Method for strengthening water-saturated soft soils

A water-saturated soft soil is efficiently improved in its strength by incorporating thereinto an additive composed of ingredients A, B and C; the ingredient A comprises gypsum, the ingredient B comprises a mixture of 40-55% by weight of a Portland cement and 60-45% by weight of a water-granulated iron blast furnace slag having a particle size almost equal to or smaller than that of the Portland cement and the ingredient C comprises a water-soluble ferrous salt. The ratio by weight of the ingredient A to the ingredient B ranges from 10/90 to 30/70 and the total amount of the ingredients A and B used for the soft soil is sufficient to strengthen the soft soil. The ingredient C is used in an amount sufficient to react with hydrogen sulfide contained in the soft soil to be treated.

BACKGROUND OF THE INVENTION 
This invention relates to a method for improving the strength of 
water-saturated soft soils. More particularly, this invention is directed 
to a method for improving the strength of such a soft soil while 
inhibiting the generation of unpleasant odor from the soft soil. 
It is generally known from the past to improve the strength of a 
water-saturated soft soil deposited on the bottom of seashore, river, lake 
and lagoon so as to enable passage of people or traffic vehicles and 
conveyance of construction machines on reclaimed soft grounds formed by 
dredging such water-saturated soft soil. In one such method, a 
strengthening agent or solidifying agent comprising a cement, quick lime, 
water glass, asphalt and organic macromolecular substances is added to the 
soft soil. However, this method is not altogether satisfactory since these 
strengthening agents are poor in strength-improving effect or are 
economically unattractive. Especially, in the case of the large-scale 
treatment of soft soil of a high water content, such as a certain kind of 
sludge or mud called "hedoro" deposited on the bottom of rivers or the 
seashore, the amount of soil to be treated in one batch may reach several 
thousand to several million cubic meters, thus requiring the addition of 
an extremely large amount of the strengthening agent, and hence the 
strengthening agent employed should be as cheap as possible and should be 
capable of remarkably improving the strength in a smaller amount. 
In Japanese Unexamined Published patent appln. No. 141459/76 there is 
disclosed a method wherein a mixture of a cement and gypsum is used as a 
strengthening agent for the soft soil. The strengthening agent disclosed 
in this reference exhibits an enhanced strength-improving effect on the 
water-saturated soft soil as compared with a cement alone or a mixture of 
a cement and quick lime, but it is still unsatisfactory in practice since 
both a large amount of the strengthening agent and a long period of time 
are required to treat a highly water-saturated soft soil so as to impart a 
practically acceptable strength. This reference suggests the optional use 
of a water-granulated iron blast furnace slag or fly ash in addition to a 
cement and gypsum as indispensable ingredients. However, both the 
water-granulated iron blast furnace slag and the fly ash are not 
indispensable but optional in this prior art method in view of the 
proportion defined therein as 0-30%. In this reference, the 
water-granulated iron blast furnace slag is regarded equivalent in 
function to the fly ash which is very poor in strength-improving effect 
and no discussion is made on the properties and technical effects of these 
optional ingredients. Thus, these ingredients are recognized in this 
reference only as a filler or the like additive for reducing the cost of 
the indispensable strengthening agents. Moreover, this prior art method 
fails to contemplate improvement in strength of such a water-saturated 
soft soil containing organic matters and generating unpleasant odor. 
In addition to the difficulty in handling and transporting of 
water-saturated soft soils, a further problem often arises in that an 
unpleasant odor is released from the soft soil, rendering the life 
environment degraded. Though some countermeasures have been proposed 
hitherto for the deodorization of various materials, an effective method 
which in the treatment of soft soil can accomplish both improvement in 
strength and deodorization has not yet been developed. 
BRIEF SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a method 
for improving the strength of a water-saturated soft soil economically in 
high efficiency. 
It is another object of the present invention to provide a method for 
improving the strength of a water-saturated soft soil without increasing 
the alkalinity thereof and without producing undesirable internal strain 
thereof. 
