JP2809496B2 - Ground improvement method for liquefaction prevention - Google Patents

Description

The present invention relates to a liquefaction-preventing ground improvement method, and more particularly to a loose sandy ground under a groundwater level, for example, saturated with water. Slow hydration and expansion reaction due to water absorption of hard-burnt quick lime, hardening reaction by sludge and gypsum with slaked lime generated by hydration of hard-burnt quick lime, and flocculation of slaked lime on clay and sand in the ground It relates to the liquefaction prevention ground improvement method to be used.

(Prior Art) There is a concern that liquefaction may occur in a sandy ground in a river basin or a coastal area where sand is deposited, and measures against such liquefaction are urgently required.

In general, use quick lime, sand, crushed stone, gypsum, granulated slag, etc. to compact the sandy ground with a high groundwater level, and improve the ground using water absorption, expansion, and water hardening,
The so-called quicklime pile construction method is known. Also, a method in which a permeable material such as sand or crushed stone is poured into the ground in a pile shape and tamped using a vibration type driving device (sand compaction pile method), or crushed stone placed on the ground surface is vibrated by a vibrating rod into the ground. And press-fitting. Furthermore, there is a crushed stone drain method in which a pile of crushed stone is formed in the ground with an auger or the like without tamping to remove excessive pore water pressure generated during an earthquake.

(Problems to be Solved by the Invention) Among the above-mentioned prior arts, in the quicklime pile method, in the case of a permeable sandy ground with abundant groundwater, hydration of quicklime occurs instantaneously, and There is a problem in that the residual pressure for compressing the ground is small, so that sufficient compaction cannot be performed.

On the other hand, in the sand compaction pile method and the like, vibration and noise are generated, which may cause an environmental pollution problem. Therefore, there is a problem in that the applicable range is limited.
In addition, the crushed stone drain method may cause clogging of the crushed stone, so there is a question about the certainty of the effect manifestation, and there is a problem in that the ground subsides after the earthquake.

The present inventors have conducted various studies on the properties of quicklime used in the quicklime pile method described above. As a result, quicklime conventionally used for ground improvement is lightly burnt quicklime obtained by firing quicklime at a relatively low temperature.
In addition, it has been found that there is a problem in that the hydration reaction of quick lime occurs too quickly as described above because the particles having a small particle diameter are used.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a ground improvement method in which the entire ground is further compacted to improve the rigidity of the ground, thereby making liquefaction less likely to occur. It is in.

(Means and Actions for Solving the Problems) According to the study of the present inventors, the sandy ground where there is a concern that liquefaction may occur has a high groundwater level, so that light burning is required as in the conventional quick lime pile method. It has been found that the use of quick lime cannot sufficiently exert the effect of ground improvement. When limestone is heated, it starts to decompose at around 900 ° C and becomes quicklime, but the rate of hydration of quicklime obtained varies depending on various conditions during production. When quicklime is heated, it gradually decomposes from the surface of the raw stone toward the center, but if the heating temperature is set high and the treatment time is prolonged, the particles of quicklime produced by decomposition grow and the hydration rate slows down. I do.

The present inventors have found that hard burned lime obtained by calcining quick lime at a high temperature of 1,200 to 1,500 ° C. has a favorable result for improvement of sandy ground. Furthermore, it has been found that hard burnt lime having a particle size of 5 mm or more has a low hydration rate, and remains in the state of quick lime for 10 to 15 minutes even when mixed with water-containing sand.

The present invention is based on these findings, the gist of which is that quicklime or dolomite ore is used at 1,200 to 1,500 ° C.
A ground improvement material pile in the ground made of hard-burned quick lime obtained by baking with a particle size of 5 mm, a water-permeable material, granulated slag, and gypsum A soil improvement method for preventing liquefaction characterized by causing expansion and hardening of a soil improvement material pile by a reaction between moisture in the ground and the soil improvement material.

