JP7133281B2 - Dredged soil modification method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for modifying dredged soil used as a water environment restoration material (for example, a material for constructing shallow waters and tidal flats).

Shallows and tidal flats are being created for the purpose of improving the water quality environment. Conventionally, shallow waters and tidal flats are created by setting up a submerged earth retaining embankment offshore using crushed stone, then placing dredged soil as filling material on the bank side (land side), and covering the surface layer with natural sand. A construction method is adopted to do so.
On the other hand, Patent Document 1 discloses a non-caking material for constructing shallow areas and tidal flats, which is composed of dredged sand and steel slag. Further, Patent Documents 2 to 4 disclose techniques for positively utilizing CaO contained in steel slag and mixing steel slag with dredged soil to improve strength. In this technique, the strength of the dredged soil is modified mainly by the pozzolanic reaction between the CaO content of the iron and steel slag and the Si, Al, etc. of the dredged soil. In this way, using iron and steel slag to improve the strength of soft dredged soil can be used in combination with CaO, which is the original characteristic of iron and steel slag. Very useful.

JP-A-2005-133309 JP 2009-121167 A JP 2011-206625 A JP 2011-208365 A Japanese Unexamined Patent Application Publication No. 2012-31618 JP-A-8-60152 JP-A-9-100470

When dredged soil mixed with iron and steel slag is used as an aquatic environment restoration material such as shallow water and tidal flat construction materials, if the strength improvement by mixing iron and steel slag is not sufficient, the installed aquatic environment restoration material may flow out or cause structural damage. problems such as the inability to ensure stability. However, the properties of the actual dredged soil vary depending on the region and sea area, and the degree of solidification (development of strength) due to the mixing of iron and steel slag varies greatly depending on the dredged soil. Therefore, when actually applying the method of mixing iron and steel slag with dredged soil, it is necessary to conduct a mixing test in advance or select the slag to be applied according to the physical characteristics of the dredged soil (for example, , Patent Document 5).

In addition, basically, the strength of the dredged soil is expressed by these measures, but as a result of experiments conducted on various dredged soils, the present inventors have found that iron and steel slag is mixed up to a ratio close to 50% by volume. However, it was found that dredged soil with little strength exists, though rare. If iron and steel slag is mixed in a ratio close to 50% by volume, the ratio of dredged soil decreases, so the original purpose of effectively utilizing dredged soil cannot be achieved. In addition, it is considered that this state is a level where steel slag grains are in contact with each other and the structure can be maintained, and it is considered that the pozzolanic reaction between steel slag and dredged soil is not sufficiently acting. For such dredged soil, it was found that even when materials other than steelmaking slag such as cement and lime were added, the strength development was poor. There was no effective method for solidifying such dredged soil, and the dredged soil could not be effectively used and had to be discarded.

Therefore, the object of the present invention is to solve the problems of the prior art as described above, and to improve the quality of the dredged soil by mixing steel slag, in which a mixing test is performed in advance and the physical properties of the dredged soil are improved. It is possible to modify the dredged soil to a property that can develop sufficient strength without selecting steel slag according to To provide a modification method capable of developing a high strength.

As a result of repeated studies to solve the above problems, the present inventors have found that even if the dredged soil does not develop strength only by mixing steel slag, the SO ₄ concentration of the interstitial water of the dredged soil is increased to a certain level or more. After that, by mixing iron and steel slag or adding a small amount of gypsum together with iron and steel slag to the dredged soil, while securing the fluidity necessary for construction, sufficient strength that does not cause outflow after construction It was found that the dredged soil can be reformed.

The present invention was made based on such findings, and has the following gist.
[1] In the method of modifying the dredged soil by mixing iron and steel slag, the iron and steel slag is mixed with the dredged soil after the SO ₄ concentration of the interstitial water of the dredged soil is 1000 mg / L or more. A dredged soil improvement method characterized by:
[2] In the method of modifying the dredged soil by mixing iron and steel slag, gypsum is mixed with iron and steel slag in the dredged soil, and the ratio of the gypsum in the mixed material is 0.5% by volume or more. A dredged soil reforming method characterized by:
[3] In the modification method of [1] or [2] above, even if the dredged soil is mixed with 20% by volume of steel slag with a free CaO content of 10% by mass, the unconfined compressive strength after 7 days is 30kN / m A dredged soil improvement method characterized in that the dredged soil does not exceed ² .