It is still another object of the present invention to provide a method for 
improving the strength of a water-saturated soft soil with unpleasant odor 
while attaining the deodorization of the soft soil.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a method for increasing the strength of 
water-saturated soft soils, which comprises admixing the soft soil with an 
additive comprising the ingredients A, B and C. The ingredients A and B 
can react effectively with soil components of the soft soil and serve to 
improve its strength, whereas the ingredient C serves to inhibit the 
generation of an unpleasant odor from the soft soil. Thus, the additive 
used in the present invention is comprised of the following ingredients: 
Ingredient A: gypsum (CaSO.sub.4.2H.sub.2 O) 
Ingredient B: a mixture of 40-55% by weight of a Portland cement and 60-45% 
by weight of a water-granulated iron blast furnace slag. 
Ingredient C: a water-soluble ferrous salt 
Any gypsum in the form of powder or granules can be used as the ingredient 
A. The term "gypsum" is used herein to mean calcium sulfate dihydrate 
exclusively. No limitation is set for the particle size of the gypsum. 
Accordingly, gypsum formed as a by-product in a process for the 
desulfurization of flue gas can advantageously be used as such for the 
present invention. 
An Ordinary Portland cement satisfying the specifications defined in JIS 
R-5210 for "Portland cements" is suitable for use as the Portland cement 
in the ingredient B. According to the nature of a water-saturated soft 
soil and the treatment conditions, the Ordinary Portland cement may be 
used alone or as a mixture with a moderate heat Portland cement, a high 
early strength Portland cement and/or a ultrahigh early strength Portland 
cement. 
The water-granulated iron blast furnace slag used together with the 
Portland cement in the ingredient B is prepared from a by-product from an 
iron blast furnace by rapidly cooling the slag with water to form 
sand-like granules of the slag having a particle size of 1-5 mm, and then 
finely dividing the granules to have a particle diameter of 100-1.mu. 
(referred to hereinafter as the water-granulated iron blast furnace slag). 
The composition of the water-granulated iron blast furnace slag varies 
according to the composition of iron ores used or on the operation 
conditions of the blast furnace but is generally as follows: 
SiO.sub.2 30-35%, Al.sub.2 O.sub.3 13-18%, CaO 38-45%, Fe.sub.2 O.sub.3 
0.5-1.0% 
MgO 3-6%, S 0.5-1.0%, MnO 0.5-1.5% and TiO.sub.2 0.5-1.0%. 
The water-granulated iron blast furnace slag to be mixed with the Portland 
cement to form the ingredient B is pulverized to have a specific surface 
area of at least 2000 cm.sup.2 /g (particle diameter of not greater than 
10.mu.) preferably 3600 cm.sup.2 /g (particle diameter of not greater than 
5.mu.) when measured according to the Blaine's air permeability method 
(JIS R-5201). Thus, it is seen that the particle diameter of the 
water-granulated iron blast furnace slag is almost equal to or smaller 
than that of the Portland cements defined in JIS R-5210, wherein Ordinary 
Portland cement is defined to have a specific surface area of at least 
2500 cm.sup.2 /g. If the particle size of the slag becomes excessively 
large, its reactivity will decrease seriously; thus, when the finely 
pulverized water-granulated iron blast furnace slag having a particle size 
almost equal to the Portland cement is used in the present invention, the 
treated soft soil is typically 3-4 times as high in strength as soft soil 
treated with an additive which includes ordinary water-granulated iron 
blast furnace slag of coarse grains. 
It is important that the water-granulated iron blast furnace slag be mixed 
homogeneously with the Portland cement prior to being added to the soft 
soil to be treated in order to promote its reactivity. 
