CA1131456A

CA1131456A - Multiple grouting process

Info

Publication number: CA1131456A
Application number: CA350,168A
Authority: CA
Inventors: Kenji Kayahara
Original assignee: Kyokado Engineering Co Ltd
Current assignee: Kyokado Engineering Co Ltd
Priority date: 1979-04-20
Filing date: 1980-04-18
Publication date: 1982-09-14
Also published as: MX152460A; GB2052604A; ES8103239A1; ES490696A0; IT1149855B; DE3014863A1; JPS55142816A; AU5762680A; JPS6117970B2; FR2454481A1; FR2454481B1; AU527860B2; NL8002265A; NL181372B; NL181372C; IT8021504A0; GB2052604B

Abstract

MULTIPLE GROUTING PROCESS

ABSTRACT OF THE DISCLOSURE
A multiple grouting process for solidifying poor ground by injecting a plurality of grouts of different penetrability through a grouting pipe inserted into the ground which comprises injecting said grouting agents into the ground through pipe lines in a multiple pipe employed as said grouting pipe while transferring said multiple pipe vertically to change an operating stage thereof, a grout of lower penetrability being injected to form a solidified seal as a packer in a gap between said multiple pipe and the ground so as to prevent a grout of higher penetrability to be injected subsequently from leaking out through the gap, and to fill up coarse-grain layers around the operating stage so as to leave said grout of higher penetrability within a predetermined region of the ground, then said grout of higher penetrability being introduced into the ground breaking through said seal.
The ground is solidified in a single step and in a short period economically.

Description

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This invention relates -to a rnultiple grouting process, and more particularly to a process for solidifying or consolidating poor ground or leaking soils uniform]y an tightly.
Generally, the poor ground includes an alternation of strata consisting of coarse-grain layers and fine-grain layers, which should be solidified uniformly in cases of construction works and the like by injecting hardeners into the ground. It is quite desirable from a stand point of preventing environmental pollutions to solidify completely the hardeners such as a water glass type grout within a predetermined region where they are injected.
There exists many conventional processes for solidifying such ground and they have more or less dis-advantages. In the case of a rod injection process, for instance, a main injecting agent and a reacting agent are mi ed at the upper end of the rod and injected into the ground. The mixture thus obtained, i.e. hardener, tends to blow out to the surface from a gap between the boring rod and the ground or leake out through a coarse-grain layer. It is difficult to solidify poor subsoils in a fine-grain layer or within a predetermined region. Further, a hardener of shorter gelation time is of-ten gelled in the injection pipe, or injected veinlikely so as no-t to penetrate arnong soil particles.

- 2 - -~ ~ 3 ~ 3 1 Higher pressure is require~l -to ef`fect the in~jection, which results in a considerable deforrnation of the ground and possibly, springing of groun~water or a landslide upon excavation.
In the case of water glass grout of liquid type using reacting agents such as esters, etc., it is possible to penetrate the grout into the fine-grain layer uniformly, but if there exists large cavities or the coarse-grain layers in the ground, the grout injected would be gathered in such region and its uniform penetration into the ground might be prevented. In order to improve this process, a double packer process using a double pipe was proposed in which a cement-bentonite grout was injected into the ground through the inner pipe thereof inserted in the fixed outer pipe, and a grout of higher penetrability is then fed repeatedly through the outer pipe line. In spite of its excellent solidifying efficiency, it takes much time to carry out this process and the pipe is inevitably left in the ground even after the solidification is completed.
The ir.ventors examined both advantages and dis-advantages of conventional injecting processes, and now we have found a novel and unexpectedly effective process for grouting.

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Accordingly, it is an object of this invention to provide a multiple grou-ting process which can be simply carried out. Another object of this invention is to provi~le a multiple grouting process which can be completed in a shorter period of time.
These and other objects of this invention have been achieved by the provision of a multiple grouting process for solidifying poor ground by injecting a plurality of grouts of different penetrability through a grouting pipe inserted into the ground which comprises injecting grouting agents into the ground through pipe lines in a multiple pipe employed as said grouting pipe while transferring said multiple pipe vertically to change an operating stage thereof, a grout of lower penetrability being injected to form a solidified seal as a packer in a gap between said multiple pipe and the ground so as to prevent a grout of higher penetrability to be injected subsequently from leaking out through the gap, and to fill up coarse-grain layers around the operating stage so as to leave said grout o~ higher penetrability within a predetermined region of the ground, then seid grout of higher penetrability being introduced in-to the ground breaking through said seal.
BRIEF DESCRIPTION OF THE DRA~ING
In the accompanying drawing:
Fig. 1 shows schematic cross-sectional views illus-trating an embodirnent of the basic concept of this invention;