[4] In the modification method of any one of [1] to [3] above, the flow value of the mixed material obtained by mixing steel slag or steel slag and gypsum in the dredged soil is 8.5 cm or more. Method for modifying dredged soil.
[5] A method for modifying dredged soil according to any one of [1] to [4] above, wherein the dredged soil has an organic carbon content of 4% by mass or more.
[6] The method for modifying dredged soil according to any one of [1] to [5] above, wherein the iron and steel slag contains 0.5% by mass or more of free CaO.

[7] In the method of modifying the dredged soil by mixing steel slag,
Dredging characterized by measuring the _SO4 concentration of interstitial water and the amount of organic carbon after drying, and performing the following (a) or (b) according to the measured _SO4 concentration and the amount of organic carbon Soil modification method.
(a) When the SO ₄ concentration is 1000 mg/L or more and the organic carbon content is less than 4% by mass, only steel slag is added to the dredged soil and mixed.
(b) When the SO ₄ concentration is less than 1000 mg / L and / and the organic carbon content is 4% by mass or more, gypsum is mixed with steel slag in the dredged soil, and the ratio of the gypsum in the mixed material is 0.5 volume % or more.

According to the present invention, in the method of modifying the dredged soil by mixing steel slag, even if the dredged soil does not develop strength only by mixing steel slag, a mixing test is performed in advance, and dredging is performed. Without selecting steel slag according to the physical properties of the soil, it is possible to modify the dredged soil to have sufficient strength to prevent runoff after construction. For this reason, dredged soil, which could not be used conventionally, can be effectively used as a water environment restoration material such as shallow water and tidal flat construction materials, and steel slag can be used without aging treatment, etc. It also has the advantage of being economically profitable.

The result of examining the strength expression (planar soil hardness) of a material (mixed material) in which a predetermined amount of steelmaking slag is mixed after replacing the interstitial water of the dredged soil collected from the seabed and changing its _SO4 concentration. graph showing Graph showing the relationship between _SO4 concentration of pore water in dredged soil before mixing with steelmaking slag and unconfined compressive strength of mixed material of dredged soil and steelmaking slag Graph showing the relationship between the mixing ratio of gypsum and the uniaxial compressive strength of the mixed material in which gypsum is mixed with steelmaking slag in the dredged soil in the range of 0 to 1.0% by volume. Graph showing the relationship between the mixing ratio of steelmaking slag and the unconfined compressive strength of the mixed material in which only steelmaking slag is mixed with dredged soil and the mixed material in which gypsum is mixed with steelmaking slag in dredged soil A graph showing the relationship between the mixing ratio of steelmaking slag and the flow value of the mixed material, which is a mixture of steelmaking slag and gypsum mixed with dredged soil.

In the dredged soil modification method of the present invention, a mixed material of dredged soil and steel slag (hereinafter sometimes simply referred to as "mixed material") is used as aquatic environment restoration material such as shallow and tidal flat construction material. It is a modification method that can improve the strength of the mixed material while ensuring a sufficient amount of mixed dredged soil when it is about to be used. In particular, it is applied to dredged soil that does not develop strength despite mixing iron and steel slag that develops strength when mixed with standard dredged soil, and develops sufficient strength. It is a modification method that can be made.

The normal hardening of steel slag mixed with dredged soil is believed to occur through pozzolanic reactions. That is, the Si and Al components of the dredged soil primarily react with the Ca of the steelmaking slag, resulting in the formation and hardening of Ca—Si—Al—H ₂ O products such as those found in cement. Conceivable. Therefore, if the dredged soil contains factors that inhibit this reaction, hardening will be inhibited. As a method of compensating for this, a typical coping method is to add cement or ground granulated blast furnace slag mainly in order to more positively generate Ca--Si--Al-- _H.sub.2O .