The proportion of the Portland cement in the ingredient B is maintained 
within the range of 40-55% by weight (the balance being essentially the 
slag). If the proportion of the cement is less than 40% by weight, the 
strength-improving effect on a highly water-saturated soft soil will be 
reduced. On the other hand, if the proportion of the cement exceeds 55% by 
weight, a significant amount of heat will be evolved during the 
strength-improving treatment, thus resulting in the formation of internal 
strain in the treated soft soil. In addition, the use of an excess amount 
of the cement results in a higher content of calcium hydroxide in the 
treated soft soil. This is undesirable because the treated soft soil is 
then strongly alkaline and is susceptible to erosion with sewer or sea 
water. Preferably, the relative proportion of the components in the 
ingredient B are 50% by weight of the Portland cement and 50% by weight of 
the water-granulated iron blast furnace slag. 
As the ingredient C, both organic and inorganic ferrous salts can suitably 
be used so far as they are soluble in water. The use of ferrous sulfate or 
ferrous chloride is preferable for reasons of economy and influence on the 
ingredients A and B. Above all, ferrous sulfate is most preferable because 
it is available in large quantities as a by-product in the production of 
titanium. In titanium-manufacturing plants, ferrous sulfate is generally 
discarded as industrial waste so that the utilization of this salt for the 
treatment of a water-saturated soft soil serves a dual purpose. 
The ingredient C, i.e. a water-soluble ferrous salt, can exhibit a 
deodorizing effect on a water-saturated soft soil having an unpleasant 
odor under neutral or weakly alkaline conditions under which the 
ingredients A and B are used, without adversely affecting the 
strength-improving effect of the ingredients A and B. The ferrous salt can 
react, under the treatment conditions according to the method of this 
invention, with hydrogen sulfide, which is a main source of the unpleasant 
odor of "hedoro" or the like soft soils and fix the hydrogen sulfide 
according to the following reaction formula: 
EQU H.sub.2 S+Fe.sup.++ .fwdarw.FeS(solid)+2H.sup.+ 
The reaction can favorably proceed even in the presence of carbon dioxide, 
which is important since in a sludge or mud emitting unpleasant odor, 
carbon dioxide is usually present in a larger amount than hydrogen 
sulfide. However, the ferrous salt can preferentially react with the 
hydrogen sulfide and, after completion of the reaction therewith, reacts 
with carbon dioxide to form harmless ferrous carbonate (siderite). The 
solubility of the ferrous salt in acidic and neutral solutions is hardly 
influenced by pH and the above H.sub.2 S-fixing reaction smoothly proceeds 
in a pH range between about 4.5 and 8.5. In contrast, ferric salts are 
found to exhibit no remarkable deodorizing effect under the treatment 
conditions of this invention. 
Hereinbelow will be described in some detail the fundamental method of this 
invention using an additive comprising the ingredients A and B but without 
ingredient C. 
The water-saturated soft soil to be treated is first admixed with the 
ingredient A so that the soft soil may be made reactive with the 
ingredient B to be added subsequently. In this case, the ingredient A is 
dispersed into the soft soil and dissolved in or wetted with the soft soil 
whereby the soft soil is converted into a preferable reaction medium for 
the ingredient B. Since the amount of the ingredient A added is small and 
since the ingredient A exhibits no increase in viscosity, unlike 
ingredient B, the homogeneous mixing of the soft soil with the ingredient 
A may be performed without difficulty. 
The water-saturated soft soil thus enhanced in reactivity is then admixed 
with the ingredient B. By adding the ingredient B to the preformed 
admixture of the soft soil and the ingredient A, various reactions for 
improving the strength of the water-saturated soft soil take place, 
including the reaction between the ingredients A and B, the hydration of 
the ingredient B and a reaction between the soft soil and the ingredients 
A and B. These reactions for improving the strength of the water-saturated 
soft soil proceed with good efficiency since the soft soil to which the 
ingredient A has been added is converted into a preferable reaction medium 
for the ingredient B. 
More specifically, in this treatment procedure, a reaction resulting in the 
formation of ettringite (3CaO.Al.sub.2 O.sub.3.3CaSO.sub.4.32H.sub.2 O) is 
believed to take place between the soft soil and the ingredients A and B. 
