Stabilizing Clay With Hydroxy-Aluminum and Base

This application is related to U.S. applications Ser. No. 281,799, U.S. Pat. No. 4,372,786 by MARION G. REED et al; Ser. No. 281,750, U.S. Pat. No. 4,377,419 by MARION G. REED; Ser. No. 281,751, U.S. Pat. No. 4,380,408 by TOR LOKEN et al; and Ser. No. 281,752 U.S. Pat. No. 4,360,599 by TOR LOKEN et al; all filed on July 9, 1981 all assigned to the assignee of this invention. Other applications related to this application are Ser. No. 438,234 and Ser. No. 438,235, filed concurrently herewith and also assigned to said assignee. The disclosures of these applications are incorporated herein by reference.

Many clay deposits often need to be stabilized before they can carry any additional load such as is applied during construction activities. This is specially true for the so-called quick clays which frequently are found, for example, in northern Soviet Union, Scandinavia, Canada, upper New York State, and New Zealand. Clays were originally deposited in marine and brackish water in a short period after the last glaciation, and later during the continental (isostatic) uplift were raised above sea level. However, only certain of these clay deposits were subsequently changed into sensitive quick clays. Two processes are mainly responsible for such a change. The original pore water salt content of the clay may have been leached by precolating ground water, or organic matter from logs or marshes which will act as dispersing agents may have been introduced into the clay. The first process has been most important in clays found in Norway, while quick clays containing high organic content formed by the second process are frequently found in Sweden and Canada.

The quick clays will in the undisturbed state exhibit a certain limited strength, but will upon remoulding completely liquefy. This same phenomena has caused several quick clay slides in the lowlands of eastern and middle Norway, often with catastrophic consequences. Heretofore, several chemical stabilization schemes have been tried for such clays. Among them were aluminum chloride (AlCl.sub.3) and potassium chloride (KCl). The quick clays have been stabilized in two ways. The clay can be mixed and remoulded with the chemicals, or the chemicals can be allowed to diffuse into the undisturbed quick clay. The disadvantage of the salt diffusion method is the long time it takes to reach the required penetration. The diffusion method has been applied only once, so far as is known, in full scale in the field, when salt wells containing KCl were installed two years prior to a major highway construction.

Heretofore, a method for deep stabilization with unslacked lime (CaO) was developed. Lime is an old stabilizing agent that has been used occasionally. In China it was used centuries ago as a construction material. In the U.S.A. in the 1940's and Europe in the 1950's lime was used as a surface stabilizing agent. The deep stabilization method involves mixing and molding the lime with the clay to form a series of piles which extend down into the clay. These piles provide lateral stabilization to the clay deposit.

Both the lime and potassium chloride methods have some disadvantages. KCl will stabilize the undisturbed circumjacent clay, but not the disturbed clay. Furthermore, CaO makes an unhomogenous stabilization. Pockets of lime cause brittle cylinders with small sideways shear strength. In addition, CaO is not useful on clays with high water content. While hydroxy-aluminum as Al(OH).sub.2.5 Cl.sub.0.5 has not been used as a clay stabilizing agent in foundation engineering before it has been applied as a cementing agent in preparing desired clay microstructures for laboratory studies. Hydroxy-aluminum solution containing KCl has also been used in wells to treat water sensitive clay containing formations and to provide sand stabilization. Relatively dilute solutions and overflushes are commnly used in the field. These methods are disclosed in U.S. Pat. Nos. 3,603,399 issued Sept. 7, 1971 and 3,827,495 issued Aug. 6, 1974 both to Marion G. Reed as assignor to Chevron Research Company.

The cited Related Applications provide for stabilizing clay soil by admixing an effective amount of hydroxy-aluminum and in some cases additional chemicals such as potassium chloride with the clay. While potassium chloride is preferred, other chemicals useful in place of potassium chloride include potassium nitrate, potassium sulfate, ammonium chloride, ammonium nitrate and ammonium sulfate. The admixture is preferably done in a manner so that the stabilized clay forms a series of piles which extend into the clay deposit at spaced apart location to provide stability for the entire clay containing deposit. The desired size, location and number of the piles are determined. An effective amount of hydroxy-aluminum is admixed with the clay in place in each location in the deposit to react with the clay and thereby form the desired piles.

