2.2.2　Soil Reinforcement Method

2.2.2　Soil Reinforcement Method

In the past，grouting method was the main method for stratum reinforcement at the starting and arriving parts of shield tunnel.However，in recent years，with the increase of buried depth and section of shield tunnel，high strength reinforcement is urgently needed.In addition，due to the time-consuming removal of underground walls such as continuous walls，it is required that the ground reinforcement can be stable for a long time.Therefore，in order to make the strata reach high strength and long-term stability，and be closely bonded with the retaining wall，the examples of high-pressure jet grouting method and freezing method are gradually increasing.Table 2.2 compares the four commonly used reinforcement methods.

2.2.2.1　Deep Mixing Pile Reinforcement Method

Deep mixing pile is one of the commonly used methods for soft soil foundation reinforcement and lateral support of deep foundation pit excavation.As early as in the 1970s，Japan pioneered and used in steel works port quay wall，highway engineering.Since 1977，China began to develop and trial，then in 1980 by the Ministry of Metallurgy presided over by the ministerial technical appraisal，applied to the foundation reinforcement project.

1）Technical characteristics

（1）The solidified pile and the original foundation form a composite foundation to improve the bearing capacity and deformation modulus of the foundation.

（2）Self-supporting retaining without support and anchor.

（3）After the pile is connected to the wall，it has the effect of waterproof curtain.

（4）There is no vibration，no noise and no pollution in the construction，which has little influence on the surrounding buildings and underground pipelines.

（5）The construction equipment is simple，easy to operate and low cost，which creates better conditions for civilized construction，especially in small construction sites.

2）Scope of application

（1）Soft soil foundation reinforcement，including shield tunnel soil reinforcement.

（2）The lateral retaining structure has a good protective effect on adjacent buildings.

（3）Curtain engineering of waterproof and anti-flow sand.

3）Process principle

Using deep mixing machinery，cement is used as curing agent and ground soil in-situ forced crushing and mixing.After curing，different shapes of piles，walls or blocks are formed.The calculation theory is calculated according to the gravity dam rigid retaining wall，and the deformation is checked according to the rigid retaining wall calculation method.

4）Construction flow

The construction site should be smoothed，rolled or compacted to ensure the positioning and movement of machine and the vertical drilling.

（1）The deep mixer is in place.

（2）Stirring and sinking.Start the motor，determine the calculation rate according to the soil quality，relax the hoist to cut the soil from top to bottom and mix and sink until the design depth.

（3）Grouting mixing and lifting.When the cement slurry reaches the mixing head，lift the mixing head，grouting，mixing and lifting according to the speed required by the calculation，so that the cement slurry and the original foundation soil can be fully mixed until the pump is closed after it has been lifted to the design elevation of the pile top.

（4）Repeat stirring down.Sink the mixer to the design elevation again.

（5）Repeated stirring lifting（no grouting）.When stirring and lifting to the natural ground，one pile is completed by closing the mixer.

5）Key points of construction

（1）All underground obstacles must be identified and removed before startup，and the parts that need to be backfilled must be filled and tamped in batches to ensure the quality of the pile.

（2）The driving track and sleeper of the pile machine shall not sink，and the vertical deviation of the pile machine shall not be greater than 1％.

（3）The cement should be 32.5 grade ordinary Portland cement，the cement mixing ratio should be 8％-16％，and different types of admixtures can be added according to different geological conditions and construction period requirements.

（4）The amount of grouting and lifting speed must be strictly controlled to prevent the occurrence of sandwich layer or pulp breakage.

（5）The speed of the two elevations of the mixing head should be controlled at the outlet pressure of the 2.5-3 m/min，grouting pump in the range of 0.4-0.6 MPa.

（6）The following matters should be taken into account in the work to be overlapped with the pile：①The overlapping time between piles shall not be more than 24 h.If it is more than 24 h，attention should be paid to increasing the grouting amount during the construction of the second pile，which can increase by 20％and slow down the lifting speed at the same time.②If the interval time is too long，the second pile cannot be overlapped，local pile replacement or grouting measures shall be adopted with the approval of the design.

（7）The pre-mixing sinking of the pile machine shall ensure that the structure of undisturbed soil shall be fully crushed according to the condition of the original soil，so as to facilitate the uniform mixing of the same cement slurry.

（8）The standard water tank is adopted，the water-cement ratio is strictly controlled，the mixing time of cement slurry is not less than 2×3 min，the slurry is poured into the collecting tank after filtration，and then the cement is continuously stirred to prevent cement segregation.The grouting should continue and cannot be interrupted.

