Soil Stabilization by Injection Techniques J O H N P. G N A E D I N G E R SOIL TESTING SERVICE. INC.. 3521 N O R T H CICERO AVE.. CHICAGO 41.
ILL.
M o s t of t h e recent work t h a t has been done on t h e altering of soil properties w i t h chemicals has pertained t o surface applications, w i t h or without physical mixing of t h e chemical with t h e soil. There are, however, a number of problems in t h e field of civil engineering t h a t could be solved effectively if chemicals could be injected into t h e soil t o react within t h e soil mass and render t h e soil impervious and cohesive. Principle problems discussed include t h e prevention of seepage into underground structures, construction of caissons and tunnels, prevention of seepage beneath d a m foundations, and sealing pervious strata i n water and oil wells.
A
NY soil mass consists of mineral grains, voids, organic matter,
and amorphous matter. The voids can contain varying amounts of water from zero t o saturation. Introduction of chemicals into the soil mass can be effected by either of two methods. The first method, generally useful for surface applications, involves the mixing of the chemical with the soil by any of several means, wherein the soil mass is actually broken down and the chemical distributed mechanically throughout the soil mass. Under certain circumstances, it is possible t o introduce the chemical into the voids in the soil by injection, without significantly disturbing the structure of the soil particles. Where water is present, the chemical displaces the water as it enters the mass. Figure 1 illustrates a typical mixing operation, in which the chemical is mixed with the soil in place. Figure 2 illustrates the gravity injection procedure, in which the chemical is sprayed on the surface of the soil, and subsequently penetrates the soil by gravity flow. Figure 3 illustrates schematically the more general concept of injection, where the chemical is forced into the soil mass through an injection pipe. There are several rather basic limitations of injection procedures. First, the soil mass must have a sufficiently high permeability so that the chemical can be introduced into the soil with reasonable pressures and in a reasonable length of time. For chemicals with the viscosity of water, injection into soils containing clay is t o all practical purposes impossible. Injection into silts is possible only with high pressures. Thus only sands, gravels, and porous rock are suitable for chemical injection treatments. A second limitation of injection techniques is that the chemicals as injected must have a low viscosity, and the chemical must have an extremely small particle size or be in solution. Even in sandy
Figure 1.
Procedure for mechanical mixing of soil and chemical
November 1955
INDUSTRIAL
AND
soils the diameter of the voids in the soil is often small, and any particles in the chemical which approach the size of the voids will tend to plug the soil mass and prevent the penetration of the chemical. Of course, the greatest limitation of chemical injection techniques lies in the cost of the chemicals, and the number of problems in which it would be economical to use chemical grouts is limited, even though the application might be a certain success. Chemical Grouts
Chemical grouts have been used for various purposes in the engineering field for a number of years, though practically all the successful civil engineering projects were limited to the application of sodium silicate grouts, except in the oil fields. The ap-
Figure 2.
Injection by gravity flow
Figure 3. Three-dimensional injection into soil mass
ENGINEERING CHEMISTRY
2249
ENGINEERING, DESIGN, AND EQUIPMENT
Figure 4.