As the water-saturated soft soil has been homogeneously admixed with the 
ingredient A in the first step of the treatment, the reaction for the 
formation of ettringite takes place smoothly allover the treated soft soil 
upon addition of the ingredient B, whereby rapid improvement in strength 
of the soft soil is attained. 
In the present invention it is of particular advantage that the 
water-granulated iron blast furnace slag used as one component of the 
ingredient B has finely been pulverized to have a particle size almost 
equal to or smaller than the Portland cement used as the other component 
and is previously mixed homogeneously with the cement before used. When, 
therefore, the ingredient B is added to the soil containing ingredient A, 
the water-granulated iron blast furnace slag is readily stimulated with 
slaked lime, formed by the hydration reaction of the Portland cement, 
thereby promoting the reaction for forming ettringite and thus accelerate 
the soil-strengthening effect. To say it in another way, whilst the 
water-granulated iron blast furnace slag itself shows no hydraulic 
property, unlike cement, it begins to exhibit such property when brought 
into contact with a stimulating agent such as a slaked lime. In case of 
the method of this invention wherein the water-granulated iron blast 
furnace slag is finely pulverized to become reactive and is previously 
mixed homogeneously with the Portland cement so as to be stimulated with 
slaked lime formed by the hydration reaction of the cement, the 
water-granulated iron blast furnace slag shows the same hydraulic property 
as shown by the cement. 
In the method of this invention, the strength of the treated soft soil is 
also believed to be improved by a Pozzolan reaction between calcium ion 
and silicate ion in addition to the reaction for the formation of 
ettringite. As the water-granulated iron blast furnace slag used in the 
present invention is in a finely pulverized form, it serves as a reactant 
also for this Pozzolan reaction. 
Anyway, the water-granulated iron blast furnace slag per se is considered 
in this invention to participate efficiently in the reactions for 
improving the strength of the water-saturated soft soil and apparently 
contributes efficient promotion of more complex chemical reactions whereby 
an improvement in strength of the soft soil is rapidly achieved, in 
contrast to the prior art methods utilizing mainly the hydration reaction 
of a cement. 
In order to attain such rapid strength improvement, it is important to 
specify the proportion of the ingredient A relative to the ingredient B. 
In the method of this invention, this proportion is so selected that the 
ratio by weight of the ingredient A, calculated as gypsum, to the 
ingredient B, calculated as a mixture of the Portland cement and the 
water-granulated iron blast furnace slag, is within the range from 10/90 
to 30/70. Use of relative proportions of the ingredients A and B outside 
this range reduces the technical merit achieved by the addition of the 
ingredients A and B. 
For improving the strength of a water-saturated soft soil, it is desirable 
that the soft soil be admixed first with the ingredient A and then with 
the ingredient B as described hereinbefore. In some cases, however, the 
soft soil may be admixed with the ingredients A and B simultaneously. 
However, admixing of the water-saturated soft soil first with the 
ingredient B and then with the ingredient A is extremely disadvantageous 
because the resulting mixture is difficult to work and also only a poor 
improvement in strength is obtained. Both ingredients A and B can be used 
either in a powdery or slurried form in the method of this invention. 
According to the method of this invention, the reactions between the soft 
soil and the ingredients A and B are promoted in an extremely high 
efficiency unlike the case of the prior art methods so that the maximal 
strength-improving effect is expected to the soft soil. Thus, only 
relatively small amounts of the ingredients A and B are needed in order to 
achieve the desired strength and, moreover, the treatment time required is 
short. In general, the strength required for the treated soft soil in the 
field is about 0.5-2 kg/cm.sup.2 in terms of unconfined compressive 
strength. In general, these levels can be achieved according to the 
present invention by admixing the soft soil with the ingredients A 
(calculated as gypsum) and B (calculated as a mixture of a Portland cement 
and a water-granulated iron blast furnace slag) in a total amount of about 
50-150 kg per cubic meter of the soft soil. In case the soft soil contains 
a large amount of organic materials and emits strong bad odors, it is 
desirable to increase the total amount of the ingredients A and B in 
comparison with the case where the soft soil contains no or a little 
organic materials. When the soft soil has a low content of organic 
materials, a total amount of the ingredients A and B within the range of 
50-100 kg per cubic meter of the soft soil is usually sufficient to 
furnish it with a satisfactory strength. 