（9）Each class must be a group of test blocks（3 blocks）.After 28 days，the unconfined compressive strength should be measured and the design mark should be reached.

6）Quality standards

In addition to referring to the relevant national standards，the following points should be noted：

（1）The vertical deviation of pile is not more than 1％，and the deviation of pile position arrangement is not more than 50 mm.

（2）The mixing pile should be mixed，the surface should be dense and smooth.

（3）Pile top elevation and pile depth should meet the design requirements.

（4）There is no abnormal and excessive amount of cement slurry filling.

2.2.2.2　High Pressure Jet Grouting Pile Reinforcement Method

High pressure jet grouting pile method has single pipe method，double pipe method，triple pipe method and multiple pipe method in recent years.It plays a good role in foundation reinforcement，improving foundation bearing capacity and improving soil quality for wall protection，retaining soil and water isolation.Fig.2.10 shows the high pressure jet grouting pile reinforcement site.

1）Technical characteristics

（1）The high pressure rotary jet grouting pile method can be used to reinforce a certain depth of strata.

（2）The soil which cannot be filled with slurry cannot be strengthened in the fine particle soil layer with small permeability coefficient，and the slurry filling is uniform and the range can be adjusted and controlled.

（3）The soil can be strengthened when the distance between the public pipelines is small or the structure has only a small slit.

（4）Combined with fixed spray method，vertical water partition wall，horizontal water partition wall or closed water insulation curtain can be effectively formed.

（5）It is convenient to use and flexible to move，which can not only form a single row pile body，but also form a multi-row pile body，and the pile diameter can be properly adjusted.

（6）The discharge mud can be recycled，the construction environment can be improved，and the transportation cost can be saved.

2）Scope of application

Suitable for sand，clay，silt and artificial fill.

3）Process principle

The engineering drilling rig is drilled to the design depth，and a certain pressure of cement slurry and air are sprayed at the same time through the special nozzle on the end side，and forced to mix with the sprayed slurry to cement and hardening.At the same time，the grouting pipe is rotated and lifted at a certain speed，that is，the mixing and solidifying with obvious diameter is formed in the soil.

The pile superimposed to form a wall with water insulation and retaining soil.

4）Construction process

（1）Drilling rig in place.It mainly refers to that grouting pipe can be installed only after drilling.If the grouting pipe is directly penetrated or sunk，it is not necessary to drill，but its verticality must be ensured.Intubation must take place immediately after drilling，not too long interval.

（2）Rotating spray lifting.Both slurry and gas must be sprayed at the same time.The lifting speed is closely coordinated with the slurry flow rate，so as not to meet the design requirements of pile diameter and pile mass，and at the same time，the lifting speed must be matched with the rotating speed.

（3）Cleaning of machinery and tools.Be sure to wash the grouting pipe，when all completed or phased pause，to mix slurry，grouting equipment to do cleaning.The concrete construction flow of high pressure rotary jet grouting pile method is shown in Fig.2.11.

5）Construction requirements

（1）Cement：the types of visual slurry（common type，early strength type，impermeable type，etc.）are selected，but slag cement is not required to be moisture-free，not caking，freshness and various indexes in accordance with national regulations，and slag cement is not suitable for impermeability requirements.

（2）Water for pulp mixing：clean，no deterioration，no corrosion，moderate acidity and alkalinity.Generally speaking，river water，well water and other clean water can be used.

（3）Admixture：in order to improve the performance of slurry，promote its early strength，improve impermeability and blowability.The admixtures available are as follows.

① Promoting coagulation，early strength，adding 2％-4％calcium chloride or 3％-5％triethanolamine，1％salt and 2％-3％calcium chloride.

② To improve the impermeability，the initial setting is fast and the final setting time is long.2％-4％sodium silicate and 10％-50％bentonite can be added.

（4）Slurry：the mixture is required to be uniform，the spray is good，and there should be no blockage of pipes and nozzles.To control the use within the initial setting time，do not time out.

6）Quality standards

（1）Excavation inspection：when the slurry reaches a certain strength，the soil can be excavated to check the verticality of the pile，the bonding condition between the piles and the diameter of the pile.

（2）Drilling inspection：the core samples were taken from the center of the pile body which had been rotated and consolidated，and the physical and mechanical properties of the standard specimens were made into standard specimens for indoor physical and mechanical properties test.

（3）The permeability coefficient of pile is measured by pressure water injection or pumping test.