It has been found that the addition of other material can improve the method described above.

The present invention provides an improved method of stabilizing clay soil by mixing with the clay soil hydroxy-aluminum having an effective amount of base mixed therewith. A method is provided for determining the amount of base to utilize with clays having less natural base than is needed for effective reaction. The carbonates and bicarbonates are useful in accordance with the invention and particularly calcium carbonate and calcium bicarbonate and mixtures thereof. The most highly preferred base for use with hydroxy-aluminum in accordance with the invention is calcium carbonate.

It is a particular object of the present invention to provide a method of stabilizing quick clay by admixing with the quick clay a mixture preferably containing hydroxy-aluminum and an effective amount of base to react with the clay. Additional objects and advantages of the present invention will become apparent from reading the following detailed description in view of the accompanying drawings.

FIG. 1 is a graph illustrating the effect of treating clay with hydroxy-aluminum containing a base in accordance with the invention; and

FIG. 2 is a graph illustrating a comparison of hydroxy-aluminum alone vs. hydroxy-aluminum plus a base.

The present invention is directed to stabilizing clay soil by admixing with at least portions of such soil an effective amount of a dry mixture of chemicals including hydroxy-aluminum and base. The base may desirably be selected from the group consisting of carbonates and bicarbonates. Calcium carbonate and calcium bicarbonate are preferred bases useful in accordance with the present invention for mixture with the hydroxy-aluminum.

In preferred form a plurality of pile-like deposits are formed in situ in the clay soil deposit by admixing with wet clay a dry mixture containing at least 5% by weight of hydroxy-aluminum and an effective amount base by weight of the pore water contained in the clay soil. Best results appear to be obtained when the dry mixture chemicals contain between 5% by weight to 70% by weight of dry hydroxy-aluminum and between 1% by weight and 5% by weight of base of the pore water contained in the clay. For different clays the optimum concentration of hydroxy-aluminum and base may, of course, vary. Optimum concentration for a given clay may be determined by simple laboratory tests as herein described.

Hydroxy-aluminum, useful in accordance with the present invention, has a hydroxyl to aluminum ratio of at least 2.0. At low pressure, hydroxyl to aluminum ratios of less than 2.2 tend to be so acidic that carbonates contained in the clay soil turn to carbon dioxide which causes bubbles that are undesired in the present invention. Therefore, in carbonate containing soils it is preferred to use hydroxy-aluminium having a hydroxyl to aluminum ratio of 2.5.

Thus, hydroxy-aluminum, useful in the present invention, has the general formula Al(OH).sub.n X.sub.3-n wherein n has a value of at least 2.0 and preferably 2.5 to 2.7 and x is an anion selected from the group consisting of chloride, bromide, iodide, nitrate, sulfate and acetate. For example, dry hydroxy-aluminum chloride having a formula of Al(OH).sub.2.5 Cl.sub.0.5 has been used successfully in laboratory demonstrations of the present invention. Hydroxy-aluminum is a commercially available chemical and can be obtained for example from Reheis Chemical Company of Berkeley Heights, N.J. or Hoechst Aktiengesellschaft, Frankfurt am Main, West Germany.

"Dry" hydroxy-aluminum refers to the dry chemical with no bulk water added. Dry hydroxy-aluminium has, of course, a substantial amount of chemically bound water. One Reheis product contains, for example, about 201/2 weight percent chemically bound water.

FIG. 1 is a legend showing a quick clay which was used in demonstrations conducted in accordance with the present invention. Hydroxy-aluminum and various chemicals were admixed with the quick clay and the results are shown in FIG. 1.

