SOIL STABILIZATION mechanical, and chemical properties which suggests a profitable future for this relatively new class of vinyl monomers. Acknowledgment
The writer is obligated to fellow associates who contributed to the work described. R. E. Zdanowski greatly assisted in obtaining much of the present information. G. J. Williamson also cooperated in providing considerable data on calcium acrylate. F. J. Glavis furnished the monomers for this work, and R. W. Auten offered many constructive criticisms on the preparation of this article. References
(1) Bacon, R. G. R., Trans. Faraday Soc., 42, 140 (1946). ( 2 ) Hauser, E. A., and Dannenberg, E. M., U. S. Patent 2,383,647
(Aug. 28, 1945).
(3) Ibid., 2,401,348 (June 6, 1946). (4) Hauser, E. A., De Mello, V. F. B., and Lambe, T. W., U. S. Patent 2,651,619 (Sept. 8, 1953). (5) Keenan, A. G., Mooney, R. TV., and Wood, L. A., J . Phys. & Colloid Chem., 55, 1462 (1951). (6) King, C. V., and Steinback, 0. F., J . Am. Chem. SOC.,52, 4779 (1930). (7) Modern‘Plastics Encyclopedia and Engineer’s Handbook, Plastics Catalogue, New York, 1950. (8) Neher, H. T., Conn, W., and Kroeker, E. H., U. S.Patent 2,502,411 (April 4, 1950). (9) Pauling, L., “General Chemistry,” p. 559, Freeman, San Francisco. 1953. (IO) Rohm & Haas Co., Washington Square, Philadelphia 5 , Pa., “Calcium Acrylate,” May 1954. (11) Simonds, H. R., Weith, A. J., and Bigelow, M. H., “Handbook of Plastics,” 2nd ed., Van Nostrand, New York, 1949. RECEIVED for review December 7, 1954.
ACCEPTED September 14, 1955.
Soil-Water Relationships in crylate Stabilized Soil V I N C E N T C. M E U N I E R , G O R D O N J. W I L L I A M S O N , A N D R O B E R T P. H O P K I N S R O H M & H A A S CO.,PHILADELPHIA,
PA.
T h e primary purpose of t h e work was t o study soil-water relationships i n calcium acrylate stabilized soil, and t h i s paper describes how triaxial figures are q u i t e useful i n achieving t h i s purpose. T h e value of t h e figures is twofold: (1) t h e y p e r m i t blocking o u t of logical and promising areas for study and (2) t h e y provide a basis for interpreting results. F r o m t h e results i t i s concluded t h a t t h e equilibrium moisture content of calcium acrylate stabilized soil, after immersion in water, depends on t h e a m o u n t of polymer present and t h a t t h e water equilibrating roleof calcium polyacrylate i s i m p o r t a n t in determining t h e w e t tensilestrength and elongation of treated soil.