（4）Standard penetration test：standard penetration test was carried out in the middle of rotary spray consolidation.

（5）Load test：do vertical or horizontal load test to determine its bearing capacity.

2.2.2.3 Chemical Grouting Reinforcement Method

Chemical grouting reinforcement is the chemical slurry with fluidity and gelation，according to a certain concentration，through the special grouting hole，compressed into the rock and soil.Slurry into the rock and soil cracks or pores，through chemical reaction to produce cementitious materials or make the surface of soil particles activated，cement solidification in the contact，in order to enhance the bonding between soil particles，improve the mechanical strength of soil，to play a reinforcement，seepage prevention，improve the physical and mechanical properties of foundation.The commonly used reinforcement methods include silicification reinforcement，alkali solution reinforcement，electrochemical reinforcement and polymer chemical reinforcement.

1）Silicification reinforcement method

By inserting a metal perfusion tube with holes，sodium silicate（commonly known as sodium silicate）solution was injected into the soil under certain pressure，or sodium silicate and calcium chloride solutions were successively injected into the soil.The former is called single liquid silicide and the latter is called double liquid silicide.

Single liquid silicification is suitable for strengthening collapsible loess with permeability coefficient 0.1-2.0 m/d and silt with permeability coefficient 0.3-5.0 m/d.When the collapsible loess is strengthened，the solution is composed of 2.5％sodium chloride mixed with 10 and 15％sodium silicate solution.After the solution is inserted into the soil，the sodium ion reacts with the calcium ion（mainly calcium sulfate）in the water-soluble salt of the soil，and forms silicic acid gel between the soil particles and their surface，which can make the unconfined ultimate compressive strength of loess reach 0.6-0.8 MPa.When the silt is strengthened，a certain amount of phosphate（specific gravity is 1.02）is added to the sodium silicate solution with lower concentration（specific gravity is 1.18-1.20）.After uniform stirring and injection，the unconfined ultimate compressive strength can reach 0.4-0.5 MPa after chemical reaction.

Double liquid silicification is suitable for strengthening sandy soil with permeability coefficient of 2-8 m/d or for preventing seepage and stopping water to form impermeable curtain.The specific gravity of sodium silicate solution is 1.35-1.44，and the specific gravity of calcium chloride solution is 1.26-1.28.After contact with soil，the two solutions produce not only general chemical reactions，but also colloidal chemical reactions，which generate silica gel and calcium hydroxide.In the adjunctive reaction，the product can also enhance the adhesion between soil particles and fill the pores.The unconfined ultimate strength of sand soil can reach 1.5-6.0 MPa.

The reinforcement radius achieved by silicification method is related to the permeability coefficient of soil，perfusion pressure，filling time and viscosity of solution，which can be determined by single hole perfusion test.Each filling hole should be arranged according to the apex of equilateral triangle on the plane，and the hole distance can be 1.7 times of the radius of reinforced soil.The reinforcement depth is determined according to the soil condition and the requirements of the building，and is generally 4-5 m.

The modulus of sodium silicate is usually 2.6-3.3，and the impurity content insoluble in water does not exceed 2％.This method must consume sodium silicate or calcium chloride and other industrial raw materials，the cost is high.The advantages are that the deformation of the foundation can be suppressed quickly and the strength of the soil has been greatly improved，which is especially suitable for the reinforcement of the existing building foundation.However，it is not suitable to strengthen the foundation soil with pH value greater than 9 for petroleum products，gum，oil and groundwater.

2）Alkali reinforcement method

The reinforcement effect of lye on soil is different from that of other chemical reinforcement methods.It does not precipitate cementitious substances from the solution itself，but after the chemical reaction between lye and soil，the surface of soil particles is activated and self-bonded，so as to enhance the mechanical strength and water stability of soil.In order to promote the reaction process，the solution temperature can be raised to 80℃and then injected into the soil.When strengthening collapsible loess foundation，the solution is generally infiltrated into the soil through the filling hole.The content of calcium and magnesium ion in loess is high，and good reinforcement effect can be obtained by using single liquid.

3）Electrochemical reinforcement method

A certain number of metal poles are introduced into the foundation soil，and the DC current is introduced through the electrode，so that the water is discharged from the cathode and the soil is consolidated.When the foundation is strengthened by electrochemical method，there are three main processes：①Electroosmosis，after electroosmosis，a large number of soil is dehydrated and consolidated；②Ion exchange，the adsorbed sodium and calcium are replaced by hydrogen and aluminum；③in the process of structure formation，the soil particle structure is formed by aluminum gel，and the soil can also be strengthened by the combination of current and chemical solution，that is，the chemical solution can be injected into the soil through the perfusion pipe network with holes，and the solution diffuses from anode to cathode with the movement of water after electrification，so as to improve the reinforcement effect.