However, the types and amounts of such reaction-inhibiting substances vary depending on the dredged soil, and in some cases, adding cement or the like has almost no effect. Therefore, the inventors of the present invention decided to study the development of strength by combining completely different reactions. Land soil is generally improved by adding cement, etc., but it is known that organic soil is the cause of difficulty in developing strength, and a method of generating ettringite-based compounds is known as a countermeasure. A method of simultaneously adding slag and gypsum to soft soil on land is proposed in Patent Documents 6 and 7 and the like. The present inventors believe that a similar mechanism may work in dredged soil, and the amount of SO ₄ contained in the solid content of dredged soil and the strength development are generally evaluated. Correlations were investigated, but no clear correlation was found in the case of dredged soil. In addition, in the case of the above-mentioned method, since it is carried out on land, there is almost no water contained at the same time. there is In such a case, the original soil does not have the property of flowing, and the fluidity of the mixed material is secured as assumed when the mixed material of dredged soil and steelmaking slag is used for underwater construction. Can not.

Therefore, as a result of further investigation, the environment is completely different between soil that is constantly exposed to air, such as that used on land, and soil that is blocked from air, such as dredged soil. It was found that it should be considered including In addition, the dredged soil contains a large amount of interstitial water unlike the soil on land, so it was found that the effect cannot be ignored. It was thought that the ion species and concentration were almost the same as seawater because it is the interstitial water of the dredged soil in the sea area. It was found that the concentrations of ionic species contained in pore water differ greatly depending on the soil. Then, among the water quality of the interstitial water, we found that when the _{SO4 concentration} is particularly insufficient, the strength development is reduced. was found to be able to proceed.
Fig. 1 shows the material obtained by replacing the interstitial water of the dredged soil collected from the seabed and changing the _SO4 concentration to 25 mg/L, 1200 mg/L, and 2400 mg/L, and then mixing 20% by volume of steelmaking slag. The results of examining strength expression (soil hardness) are shown. The strength (soil hardness) was evaluated using a Yamanaka soil hardness tester (flat type). According to FIG. 1, it can be seen that the strength of the mixed material is greatly improved by increasing the SO ₄ concentration of the dredged soil before mixing with the steelmaking slag.

In the actual water area, the reason why the _SO4 concentration of the pore water in the dredged soil decreases is not necessarily clear. One possible cause is that sulfates mutate into sulfides.
From the above investigation results, by sufficiently increasing the concentration of SO ₄ in the pore water of the dredged soil mixed with iron and steel slag, iron and steel slag can be mixed even in dredged soil that does not develop strength only by mixing iron and steel slag. It was found that the strength of the mixed material can be expressed, and the strength can be further increased in the dredged soil in which the strength is expressed by mixing steel slag.

Therefore, in the first reforming method of the present invention, the interstitial water of the dredged soil has a _SO4 concentration of 1000 mg/L or more, preferably 1800 mg/L or more, and then iron and steel slag is mixed with the dredged soil. is. The SO ₄ concentration of pore water is measured, for example, by centrifuging the dredged soil and measuring the SO ₄ concentration in the supernatant.
In order to increase the _SO4 concentration of the interstitial water of the dredged soil, a solution with a _SO4 concentration higher than the _SO4 concentration of the interstitial water of the dredged soil can be added to the dredged soil, but standard seawater has _SO4 contains about 2400 mg/L, a simple method to increase the SO ₄ concentration in the interstitial water is to add seawater when adjusting the moisture content of the original dredged soil. Also, a SO ₄ ion-containing solution other than seawater may be added. The SO ₄ ion-containing solution such as seawater can be added to the dredged soil at any time after the dredged soil is excavated and before the steel slag is mixed.
When increasing the _SO4 concentration of interstitial water in the dredged soil, there is no particular upper limit for the _SO4 concentration, but if the _SO4 concentration becomes too high, ettringite will be excessively generated. is likely to occur, it is preferable to set the upper limit to about 2800 mg/L.