The method of the present invention is not influenced by the water content 
of the water-saturated soft soil and is applicable to water-saturated soft 
soils having a wide range of water content, for example a water content of 
50-200%, or even to highly water-saturated soft soils having a water 
content as high as 500-1000%. When the method of this invention is applied 
to a highly water-saturated soft soil, excess water will be separated from 
the treated soft soil onto the surface thereof. 
In the method of the present invention, the amount of the Portland cement 
used is relatively small, so that the generation of heat resulting from 
the hydration reaction of the cement is generally insufficient to cause an 
undesirable development of strain in the treated soft soil. Moreover, the 
quantity of residual alkalis in the treated soft soil is small, so that no 
significant increase in alkali concentration is found in the treated soft 
soil, and the risk of erosion of the treated soil by e.g. sewer or sea 
water, is minimized. The present invention is economically very 
advantageous in that the total amount of the ingredients is small and the 
amount of the Portland cement is decreased with increase in the amount of 
the water-granulated iron blast furnace slag. 
The method of this invention is thus advantageously applied not only for 
improving the strength of reclaimed soft grounds but also for improving 
the nature of soft soil or sludge deposited on the bottom of sea, river 
and the like. 
The method wherein the ingredients A and B are used is effective to furnish 
a water-saturated soft soil with a satisfactory strength but is not 
sufficient in deodorization of the soft soil. Thus, the above method 
cannot be applied to treat a water-saturated soft soil having a strong 
unpleasant odor. To achieve both improvement in strength and deodorization 
of water-saturated soft soils, a combination of the ingredients A, B and C 
is used in accordance with the present invention. 
The ingredient C may be added to the soft soil being treated at any time 
prior to the addition of the ingredient B. Concerning the order of adding 
the ingredients A, B and C to the soft soil, therefore, the following 
possibilities exist: the ingredients are added in the order (C-A-B); the 
ingredients are added in the order (A-C-B); ingredients A and C are first 
added simultaneously and then the ingredient B is added; and the 
ingredient C is first added and then ingredients A and B are added 
simultaneously. In case of adding the ingredients A and C or A and B 
simultaneously, these ingredients are added separately or in the form of a 
mixture. If the soft soil to be treated has a strong unpleasant odor, it 
is preferable to add the ingredient C first to effect deodorization 
preferentially, and therefore to facilitate its subsequent handling, and, 
thereafter, to add ingredients A and B simultaneously to effect the 
strength improvement. The ingredient C should not be mixed or added 
simultaneously with the ingredient B since the strength improvement effect 
is then found to be impaired. 
In order to facilitate homogeneous dispersion of the ingredient C in the 
soft soil, it is preferred that the ingredient C be added as an aqueous 
solution. Alternatively, a method wherein the ingredient C is first mixed 
homogeneously with the ingredient A and the resulting mixture is dispersed 
in the soft soil is also preferred. In the latter case, the homogeneous 
mixture can suitably be obtained by mixing the ingredient A in a solid or 
slurried form with an aqueous solution of the ingredient C. The resulting 
mixture can be added to the soft soil directly or after being dried. 