Electrochemical method is generally used to strengthen silt foundation with permeability coefficient less than 0.1 m/d.However，this method is expensive and must be tested by special equipment and confirmed to be effective before it is adopted.

4）Polymer chemical reinforcement method

It is a method of ground treatment by pressing polymer chemical solution into soil.It is suitable for reinforcement of sand soil foundation，curtain grouting and water stop and leakage of underground works；the reinforcement of the mud interlayer and fault fracture zone of dam foundation works is also effective.For example，the compressive strength after the cyanide is poured into the sand soil can reach 10.0 MPa.

There are many kinds of polymer materials used in foundation reinforcement，such as urea-formaldehyde resin，acrylamide（also known as propioning），polyurethane（also known as polycarbamate or cyanide），among which polyurethane is better.Polyurethane is made of polyisocyanate and polyether resin as the main materials，and then adding Plasticizer，diluent，Surfactant，Catalyst and so on to form slurry.After pouring into the formation，the gel is formed when the water is encountered，which plays the role of strengthening the foundation and preventing seepage and plugging leakage.In the late 1960s，the TACSS grouting material first developed by Japan and the cyanide condensation material developed by China in the early 1970s were obtained by the reaction of excessive isocyanate with polyether，which was called prepolymer.The prepolymer contains a certain amount of free isocyanate（NCO）can react with water.When the slurry is poured into the soil，the NCO base is further polymerized and crosslinked under the action of catalyst，the viscosity of the reactant increases gradually and solidifies，and the polymer insoluble in water is formed to strengthen the foundation.

The characteristics of cyanide grouting：Since water is an integral part of the reaction，it is less likely that the slurry is diluted or lost by water.And the carbon dioxide gas released in the process of water reaction makes the slurry expand，diffuse around，and expand the scope of reinforcement.Polymer materials are expensive，limiting their use.Cyanide coagulation is highly toxic，anti-toxic measures should be taken in construction，and environmental pollution should be considered.

Polyurethane pulp is divided into water-soluble and insoluble，the former has low strength，but has good impermeability，the latter has high strength，good seepage prevention and widely used in engineering.Among them，the two-step method（also known as prepolymerization）is the best pulping method，which is to synthesize the main agent into polyurethane prepolymer first，and then to mix the prepolymer and admixture into slurry according to the need.

The characteristics of polyurethane slurry are as follows：

（1）The slurry has low viscosity，good filling property and high strength of stone，which can be combined with cement grouting to establish reliable anti-seepage curtain.

（2）After the slurry is treated with water，the viscosity of the slurry increases rapidly and will not be diluted and washed away.It can be used for waterproofing and leakage under the condition of moving water，blocking various forms of underground，ground and pipeline leakage，sealing firmly，and stopping water quickly.

（3）Good durability，safety and reliability，no pollution to the environment.

（4）The operation is simple and the economic benefit is good.

5）Key points of construction

（1）Grouting method can be divided into single liquid and double liquid.The single liquid grouting method is to mix the components of the slurry evenly first，prepare them once，and then pump the grouting into the hole；The double liquid grouting method is to load the two kinds of slurry in their respective slurry grooves respectively，and then use the proportional pump or two grouting pumps to press the two kinds of slurry into the hole according to the proportion，so that the two liquids can be mixed in the mixer near the hole，and then sent to the hole section，or the double liquid will be uniformly and directly pressed into the hole and mixed in the hole.

（2）Grouting order：from the center to the outside，from the bottom up.The length of grouting section is generally about 5 m，if the size of cracks in grouting part is very uneven，the rock and soil are loose and heavy，and there are many cracks，then the length of the section is 1-3 m.The hole distance is generally 0.5-1.2 m，and the bottom end of the slurry pipe should be kept 0.3 m from the bottom of the hole.The grouting pressure is generally 0.05-0.2 MPa.For water curtain grouting，the surface section should not be less than 1-1.5 times water head，and the bottom should be 2 times 3 times water head.

（3）During grouting，attention shall be paid to stop grouting after grouting is no longer carried out，and the valve of the grouting pipe shall be closed to prevent backflow.