In addition, in the second modification method of the present invention, gypsum is mixed with steel slag in the dredged soil, and in this method, gypsum is added and dissolved or reacted more directly. . The ratio of gypsum in the mixed material (dredged soil + steel slag + gypsum) is 0.5% by volume or more, preferably 1.0% by volume or more. If the proportion of gypsum is less than 0.5% by volume, the effect of addition cannot be sufficiently obtained. On the other hand, when the ratio of gypsum increases, the time required for homogeneous mixing increases, and readjustment of the water in the dredged soil is required to ensure fluidity. The proportion of gypsum in (steel slag + gypsum) is preferably 3.0% by volume or less. The amount of such gypsum added is extremely small compared to the amount of gypsum-based material added in normal soil improvement on land (for example, 5 to 20% by volume (literature value)). With such a small amount of gypsum addition, even if the dredged soil and steel slag are mixed together, the fluidity of the mixed material, in other words, the dredged soil can be improved without hindering the workability of the mixed material. can be done.

Although the type of gypsum used in the present invention is not particularly limited, gypsum dihydrate is particularly preferred among gypsum. The mixed material of dredged soil and steel slag in the present invention is constructed in a fluid state, but gypsum hemihydrate, which is mainly used as a soil conditioner on land, does not absorb water into the gypsum hemihydrate. This is because it is necessary to add water in excess.
The addition and mixing of the gypsum may be performed simultaneously with the addition and mixing of the steel slag, or may be performed before and after the addition and mixing of the steel slag.

As in the first modification method and the second modification method of the present invention, the mechanism by which the solidification of the mixed material of dredged soil and steel slag proceeds by adjusting the water quality or adding gypsum in a small amount is as follows. Although it is not always clear, when sulfur-containing crystals such as ettringite and taumulite are formed, the influence of reaction inhibitors is reduced by incorporating or adsorbing reaction inhibitors into the crystals, and as a result, iron and steel slag It is thought that the pozzolanic reaction in which the

Other conditions of the method of the present invention are described below.
Iron and steel slag (slag generated in the steel manufacturing process) mixed with the dredged soil in the present invention includes blast furnace slag, steelmaking slag, ore reduction slag, and the like. Blast furnace slag includes slow-cooled blast furnace slag and granulated blast furnace slag. Steelmaking slag includes steelmaking slag generated in processes such as hot metal pretreatment, converter blowing, and casting (for example, decarburization slag, dephosphorization slag, desulfurization slag, silicon removal slag, ingot-making slag, etc.), Examples include electric furnace slag. Among these, steelmaking slag is preferable from the viewpoint of effectively acting the pozzolanic reaction, and converter decarburization slag is particularly preferable.
Generally, when steel slag is used, it is pre-aged. Aging is to secure volume stability of slag and stability of surface minerals by weathering in the atmosphere or placing in a steam atmosphere for a certain period of time. However, in the case of the present invention, the chemical reaction on the slag surface also contributes to the development of strength, so aging is not particularly required.

The iron and steel slag preferably has a free CaO content of 0.5% by mass or more. If the free CaO content is less than 0.5% by mass, the pozzolanic reaction as described above may not occur sufficiently. However, if the free CaO content exceeds 10% by mass, the alkali elution amount increases, so the free CaO content is preferably 10% by mass or less.
The particle size of the steel slag is not particularly limited, but if the specific surface area is small, the reactivity will decrease. A particle size of 40-0 mm to 20-0 mm is preferred, especially in order to obtain an improved effect by mixing the gravel-like material.

Further, the ratio (mixing ratio) of iron and steel slag in the mixed material is preferably 10 to 50% by volume. If the mixing ratio of iron and steel slag is less than 10% by volume, the advantage of mixing gravel-like iron and steel slag is reduced, and the effect of iron and steel slag that causes the pozzolanic reaction to modify the dredged soil is reduced. On the other hand, if the mixing ratio of iron and steel slag exceeds 50% by volume, the ratio of dredged soil decreases, so the significance of effective use of dredged soil decreases, and workability management and pH control become difficult. As will be described later, the flow value of the mixed material is preferably 8.5 or more, but from the viewpoint of securing this flow value, the mixing ratio of steel slag should be less than 50% by volume (especially 48% by volume or less). is preferred.
In view of the above, a more preferable range of the mixing ratio of iron and steel slag is 15 to 30% by volume.

