NO20160273A1 - Method for stabilizing grounds - Google Patents

Description

Process for stabilizing grounds

The invention relates to a process for stabilizing and consolidating grounds, especially soft and sensitive clay grounds, to improve their mechanical strength. Stabilisation of such grounds may be useful for the construction of roads, railways, airports, buildings, etc, but also preventive to reduce risks for landslides and the similar.

Soft and sensitive grounds may include marine 'quick clays' and other sensitive clays; and by "sensitive marine clays" and "quick clays", it is here meant clays mainly found in the glaciated areas of Canada and Scandinavia. They were deposited in marine environments during the retreat of the inland glaciers 10 to 13 thousand years ago. The fine material in these marine sediments is dominated by phyllo-silicates, i.e. mica and clay minerals. Small distortions of 'quick clays' may cause collapse of the particle aggregates, resulting in a complete loss of sediment shear strength, which can lead to large landslides. This phenomenon has caused several quick clay slides in the lowlands of Norway, often with catastrophic consequences. These types of clays are more thoroughly described in Rosenqvist, I.T. (1978), "A general theory for the sensitivity of clays". Proceeding of the interdisciplinary conference held at the University of Luleå. Permont Press.

Soft grounds such as clay deposits often need to be stabilized before they can carry any additional load such as applied during filling and construction activities, and thus, several chemical stabilization schemes have been tried for such clays. Stabilization of quick clays has mainly been performed in two ways; mixing and remoulding the clay with theChemicals, or theChemicals have been allowed to diffuse into the undisturbed quick clay. The disadvantage of the diffusion method is the time; it may take long time to reach the required stabilization.

A method according to prior art, for stabilization deep into the ground, is the infusion of unslaked lime (CaO). Lime is an old stabilizing agent that has been used for centuries as construction material. The deep stabilization method involves mixing and moulding the lime with the clay to form a series of piles which extend down into the clay. These piles provide stabilization to the clay deposit.

The use of hydroxy-aluminium alone and/or in combination withChemicals such as potassium chloride (KCI) is disclosed as a clay stabilizing agent in U.S. Pat. No. 4,360,599 issued to Tor Loken and Odd R. Bryhn on Nov. 23,1982. The same patent describes the need for proper admixing of the stabilizing agent into the clay in order to achieve the desired results.

Another method for infusingChemicals such as hydroxy-aluminium mixture, into a quick clay formation, and an in-situ mixing device for introducing and mixing hydroxy-aluminium into quick clay formations is described in Norwegian patent NO 168124. The method comprises the steps of: (a) rotating a hollow torque tube with a helical blade håving convolutions of at least one revolution attached thereto down into the formation, said convolutions håving substantial thickness adjacent said torque tube and tapering to an outer edge; (b) creating a disturbed column in the formation and flowing the hydroxy-aluminium down the torque tube to the helical blade and into the formation; (c) rotating said torque tube and helical blade in place thereby mixing the hydroxy-aluminium into the formation; (d) rotating out said helical blade from the formation and allowing the formation to stabilize; and (e) permitting backflow through bypass channels in the convolutions of said helical blade. NO 168124 also describes an in-situ mixing device for introducing and mixing hydroxy-aluminium into quick clay formations.

Yet another method for stabilizing clay deposits (particularly quick clay) by creating a disturbed column and infusing stabilizingChemicals by means of an in-situ mixing device, is for instance described in a brochure published by NORCEM in collaboration with NGI, 2006: "Grunnforsterkning med kalksement", and G. Holm, 2006: "Svensk Djupstabilisering FoU 1995-2006", Report 18. According to this method, large drill rigs are used for carrying out this consolidation jobs, by drilling down to e.g. 25 meters depth (or deeper if needed). A "wing" or bucket with nozzles is mounted onto the drill string (close to the drill bit) where the cementing substances are blown out with a propellant gas at 4 to 9 bar pressure. The high pressure is necessary to overcome the pore pressure in the ground, and at depths of 15 to 25 meters commonly applied pressures are 7-9 bar. This takes place while the drill is rotated upwards in the bore hole. The diameter of the treated zone is determined by selecting "wings" of different sizes, but has a maximum diameter of about one meter.

By these known methods one achieves a stabilisation of the ground, but it takes time before the addedChemicals are completely cured. Therefore, the stabilisation must take place in sufficient time before the ground may be used. Traditionally the object has been to achieve a shear strength of at least 500 kPa after one year of curing.