The method according to the present invention is different from light burnt lime conventionally used for the chemical industry, steelmaking and civil engineering, and is obtained by firing a rough stone having a particle size of 20 to 40 mm in a vertical furnace at a high temperature range. It has a fundamental feature in that it uses hard burnt lime. The quality of the hard-burnt quick lime is determined by a test method based on the Japan Lime Association method, that is, after sizing a sample to 1-5 mm and pouring it into water at a constant temperature,
It is determined based on the required consumption of the 4N-HC1 at the time of neutralization with, and the consumption of HC1 in the case of the above-mentioned calcined lime is 100 ml at 5 minutes after the start of titration and 150 ml or less at 10 minutes. Such hard burnt lime (particle size: 5-35 mm) is stable without hydration for 10-15 minutes even when mixed with wet sand, and keeps its true shape. Incidentally, the HC1 consumption of light burnt lime, which has been used for civil engineering, is 5 minutes value of 2
The hydration rate is extremely high, with a value of about 00 ml and a value for 10 minutes of about 300 ml.

In the method according to the present invention, the time required for forming the soil improvement material pile on the ground is several minutes, and thus the hydration reaction of the hard-burnt quick lime starts after the work is completed. By changing to slaked lime, the expansion action works and the compaction action can be effectively exerted on the ground near the ground improvement material pile.

Hard burnt lime gradually hydrates to slaked lime, and this slaked lime fills the space between the sand particles in the ground well and replaces the negative charge on the surface of the clay attached to the sand with calcium, which is a positive charge. Since the clay particles are agglomerated by replacement with ions, liquefaction of the ground can be effectively prevented.
Furthermore, since the ground improvement material used in the construction method according to the present invention contains gypsum in granulated slag, a pozzolanic reaction occurs due to the activity of slaked lime to generate a hydraulic compound,
Therefore, the ground can be hardened and modified. The ground improvement by the conventional sand compaction pile method is to mechanically fill crushed stone, sand, etc. similar to local sands while applying vibration, but the method according to the present invention is to slowly hydrate hard burnt lime. And the dual effect of the formation of hydraulic compounds.

Among the ground improvement materials used in the method of the present invention, sand or crushed stone can be exemplified as the water-permeable material, and as the gypsum, natural gypsum, chemical gypsum, anhydrous gypsum, hemihydrate gypsum or a mixture thereof is used. be able to. It is preferable that this ground improvement material is blended in a ratio of 20-50 parts by weight of hard burned lime, 30-70 parts by weight of a water-permeable material, 10-50 parts by weight of granulated slag and gypsum.

Next, the method of the present invention will be described in the order of steps with reference to FIG.

(A) First, the pile driver 1 is set upright at a predetermined position, and the casing 2 is rotated with the shoe 3 provided at the tip of the cylindrical casing 2 closed, and the casing 2 is soiled by the spiral blade 4. Let go inside.

(B) When the tip of the casing 2 reaches a predetermined depth L ₀ , the rotation of the casing 2 is stopped, and the casing 2 is previously mixed and prepared from the side hopper 5 installed above the casing into the casing 2. It ejected the soil improvement material, loading the soil improvement material as soil improvement material of the inner casing reaches a level below the ground surface by a distance L ₁ to the inner casing.

(C) While the shoe 3 is opened, compressed air is sent from the compressor 6 to pressurize the inside of the casing 2 to a predetermined pressure, and rotate the casing 2 in the opposite direction to the step (a) to put the casing 2 in the soil. Pull out from.

(D) By the above operation, the pile 7 made of the ground improvement material is formed in the soil. At this time, the hard burned lime in the soil improvement material does not hydrate, and therefore, the soil improvement material pile 7 has not expanded or hardened, so that moisture in the ground permeates into the pile. In the space from the top of the ground improvement material pile 7 to the ground surface, measures such as stacking soil at the site are taken so that the ground surface does not rise later. Such a soil improvement material pile 7 is formed at predetermined intervals in an area where the ground improvement is to be performed.