C
ALCIUM acrylate is a white powder soluble in water to the extent of 30% a t room temperature. Clear and colorless solutions can be prepared from typical product without filtration and without carbon treatment. When catalyst and activator are added t o solutions, polymerization occurs, eventually producing water-insoluble gel. Polymerization will take place a t temperatures as low as 32’ F. When it is made to occur in situ in soil, solidification is achieved. The term “redox system,” commonly used to describe the combination of catalyst and activator for free radical-type polymerizations, appears frequently in this paper. The particular redox system applied t o calcium acrylate consists of equal parts by weight of ammonium persulfate as catalyst, and sodium thiosulfate as activator. Personnel of the Massachusetts Institute of Technology, working under a soil solidification contract with the U. S. Army Corps of Engineers, have studied calcium acrylate extensively and much of their work has been published (1-6). Two U. S. patents related to clay modification with acrylate salts are assigned to Hauser and Dannenberg, granted in 1945 and 1946. A third U. S. patent, involving soil solidification with calcium acrylate, granted in 1953, is assigned t o Research Corp. by De Mello, Hauser, and Lambe. A review of soil stabilization by Lambe and Michaels was published in Chemical and Engineering X e w s in 1954. November 1955
Triaxial Figures
As part of a study of the fundamental properties of monomer, polymer, and stabilized soil, soil-water relationships in calcium acrylate stabilized soil have been investigated. The work, carried out with the aid of triaxial figures is described in this paper. Although several diff’erent; types of soil were included, the present paper is limited to Fort Belvoir soil, with the approximate composition Clay Sand Silt
% 20 40
40
Treated soil can be studied in relation t o a number of soil properties-e.g., density and optimum moisture for compaction. In this work Atterberg limits were selected which define nonplastic, plastic, and liquid behavior of soil in terms of soil moisture content. In Figures 1 to 7 the Atterberg limits for the system Fort Belvoir soil-water-calcium acrylate monomer are plotted from published information (4). Alsoshown are curve8 representing upper water limit for homogeneity. This is not a true Atterberg limit. It is here defined as the highest water content at which a separate liquid phase does not form on standing
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ENGINEERING, DESIGN, AND EQUIPMENT for one hour. The extent of curvature of these lines is determined by the third component, which in this case is calcium acrylate. The monomer acts as a dispersant and aids in its own mixing. The important materials under study were soil, water, and calcium acrylate, which appear in Figures 1 to 7 as the major variables. I n these experiments soil, water, and monomer containing redox system were mixed and the initial compositions were shown. Since polymerization does not occur immediately, the initial compositions had the same Atterberg limits as ternary compositions without redox system. First the area between the liquid limit and upper water limit for homogeneity was studied. This area is given in Figure 1. Inasmuch as the solidification of soil with calcium acrylate appears to be brought about by the gelation of aqueous solutions of the salt on polymerization, it was assumed that solidification could not occur if the acrylate content of the solutions was too low to yield a gel on polymerization, and calcium acrylate in excess of that soluble in the water present would not contribute efficiently to solidification. In Figure 2 a line was drawn representing maximum solubility, which at 100" F. is about 4Oy0. Fromexperiments with monomer without soil, a minimum monomer concentration of approxi-
mately 5y0 was necessary to give typical rubberlike polymer. Because of the fundamental nature of this work, the cost of treatment is not emphasized, but, if it is desirable to establish an economic limit for calcium acrylate treatment, this can be conveniently shown. I n Figure 2, for example, a limit of 15% acrylate is assumed, shown by the broken line. The maximum solubility line represents the fixed ratio 4 parts monomer to 6 parts water, while the minimum line represents 5 parts monomer t o 95 parts water. The broken line relates monomer and soil, giving the ratio 15 parts monomer to 85 parts soil. I n succeeding figures the broken line representing economic limit is not shown. Experim e nt a I
Soil solidification experiments were conducted with compositions representing the four corners defined by the intersecting lines of Figure 2 and the geometric center of the area. These are points 1, 2, 3, 4, 5. The ingredients were dry mixed in a closed glass bottle. Water was then added and the contents mixed thoroughly with a spatula. The mixture was used to fill an aluminum two-piece mold of the dimensions illustrated in Figure 8. Specimens were allowed to stand overnight in the
CoA
C04
Figure 1.
Atterberg l i m i t s
Fort Belvoir soil and monomerlc calcium acrylate
W4TER
co 4
Figure 3.
Atterberg l i m i t s
Fort Belvoir soil and monomeric calcium acrylate
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Figure 4. Selection of second area t o investigate soil water relationships of calcium acrylate stabilized soil
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47,No. 11
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SOIL STABILIZATION Table I.
Tensile Strength Results Applying t o Compositions Results Ratio. tensile strength
Composition, % Point in Figure 2
Calcium acrylatea
Watera
Redox systemb
18
70
10
18 4 4 15
35 35 70 50
5 15 20 5
6
10
23
10
7
5
11.8
10
8
6
23
15
9
3
11.8
15
a
Basis, dry weight of soil.