As illustrated graphically in FIG. 1 hydroxy-aluminum-containing calcium carbonate (CaCO.sub.3) was very effective in improving the compressive strength of the quick clay. None of the other additives to the hydroxy-aluminum came close to the result achieved with hydroxy-aluminum plus calcium carbonate. The particular clay used in the demonstrations shown in FIG. 1 was Ho/ nefoss. Ho/ nefoss clay does not contain much calcium carbonate naturally while many other quick clays do. The concentration of hydroxy-aluminum in the effective demonstration was 0.57 gram per gram of water and the calcium carbonate concentration was chemically equivalent to 0.37 KCL. The results are shown in Table 1.

FIG. 2 shows a comparison of hydroxy-aluminum alone vs. hydroxy-aluminum plus a base. In this case the base was 1.3% calcium carbonate.

Thus the present invention provides a method of stabilizing quick clay soil by mixing hydroxy-aluminum containing an effective amount of base with the clay. The base is selected from the group consisting of carbonates and bicarbonates and preferably is either calcium carbonate and calcium bicarbonate. Calcium carbonate is the preferred base.

An effective amount of base for stabilizing quick clay containing less natural base than required for effective reaction with hydroxy-aluminum is determined in accordance with the invention. The natural base content of the quick clay is determined. A minimum amount of base needed for effective reaction is determined. A mixture of at least the minimum amount of base needed for the clay and hydroxy-aluminum is mixed with the quick clay to stabilize the clay.

One of the mechanisms by which OH-Al stabilizes quick clays is the gellation of water. Gellation requires that the OH-Al polymerize to form larger inorganic structures. This polymerization in turn requires hydroxide groups may be supplied by the natural bases in the clay deposit. Where natural bases are insufficient in quantity. In accordance with the invention solid base is mixed with the solid OH-Al during application of the treatment. The reaction can be written empirically as follows:

Solid forms of the chemicals must be used when base is added to the OH-Al during application because the reaction must be retarded until they are in contact with the water to be gelled.

In those thick clays that contain natural bases such as carbonates, there may be enough natural base to satisfy the total requirement. In other cases, all or part of the requirement must be met by adding a solid form of base with the OH-Al. To make these judgments, one must determine the natural base content of the quick clay. A procedure is as follows:

(1) Weight out 20 grams of overdry soil (110.degree. C.) for 48 hours) or an amount of moist soil that is equivalent to 20 grams of overdry soil.

(2) Add 50 millimeters of distilled water and stir with magnetic stirrer for 30 minutes.

(3) Place calibrated pH electrode into soil-water mixture and slowly stir.

(4) Titrate the mixture slowly to pH 4.5 using 0.2 N HCl. Record volume of acid used.

(5) Calculate the natural base content as follows: ##EQU1##

(6) Example Calculation: ##EQU2##

In the reaction as written, a total of 0.2 equivalents of base are needed for each mole of added Al. If we subtract from this the amount of base supplied by the soil, the remainder is that which must be added. A step-by-step procedure for making this calculation is as follows:

(1) Calculate the amount (moles) of Al to be adder per 100 grams of dry soil.

(2) Assume a total of 200 milli-equivalents of base are needed for each mole of Al added (as Al(OH).sub.2.5 Cl.sub.0.5) to the soil.

(3) Subtract the natural base content (determined by the previously described titration method) from this total requirement to determine the additional base required for the stabilization process.

(4) Example Calculation:

Specific Case Assumptions

(a) Water content of soil is 0.37 gm per gram of dry soil.

(b) 0.5 gm of solid hydroxy-aluminum added per gram of pore water.

(c) Total of 200 meg. base required per mole of Al added.

(d) Natural base content of soil is 0.2 meg. per gram of dry soil.

(e) Solid CaCO.sub.3 is used to make up base deficits.

(f) Solid hydroxy-aluminum is 24.9% Al. ##EQU3##

From the above description it is evident that the present invention provides a method of stabilizing quick clay by admixing dry hydroxy-aluminum and an effective amount of base with the clay. Although only specific embodiments of the present invention have been described in detail, the invention is not limited thereto but is meant to include all embodiments coming within the scope of the appended claims.