Soil Stabilization for Pavements
John Poullain, P.E.
This three hour
online course discusses criteria and methods for improving soil characteristics
by the alteration or modification of one or more of a soil properties for improved
performance of roads and airfield pavements. Stabilization can be used for sub-grade
materials ranging from expansive clays to granular materials. A wide selection
of processes and materials are available for the engineer. The course will describe
the types of available additives used for different types of soil, the quantity
to use and the methods of construction used to blend the additives and soil.
Soil additives discussed here include portland cement, lime, flyash, bitumen
and combinations of two or more these additives.
This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.
At the conclusion of this three-hour course, the student will:
This course is intended primarily for civil engineers.
Compaction or mechanical stabilization is one of the oldest means of stabilization. Soil particles are rearranged and densified to improve the soils' engineering properties of strength, permeability and compressibility. Mechanical stabilization may achieve the desired results by blending two soils. An existing soil may have poor strength or stability perhaps because of excess clay, silt or fine sand. If a suitable soil was located within a reasonable haul distance, blending the soils together could effect an improvement in the existing soil. However the blending would introduce ROW, material hauling and handling issues to consider. The benefits of using chemical or bitumen additives are to improve the soils' strength, workability, durability and gradation, reduce plasticity, reduce pavement thickness, for dust control during construction and for soil waterproofing to preserve the soils' natural strength by retarding ingress of surface water. Additives have a wide range of applications including road and airfield pavement foundations, construction staging areas, roads and levees.
There are problem
soils such as loess, hydraulic fills and tailings, which have collapsing or
low-density structures, and when saturated have large decreases in volume and
loss of strength. Other soils, which contain high plasticity clays like bentonite,
are highly expansive and will increase in volume when exposed to water. Expansive
soils however will shrink in volume when water is not present. There are also
dispersive clays so named because the soil particles are not structurally sound
and can easily disperse or detach and erode in still water.
Expansive clays and shales are a widespread problem around the nation. Surveys show as many as 36 states are affected. Expansive clays cause severe damage to buildings and roads. Foundation systems are used for building but are not viable solutions for roads and streets. Some roads built on expansive clays will give the motorist a roller coaster ride. There are too many regions and it would be too expensive to avoid them or haul borrow material. Regions in Texas for instance, are noted for having highly expansive clay soils and/or sulfate bearing clays. Lime treatment of most clay, but not all, is improved with decreased plasticity and reduced volume changes. In 1993 dual component liquid stabilizers were developed to deal with sulfate clay problems. Benefits of some chemical soil stabilizers include the ionizing action in water, which includes +ions exchange at the surface of clay articles in gravel. This process of ionic exchange helps to breakup the affinity between water and clay allowing water to become free water. Also happening during this interaction is the coagulation of clay particles after compaction and some low degree of cementing action occurs.
Injection systems using lime and lime/fly ash or potassium chloride have been used for 30 years. The treatments stabilize subgrades by increasing dry density and shear strength. When lime/fly ash is injected it displaces the water and a chemical reaction occurs. The silica and alumina in the clay and the lime form a cementing compound that is non-expansive. An injection of water may follow to disperse and mix the lime and fly ash into the subgrade soils. Lime treatment of other expansive clays is suitable and beneficial for highway subgrade treatments, RR embankments, runways, bridge approaches and also temporary yard and pads needed for highway construction projects.
For instance in Jefferson County, Texas a soil-stabilizing agent, base-seal liquid soil stabilizer, was used to waterproof about 7 miles of road subgrade. Soils were "black gumbo"; high PI clays that swell when wet and shrink when dry. In this case lime and fly ash additives were not injected as is commonly done but instead were mixed along with the recycled flexible road pavement. The flexible road pavement was removed to the problem subgrade; bitumen liquid soil stabilizer was sprayed for waterproofing. Then the blended recycled asphalt pavement, lime and fly ash mixture was applied to the subgrade in 3-4 inch lifts. A wearing surface flexible pavement completed the road project.
In other applications soil stabilization is used to construct equipment and materials yards required for highway construction. Paved or unpaved areas are required for equipment parking, maintenance, portable batch plants and aggregate stockpiles. Usually thick aggregate courses or stabilized earth pads are used. Lime applications to sulfate bearing clay soils however were not as effective and even counterproductive because gravel had to be placed from heavy rutting caused by trucks and construction equipment loading and unloading stockpiled materials. When sulfate conditions are detected other chemical stabilizers are added to counteract the adverse reactions of silica and sulfates.
Certain soils because
of their chemical nature, organic and high acid soils, may be encountered that
are not responsive to normal stabilization methods. Often these soils are not
readily distinguished by their classification or physical properties. A pH test
will determine organic content of the soil if they are suspect.
The purpose of this course is to provide the student with a general
knowledge and understanding of stabilization of soils and in particular the
most beneficial type or combination of additives and the quantities to use to
achieve the design requirements. The most commonly used additives, portland
cement, lime, fly ash and bitumen, are discussed.
Portland cement is regarded as a suitable stabilizer for many different soils but its effectiveness diminishes for CH and CL soils with a PI greater than about 20. This is due basically because finer soils have greater surface areas and wet soil is difficult to blend with cement.
Hydrated lime is most effective for clay soils or coarse soils with excessive clay fines, especially for expansive soils having high swell potential. Lime has limited use in silty soils and none for sandy soils unless mixed with cement and fly ash (LCF). Although hydrated lime is commonly used, quicklime when used with safety precautions has advantages over hydrated lime; 25% less is needed, strength of wet soils increases faster and its curing time is shorter.
Bituminous additives are not satisfactory to use for clays, silts or granular materials with PI greater than 10 and/or 30% of particles by weight passing the No. 200. Some of the major disadvantages of bituminous materials are: a) large amounts are required, b) the range of soil types is limited, c) the mix design is critical; too little - no change and too much - too "rich" and d) long curing times (2-5 days).
The basic methods used to apply stabilizing additives are admix applications, which include in-place mixing or off-site mixing and surface penetration applications.
You are required to study the Technical Manual, Soil Stabilization for Pavements TM 5-822-14/ AFJMAN 32-1019 of the joint departments of the Army and the Air Force, 1994 edition, 57pages, PDF file format.
The link to the Technical Manual (in PDF format) is:
Chapter 1. Introduction
Chapter 2. Selection of Additive
Chapter 3. Determination of Stabilizer Content
Chapter 4. Construction Procedures
Chapter 5. Quality Control
You need to open or download this document to study this course.
There are three
primary reasons to chemically stabilize a soil when mechanical stabilization
is not adequate: a) improve the strength or increase the soil stability, b)
waterproof the soil and c) for dust control. The course should help determine
the type and amount of additive necessary and the most feasible for improving
an existing soil. The advantages and disadvantages of the commonly used additives
are discussed along with various methods for application.
For additional technical information related to this subject, please refer to:
Information and applications describing construction methods for ground improvement, structural support, earth retention and slurry grouting and necessary materials. Provides solution tools for problem sites and project applications.