the soil to lift and stabilize.

One advantage to mudjacking is the more or less uniform bearing pressure that can be obtained. Another advantage is the introduction of an easy method to fill voids under the slab when the perimeter of a building is lifted, thus producing a uniform bearing surface for the interior of a slab.

There are four major disadvantages to mudjacking.

  1. It fills all of the voids under the slab. If there are open pipes under the slab, they too can be filled with the slurry.

  2. If mudjacking is introduced under a uniform slab and under beams or other heavy areas, the slurry will migrate to the area of least resistance, i.e. the thin slab. This can result in an initial leveling of the floor, but very shortly thereafter heaving of the lightly loaded slab, with settling of the heaver areas. This is the result of the liquidity of the slurry and the tendency to seek a hydraulic balance. It lifts areas with heavy loads less than those with light loads. For example, if used to lift a fireplace and an adjacent thin slab, the thin slab will be lifted and the fireplace will remain the same. This is due to the hydraulics of the lifting process. The upward force will be the same i.e. pounds per square inch on each area.

    1. If the downward force is 500 pounds per square inch in one area and the adjacent downward force is 100 pounds per square inch, the lighter area will be raised and the heavier area will not. There may be an immediate lifting of the heavier area due to the velocity of the pumped material, but in the end, it will ultimately settle.

  3. It is uncontrollable unless in a confined area.

  4. The slurry is injected between the supporting soil and the slab. It relies on the supporting soil being stable. If the soil is expansive, any change in moisture in the soil will result in the soil expanding or shrinking, thus the support mechanism, having moved, can result in the slab moving.

If the soil is in a relatively dry condition when mudjacking, the introduction of additional moisture into the soil after mudjacking results in heaving of the interior. Mudjacking is particularly hazardous when a seasonal moisture change in the soil is a normal situation, such as in the Dallas/Fort Worth Metroplex.

THE EFFECT OF SOIL ON FOUNDATIONS

SOIL ACTIVE ZONE

The active zone is the layer of the soil that is subjected to seasonal volume changes. The extent and condition is particularly important in clay soils that support structures. Clay soils expand when moisture increases, and conversely, shrink when moisture decreases. Structures that are supported on these soils are subject to differential movement. When it is necessary to provide structural support to a building subject to differential movement, the primary support system must be below the active zone and must be designed to be stable when the active zone attempts to move the support system.

The soils in the Dallas/Fort Metroplex are highly expansive. Montmorillonite is the predominate clay soil with a high plasticity index, ranging from 25 to 60 1 . Montmorillonite can expand up to 3- 1 / 2 times its dry volume with the addition of water.

BEDROCK

Geologically, bedrock is Igneous Rock, such as Granite. Igneous rock 2 (derived from the Latin word ignis meaning fire) is one of the three main rock types , the others being sedimentary and metamorphic rock . Igneous rock is formed through the cooling and solidification of magma or lava . Igneous rock may form with or without crystallization , either below the surface as intrusive ( plutonic ) rocks or on the surface as extrusive ( volcanic ) rocks. This magma can be derived from partial melts of pre-existing rocks in either a planet's mantle or crust . Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them having formed beneath the surface of Earth 's crust.

Locally, bedrock is used to describe Sedimentary Rock such as Austin Chalk, Limestone and Shale, all of which are minimally plastic 3 . Sedimentary rocks 2 are types of rock that are formed by the deposition of material at the Earth 's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles ( detritus ) to settle and accumulate or minerals to precipitate from a solution . Particles that form a sedimentary rock by accumulating are called sediment . Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water , wind , ice , mass movement or glaciers which are called agents of denudation .

The sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust. [1] Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks . Sedimentary rocks are deposited in layers as strata , forming a structure called bedding .


Footnotes:

1 Anything over 15 is considered an expansive soil. The higher the PI, the more the tendency of the soil to expand with the addition of moisture.

2 Wikipedia definition.

3 PI’s of less than 10.

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