The type of dredged soil to which the present invention is applied is not particularly limited, but it is useful to apply it to dredged soil with poor pozzolanic reactivity. In particular, dredged soil whose unconfined compressive strength after 7 days does not exceed 30 kN / m ² even if 20% by volume of iron and steel slag with a free CaO content of 10% by mass is mixed does not develop strength just by mixing iron and steel slag. Since it can be said to be dredged soil, it is useful to apply it to such dredged soil. However, the dredged soil with high pozzolanic reactivity (especially dredged soil with a uniaxial compressive strength exceeding 30 kN / m ² after 7 days when iron and steel slag with a free CaO content of 10% by mass is mixed with 20% by volume) It can also be applied to , which can further increase the strength.

In addition, if the dredged soil has a large amount of organic carbon, it becomes difficult to develop strength. Useful. Although the effect of the present invention can be obtained even with dredged soil having an organic carbon content of less than 4% by mass, the necessary strength can be secured basically by adjusting the amount of steelmaking slag within the organic carbon content range. . On the other hand, dredged soil with an organic carbon content of 4% by mass or more can increase its strength to some extent by mixing only iron and steel slag, but when trying to obtain the desired strength, a large amount of iron and steel slag (for example, the ratio in the mixed material amount exceeding 50% by volume), and the usage ratio of the dredged soil, which is originally intended to be effectively used, is greatly reduced.

The mixed material of dredged soil and iron and steel slag (mixture of iron and steel slag or iron and steel slag and gypsum mixed with dredged soil; the same shall apply hereinafter) has fluidity, and there is no particular limitation on the degree of fluidity, but workability And considering the effect of stably covering the steel slag particles with the dredged soil, the flow value of the mixed material is preferably 8.5 cm or more. On the other hand, if the flow value of the mixed material is too large, the dredged soil and the steelmaking slag will separate, and problems such as not being able to secure the quality after construction or flowing out immediately after construction are likely to occur. Therefore, the flow value is preferably less than 23.0 cm. Here, the flow value is measured according to JHS-A-313 (Japan Highway Public Corporation Standard) "Cylinder Flow Test". The flow value of the mixed material of dredged soil and steel slag can be adjusted by the water content of dredged soil, the mixing ratio of dredged soil and steel slag, and the like.

In carrying out the present invention (second reforming method), for example, the SO ₄ concentration of interstitial water in the dredged soil and the organic carbon content after drying are measured, and depending on the measured SO ₄ concentration and organic carbon content Then, the following (a) or (b) can be performed.
(a) When the SO ₄ concentration is 1000 mg/L or more and the organic carbon content is less than 4% by mass, only steel slag is added to the dredged soil and mixed.
(b) When the SO ₄ concentration is less than 1000 mg / L and / and the organic carbon content is 4% by mass or more, gypsum is mixed with steel slag in the dredged soil, and the ratio of this gypsum in the mixed material is 0.5 volume % or more.
The dredged soil (mixed material) modified by the present invention can be applied to various uses in water areas, including materials for building shallows and tidal flats.

The dredged soil collected from the industrial area of Tokyo Bay was modified by mixing it with steelmaking slag. Table 1 shows the basic physical properties of the dredged soil. This suggests that the ignition loss is relatively high and the organic content is high. In addition, this dredged soil contains 2.6% by mass of SO ₄ in dry soil, which is higher than general dredged soil (0.8% by mass or less), but the SO ₄ concentration in pore water is 780 mg / L and were at a low level. The SO ₄ concentration of the interstitial water is obtained by centrifuging the dredged soil and measuring the SO ₄ concentration of the supernatant.
As the steelmaking slag, converter decarburization slag having a grain size equivalent to CS20 in JIS A5015 "Iron and steel slag for roads" was used. This converter decarburized slag has a surface dry density of 3.11 and an amount of free CaO of 3% by mass.

[Example 1]
After adjusting the SO ₄ concentration of the interstitial water of the dredged soil by replacing it with artificial seawater in which only the SO ₄ concentration is changed, these dredged soil and steelmaking slag are vol% 70 (dredged soil): 30 (steelmaking slag), and investigated the strength of the mixed material after curing. In the investigation of the strength of this mixed material, the mixed material was placed in a mold of φ5 cm×10 cm high, cured, dredged soil and steelmaking slag were mixed, and the unconfined compressive strength was measured 7 days later. The unconfined compressive strength was measured according to the concrete compressive strength test method specified in JIS A1108.