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1 Fig. 2 shows schematic cross-sectional vie~ls illustrat-ing an embodiment of this invention in which a position of the discharge end is changed ver-tically;
Fig. 3 shows schematic cross-sectional views illustrat-ing an another embodiment of this invention in which the inner and outer pipes of the multiple pipe go up and down independent-ly parallel to their axes.
DETAILED DESCRIPTION OF THE INVENTION
In this invention, a multiple pipe is employed as an injecting pipe which is inserted into the ground to be solidified. A plurality of grouts of different penetrability are injected through the multiple pipe while it is transferred vertically to change its operating stage. At first, a grout of lower penetrability is injected into the ground, which fills up a gap between the multiple pipe ard the ground thereby a solidified seal as a packer is formed to prevent a leakage of a grout of higher penetrability to be injected subsequently. The grout of lower penetrability is simul-taneously injected into boundaries of coarse-grain layers or poor layers around the operating stage, which is used to remain the grout of higher penetrability within a predeterm-ined region. After these pretreatments of the ground are - completed, the grout of higher penetrability is injected which penetrates into fir.e-grain layers where the pretreating grout is impossibly reached. This grout is effectively injected without any leakage through the periphery of the multiple pipe or the coarse layers.

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~'' i ;''` ' : ' s6 1 In order to prevent such ]eakage, grouts which are gelled in a relatively short period of time are eMployed in conventional processes, but they do penetrate into the coarse-grain layer solely, and sometirnes lead to undesirable springing of water or a landslide. On the other hand, in order to penetrate the grout among soil particles thoroughly, it is necessary to solidify over a long gelation time, which causes its leakage through the periphery of the grouting pipe. After all, it is impossible to solve problems as point out above by conventional processes using a single grout.
In this invention, these conflicting disadvantages are able to be overcome by means of the multiple grouting pipe and a plurality of grouts. The grout of lower penetrability is used to fill up the gap around the pipe and to form the solidified seal as a packer. The grout of higher penetrability is used to penetrate arr.ong fine soil particles effectively and to solidify them. The multiple grouting process of this invention may be carried out in a single step continuously. Since the present process is simply practiced and the grouting pipe is lifted up gradually until it is pulled out completely at the end of the process, it is possible to inject the grout without any trouble and solidify the ground easily and economically in a short period.
In the process of this invention, following technical ideas are combined:

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1 1) A multiple pipe is used as the grouting pipe;
2) A plurali-ty of grouts of different penetrability, i.e. combination of grouts ha~ring higher penetrability and lower one, are employed as the grou-t;

3) The plurality of grouts are injected into the ground through pipe lines formed in the multiple pipe, thereby, as first, the grout of lower penetrability (preferably, the gelation time thereof is less than 30 seconds) and then the grout of higher one are fed;

4) The grout of higher penetrability is injected into a region where the grout of lower penetrability has already been injected and coarse grain layers around has pretreated.
This invention will be fully described in the following basic and preferred embodiments. Some comments common to these embodiments are mentioned first to avoi~
repetition thereof.
1. Structure of the multiple pipe a) The multiple pipe used in this invention may be double or triple pipe, or those one in which parallel inner pipes are contained in an outer pipe.
b) The inner pipe and/or the outer one may be rotated during the injection.
c) The inner pipe or the outer one may be pro~ided with a metal crown at its bottom end so that the grouting pipe itsel~ bores the ground to a predetermined depth.

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1 2. Combination of the grouts Basically, a plurality of grouts rnay be combinations of following grouts of higher and lower penetrability:
a) A combination of grouts having a shorter gelation time and a longer one.
b) A combination of grouts of a suspension type and a solution type (both of these grouts may have shorter gelation time). These grouts are prepared by rnixing or combining grouting agents, for example, a water glass type grout is obtained by mixing or combining a water glass solution and an aqueous solution of reacting agent.
c) The plurality of grouts having a shorter gel-ation time and a longer one may be obtained by employing the water glass and a same kind of reacting agent of different concentrations, or combining the water glass with a more rapidly reacting agent such as inorganic compounds and a more gently reacting agent such as organic compounds.
3. Mixing of grouting a~
a) The grout of lower and/or higher penetrability may be prepared prior to feeding to the multiple pipe and injected into the ground through the inner and/or outer pipe.
b) A plurality of grouting agents may be fed to the inner and outer pipes separately, and then mixed at the bottom end of the multiple pipe to form and inject the grout of lower and/or higher penetrability.