The ingredient C is used in an amount sufficient to fix hydrogen sulfide 
contained in the soft soil, but since the amount of hydrogen sulfide 
present varies according to the origin of the soil, the amount of the 
ingredient C cannot specifically be defined. In general, however, the 
ingredient C is desirably added at least in a stoichiometric amount, 
preferably in a 1-3 molar proportion with respect to the amount of the 
total hydrogen sulfide contained in the soft soil to be treated. The term 
"total hydrogen sulfide" is used herein to mean all forms of hydrogen 
sulfide, dissociated and non-dissociated (free), in water contained in the 
soft soil and bound by sorption to solids in the soft soil but does not 
include the sulfur moiety of water-insoluble metal sulfides. The amount of 
such total hydrogen sulfide contained in the soft soil can be determined 
by subjecting a sample of the soft soil to steam distillation and 
quantitatively analyzing the distilled hydrogen sulfide. Insoluble 
sulfides combined with metals can be determined by adding concentrated 
sulfuric acid to the distillation residue in the analysis of the total 
hydrogen sulfide and subjecting the mixture again to steam distillation 
and analyzing the quantity of hydrogen sulfide evolved. Unreacted 
ingredient C added in an excess amount is reacted, as described 
hereinbefore, with carbon dioxide to form siderite or is captured by 
cationic exchangers contained in the soft soil whereby the soluble ferrous 
salt is all fixed. Consequently, addition of the ingredient C in an 
excessive amount to the soft soil to be treated does not adversely affect 
the strengthening effect of the ingredients A and B so far as the amount 
of the ingredient C does not exceed the ferrous salt-fixing capacity of 
the soft soil. 
The present invention will now be illustrated by the Examples which follow. 
In Examples 1-4, powderly gypsum (average particle diameter: 53.mu., water 
content: 9%, composition: CaO 31.2% and SO.sub.3 44.1%) produced as 
by-product in the desulfurization treatment of waste gas was used as the 
ingredient A, and a homogeneous mixture of (X) Ordinary Portland cement 
(specific surface area: 2500 cm.sup.2 /g measured according to the 
Blaine's air permeability method) and (Y) a water-granulated iron blast 
furnace slag (specific surface area: 3600 cm.sup.2 /g measured according 
to the Blaine's air permeability method, composition: SiO.sub.2 33.5%, 
Al.sub.2 O.sub.3 15.7%, CaO 42.5% and Fe.sub.2 O.sub.3 0.7%, a vitreous 
substance free from crystalline substances as a result of an X-ray 
diffraction test) was used as the ingredient B. In Examples 1-3, a muddy 
marine deposit having a water content of 260% (particle size distribution: 
0-20.mu. 72%, 20-50.mu. 17% and 50-150.mu. 10%; an average particle 
diameter: 7.mu.) and a density of 1.20 g/cm.sup.3 at a water content of 
260% was used as a water-saturated soft soil to be treated. In Example 4, 
a river sediment (soft soil) having a water content of 384.4% (particle 
size distribution: 0-5.mu. 46%, 5-20.mu. 49% and above 20.mu. 5%; an 
average particle diameter: 5.2.mu.), a density of 1.148 g/cm.sup.3 at a 
water content of 348.4%, a pH value of 8.0 and an ignition loss of 23.7% 
was used as a water-saturated soft soil to be treated. This soft soil had 
unpleasant odor and the gas evolved therefrom contained 1800-2000 ppm of 
hydrogen sulfide. This soft soil had a total hydrogen sulfide content of 
430 mg (12.6 m-mol)/kg, an insoluble sulfide content of 2386 mg (70.2 
m-mol)/kg and a total organic matter content of 21.2 wt.%/kg. The total 
hydrogen sulfide content was determined by measuring by way of iodometry 
the quantity of free hydrogen sulfide distilled during the steam 
distillation of the soft soil to be treated. The insoluble sulfide content 
was determined by adding concentrated sulfuric acid to the distillation 
reside in the analysis of the total hydrogen sulfide, subjecting the 
mixture again to steam distillation, and analyzing the quantity of 
hydrogen sulfide distilled. The organic matter content was measured by a 
testing method using chromic acid. 