Stabilizer AM-955 i n cohesionless sand
plications of silica grouts have, however, been somewhat limited by the relatively high viscosity of the silicate, and by the necessity of injecting the chemical and a reacting solution in two successive shots on most applications. Information concerning other chemicals will be available in the next year in a report of the chemical grouting task committee of the Soil Mechanics and Foundation Division of the American Society of Civil Engineers. Aqueous solutions of stabilizer AM-955, acrglamid methylenebisacrylamide, a product of the American Cyanamid Co., either singly or mixed with calcium acrylate, a product of Rohm & Haas Chemical Co. and other companies, with suitable redox catalyst systems, were employed in the injection applications. These chemicals are readily soluble in water. Control of the temperature of the water, the amount of catalyst, and the basic chemical concentration permit control of the length of time that will be required for polymerization of the chemical, thus allowing enough time for the physical aspects of chemical injection. The calcium acrylate and stabilizer AM-955 have the further advantage that they can be mixed with each other and with other chemicals to form copolymers and to vary the physicaI properties of the resulting gel. Figure 4 illustrated the cohesion that is imparted to a mass of sand by polymerization of stabilizer AM-955 in the sand voids. The compressive strength of the sand after polymerization of the chemical in the voids is approximately 20 pounds per square inch. Although this strength is not great enough to justify the use of the chemical grout t o improve the bearing capacity of sand, it does provide enough strength to facilitate excavation through the sand with vertical cuts and in tunnels. The polymer also renders the soil mass impervious, thus preventing the flow of ground water. This impermeability is highly desirable for many construction purposes. Seepage into Underground Structures
The first field applications of chemical grouting procedures performed by the author involved seepage in underground structures. Whenever a basement, boiler room, or tunnel is constructed below the ground water table, the walls and floor of the structure are normally under hydrostatic pressure. If the soil mass is relatively pervious, and if there are any cracks, open joints, or porous areas in the structure, the ground water ultimately finds its way inside the structure, resulting in unsightly appearance, slippery conditions, and rapid deterioration of equipment and of the structural concrete and steel.
Figure 6.
2250
Seepage through boiler room slab
Figure 7.
Injection detail through concrete
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 11
SOIL STABILIZATION utilized because of the ease of cleaning. An air operated pistontype pump has also been successfully used on various projects, and experimentation is under way using a spiral displacementtype pump. In practically all cases, the temperature and amount of catalyst have been established so as t o cause polymerization in approximately 20 minutes after the time the catalyst is added. This period has always been long enough t o complete the injection operation. A jar sample of the chemical is taken from the batching tub as a control, though the normally large difference between room temperature and temperature within the soil mass eliminates part of the usefulness of the control. In each case in which the treatment has been effective, the seepage has been observed t o stop in less than 24 hours; the delay usually is caused by the low temperature in the soil. There have been several locations where the treatment has been unsuccessful. In most cases the reasons for the failure have been determined, and changes have been made in the procedure in order to obtain satisfactory results. I t is not always obvious from observation alone where the water is coming from. The wet area may be observed a t one point on the inside of a wall, and i t may later be discovered that the water is penetrating the wall on the far side some distance away. The chemical grout cannot be pumped into a large cavity containing water and be expected t o polymerize, as the chemical
Figure 8. Water seepage through construction joint a t stairwell
Figure 5 , shown with symposium title page, illustrates a typical case where a boiler room was constructed below the ground water table. The soil beneath and beside the boiler room was sand, and the supply of water almost unlimited. Ground water seeped into the boiler room through cracks and joints in the mall and resulted in an almost permanent wet condition within the room. Figure E illustrates the condition in the boiler room. The approach that was used to prevent seepage in the boiler room was to drill a number of 2-inch-diameter holes a t each of the areas in which seepage was observed. Iron pipe inserts were placed in these holes, and the space between the pipe and the concrete was calked t o prevent baclrflow of the chemical during pumping. Figure 7 illustrates how the chemical was pumped through the insert into the underlying sand. It subsequently polymerized in the sand mass, rendering it impervious and stopping the flow of water. During the first application, much was learned about the importance of various factors, such as control of the temperature of the chemical grout, protection of metallic mixing drums and pipes from contact with the chemical-which was found to accelerate polymerization-selection of the proper catalyst concentration and quantity of chemical per injection shot, and selection of a suitable pump that would be easy t o clean if the chemical should polymerize prematurely. The second major project with which these chemical grouts were used was in conjunction with seepage of ground water into a large bus terminal in downtown Chicago. The floor of the lower level in the structure was approximately 20 feet below the water level in the nearby Chicago River, ensuring that the water level outside the structure would remain rather constant throughout the years. Figure 8 indicates a typical stairwell into which water was seeping through construction joints between the slab and the walls. Figure 9 illustrates the method whereby the chemical was pumped into the soil beneath the concrete through an injection point. Figure 10 shows the chemical mixing tub, which had been coated with a vinyl lacquer, and the hand pump,
November 1955
Figure 9.
