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Expansive Soil Stabilization

John Poullain, P.E.


Course Outline

This four hour online course discusses guidelines and criteria for modifying and stabilizing expansive clay soils by various methods. The methods covered include injections with water, lime, lime/ fly ash and potassium chloride and mixing soil with lime, fly ash, lime-cement and Portland cement admixtures. Other methods considered are asphalt stabilization and soil encapsulation. These methods modify expansive soils and unstable rock materials in order to improve performance for slope stability, bearing capacity, road pavements, and excavations in restricted spaces, and to reduce differential settlements. A wide selection of processes and materials are available for the engineer and the course describes suitability for given site conditions. Other measures, which protect expansive soils from wide swings in moisture or water movement, such as geotextiles, geomembranes, slurry walls, and surface water and ground water control, are not covered in this course.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will have reviewed the following topics:


Intended Audience

This course is a general guide intended for civil engineers, inspectors, and project engineers.


Benefit to Attendees

The student will become familiar with the guidelines and criteria for modifying and stabilizing expansive clay soils


Course Introduction

If the existing subgrade has poor strength or instability due to excess clay, expansive clays, or other problems such as silts, fine sands, collapsing soils or high watertables, ground improvements will protect from potential settlement and provide the required bearing capacity. 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 clays such as bentonite or montmorillonite can expand and increase in volume when exposed to water. Expansive soils can shrink or decrease in volume when water is not present.

Unstable soils in the United States, which includes expansive clay soils cause billions of dollars of damage to property each year and may exceed the total costs of natural disasters. Expansive soils are a problem in over 30 states especially in Texas, Colorado, Virginia, North Dakota, Oklahoma and Montana. Expansive clay soils are affected by the shrink/swell cycles caused by changes in moisture content during the year. The amount of swell depends on the amount of clay, relative density, location of water table and overburden stress. Affected sites include building foundations, roads and highways, parking lots, building pads, driveways, houses, pools and decks. Damage may range from sticking doors and cracked walls to foundation failures and building condemnations.

Stabilization treatments for expansive soils include:

  1. injection with lime, water, potassium chloride
  2. lime, cement, fly ash and LKD admixtures
  3. groundwater control with slurry wall drains at footings
  4. surface water control
  5. soil removal and replacement
  6. dewatering and drain systems

Selection of the most suitable method for stabilization will depend on the type of soil, degree of improvement and depth and extent of treatment required. Another factor to consider is whether the ground improvement is required for a new or existing structure.

Mechanical stabilization may achieve the desired results by blending two soils and/or mixing with admixtures. If suitable soil was located within a feasible haul distance, blending the soils together could effect an improvement in the existing soil. However the soil blending would introduce ROW, hauling and handling issues to consider. Using chemical or bitumen additives to improve a soil is another possibility but handling and excavation of the existing soil would also have to be considered.  Certain soils because of their chemical nature, organic or high acid compounds may not be responsive to these stabilization methods. Often the soils are not readily distinguished by their classification or physical properties. A pH test will determine organic content of the soil if it is a suspected problem.
     
Other Ground Improvement Methods -Grouting
Grouting is a high-cost treatment method and should be used where there is adequate confinement to handle the injection pressures. The typical applications include control of groundwater during construction, filling voids to prevent larger amounts of settlement, soil strengthening, and stabilization of loose sands, foundation underpinning, filling voids in calcareous formations and strengthening soils for protection during excavation.

Grouting especially with some chemical grouts may present risks to the public health and environment which should be considered. Considerations for utilizing a treatment method include energy use, maintenance costs, requirements for excavation and adequate treatment performance. Environmental risks include mismanagement of surface and groundwater drainage and incomplete treatment. Leachates and migration of contaminants can contaminate subsoil, groundwater, water wells and nearby surface water unless properly managed. There are several ground barrier methods used to control seepage, which include slurry-trench cutoff walls and grout curtains.