Example 1 
To 1 m.sup.3 of the soft soil was added 20 kg of the ingredient A and the 
mixture was homogeneously mixed in a kneader. To the mixture was then 
added 80 kg of the ingredient B (X/Y=50/50) and the whole was thoroughly 
mixed in the kneader. A sample of the mixture was then injected into a 
cylindrical mold of 50 mm in inside diameter and 100 mm in height, 
maintained at 20.degree. C. in a constant temperature and humidity box for 
a given period of time to effect curing the sample, and then released from 
the mold for the measurement of its unconfined compressive strength. 
For the purpose of comparison, a similar test was performed except that a 
water-saturated iron blast furnace slag in the form of coarse granules 
(Y') was used. 
The result of these tests is shown in Table 2 and as a graph in FIG. 1. The 
particle size distribution of the water-granulated iron blast furnace 
slags Y and Y' used is shown in Table 1. 
TABLE 1 
______________________________________ 
Sort 
of the 
water- 
gran- 
ulated 
iron 
blast Passing weight (%) 
furnace 
Size of sieves (mm) 
slags 0.01 0.03 0.088 
0.15 0.3 0.6 1.2 2.5 5 
______________________________________ 
Y 40.5 84.0 100 -- -- -- -- -- -- 
Y' 5.0 17.5 40.0 60.5 87.0 99.5 
______________________________________ 
TABLE 2 
______________________________________ 
Unconfined -compressive strength (kg/cm.sup.2) 
Age in days 
Examples 7 14 28 
______________________________________ 
The present invention 
1.9 2.8 4.0 
Comparative Example 
0.4 0.8 1.1 
______________________________________ 
In the graph of FIG. 1, the abscissa stands for the age in days after the 
treatment and the ordinate for the unconfined compressive strength of the 
treated soft soil (kg/cm.sup.2). The line 1 stands for the result of the 
test in accordance with the present invention while the line 2 for the 
result of the Comparative run using furnace slag Y'. 
Example 2 
A test was performed in the same manner as described in Example 1 except 
that the proportion of the ingredient A to the ingredient B was varied. 
The result of the test is shown in FIG. 2. 
In the graph of FIG. 2, the abscissa stands for the percent ratio by weight 
of the ingredient A to sum of ingredients A and B (A/A+B.times.100) while 
the ordinate for the unconfined compressive strength of the treated soft 
soil. In this graph, curves 1 and 2 show results obtained for the treated 
soft soil 4 weeks and 2 weeks after the treatment, respectively. 
Example 3 
A test was performed in the same manner as described in Example 1 except 
that the method of adding the ingredients A and B to the soft soil used in 
Example 1 was modified. 
For the purpose of comparison, a test was also performed in such manner 
that the ingredient B was first added to the soft soil and then the 
ingredient A was added thereto. 
The result of these tests is shown in Table 3. 
TABLE 3 
______________________________________ 
Order of addition 
of the ingredients Cost for Comparison 
Test (first) (second) Workability 
work in strength 
______________________________________ 
I A B Good Small 100 
A and B at the 
Generally 
II same time good Medium 98 
III B A Bad Great 87 
______________________________________ 
Example 4 
To 1 m.sup.3 of the soft soil was added two or three of the ingredients A-E 
as shown below in the order as indicated in Table 4 and the mixture was 
thoroughly mixed in a mill. The mixture was then molded in the same manner 
as described in Example 1 to obtain a mold. Each mold was then subjected 
to a series of tests to measure its unconfined compressive strength, the 
amount of hydrogen sulfide evolved therefrom and the pH value according to 
the elusion test stipulated in Notification No. 2 of the Ministry of 
Environment. The result of the tests is shown in Table 4. 