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5~i 1 c) Ultimately, the mixing of grouting agents may be carried out either in the grou-ting pipe or in the ground after they are discharged from it.
4. Structure of the bottom end of the ~routing pipe a) The grouting pipe employed in this invention includes a multiple pipe of standard type (provided with nothing particular), or those which provided with a check valve at its bottom end or a plurality of discharge holes disposed axially thereon. Further, a multiple pipe in which the inner pipe is protruded beyond the outer one in the ground is also usable.
b) The plurality of grouts may be injected into the ground through either the inner pipe or the outer one, or through both of them. Cutting off the connection between both pipes, the grout of higher penetrability may be injected through the inner pipe.
c) The discharge holes for the grouting agent or the grout(s) may be disposed on a wall of the inner pipe or the outer one, or at the bottom end thereof.
c) The check valves may be furnished at the dis-charge holes of the inner or outer pipe, or between both pipes.
e) Conventional bottom end parts of the multiple pipe is also used herein.
Referring to the drawing, the present process will be described in detail in the following. In Fig. 1, an embodiment based on the concept of this invention is shown, " . ,, , .. :, , : :.- : . : :
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5~j : 1 in which a flash setting grout, having a gellation time of less than 30 seconds, is used as the grout of lower penetr-ability. In Fig. 1, 1 and 2 mean an ou-ter pipe and an inner pipe, respectively, 3 is a metal crown, and 4' is a check valve. A ball check or a structure in which a discharge hole, disposed near the bot-tom end of the inner pipe, is covered with a gum sleeve is preferably used herein as the check valve. Pipe lines I and II are formed by -the outer pipe 1 and the inner pipe 2, res~ectively. Pipe lines I' and I" are used for feeding grouting agents to the outer pipe 1 and pipe lines II' and II" are similarly used to the inner pipe 2. In this embodiment, water glass is fed from the pipe line I' and aqueous solutions of different kinds of reacting agents are independently fed from the pipe lines II' and I". The reacting agent in the pipe line II"
is mixed with the water glass to form the flash setting grout, and another agent in the pipe line I" is mixed similarly to form a grout o~ longer gelation time. In Fig.
1 (a), the boring is carried out by the grouting pipe itself while feeding water, which is then stopped at a predetermined depth. In Fig. 1 (b), grouting agents from the pipe lines I' and II' are mixed at the bottom end of the grouting pipe to form the flash setting grout, which results in solidifed soil D around the grouting pipe thereby a packer is completed.
At the same time, the flash setting grout penetrates veinlikely into coarse or poor layers where a solution type grout leakes out freely, and fills up the layers so as to . ~ . ~ . - .; ... ::

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1 leave such grout to be in;jected subsequen-tly ~llthin a predetermined region of -the ground. In Fig. 1 (c), the feeding of the grouting agents Prorn t~le pipe line LI' has been stopped and a grou-t of high pene-trability prepared by Mixing the grouting agents from the pipe lines I' and ~" is inject-ed into the ground through the outer pipe 1. Breaking through the packer formed above, this grout penetrates among fine-grain layers where the flash setting grout cannot penetrate into, solidifies a region E to be treated and makes it watertight. In Fig. 1 (d), the grouting pipe is lifted up to inject the flash setting rout, and the grout of high penetrability is then injected as shown in Fig. 1 (e).
In Fig. 2, the plurality of grouts having difPerent penetrability are injected into the ground through discharge holes disposed axially on the multiple pipe. Injected grout generally tends to be concentrated around the grouting pipe where the penetrating resistance is lower and leak out toward the ~surface, therefore, it is important to inject the grout of lower penetrability Prom an upper discharge hole so as to prevent the grout of higher penetrability to be injected subsequently from leaking out, then to inject the latter.
While transferring its operating stage upwardly, the latter is always injected into a region already pretreated by the former. Elevating the operating stage, the discharge hole for the latter may be adjusted at a level where the ~ormer has been injected or it may be placed between bo-th discharge holes. In either case9 the latter penetrates into the ~ , :, .,.,. : . .

