Ground Improvement Methods

Ground Improvement Methods - Grouting 

Course Outline

This three hour online course discusses guidelines and criteria for improving ground conditions by chemical and particulate grout injections. Grouting methods include permeation, compaction, slurry, jet grouting, deep soil mixing and mini-piles. Grouting can be used to modify a soil properties in order to improve performance for slope stability, bearing capacity, seepage instability, groundwater control during construction, excavations in tight places, tunneling and to create subsurface groundwater barriers. Grouting can be used to stabilize sub-grade soil ranging from expansive clays to granular materials. A wide selection of processes and materials are available for the engineer. The course will describe the grouting methods and types of grouts used, suitability for the ground problems and degree of improvement attainable for different soils. 

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:

·Will know how to select the type of particulate grouts and methods suitable for improving the construction quality of existing soils;

·Be aware of the distinguishing characteristics, advantages and limitations between particulate (cement) and chemical grouts;

·Have a better understanding of the methods and objectives of various grouting techniques;

·Understand the conditions where chemical grout is used to reinforce structures and to improve the construction qualities of a soil;

·Be aware of grout mixtures and components used for sound and durable grout applications;

·Be familiar with the methods used to improve construction activities at sites having high water tables;

·Be familiar with various types of soil, problem soils and ground conditions and suitable methods to use for treatment;

·Have a better understanding of the limitations and benefits of grout curtains and slurry walls and the materials commonly used; and

·Understand the hazards of certain grouts and precautions for the protection of workers and the environment.

Intended Audience

This course is intended for civil 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 or seepage 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 they are suspect.

In addition to ground improvement methods, which include compaction, admixture stabilization, soil replacement, dewatering and drain systems, there are deep densification, explosive compaction, and soil reinforcement and grout injection methods. 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. 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 treatment is required for a new or existing structure.

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 installed adjacent to existing walls

g. Can be used to control seepage, groundwater flows and hazardous waste plumes

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. They may not protect from attack from acids, strong salts and some organic compounds. Compatibility of the slurry mix with the contaminants and groundwater must therefore be tested to safeguard against deterioration by groundwater contaminants. Both organic and inorganic contaminates can adversely impact bentonite. For instance bentonite slurry may thicken or flocculate if it is not compatible with contaminants in groundwater. Fly ash can reduce the degradation of sulfate attacks. Other materials such as ground-blast furnace slag and plastic fines may be added to improve the performance of basic slurry mixtures and the permeability of C-B slurry. 

Cement-bentonite walls are similar to S-B walls. However because cement is added to the slurry mix, C-B slurry walls have the following advantages and disadvantages:

a. Used where working room for mixing and placing S-B backfill is restricted

b. Used where ground slopes are too steep to perform site work and grading necessary for S-B walls

c. Used where existing soils require greater stability or have questionable stability since C-B walls are much stronger than S-B walls and have relatively quicker setting times 

d. Backfilling of the trench is eliminated and borrow is not required. This is important if the available soil is unsuitable or is insufficient for the project 

e. C-B walls are limited by its higher permeability and become more porous over time

f. C-B walls have a narrow range of chemical compatibility and are less resistant to attack by sulfates, strong acids and bases

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.

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. 

Permeable Reactive Barrier PRB walls are passive treatment walls because their underground construction intercepts contaminated groundwater and funnels flow through paths of reactive material or "gates". As groundwater flows through the reactive material, chemical, biological or physical processes treat contaminants, which transforms it into harmless byproducts. They can be constructed by excavation and backfill methods or as in most cases by biopolymer trenching. A narrow trench is excavated and filled with biodegradable slurry. Shoring or dewatering is not necessary since the slurry acts as shoring by exerting hydraulic pressure against the trench walls. Sand, zero-valent metals, chelators, sorbents or microbes are mixed at the proper ratios and usually tremied into the excavation. 

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.

Horizontal Subsurface Barriers Until recently only vertical subsurface barrier techniques were available. There have been some testing and installations of the horizontal barrier technology. In one patent pending method, HSSB, air is injected into boreholes under increasing pressure causing the soil to fracture on horizontal planes. A fluid such as a Bingham fluid, a substance that has true plastic behavior, is then injected through the boreholes and spreads along the horizontal plane of soil fractures. Vertical barrier walls may also be used in combination with the HSB to envelop a waste site. There are uncertainties over continuity of the HSB and it would be expensive for large-scale remediation. The barrier however requires minimal excavation, disposal of soil and handling of contaminated soils. 

Course Content

This course is based primarily on Chapters 1 to 3 of the US Army Corps of Engineers Manual, "Chemical Grouting", EM 1110-1-3500 (1995 Edition).

The course is also based on selected paragraphs of Chapter 3 of the US Army Corps of Engineers Technical Letter, "Guidelines on Ground Improvements for Structures and Facilities