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Ground Improvement Guidelines

John Poullain, P.E.


Course Outline

This three hour online course discusses guidelines and criteria for improving and modifying ground conditions by various methods including deep dynamic compaction, vibrocompaction, stone columns, gravel drains and prefabricated drains. These methods are used to modify a soil properties in order to improve performance for slope stability, bearing capacity, excavations in restricted spaces, increase resistance to liquefaction, differential settlement, rate of consolidation settlement as well as reduce settlement. They are used to stabilize sub-grade soils ranging from expansive clays to granular materials. A wide selection of processes and materials are available for the engineer. The course will describe their suitability for ground problems and degree of improvement attainable for different soils. Grouting and grout injection methods 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:

Intended Audience

This course is intended for civil engineers and project engineers.


Course Introduction

There are many methods used to modify the engineering properties of soils for ground improvements. Compaction or mechanical stabilization is one of the oldest means of soil stabilization. Soil particles are rearranged and densified to improve the soils' engineering properties of strength, permeability and compressibility. The existing subgrade may have poor strength or instability due to excess clay, expansive clays, silts, fine sands, voids, 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 however can shrink or decrease 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.

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.

Ground improvement methods, which include compaction or densification, admixture stabilization, soil replacement, dewatering and drain systems and also deep densification, explosive compaction, soil reinforcement and grout injection methods. 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.

Dynamic Compaction

This method also called heavy tamping consists of repeatedly dropping a heavy weight or pounder in free fall from a crane. Soil particles are densified by the energy transmitted from the ground surface to the deeper layers, which cause shear and compression waves to force the particles into denser states. The compaction is carried out in several passes along a design grid pattern with each grid point receiving several blows in each pass. A one to two meter thick stiff layer of soil coverage indicates the energy has been transferred to the ground. The distance between compaction pints may be decreased for subsequent passes with lower energy pounding. Compaction is performed between the points already compacted with a final pass or "ironing pass" with reduced dropping heights and lower energy.

The maximum effective depth achieved depends on several factors such as the properties of the soil being compacted and the soil layers above and below, the water table and the height and weight of the pounder being dropped. D the maximum depth of improvement is estimated by the formula D = n (WH) 1/2. The value for the "n" coefficient is determined for each site and varies between 0.3 and 0.7. Because each site will respond differently to compaction efforts due to the physical differences in the soil layers being compacted, compaction trials are made to optimize the process. Deep dynamic compaction has limitations unless it is monitored during compaction to provide data for evaluation of the compaction. Monitors may be installed on the pounder to determine the applied energy and the then correlate these measurements with the recorded dynamic response of the soils.

Vibrocompaction

Vibrocompaction is used to densify loose sands and silty sands, which contain less than 15% for stable foundations. Vibration and water saturation with jetting rearranges and densifies sand particles. The vibration probes are cylindrical often with 12-16 inch diameters and vibrate at about 1600 cycles per minute. The vibrator penetrates the ground to the level necessary by vibration and air or water jetting or vibration alone. Densities above 85% with bearing pressures of 8000 psf are possible.

Stone Columns

Stone columns also called vibro-displacement or vibro-replacement is performed with vertical columns of compacted aggregate in the soils to increase load-bearing capacity and shear resistance. Soils with large amounts of silt or clay do not densify readily with vibratory compaction so columns of aggregate are formed. Vibrating probes create 2-5 foot diameter columns and can be used adjacent to existing buildings with out vibration damages. They are usually used under building foundations.

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, 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 that must 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. Can be performed on almost any ground condition
b. It doesn't induce vibration and can be controlled to avoid structural damages
c. Improvements to ground formations can be measured
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 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 selected paragraphs of Chapter 3 of the US Army Corps of Engineers Technical Letter, "Guidelines on Ground Improvements for Structures and Facilities", ETL 1110-1-185 (1999 Edition, 27 pages), PDF file and the course paragraph "Course Introduction". Please note, pages 58 - 63 covers topics that are not included in this course and were omitted.

The link to the course materials are:

Guidelines on Ground Improvements for Structures and Facilities

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 primary uses for modifying ground formations are to: a) increase the strength and bearing capacity or the soil stability, b) reduce seepage and control groundwater during construction, c) form groundwater barriers and d) rehabilitate or reinforce structures. This course should serve as a guide for determining the method to use for improving a subgrade soil in order to protect against settlement, liquefaction, and ground movement. The advantages and limitations of the methods commonly used are discussed along with methods of application.


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.terrasystems-inc.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.