In order to stabilize the ground with the method above, one traditionally uses engines with combustion motors, and C02 will be released to the environment during the operation. This might cause environmental problems and should be avoided.

The object of the present invention is to develop a method for stabilizing ground fast, and that sufficient shear strength of the ground is established within hours rather than weeks. This will enable an operator to move a drill rig for drilling holes to stabilize the ground forward into the sensitive region, without the need for temporary enforcement of the ground.

Another object of the present invention is to achieve a shear strength greater than 200 kPa within a day, preferably 2-5 hours. This means that stabilisation of an area of soft and sensitive grounds will be quicker and less expensive than corresponding methods according to prior art.

Yet another object of the invention is to inject C02 for permanent storage into the ground. The C02 to be injected may be produced by a chemical processes, combustion motors or the similar.

SUMMARY OF THE INVENTION

The present invention concerns a method for stabilizing ground by infusing stabilizingChemicals into the ground. TheChemicals are pumped into the ground by means of a propellant gas, and the propellant gas contains an amount of C02 that is higher than the amount of C02 in the air surrounding the air intake of a pump or compressor being used to pump theChemicals. The amount of C02 is decided by the amount and type of injectedChemicals.

The method is especially suitable for stabilizing soft grounds and clay deposits (particularly quick clay) under ground, such as from 1-25 m deep. Clay deposits and quick clay are normally covered by a crust of 1 -5 meters, and this method is especially suitable for stabilizing clay deposits below the crust. A person skilled in the art would know how to measure the thickness of the upper crust in an area to be stabilized. The method comprises steps for creating a disturbed column and infusing stabilizingChemicals by means of an in-situ mixing device, for instance corresponding to the process described in the brochure published by NORCEM mentioned above. As this process is known to a person skilled of the art we will not describe all features and technical details of the known parts of this process.

The invention relates to a method for stabilizing ground by infusing stabilizingChemicals comprising calcinated carbonate rocks, into disturbed columns of the ground, wherein theChemicals are pumped into the ground by means of a compressed gas, hereinafter referred to as a propellant gas. The propellant gas may be any suitable gas supplied at sufficient pressure. In a preferred embodiment of the invention, the propellant gas is the air surrounding the air intake of a compressor, enriched with C02, and the compressor compressing the gas should be strong enough to release the propellant gas with sufficient pressure to transport the chemicals into the desired depth of the hole, and still overcome the pore pressure. By adding C02, the chemicals will react and cure faster and thus one will achieve a sufficient stabilization of the ground faster.

In a preferred embodiment, the method uses a device which creates a disturbed column in a sensitive ground formation and the stabilizing chemicals are infused and properly mixed into the disturbed column. The column should be as deep as necessary, such as 25 m, or even more. The upper part of the column, such as the upper 1-5 m, is normally the crust, and does not need to be stabilized. The chemicals are thus not infused in this area. In a further preferred embodiment, several holes are drilled and columns of stabilized material are made in the same area, and by making a sufficient number of such columns, the whole area will be stabilized.

In a further preferred embodiment, a drilling rig is used to create a hole into the ground, and the stabilizing chemicals are pumped through a drill string of the rig and out of a lower end of the drill string and into the surrounding ground, while the drill string is moved vertically, preferably upwards.

In a preferred embodiment, the carbonate rocks to be calcinated and used as stabilizing chemicals are limestone, dolomite and/or magnesite, preferably limestone. In another preferred embodiment the stabilizing chemicals further comprises portland cement.

In a preferred embodiment, the amount of C02 added to the propellant gas depends on the amount of calcinated carbonate rocks. In a further preferred embodiment, the amount of C02 is about 80 weight% of the calcinated carbonate rocks. In another embodiment, the amount of C02 is about 40 weight% of the calcinated carbonate rocks. In yet another embodiment, the amount of C02 is between about 40-80 weight% of the calcinated carbonate rocks. In another embodiment, the added amount of C02 is less than the removed amount of C02 during the calcination of the carbonate rock. By adding such an amount of C02, one achieves that most of the calcinated carbonated rocks are carbonated, for instance quicklime is carbonated to calcite, while the pH is kept at a desirable level.

The invention also relates to use of such a method for stabilizing ground, particularly soft ground and clay such as quick clay.