Curing Conditions a t 750 F. Time, days Medium
Water content, % a t time of tensile testa
Tensile strength, lb./sq. inch
Figure 2 Compositions air 3 1 water 3 a!r 3 a!r air 3 3 water 1 air 1 water Figure 4 Compositions 3 days in air, immersion in water t o Constant weight (2 t o 3 days) 3 days in air, immersion in water t o constant weight (2 t o 3.dayp) 3 days in air immersion in water to cbnstant weight (2 t o 3 days) 3 days in air, immersion in water to constant weight (2 t o 3 days)
t o concentration
of calcium acrylate
48
30
2.7
75 14 12 38
25 20 27 29
4.2 3.5 3 2.5
43
23
4.3
38
17
7.6
32
18
5.4
10
15
3.3
b Basis, calcium acrylate.
molds at room temperature. They were then removed and subjected to various conditions of cure. The tensile strength was determined with a Scott tensile tester in a constant temperature room at 25" C. Compositions, curing conditions, and tensile strength data appear in Table I. In Table I considerable variation in the concentration of redox system is noted. If the same redox concentration had been used throughout, in some cases gelation would have occurred before the specimens were molded, because of variations in water content and soil-acrylate ratio. Since the redox concentration and gelation time both could not be held constant, it was decided to keep gelation time about the same for convenience in molding. While redox concentration affects water sensitivity of polymer films, a redox concentration effect on water sensitivity of soil systems has not been observed. The relative massive quantities of soil extender may minimize or obscure the differences observed with 100% polymer. Untreated soil mixed with water and put through the cycles would have zero tensile strength.
Con
Figure 5. Equilibrating effect of calcium polyacrylate on water absorption of solidified Fort Belvoir soil a t 75' F. 0
0
November 1955
Before polymerlzatlon After polymerization and conditioning
I/;
0'
I
20
I
40
I
I 60
I
I
80
I
I I 100
SOIL M O I S T U R E C O N T E N T , ( B A S I S , DRY
Figure 6.
WEIGHT
I
I
120
I I 140
I
160
%
OF SOIL1
Relationship of soil moisture content to polymer content of treated soil
Higher tensile strength was obtained a t higher calcium acrylate concentrations and a t lower initial water content. Although the results for the area 1,2,3,4 show marked increases in strength for higher acrylate concentrations, in later work higher acrylate concentrations led to diminishing returns in wet strength. Wet strength may be defined as the tensile strength a t the equilibrium water content, which is obtained by soaking specimens to constant weight. The effects to be expected in going outside the area were considered. Above the line 1,4 mixing would be with higher water concentrations. This would not improve ease of mixing and would yield lower strength ultimately. T o the right and past 3,4 the monomer would not be expected to polymerize to rubberlike polymer and lower strength values would result. To the left and past 1,2 higher wet strength would result, but with high cost of treatment. The promising location is near the liquid limit line. Directly below the liquid limit, soil-water mixtures become sticky and i t is very difficult to mix in materials with which the soil is to be treated. As the water concentration is lowered, the lower sticky limit is approached, and finally, below the sticky limit, an area of relatively easy mixing is reached. This is the plastic area or low water test area. Figure 3 presents these changes with diminishing water concentration. The area be-
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ENGINEERING, DESIGN, AND EQUIPMENT tween the plastic limit and the liquid limit was investigated. I n Figure 4 the limiting lines for calcium acrylate are shown, and points 6, 7, 8, 9 were chosen for study. Composition data, curing conditions, and tensile strength results are given in Table I.