The advantages of grouting include:

a. performed on almost any ground condition  
b. vibration is not induces and can be controlled to avoid structural damages
c. improvements to ground formations is measureable
d. very useful for confined spaces and low headroom applications
e. used for slab jacking to lift or level distorted foundations
f. can be performed adjacent to existing walls
g. can be used to control seepage, groundwater flows and also hazardous waste plumes

a. Slurry-Trench Cutoff Walls. Slurry trenching is a method used to retard or redirect the flow of ground water by trenching around a construction area or contaminated site or to contain the groundwater at a contaminated site. The upgradient side of a slurry wall will divert groundwater flow around the site. It is a successful and relatively inexpensive method, compared to sheet pile walls and grout curtains, which has served to make it a replacement method for those methods in some cases. The slurry is either a soil and bentonite (S-B) or cement and bentonite C-B) mixture with water. C-B walls can not completely stop groundwater movements.
S-B slurry walls have been used for decades for cut off walls at dams, at contaminated sites by the petroleum industry and recently at the Boston “Big Dig” project. For this project however the slurry, a clay-water mixture, was displaced with concrete instead of C-B or S-B backfill. Concrete was pumped into the trench and the displaced slurry was re-used. The slurry trench method was an ideal use for the confined spaces and restricted headroom of the densely developed city

b. Grout Curtains Grout curtains are constructed by injecting particulate or chemical grouts under pressure. The types of grout most commonly used are particulate grouts such as Portland cement.  Grout curtains reduce the permeability and increase the mechanical strength of the soils but can be three times more expensive than slurry walls. Because of the expense, grouting is best suited to seal unsound rock and for situations where other barrier walls are impractical. In addition to cost considerations some chemical grouts such as phenolic, acrylamide and polyester are not often used or are not available because their toxicity requires special care in handling and for safeguards after implementation.

c. Microfine Cement Thick slurries can not penetrate fine cracks and higher injection pressures would cause fracturing of ground foundations. Because of the higher water requirements of microfine cement, the slurry remains fluid enough to flow into and penetrate fine sands and small cracks in rock. These cements can treat finer grained sands not possible to treat with Portland cement alone. They are also used to stabilize waste plumes. They are composed of ground slag and Portland cement mixed with large quantities of water or dispersants to become more fluid. Microfines can develop early strength and the thickening time is optimized with retarders. 

d. Jet Grouting Jet grouting is performed with high-pressure jets which discharges cement grout sideways into the borehole to replace most types of soils. The soil is eroded and grout is mixed with the soil during the process. Jet grouting or soil mixing can also place reactive materials. There are single and multi-axis-drilling equipment used to inject reagents suspended in biopolymer slurry into the soil with out excavating trenches. Special mixing tools have been designed for the drills. The grouting has been used for underpinning of structures, cutoff walls for tunnels and open cuts and to consolidate soft foundation soils. The advantages include no material disposal and less working room is required.

Course Content

The course is based on Chapter 7 of the US Department of Transportation – Federal Highway Administration manual, “Geotechnical Aspects of Pavements”, FHWA NHI-05-037 (2006 Edition, 23 pages), PDF file. The course is based on Chapter 7 of the US Department of Transportation – Federal Highway Administration manual, “Pavement Design and Construction Considerations”, FHWA-IF- 03- 019 (2003 Edition, 5 pages), PDF file. The course is also based on Chapters I - III and V of the National Lime Association Bulletin 326, “Lime-Treated Soil and Construction Manual”, (2004 Edition, 30 pages) and the “Injection Systems for Expansive Soils” publication of Hayward Baker. (2005 Publication, 4 pages), PDF file.    

The links to the those documents are:

"Geotechnical Aspects of Pavements”, FHWA NHI-05-037

“Pavement Design and Construction Considerations”, FHWA-IF- 03- 019

Chapters I - III and V of the National Lime Association Bulletin 326, “Lime-Treated Soil and Construction Manual”

“Injection Systems for Expansive Soils” publication of Hayward Baker

Engineering Uses for Fly Ash FHWA-IF-03-019

Please click on the above underlined hypertext to view, download or print the document for your study. Because of the large file size, we recommend that you first save the file to your computer by right clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader. If you still experience any difficulty in downloading or opening this file, you may need to close some applications or reboot your computer to free up some memory.

Course Summary

The course describes methods used to modify or stabilize expansive soils which include a) injections with lime, water or potassium, and b) admixtures with lime, Portland cement, fly ash and combinations. It describes the characteristics of expansive soils and the mechanism of soil injections and admixtures to provide the necessary strength, bearing capacity and stability of soils. This course should serve as a guide for determining a treatment method to protect against settlement, and ground movement. The advantages and limitations of the stabilization methods used are discussed.


Related Links

For additional technical information related to this subject, please refer to:
 

http://www.haywardbaker.com/
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.

http://www.terrasystemsinc.com  
Describes features and applications of dynamic compaction, vibrocompaction, and stone columns, wick drains and other ground modification methods.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.