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1 region where the former has not thoroughly pene-trated although its periphery has already solidified tigh-tly. In Fig. 2(a), the grout of lower penetrability is discharged from an upper discharge hole B disposed on a double pipe A to form solid D
around the grouting pipe and an upper periphery of the layer.
In Fig. 2 (b), the grout of higher penetrability is dis-charged from a lower discharge hole C to solidify the ground in the predetermined region E i~Jithout concentrating around the grouting pipe or leaking out toward the surface. In Fig. 2 (c), the grout of lower penetrability is in'ected into the ground after the operating stage is elevated, and then the grout of higher penetrability is discharged as shown in Fig. 2 (d).
The structure and function of the bottom end of the double grouting pipe will be described in the following.
The outer and inner pipes, for instance, are desirably not connected at the bottom end of the double pipe to dis-charge the grout of higher penetrability from the bottom end of the inner pipe and the grout of lower penetrability from a discharge hole disposed on the outer pipe at one meter above its bottom end. In a case of another type of the bottom end, it bores the ground ~hile feeding water into the ground, then an lower discharge hole is closed and an upper one disposed on the wall of the grouting pipe is opened to feed wa-ter glass instead of water for boring through the same pipe line, and a reacting agent is fed through an another pipe line and mixed with the water glass to discharge the flash setting , , , , -, :: ., .: :: ~

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1 grout into the ground through the discharge hole on the grouting pipe. These discharge holes may be closed and opened by sliding the inner pipe or the outer one axially or vertical-ly, or by means of a va]ve disposed in the double pipe.
Further, a triple pipe is also employed herein. Using t~,io pipe lines thereof, the grout of lower penetrability may be prepared and injected into the ground through its upper discharge pipe, then the grout of higher penetrability is fed through the third pipe line and discharged from its lower discharge hole.
In Fig. 3, the outer pipe and the inner one slide axially each other, and in this case, the inner pipe is protruded downward and left a certain length thereof un-covered~ The bottom end of the multiple pipe may be modified widely. A check valve may be provided at the bottom end of the inner pipe or between the outer and inner pipes. The outer and inner pipes, fcr instance, are desirably not con-nected at the bottom end of the multiple pipe and the grout is discharged from the bottom end of the inner pipe. The discharge hole for the grout or the grouting agents may be disposed either on the bottom end or the wall of the inner pipe. A triple pipe or a multiple pipe included two parallel inner pipes is also usable. The grouts of lower and higher penetrability may be injected into the ground through the outer pipe and the inner one, respectively. Also, a plurality of grouting agents may be fed through the outer and inner pipes and discharged from the bottom ends of both pipes to 4SÇ~
1 prepare the grouts outside of -the double pipe or around the bottcm end of the inner pipe. These grouts may be formed by mixing the grouting agents at the bot-tom end of the double pipe. In either case, the grout of higher penetrability is injected from the bottom end of the inner pipe into the ground already pretreated by the grout of lower penetrability. Such preferred modifications of the bottom end are simply shown in Fig. 3. In Fig. 3 (a), the double grouting pipe inserted in the ground is shown. In Fig. 3 (b), the outer pipe is lifted up to inject the grout of lower penetrability, then the grout of higher penetrability is being injected from the inner pipe. In Fig. 3 (c), the same process is repeated after the operating stage is elevated.
The grouts of different penetrability employed in this invention are selected from following group:
(a) A suspension type grout containing cement or clay as the effective component;
(b) A grout which contains suspending cement, water glass, etc. and gels as a whole suspension;
(c) A solution type grout which does not contain an suspending agents and has relatively long gelation time;
(d) A solution type grout which does not contain any suspending agents and has shorter gelation time;
(e) A grout containing an aqueous solution of non-alkaline silicate prepared by adding water glass into an acidic solution.
It is especially preferred in this invention to employ a grout which gels within 30 seconds to form an - : .. :, .:. . , ;:

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1 effectively gelled seal as a packer in a gap bet~leen the multiple pipe and the ground. This seal preven~s a grout of longer gelation time from leaking out toward the surface along to the pipe and allows the grout injec-ted subsequently to penetrate thoroughly to soils around the operating stage.
It is impossible to employ such grout of shorter gelation time in the conventional rod injection process because it clogs the rod when it is mixed in a Y shaped pipe disposed at an upper end of the rod. This problem, however, is easily solved in the present process by mixing such grout at the bottom end of the multiple pipe.
When the gelation time exceeds 1 minute, the grout tends to gush out toward the surface and it is difficult to form the solidified seal. If the grout of longer gelation time is subsequently injected into the ground pretreated undesirably as described above, it also gushes out like the first grout and solidifies the ground unsatisfactorily.
In each operating stage, it is desirable to inject the grout of shorter gelation time and after its gelation ig completed, i.e. its flow is stopped entirely, to introduce the grout of longer gelation time. For instance, if a grout gelling within 5 seconds is injected and, at an interval of 10 seconds, a grout of longer gelation time than the first one (e.g. 10 minutes) is introduced, it is possible for the second grout to penetrate into the ground desirably around . each operating stage breaking through the gelled seal.
The reacting agent used hereirl for the water glass ~- grout includes water soluble acids such as inorganic or .

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1 organic acids; salts such as inor,ganic, or~anic, basic, neutral or acid sal-ts; esters, aldehydes, amides, alcohols, and the like. The following suspending agents are also emloyed: oxides of aluminium, iron an~ rnagnesiurn such as alumina, iron oxide and magnesiuM oxide; salts of calcium, aluminium, magnesium an~ the like such as slag, fly ash, calcium silicate, cement (portland cement, blast furnace, etc.) and clay. They are not soluble in water, but form suspensions in which free Ca, Al, Fe, etc. are possible to react with silicate in the water glass.
The grout desirably used in this invention, which containes non-alkaline silicate as the main component, in-cludes a solution type grout prepared by adding alk.aline grouting agents such as water glass, carbonates, hydrogen-carbonates, sodium phosphate, magnesium oxide, and the liketo an acid silicate solution, or a suspension type grout prepared by adding one or two kinds of suspending agent selected from cement, lime, gypsum, sodium carbonate, slag, and the like to the acid silicate solution. A neutral water glass grout or that one containing clay is also used in this invention.
The water glass used herein has a mol ratio of SiO2/
M20 ranging from 1.5 to 5.0 and inciudes alkali salts of silicic acid or their mixtures with silicic acid such as liquid, anhydrous, hydrated, or crystalline water glass.
This invention will be more fully ~escribed in the following examples.

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Examele 1 Using the multiple pipe shown in Fig. 1, following grouts were injected into the ground in Tokyo which included an alternation of strata consisting of a fine sand soil and a gra~el soil~

kiquid I': #3 water glass 30~ (per 50~ of the liquid) water Liquid II": ethylene glycol diacetate 3~1(per 50 ~ of the ln liquid) sodium hydrogencarbonate 2 kg Liquid II': 75% phosphoric acid 3~ (per 50~ of the liquid) water 47 R

Equivalent Liquids I' and I" gave a grout gelling within 5 minutes, and equivalent Liquids I' and II' gave a grout gelling within 2 seconds.
The double pipe was inserted into the ground down to a predetermined depth, and Liquid I' from the outer pipe and Liquid II' from the inner pipe were mixed equivalently to form the grout of shorter gelation time, which was then injected. Discontinuing the mixing of Liquid II', Liquid I"
was mixed into Liquid I' equivalently to form the grout of longer gelation time, which was then injected into the ground.
While elevating the operational stage, the procedure describ-ed above was repeated. After all injections in each stage were completed, the solidified soil was excavated and examined.

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1 It was found -that -the grout of shor-ter gelation time ~/as filled around the g~routing pipe and between the layers, and the grout of longer gela-tion time ~/as penetra-ted into the soil particles to solidify them uniforrnly.
Example 2 Using the double grouting pipe of Fig. 1 inserted in the ground down to a predetermined depth, following grouts were injected into the alternation of strata consisting of a fine sand soil and a gravel soil located in Tokyo.
Liquid I' was prepared by adding ~3 water glass into an aqueous sulfuric acid. The grouting agent thus prepared had a ~sio23 mol concentration of 7, pH of 1.0 and the gelation time of 15 hours.

Liquid II " : a 10% solution of sodium hydrogencarbonate;
Liquid II' : a 20% solution of #3 water glass.