As said above, the chemicals are blown out of the drill string and into the ground by means of a propellant gas at 4 to 9 bar pressure. The high pressure is necessary to overcome the pore pressure in the ground, and at depths of 15 to 25 meters commonly applied pressures are 7-9 bar.

During the rotation of the drill string, the particle structure of the clay will collapse, and a substantial part of former capillary bound pore water will be mobilized and displaced by the propellant gas, thus giving a lower bulk density of the sediment. This will contribute to the agitation of the sediment and facilitate the in-mixing of chemicals.

Stabilizing chemicals commonly used are quicklime (CaO) and Portland cement. The European Standard EN 197-1 defines Portland cement as: Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3 CaOSi02 and 2 CaO-Si02), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of CaO to Si02 shall not be less than 2.0. The magnesium oxide content (MgO) shall not exceed 5.0% by mass.

In the following we will describe the invention when using quicklime as a stabilizing chemical, but other carbonate rocks (e.g. dolomite and magnesite) which may be calcinated to CaMg02 and MgO, may also be used instead of, or together with quicklime. Any mixtures of these may also be used, and should be included when reading the following description.

Once the quicklime and possible other stabilizing chemicals are infused into the ground, the quicklime will spontaneously interact with pore water and undergo various hydration reactions. The quicklime will dissolve in the pore water in an exothermic hydration reaction, with significant heating of the treated zone. The heating is important because it accelerates the cementing reactions; particularly when Portland cement is being used. Several stabilizing processes of the ground take place simultaneously: • Water in the ground will be displaced by the propellant gas, and thus the amount of water in the ground to be treated, is reduced.

• Water consumed by hydration of quicklime entails a drying effect.

• The hydration results in high pH (above 11), which causes precipitation and crystallization of the cementing mineral calcium hydroxide:

CaO + H20 = Ca (OH)2.

• Quicklime will dissolve in the water phase, and ion exchange reactions will occur between the sodium ions adsorbed on the surfaces of clay minerals in the ground and the calcium ions. Calcium ions are bivalent and will stabilise the negative chargés of clay minerals better than the sodium ions. Thus, the ion exchange results in a significant stabilizing effect on clay sediments. • When Portland cement is used, it will also react with the pore water to form calcium hydroxide and a variety of concrete minerals such as calcium silicates and calcium-aluminium silicates, • The high pH in the treated zone may lead to dissolution of the finest quartz particles in the ground, with the formation of calcium silicates (concrete minerals) as a result. This may also contribute to the stabilization of the clay.

As mentioned above, it is an object of the invention to establish a fast stabilization of the ground. According to methods of prior art, it takes weeks before the full impact of concrete mineralization is expressed from Portland cement. Formation of calcium hydroxide (Ca(OH)2) is fast, but this is a relatively weak cementing mineral. It is therefore desirable to replace calcium hydroxide by the stronger calcium carbonate mineral (calcite) in a carbonation process. Carbonation of quicklime is fast - within a few hours - in an aqueous environment with excess C02.

According to the present invention C02 is added to the propellant gas, and thus the formation of calcite from the injected quicklime will be fast:

The present invention differs from prior art in the composition of the propellant gas, and further the method and pressure used to pump and blow the chemicals into the ground. Traditionally the chemicals have been pumped into a transport line by a regular pump or an air compressor, using air from the surroundings of the air intake of the compressor, as propellant gas. However, according to the present invention, the propellant gas has a higher content of C02 than the surrounding air. This may for instance be achieved by adding concentrated C02 gas directly to the drill line, or the gas used to pump the chemicals is enriched in C02. The C02 to be added may for instance be stored in a high-pressure container besides the air compressor, and the C02 may be conducted into an injector tube for the air and stabilizing chemicals.

The C02 gas may originate from an environmentally unfriendly process, and since the method according to the present invention consumes the C02, it is a safe way of disposing C02. In a preferred embodiment the added C02 is the C02 released during calcination of the limestone to quicklime, and thus the total process releases less or no C02 to the environment. Thus, this method includes a permanent storage of C02 in the treated zone, which is regarded as environmentally beneficial.

The added amount of C02 may be adjusted by adjustable valves on the pressure vessel, for instance by the operator. The air pressure needed to infuse the chemicals is adjusted according to the depths in the borehole and the pore pressure in the ground at any depths. This means that the concentration of C02 in the injected air stream will vary accordingly, but the amount of C02 should be adjusted according to the amount of chemicals injected.