Ingredient A: 26 kg 
Ingredient B: (X/Y=50/50) 104 kg 
Ingredient C: 52 l of an aqueous solution of ferrous sulfate having a 
Fe.sup.++ concentration of 15.6 g (0.279 mol)/l 
Ingredient D(1): 100 kg of slaked lime 
Ingredient D(2): 15 kg of slaked lime 
Ingredient D(3): 1 kg of slaked lime 
Ingredient E: 52 l of an aqueous solution of ferric sulfate having a 
Fe.sup.+++ concentration of 15.6 g (0.279 mol)/l 
In Table 4, C+A, A+B and C+A+B mean simultaneous addition of the 
ingredients C and A, A and B, and C, A and B, respectively. The marks 
given with respect to evaluation of unpleasant odor mean as follows: 
+ . . . very weak 
++ . . . moderate +++ . . . strong 
TABLE 4 
__________________________________________________________________________ 
Unconfined 
compressive 
strength Evaluation 
Order of addition 
(kg/cm.sup.2) 
of 
Exp. 
of the ingredients 
Age in days 
H.sub.2 S 
unpleasant 
No. 
(1) (2) (3) 
3 7 14 (ppm) 
odor pH 
__________________________________________________________________________ 
1 C A B 0.30 
0.91 
1.10 
trace 
+ 9.46 
2 C + A B -- 
0.29 
0.94 
1.02 
" + 9.44 
3 C A + B 
-- 
0.29 
0.92 
1.11 
" + 9.46 
4 A C B 0.30 
0.93 
1.01 
" + 9.45 
5 A B -- 
0.40 
0.80 
1.01 
5 +++ 9.69 
6 C + A + B 
-- -- 
0.12 
0.50 
0.71 
2 + + 9.68 
7 D(1) -- -- 
* * * trace 
+ 11.20 
8 D(2) A B 0.19 
0.84 
1.09 
" + 10.45 
9 D(3) A B 0.20 
0.87 
1.12 
" + 10.02 
10 E A B 0.11 
0.43 
0.68 
7 ++ 9.50 
__________________________________________________________________________ 
(Remarks) 
*The strength was not improved 
In Experiment Nos. 1-4 according to the present invention, hydrogen sulfide 
contained in the soft soil was fixed by addition of the ingredient C and 
no free hydrogen sulfide was evolved. In these experiments, the measured 
values of the unconfined compressive strength were almost equal. In 
Experiment No. 5 given for comparison wherein the ingredient C was 
missing, the strength of the soft soil was improved but generation of 
hydrogen sulfide in an amount of 5 ppm was detected. A result of 
Experiment No. 6 wherein the ingredient C was used is superior in 
deodorizing effect to a result of Experiment No. 5 but is inferior in 
unconfined compressive strength to results of Experiment Nos. 1-4 of the 
present invention and Experiment No. 5 given for comparison. This is 
ascribable to the reason that the ingredient C added simultaneously with 
the strength-improving ingredients A and B adversely affected the reaction 
between the soft soil and the ingredients A and B. In Experiment No. 7 
given for comparison wherein the ingredient D(1) alone was used, the 
deodorizing effect was found excellent but no improvement was attained in 
strength in terms of the unconfined compressive strength. Accordingly, the 
ingredient D(1) is unsuited as a strengthening for the soft soil and, 
moreover, it tends to give a harmful effect to the environment because of 
its high alkalinity shown by the tabulated pH value. Experiment Nos. 8 and 
9, also given for comparison, wherein the ingredients D(2) and D(3) were 
used, respectively, exhibited excellent deodorizing effect but had a 
considerable disadvantage in that the treated soft soil had a high pH 
value. Experiment No. 10 wherein the ingredient E was used, gave extremely 
inferior results in both improvement in strength and deodorization. Thus, 
the ingredient E is quite unsatisfactory for practical use. 
Experiment Nos. 1-4 according to the present invention were each good in 
workability and low in cost. On the other hand, in case of Experiment Nos. 
8-9 given for comparison wherein improvement in both strength and 
deodorization was remarkable, the viscosity of the soft soil was extremely 
increased when the ingredient D (slaked lime) was added thereto and thus 
workability of the soft soil became seriously bad. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.