Liquid I' from the outer pipe and Liquid II' from the inner pipe were mixed to form a grout of pH 7.5 having a gelation time of 2 seconds, which was then injected into the ground. After that, discontinuing -the mixing of Liquid II', Liquid II" was mixed with Liquid I' to form a grout of pH 5.5 having a gelation time of 10 minutes, which was then injected into the ground. While elevating the operating stage, the procedure described above was repeated. After all injections in each stage were completed, the solidified soil was excavated and examined. It was found that the grout `

1~3~L~5~3 1 of shorter gelation tiMe was filled arouncl t~e grouting pipe and between the layers, and the grou-t of ]onger gelation tirr,e was penetrated in-to -the soil particles to solidify thern uniformly.
Example 3 According to the process of this invention, an experimental engineering work was carried out in a riverbed gravel containing groundwater. The grouting agents were;

Liquid I': a grouting agent of pH 1 prepared by adding water glass to an aqueous sulfuric acid;
Liquid II": aqueous water glass solution.

Liquid I was prepared by mixing Liquids I' and I" so as to form a grout which was gelled within 3 seconds.
Liquid II was prepared by adding the water glass to sulfuric acid to form a grout of pH 3 having a gelation time of 1 hour.
Liquid I was injected according to the process shown in Fig. 2, then after Liuqid II was injected, the operating stage was elevated. The injection procedure was repeated. After all injections in each stage were completed, the solidified soil was excava-ted and examinec]. It was found that Liquid I was injected into the coarse-grain layers to solidify them, and Liquid II was penetrated into the fine-grain layer to stabilize them. No leakage of Liquid II
was observed.

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1 Example 4 The process of Exarnple 2 was repeated using following Liquids.

Liquids I' and II": same as Example 2;
Liquids II': a suspension containing 25%, by weigh, of blast furnace cement, 10% of slaked lime and 2% of bentnite.

Liquid I' from the outer pipe and Liquid II' from the inner pipe were mixed to form a grout of pH 5 having a gelation time of 2 seconds, which was then injected into the ground. After that, discontinuing the mixing of Liquid I, Liquid II" was mixed with Liquid I' to form a grout of pH 5.5 having a gelation time of 10 minutes, which was then injected into the ground. While elevating the operating stage, the procedure described above was repeated. After all injections in each stage were completed, the solidified soil was excavated and exarnined.It was found that the suspen-sion of shorter gelation time was filled around the groutingpipe and between the layers, and the grout of longer gelation time was penetrated into the soil particles to solidify them uniformly.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A multiple grouting process for solidifying poor ground by injecting a plurality of grouts of different penetrability through a grouting pipe inser-ted into the ground which comprises injecting grouting agents into the ground through pipe lines in a miltiple pipe employed as said grouting pipe while transferring said multiple pipe vertically to change an operating state thereof, a grout of lower penetrability being injected to form a solidified seal as a packer in a gap between said multiple pipe and the ground so as to prevent a grout of higher penetrability to be injected subsequently from leaking out through the gap, and to fill up coarse grain layers around the operating stage so as to leave said grout of higher penetrability within a predetermined region of the ground, then said grout of higher penetrability being introduced into the ground breaking through said seal.

2. A process claimed in claim 1, in which at least an inner pipe of said multiple pipe is provided with a check valve at a discharge hole thereof.

3. A process claimed in claim 1, in which discharge holes for discharging the plurality of grouts of different penetrability or the grouting agents into the ground are disposed at axially different positions on said multiple pipe.

4. A process claimed in claim 2, in which discharge holes for discharging the plurality of grouts of different penetrability or the grouting agents into the ground are disposed at axially different positions on said multiple pipe.

5. A process claimed in claim 3 or 4,in which said grout of lower penetrab-ility is injected through the discharge hole disposed at upper psotion of the multiple pipe and said grout of higher penetrability is injected through the discharge hole disposed at lower position thereof.

6. A process claimed in claim 1, in which said grouting pipe is a multiple pipe consisting of an inner pipe and an outer pipe which slide axially of each other so as to leave the inner pipe uncovered at an end part thereof.

7. A process claimed in claim 6, in which said grout of higher penetrab-ility is injected through said uncovered end part of the inner pipe into a region pretreated by said grout of lower penetrability.

8. A process claimed in claim 1, 2 or 3, in which said plurality of grouts of different penetrability is a combination of a grout gelling within 30 seconds and a grout of longer gelation time.

9. A process claimed in claim 4, 6 or 7, in which said plurality of grouts of different penetrability is a combination of a grout gelling within 30 seconds and a grout of longer gelation time.

10. A process claimed in claim 1, 2 or 3, in which said plurality of grouts is a combination of a suspension type grout and a solution type grout.

11. A process claimed in claim 4, 6 or 7, in which said plurality of grouts is a combination of a suspension type grout and a solution type grout.