The amount of added C02 depends on the amount of quicklime used in the stabilizing process. Injection of C02 influences the stabilization of the ground and the cementing process itself, in several ways. C02 under high pressure (4 to 10 bars) dissolves quickly in the pore water, and the solubility of C02 in water, e.g. at 10 degrees C and 8 bar pressure is 0.75 mol%. Clays and silty sediments arecharacterized bybeing water wet and håving small pores with high capillary pressure, and much of the residual pore water (after injection of the chemicals with propellant gas) will be found as bound moisture films on mineral surfaces and in water menisci between the mineral particles. C02 will dissolve in this residual water at the high pressures, and ensure that the formation of carbonate cement will also take place in water menisci between the mineral particles. In this way, strong cement bonds are formed directly in contact with the mineral particles of the ground, resulting in a far better stabilizing effect than if the cementing minerals were formed in the middle of the pores; without any contacts with sediment particles.

The formation of carbonate minerals such as CaC03 requires a pH above 8 in the water phase, and when Portland cement is being used, a pH above 11 is required for the formation of concrete minerals (portlandite, calcium silicates and calcium-aluminium silicates). The injection of Portland cement and/or quicklime will lead to a rise of pH in the pore water to above 12. An injection of C02 will lower the pH of the pore water. The amount of injected C02 must therefore be balanced against the amount and type of added stabilizing chemicals. If it is used only quicklime and the like, the pH should be above 8, if Portland cement is used, the pH should be kept above 11.

In order to achieve the correct pH range, the amount of quicklime should be equal to or less than the amount of C02 that was removed during the calcination of the carbonate rock in the first place. During the production of quicklime the amount of removed C02 corresponds to a weight loss of 44% for limestone. The amount of C02 that should be added for full carbonation of quicklime corresponds thus to addition of about 80 weight% C02 related to the weight of quicklime. In order to achieve full carbonation of 50 kg quicklime, one should add a maximum of 40 kg C02.

As seen from the equations above, hydration water is released in the carbonation reaction. In a preferred embodiment the stabilizing chemicals comprise both Portland cement and quicklime, and thus the release of hydration water during carbonation may be beneficial, as it ensures supply of hydration water for the more time consuming formation of concrete minerals from Portland cement. The main issue is to establish strong cementing bonds in the treated borehole profile fast and preferably before the water displaced by the propellant gas seeps back into it.

A preferred process according to the present invention which may be used for stabilizing a ground such as quick clay, is to create a disturbed column in the ground and infuse stabilizing chemicals by means of an in-situ mixing device. The mixing device is rotated through the upper crust and down to the bottom of the soft ground, or as far down as it is desired to stabilize. The device is then rotated and moved up through the ground at a vertical travel speed while the stabilizing chemicals are pumped down the drill string; through exit ports and into the ground where it is mixed with the ground by a rotating mixing device. Vertical travel speed, rotational speed and direction of the mixing device may be varied to optimize mixing. Means for applying and controlling vertical and rotational motion are supplied from a surface equipment. Such surface equipment is for instance available from Linden-Alimak AB, 5-93103 Skellefteå, Sweden.

The stabilizing chemicals may be infused while the mixing device is rotated in place, or moved down through the formation, but it is preferred to infuse them while the mixing device is rotated and moved upwards through the formation. When infusing the chemicals while the mixing device is moved upwards through the formation, the stabilizing processes, which start immediately once the chemicals are infused, will not be disturbed significantly by the mixing. This is particularly important when stabilizing clay formations with quicklime and C02 as the chemical reaction is almost immediate.

Laboratory experiments with quick clay under controlled conditions show that possibly up to 50 percent of the pore water may be displaced with compressed air at 7 bar pressure. Therefore, it is expected that the injection process will result in a significant reduction of the water saturation in the treated zone. This is observed by elevated pore pressures measured in the ground outside the zone being treated. In a preferred embodiment a drain hole or drain channel is made in close relation to the area to be treated, in such a way that water being displaced by the propellant gas may be drained and removed from the area.