Fig. 2 shows the measurement results (the horizontal axis in Fig. 2 is the _SO4 concentration of the interstitial water of the dredged soil before mixing with the steelmaking slag). According to this, dredged soil having a SO _{4 concentration in pore water (SO 4 concentration} in pore water before mixing with steelmaking slag) of less than 1000 mg/L does not exhibit sufficient strength of the mixed material. _On the other hand, dredged soil with a _SO4 concentration of 1000 mg/L or more in pore water exhibits strength of the mixed material. The strength is about 200kN/m ² , and sufficient strength is secured for use as seabed ground. In this way, by adding and mixing steelmaking slag in a state where the SO ₄ concentration of the interstitial water of the dredged soil is sufficiently increased, even dredged soil that does not develop strength simply by mixing steelmaking slag , strength can be expressed stably.

[Example 2]
Gypsum was mixed with steelmaking slag in the dredged soil, and the strength of the mixed material after curing was investigated. The dredged soil, steelmaking slag, and gypsum mixture ratios were fixed at 70% by volume for dredged soil and 30% by volume for steelmaking slag + gypsum, and the ratio of gypsum was varied within the range of 0 to 1.0% by volume. In the investigation of the strength of this mixed material, as described above, steelmaking slag with a grain size of 20-0 mm was used, and the mixed material was cured in a mold of φ10 cm × 20 cm height, dredged soil and steel slag (and gypsum). The unconfined compressive strength was measured 7 days after mixing and 28 days after mixing. As the gypsum, gypsum dihydrate (manufactured by Yoshino Gypsum Co., Ltd., surface dry density: 2.32) was used. The unconfined compressive strength of the mixed material was measured according to the concrete compressive strength test method specified in JIS A1108.
FIG. 3 shows the measurement results. According to this, almost no change is seen until the amount of gypsum added is 0.3% by volume, but when the amount is 0.5% by volume, the increase in strength becomes clear. The strength has increased to a level that can be used as improved soil on land.

In addition, the dredged soil with the basic physical properties shown in Table 2 collected at a location (coastal area of Tokyo Bay) different from the dredged soil in Table 1 is targeted, and a mixed material with gypsum (addition amount 0.5% by volume) added 4 shows the results of measuring the unconfined compressive strength after 28 days for mixed materials with no gypsum added and varying the mixing ratio of steelmaking slag (proportion in the mixed material). According to this, the addition of gypsum increases the strength regardless of the mixing ratio of steelmaking slag. FIG. 5 shows the measurement results of the flow value at that time. According to this, if the mixing ratio of steelmaking slag is high, the flow value will be small, and the workability will decrease. It has been confirmed that the above has been ensured and that stable workability has been ensured.

Claims (6)

In the method of improving the quality of dredged soil by mixing iron and steel slag (excluding granulated slag),
As iron and steel slag, using one or more selected from steelmaking slag and ore reduction slag,
Dredged soil characterized by mixing gypsum with iron and steel slag containing gravel grains with dredged soil excavated from the bottom of the water so that the ratio of the gypsum in the mixed material is 0.5% by volume or more. (However, the method of mixing one or more of alumina-based material, coal ash, ground granulated blast furnace powder, and hydrated lime with iron and steel slag and gypsum with respect to dredged soil is excluded.). Even if the dredged soil is mixed with 20% by volume of iron and steel slag with a free CaO content of 10% by mass, the unconfined compressive strength after 7 days is dredged soil that does not exceed 30 kN / m ² . A method for modifying dredged soil as described. 3. The dredged soil reforming method according to claim 1 or 2, wherein the flow value of the mixed material obtained by mixing the dredged soil with steel slag and gypsum is 8.5 cm or more. The dredged soil reforming method according to any one of claims 1 to 3, wherein the dredged soil has an organic carbon content of 4% by mass or more. The method for modifying dredged soil according to any one of claims 1 to 4, wherein the iron and steel slag contains 0.5% by mass or more of free CaO. The dredged soil according to any one of claims 1 to 5, wherein the ratio of iron and steel slag in the mixed material is 10 to 50% by volume and the ratio of gypsum is 0.5 to 3.0% by volume. modification method.

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