Injection point in concrete slab
Figure I O .
Injection operation
INDUSTRIAL AND ENGINEERING CHEMISTRY
225 1
ENGINEERING, DESIGN, AND EQUIPMENT the cost of injection will vary between $25 and $50 per cubic yard of soil, for average conditions. With the expectations of reduced chemical costs as production increases and development of new injection techniques, prospects are good that mass injection costs will be reduced.
Construction Applications
Figure 11.
Figure 12.
Injection f o r tunnel excavation
Injection i n t o deep sand strata to expedite caisson construction
immediately disperses in the water without polymerization. Experiments are currently under way to develop a satisfactory procedure for handling such problems. I n most work in which a chemical has been pumped through a wall or floor, the chemical tends t o flow along and parallel to the far side of the concrete. This is, of course, a fortunate engineering experience, as it eliminates the necessity of injecting on rather close spacings. Although every problem must, of course, be treated separately, i t appears that one injection point can effectively seal approximately 25 square feet of area.
Although experience has been limited in the application of chemical grouts for construction purposes, the possibilities in this field are encouraging. Usually the contractor who builds a tunnel through clay is fortunate, because the clay can be removed and the clay facing will stand in place until the tunnel rings are erected. However, tunneling through saturated sand is not nearly so simple. The sand, particularly when below the water table, runs into the already completed excavation, usually hampering progress and in many cases endangering personnel and equipment. Under such circumstances, the contractor usually is forced to install air locks and to complete the work under compressed air. Injection of sand by chemical grouts could render the sand impervious and a t the same time cohesive. This injection could be performed either through the tunnel heading, or from the the ground surface in advance of the tunnel. Sodium silicate grouts have been used with some success under such circumstances, though stabilizer AM-955 or calcium acrylate provide more satisfactory but more expensive results. Figure 11 illustrates how the stabilizer sand layer expedites construction of the tunnel by preventing sand from washing into the tunnel. Sand below the water table often complicates the construction of caissons. Normally the caisson is advanced dry and quicksand effects develop as soon as it encounters a saturated sand layer.
I
\ \
..
.
Figure 13. Injection into pervious strata beneath d a m to prevent loss of water and undermining
Cost Estimates Although the actual cost of chemical injection work will vary with the type of chemical, the size and accessibility of the project, and the soil conditions, certain general cost figures are cited that would be useful in establishing the feasibility of chemical grouting work on any particular project. For waterproofing applications, using Stabilizer AM-955, the cost has varied from $50 to $150 per injection hole, or from $2.00 to $6.00 per square foot of surface injected. Since most applications involve linear joints or cracks that are leaking, the best means of estimating the cost of such work would be to calculate the number of injection holes required, and use the average cost per hole for the estimate. Where the greater strength and resilience of the calcium acrylate polymer is desired, the chemical cost rises considerably because a larger percentage of the chemical is required in the grout, with perhaps a 50% increase in the cost per hole. Where chemical grouting solidifies or impermeabilizes large volumes of soil, for construction purposes in tunnels and caissons
2252
I
I Figure 14.
Injection of chemicals into pervious strata in oil well
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 11
SOIL STABILIZATION
.