(E) After a lapse of a predetermined time, the hydration reaction of the hard-burnt quick lime in the soil improvement material pile 7 starts, and when it changes to slaked lime and expands, it exerts a pressing force on the ground portion around the pile. Is compressed and compacted, and slaked lime acts on granulated slabs and gypsum in the soil improvement material pile 7 to form a hydraulic compound, so that the expanded pile hardens, and slaked lime is clay particles in the ground. To form a composite ground composed of a ground whose structure has been improved and a hardened columnar body, compared with the original ground.

As described above, the construction method according to the present invention can be performed with low vibration and low noise on sandy ground, and according to the present invention, the ground between the piles formed is compacted by the expansion of the pile and Since the pile itself also exhibits strength by hardening, a rigid ground, which is a composite ground of a strong pile and a tight sand layer, is formed. Therefore, the ground has high resistance to liquefaction.

Next, Example 1 is also a test for confirming the mixing ratio of each material component in the ground improvement material used in the construction method according to the present invention and the expansion rate of the formed pile, and a construction experiment on an enlarged scale. Embodiment 2 The present invention will be described in more detail and specifically with Embodiment 2.

Example 1 The compounding ratio of various materials used for preparing the ground improvement material is based on the sand compaction pile method among the conventional methods, and the method of constructing a pile in the ground is also performed by the method. The law was taken into account. In this case, vibration compression is not required to create the pile, so that the total construction cost is the same as that of the sand compaction pile method,
The weight-based unit price of the ground improvement material is determined, and the blending ratio of the blended material is determined based on the unit price.

Assuming that the unit price of the ground improvement material suitable for the sand compaction pile method is 1, the unit price of each compounded material is set to be the ratio shown in Table 1 below.

The unit price A of the ground improvement material is expressed by the following formula using the symbol of the mixing ratio in Table 1 above, in which the mixing ratio is defined on the basis of weight division.

A = 2.0s ₁ +0.5 (s ₂ + s ₃ ) +0.16 (s ₄ ) The unit price A is set to 1 or less, which is the competitive unit price with the ground improvement material for the sand compaction pile method.
The mixing ratio of each material was set variously.

The soil improvement material piles 7 were formed in the experimental soil tank using the soil improvement materials of various set mixing ratios, and were expanded and hardened. The improvement effect of the ground was judged from the data of the expansion rate of the pile and the penetration resistance of the ground between the piles.

According to the data obtained, the expansion rate of the pile varied, but the expansion rate that reduced the gap in the sandy ground and provided the required compaction strength was higher than the diameter of the pile before expansion. 1.3 times and the volume expansion rate was 1.69.

The compounding ratio of each material that gives an expansion rate not less than this value and the material unit price A is 1 or less is determined by a simple preliminary test, and further examined using a soil improvement material having these compounding ratios and using an experimental soil tank. Was piled up. The improvement effect of the ground depends on the diameter of the pile which is initially formed before inflation and the area ratio of the pile to the ground. In this test, these were set to 9 cm and 0.1, respectively.

The relationship between the unit price ratio of the ground improvement material and the volume expansion rate of the formed pile, in which the mixing ratio of each material for preparing the soil improvement material was set variously and the unit price ratio was 1 or less, is shown in the graph of FIG. As shown, it was found that the volume expansion coefficient of the pile exceeded the above-mentioned set requirement of 1.7 in any of the prepared soil improvement materials.

In addition, since the penetration resistance of the ground between piles increased 2-3 times as compared with the expansion surface of the pile, it was clarified that the ground had a compaction effect.

Example 2 Based on the results of the laboratory test in Example 1, the mixing ratio of the ground improvement material, which is estimated to bring about a result that is economical and the expansion rate of the pile is not inferior to that of the sand compaction pile method, is as follows. An outdoor experiment with the scale determined and set as shown in Table 2 was conducted as follows.