CoA
Figure 7. Summary of relationship between soil moist u r e content and wet tensile and per cent elongation properties of treated soil (after polymerization and conditioning)
1
standpoint of mixing and since the ratio of acrylate t o soil was expected to influence strength, these factors were also studied. Lines were plotted so that maximum information could be obtained from the intersecting lines of the three variables. Figure 5, therefore, permits comparison between three different initial monomer solution concentrations, five different soil water ratios, and seven different levels of acrylate treatment. Points were included which were initially under the plastic limit, over the liquid limit, and in between. Initial compositions were chosen corresponding to the dots in Figure 5. Samples were made up and hand mixed thoroughly a t room temperature. Standard molds, such as the one in Figure 8, were filled with the mixtures and held until gelation occurred. The samples were then removed and held a t 75" F. a t 55% relative humidity for 3 days. They were then soaked in water until they had come to equilibrium. Tensile strength determinations were made with a Scott tester. Elongation was also measured. The first effect observed was the influence of the amount of calcium polyacrylate present on the amount of water absorbed by the stabilized soil. The initial points (dots) in Figure 5 took up new positions (open circles) to form an equilibrium line. By plotting on a figure the equilibrated compositions for a t least four reasonably different sets of conditions, it should be possible to draw the equilibrium line for the solidified system. The line A 0 in Figure 5 represents a fixed percentage of initial water, which is 46% based on dry weight of the soil. Solidified samples made up at initial compositions above line A 0 were observed to shrink excessively on outdoor exposure (6).
Wet tensile
%
Elongation
The absolute strength values are less than those obtained in the higher water test area. However, there are advantages in operating near the plastic limit line. Better utilization of calcium acrylate is achieved, giving more strength per unit weight, as indicated in the last columns to the right in Table I. Mixing is easier and specimens weather better from the standpoint of resistance t o moisture pickup and cracking, and retaining tensile strength (6). Further Studies of Soil-Water Relationships w i t h Calcium AcrylateStabilized Fort Belvoir Soil
In the preliminary work triaxial figures did not permit pinpointing optimum conditions and compositions for solidification with Figure 8. A l u m i n u m two-piece mold calcium acrylate. They did suggest attractive areas in which to work, and, conversely, eliminated unattractive areas. An attempt will be made to illusTable I I. Tensile Strength and Elongation Data Applying t o Compositions of Figure 5 trate how the figures can be used to aid in interpreting data applying to soil-water relationships of stabilizing soil. 58 150 1 0.13 100 12.5 One factor that influences 80 90 28 0.48 59 6 the amount of water absorbed 45 83 Flocculation limit 70 22 50 0.44 2 by calcium polyacrylate stabil280 59 7 30 41 1.4 300 51 1.7 11 23 40 ized soil is the initial amount 200 48 3.4 12 13.7 47 100 36 Liquid limit 36 2.0 3 20 40 of water in calcium acrylate 30 23 8 12 43 3.6 solutions used to treat the 43 4.3 25 23 Sticky limit 22 4 10 soil. With Figure 5 points 32 5.3 10 18 9 6 38 7 6 5 17 Plastic limit 16 5 5 were selected corresponding 10 3.3 2 15 10 3 to 10, 20, and 30% aqueous Basis dry weight of soil. solutions. Since initial soil 6 Atterberg limits of soil containing monomer expressed in terms of soil moisture contents, taken from Massato water ratios have pracchusetts Institute of Technology data. tical significance from the ~~
~
~
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 11
SOIL STABILIZATION Nechanical strength of calcium polyacrylate stabilized systems appeared to be associated with the Atterberg limits initially measured on the monomer-Fort Belvoir soil. The most important, factor affecting strength was the position of the equilibrated samples with respect to the Atterberg limits. Mechanical strength data are presented in Table 11. Samples 1, 2, and 6 were heterogeneous, causing poor strength. They were deliberately prepared above the flocculation limit, and they were useful in water equilibration determination. Samples 3, 7, 11, and 12 with final soil moisture contents above the liquid limit of the monomeric system, showed high tensile and high elongation, but poor utilization of calcium acrylate. Samples 4 and 8 gave maximum tensile and minimum elongation with good utilization of acrylate. The most noticeable evidence of the importance of Atterberg limits on properties of solidified soil appears in the comparison of points 5 and 10, which were on either side of the plastic limit line. The relationships between equilibrium water content, tensile strength, and elongation of calcium acrylate stabilized soil are further illustrated with Figure 7 . Conclusions
1. The equilibrium moisture content of calcium acrylate stabilized soil after immersion in water, depends principally on the amount of)polymerpresent.