Example

In a preferred embodiment of this patent, the stabilizing chemicals are standard Portland cement and quicklime. Recommended quantities of stabilizing chemicals correspond to recommended quantities of prior art, and recommended quantities of quicklime and Portland cement mixtures for the stabilization of soft clay grounds, are 80 to 130 kg per m3. In 'quick clays' it has become common practice to use a mixing ratio of 50% quicklime and 50% Portland cement. Portland cement normally provides the highest shear strength of the ground after stabilization, while quicklime provides more tenacious stabilized clay. In grounds håving high content of organic matter (humic substances) it is recommended to use mixtures with higher proportions of Portland cement; e.g. 25% quicklime and 75% Portland cement. The preferred amounts of quicklime and cement, and the ration between them in relation to the type of ground to be stabilized, is known in prior art and will be familiar to a person skilled of the art.

According to the present invention, C02 should be added, and in the enclosed

Figure 1, the proportions of C02 that should be added to quicklime and mixtures of quicklime and Portland cement to produce calcium carbonate from the quicklime, is illustrated. A maximum and minimum addition of C02 is indicated (C02 max and C02 min). The amount of C02 added is determined by the amount of quicklime, wherein the amount of C02 should not exceed about 80 weight% of quicklime and should not be less than about 40 weight% of quicklime. This is regardless of how much Portland cement is injected into the ground.

Complete conversion of quicklime to calcium carbonate requires an addition of C02 amounting to approximately 80 weight% of quicklime. E.g. to fully convert 50 kg quicklime to calcium carbonate it should be added about 40 kg C02. This is illustrated by the red and green columns in Figure 1.

In sensitive and quick clays, a 1:1 mixture of quicklime and Portland cement is

preferred for stabilizing the clay, and in Figure 1 this mixture is illustrated by the blue columns. In order to ensure the crystallization of concrete minerals from the Portland cement the pH of the system must be kept above 11. For this reason, the addition of C02 should not exceed the amount given by the green columns; i.e. it is the amount of quicklime injected that is determining the amount of C02 to be injected,

regardless of the amount of Portland cement injected.

The minimum addition of C02 (about 40 weight% of quicklime weight; illustrated by the yellow columns in Figure 1) is related to the goal of achieving sufficiently high shear strength (200-500 kPa) in the ground, in such a way that the operations may move fast forward into the un-stabilized ground, without creating temporary enforcements that can carry the heavy machinery.

Even though quicklime has been used as an example of stabilizing chemicals in the description above, other carbonate rocks (e.g. dolomite and magnesite) which is calcinated in a lime kiln to CaMg02 and MgO, may also be used. Any mixtures of these may also be used, and is to be included in the invention.

Although only specific embodiments of the present invention have been described in detail, the invention is not limited to this, but is meant to include all embodiments within the scope of the appended claims.

Claims (12)

1. Method for stabilizing soft clay grounds by infusing stabilizing chemicals comprising calcinated carbonate rocks into the ground, wherein the chemicals are pumped into the ground by means of a propellant gas,characterized in thatthe propellant gas contains more C02 than the air surrounding the air intake of a compressor being used to infuse the chemicals.

2. Method according to claim 1,characterized in thatthe method uses a device which creates a disturbed column in a sensitive ground formation and that the stabilizing chemicals are infused and properly mixed into the disturbed column.

3. Method according to claim 2,characterized in thata drilling rig is used to create a hole into the ground, and that the stabilizing chemicals are pumped through a drill string of the rig and out of a lower end of the drill string and into the surrounding ground, while the drill string is moved vertically, preferably upwards.

4. Method according to any of the preceding claims,characterized in thatseveral holes are drilled in the same area, in order to stabilize the ground.

5. Method according to any of the preceding claims,characterized in thatthe carbonate rocks are limestone, dolomite and/or magnesite, preferably limestone.

6. Method according to any of the preceding claims,characterized in thatthe stabilizing chemicals further comprises portland cement.

7. Method according to any of the preceding claims,characterized in thatthe amount of C02 added to the propellant gas depends on the amount of calcinated carbonate rocks.

8. Method according to claim 7,characterized in thatthe amount of C02 is about 80 weight% of the calcinated carbonate rocks.

9. Method according to claim 7,characterized in thatthe amount of C02 is about 40 weight% of the calcinated carbonate rocks.

10. Method according to claim 7,characterized in thatthe amount of C02 is between about 40-80 weight% of the calcinated carbonate rocks.

11. Method according to claim 7,characterized in thatthe added amount of C02 is less than the removed amount of C02 during the calcination of the carbonate rock.

12. Use of a method according to one or more of claims 1-11, for stabilizing grounds, preferably quick clay and soft clay grounds.