A concrete pit with a length of 4.6m, a width of 3.7m and a depth of 5m was constructed outdoors, and water was poured into the pit and sandy soil was poured in to create a loose saturated sandy ground. For this artificial ground, soil improvement material piles were formed in a regular triangle arrangement with a depth of 5 m and a pitch of 1.2 m as described above using a casing having an inner diameter of 400 mm.

FIG. 3 shows the arrangement of the piles formed in this example and the N value, which is the number of impacts by the iron-penetration penetration test, before and after the ground improvement by forming the piles.
According to this, the N value is improved immediately after and 28 days after the pile is formed, so that the effect of improving the ground at each depth is recognized.

FIG. 4 is a graph showing the change over time in the horizontal earth pressure and the water pressure between the piles measured by an earth pressure gauge and a water pressure gauge installed between the piles. It can be seen that even if excess water pressure due to pile expansion occurs, it dissipates immediately and becomes hydrostatic pressure because of the permeable sandy ground.

The ground improvement effect of the present embodiment is comparable to the results of the relationship between the increase in the N value and the pile area ratio in the case of the conventional sand compaction pile method. Regarding the effect of preventing liquefaction of the ground, the safety factor against the occurrence of liquefaction indicated in the Building Foundation Structure Construction Guideline was calculated and found to be 1 or more after construction. It was confirmed that it provided preventive measures.

(Effect of the Invention) According to the soil improvement method for preventing liquefaction according to the present invention, a pile is formed in the ground with a ground improvement material mainly composed of hard-burned lime, and the pile is expanded and hardened by moisture in the ground. By doing so, it is possible to form a ground where the whole is well-tightened and has high rigidity and hardly causes liquefaction. The construction method according to the present invention can reduce the price of the conventional sand compaction pile if the mixing ratio of the hard ground lime, the permeable material (sand, crushed stone, etc.), the granulated slag, and the gypsum, which are the ground improvement materials to be used, are properly adjusted. It can be made cheaper than the ground improvement material used in the construction method, and does not cause environmental pollution such as vibration and noise during construction.

[Brief description of the drawings]

1 (a) to 1 (e) are elevation views schematically showing construction steps of a liquefaction-preventing ground improvement method according to the present invention, in which the ground portion is shown in cross section, and FIG. Has various ratios of hard lime, permeable materials (sand, crushed stones, etc.), granulated slag, and gypsum so that the unit price is lower than that of the ground improvement material used in the conventional sand compaction pile method. FIG. 9 is a graph showing the relationship between the volume expansion coefficient of the pile and the unit price ratio of the soil improvement material when a soil improvement material is prepared by setting the mixing ratio and a pile is formed using these soil improvement materials. FIG. 3A is an N-value distribution chart obtained by a ground penetration test performed in Example 2, and FIG. 3B is a diagram illustrating the arrangement of piles formed in Example 2 and the site of the penetration test. Fig. 4 shows the horizontal earth pressure between the piles created. Is a graph showing temporal change of water pressure. 1: pile driver, 2: casing, 3: shoe, 4: spiral wing, 5: side hopper, 6: compressor, 7: pile

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) E02D 3/08

Claims (2)

(57) [Claims] (1) a limestone or dolomite ore of 1,200-
Hardened calcined lime obtained by firing at 1,500 ° C and having a particle size of 5 mm or more, a permeable material, granulated slag, and a ground improving material containing gypsum. A soil improvement method for preventing liquefaction, characterized by forming a pile and causing expansion and hardening of the soil improvement pile by a reaction between moisture in the ground and the soil improvement material. 2. Among the ground improvement materials, 20-50 parts by weight of hard burned lime, 30-70 parts by weight of a permeable material, and granulated slag and gypsum are used.
The liquefaction-preventing ground improvement method according to claim 1, wherein the amount is 10 to 50 parts by weight.