If the contractor attempts to pump the water from the caisson, he finds that he is removing sand with the water, often endangering adjacent buildings by undermining their foundations. Figure 12 shows how the chemical can be injected into the sand from the ground surface, in advance of caisson construction, thus preventing quicksand action. Chemical grouts would also be useful in expediting construction of open trenches throughout sandy soils before the water table. However, the cost of such treatment is generally large, so that the potential usefulness of the method for such work is limited. Figure 13 illustrates a pervious sand stratum extending beneath a dam foundation. Such a stratum often causes considerable loss of water from reservoirs, as well as endangering the stability of the dam in some cases. Cement grouts have been used for years in dam foundations, though chemical grouts, injected before or after construction of the dam in the pervious strata, would more effectively seal the strata, because of the difficulties encountered in grouting fine sand formations with cement. Chemical grouts would also be used in sealing fractured or cavernous rock formations, but there is a danger that the chemical would disperse and fail to polymerize should it encounter voids of any size in the rock. I n drilling water and oil wells, difficulties are often encountered because the expensive drilling mud is lost through pervious strata encountered a t various depths. Also, in drilling oil wells, it is often necessary to seal off the salt water encountered beneath
the oil in the oil-bearing sand stratum. It is believed that chemical grouts, used with special injection equipment and packers inside the wells, as pictured in Figure 14, could effectively prevent the loss of drilling mud, and could likewise prevent salt water from flowing into the well. Such grouting has been used successfully in a water well by the author, but no work has been done in the oil fields. Conclusions
There are numerous engineering problems concerning soil that could be eliminated by modifying the properties of the soil mass with the injection of chemicals. The cost of such treatment is in many cases reasonable, and with continuing development of injection techniques, some day it will be a valuable tool for the construction industry. Ac k now Ied gment
The author wishes to acknowledge the contributions of Clyde N. Baker of Soil Testing Services, Inc., and Robert J. Gnaedinger, Sr., engineering consultant. He is also indebted to J. G. Affleck of hmerican Cyanamid Co., V. C. Meunier of Rohm and Haas Co., and T. William Lambe of Massachusetts Institute of Technology for their cooperation in various aspects of the work. RECEIVED for review April 6, 1955.
ACCEPTED August 17, 1956.
Effect of Fatty Quaternary Ammonium Salts on Physical Properties of Certain Soils F R A N K X. GROSS1 A N D J O H N L. WOOLSEY UNION STARCH A N D REFINING CO.. GRANITE CITY. ILL.
T h e apparent limit t o t h e quantity of water adsorbable by a Putnam silt loam treated with 0.1% dimethyldioctadecylammonium chloride (DDAC) i s directly responsible for its hydraulic stability. This alsoexplains t h e ability of the treated soil t o withstand rupture from cyclic wetting and drying and also from freezing and thawing. T h e effectiveness of t h e treatment of t h e soil will vary with t h e concentration of DDAC, t h e magnitude of t h e electrokinetic potential of t h e soil, t h e p H of t h e soil, and t h e concentration of indifferent salts. Soils with p H above 8 and with abnormally high concentration of salts do not respond t o t h e described treatment. T h e capacity of t h e treated soil t o reaggregate on being reworked should prolong t h e duration of t h e treatment. T h e virtual lack of capillarity of t h e treated soil demonstrates t h e weakened forces of attraction between t h e treated soil particles and t h e water molecules. T h e compressive strength of t h e treated soil appears t o reach a constant value above a moisture content of 15% while t h e compressive strength for t h e untreated diminishes rapidly above t h e same moisture level.
C
ONTEMPORARY workers (3, 6, 7 , 8) in the field of soil aggregation have confined their efforts largely to aggregating agents of the polymeric anionic type and have emphasized the benefits to soil structure by their incorporation in the soil. Michaels (6) has studied the mechanism by which the anionic polyelectrolytes function as soil aggregants and has described how the effectjveness of polyacrylamide as a soil flocculant is related to the degree of hydrolysis. Michaels and Lambe (7) have also evaluated the use of a cationic polyelectrolyte, a copolymer of styrene and AT-methyl-2-vinylpyridine methosulfate, November 1955
on three widely separated soils, but the work of recent investigators with soil flocculants has dealt chiefly with polymeric soil additives with molecular weights as high as 200,000. The capacity of these compounds to aggregate soil particles is attributed ( 6 ) to their chain like structures, their adsorption on soil particles, and their ability to link to other soil particles. Attention has centered on hydrophilic polymers, the majority of which are negatively charged, and their effects on the physical properties of soils as they relate to crop response. This paper will deal with a markedly different type of soil
INDUSTRIAL AND ENGINEERING CHEMISTRY
2253