2. The water equilibrating role of calcium polyacrylate in the soil is important in determining wet strength of treated soil. 3. Triaxial figures are useful in studies of chemical soil solidification. Acknowledgment
We are indebted to the Massachusetts Institute of Technology and to the U. S. Army Corps of Engineers for much of the background information on calcium acrylate treatment of soil. The Corps of Engineers has conducted considerable field work and has demonstrated the ability of calcium acrylate to impart mechanical strength to soil and to reduce its water permeability. Literature Cited (1) Dannenberg, E. M., and Hauser, E. A,, U. 8. Patent 2,383,647
(August 28,1945). (June 0,1940). (3) De hlello, V. F. B., Hauser, E. A,, and Lambe, T. W., U. S, Patent 2,651,619 (Sept. 8,1953). (4) Lambe, T. W., Boaton SOC.C$viZ Engrs., 38, No.200 (April 1951). (6) Lambe, T.W., and Michaels, A. B., Chem. Eng. News,32, 488(2) Ibicl., 2,401,348
92 (1954). (0) Rohm & Haas Co., unpublished data, August 1953. RECEIVED for review May 16. 1955.
ACCEPTED August 10. 1955,
Aluminum Sulfate and Iron Sulfates as Auxiliaries in Bituminous Stabilization of Soils C. KINNEY H A N C Q C K TEXAS ENC3INEERING EXPERIMENT STATION, TEXAS A. & M. COLLEGE SYSTEM, COLLEGE STATION. T E X .
Even t h o u g h there are m a n y literature references t o t h e use of metallic salts as auxiliaries in t h e b i t u m i n o u s stabilization of soils, few quantitative data resulting f r o m economically feasible methods of application are t o be found. Data presented on eight abnormally hydrophilic soils indicate t h a t increased stabilization of t h e m a j o r i t y of these soils can be attained by using cutback asphalt w i t h 0.5 t o 4% ferrous, ferric, or a l u m i n u m sulfate as auxiliary. Ferric chloride glves similar results if t h e excess salt i s washed out, b u t t h i s is n o t economically feasible on a large scale. Ferric and a l u m i n u m sulfates have about t h e same auxiliary stabilizing action while ferrous sulfate has a slightly smaller effect. For seven of t h e soils, use of 2% auxiliary salt resulted in a n approximate t w o t o tenfold increase in modified bearing value accompanied by a corresponding significant decrease in water absorption. In many cases, auxiliary salt treatment reduced t h e q u a n t i t y of asphalt needed f o r satisfactory stabilization. The auxiliary effect of these salts in bituminous soil stabilization is of interest in highway construction for it m a y lead t o t h e use of otherwise unusable right-of-way soils.
I
N THE construction of modern highways, the use of soils from the immediate right of way is an important economic consideration since it eliminates both the purchase and transportation costs of materials brought from outside. Certain such soils, if wetted to optimum moisture content, compacted to maximum density, partially dried, and covered with a wearing surface, will have good load-carrying characteristics; however, this will be true only over a rather narrow range of moisture contentLe., if the moisture decreases significantly, crumbling will occur, and, if the moisture content increases considerably, the soil will become plastic and undergo severe deformation under traffic loads. November 1955
As a result, it has become rather common practice to add a stabilizing agent a t some point in the process of wetting, compacting, and drying the soil. Among the widely used stabilizing agents are asphalt, portland cement, and lime. The use of asphalt as stabilizing agent has become quite common in Texas. Recently, many significant developments, including the use of trace chemicaIs, have occurred in the field of soil stabilization. Many of these new methods have been reported or reviewed by Lambe ( 6 4 ,Michaels (8, 11), and Murray (12). The present article is concerned with the results of a 194142 study of the auxiliary action of several metallic salts in bituminous stabilization of soils. The results of modified bearing
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