（4）After grouting，the pipe should be extubated in time，and the height of each extubation is 350 mm.Brush all equipment，pipe lines，and eliminate waste slurry pollution to the environment in order to keep it unobstructed and clean.Holes left by extubation are filled with cement mortar or earth.

6）Quality control

（1）Calculate the grouting quantity statistically and analyze the grouting effect by using the automatic flow and pressure curve in the grouting process.

（2）The mechanical index of soil before and after reinforcement is tested by static penetration test to understand the reinforcement effect.

（3）The field load test was carried out to determine the bearing capacity and deformation modulus of the strengthened soil.

（4）The mechanical energy of soil before and after reinforcement is measured in situ by standard penetration test or portable penetration test，or laboratory test is carried out to determine the physical and mechanical properties of soil before and after reinforcement，and to compare and judge the reinforcement effect.

（5）The test point is generally 2％-5％of the number of grouting holes.If the unqualified rate of the inspection point is greater than or equal to 20％，or less than 20％，but the average value of the inspection point does not meet the design requirements，after confirming that the design principle is correct，repeated grouting should be carried out in the unqualified grouting area.

2.2.2.4　Freezing Reinforcement Method

In subway construction，because of the abnormal congestion of the ground building，the complexity of the shallow pipe network on the surface，the soft and water-bearing strata of the subway tunnel，and the poor stability，it is necessary to study a reliable method of formation reinforcement.In subway construction，freezing reinforcement technology has achieved some results，which will be further popularized and applied.

Frozen reinforcement method（hereinafter referred to as freezing method）refers to a special method of geotechnical construction for underground engineering construction under the protection of the frozen soil wall by freezing the soil surrounding the underground space to be excavated into ice and cemented with the soil，forming a frozen soil wall or a closed frozen soil body according to the design profile，which is used to resist the earth pressure，isolating the groundwater，and carrying out the underground engineering construction under the protection of the frozen soil wall（Fig.2.12）.

The method was first used for gold mining in Russia，and then German engineers used for coal mine construction to obtain patented technology.It has been widely used in subway，deep foundation pit，mine construction and other projects.Its advantages are good reinforcement effect，and the frozen soil wall can also be controlled by temperature，which can ensure long-term stability.The discussion on the reinforcement range is the same as that of the high pressure jet grouting method，which is calculated according to the general temporary buildings.

1）Basic principles

The principle of ground consolidation by freezing method is to use artificial refrigeration method to send cryogenic refrigerant into the stratum，freeze the stratum around the excavation body into a closed continuous frozen soil wall to resist pressure and isolate the connection between groundwater and excavation body，and then excavate and make permanent support under the protection of the closed continuous frozen soil wall.It is a special method of stratum reinforcement.The coolant entering the formation is connected with the formation through the inlet and return pipeline，and the heat exchange is carried out with the formation through the frozen pipe，and the cooling capacity is transferred to the surrounding formation，and the heat in the formation is taken away.As a result，the strata around the freezing pipe are cooled from near to far，and the water in the formation is gradually turned into ice，and the original loose or porous strata are glued together through ice to form impermeable permafrost columns.Several such frozen tubes are arranged and the refrigerants in the frozen tubes are recycled to freeze the soil around the frozen pipes into permafrost columns.With the continuous expansion of the radius of the permafrost column，the adjacent permafrost columns will be connected and tightly bonded with each other through ice to form a sealed continuous wall.

2）Refrigeration system

Compression refrigeration consists of three cycles：ammonia cycle，brine cycle and cooling water cycle.

（1）Ammonia circulation system.

Ammonia cycle plays a leading role in the refrigeration process.In order to transfer geothermal energy to cooling water and then release it to the atmosphere，saturated steam ammonia in evaporator must be turned into superheated steam at high pressure and high temperature，resulting in temperature difference with cooling water，heat transfer to cooling water in condenser，and superheated steam ammonia condensation into liquid ammonia to realize the transformation from gas to liquid.Liquid ammonia evaporates into evaporator through throttle valve，and then absorbs heat from brine around it to saturated steam ammonia.In this way，over and over again，constitute the ammonia cycle.

Ammonia circulating system equipment consists of evaporator，ammonia compressor，condenser and throttle valve.

（2）Brine circulation.

Brine cycle plays an important role in cooling capacity transfer.The circulation system is driven by the flow of salt water.The freezer is a heat exchanger in which low temperature brine exchanges heat with the formation.The faster the flow rate of brine is，the greater the heat transfer intensity is.The freezer consists of a freezing tube，a liquid supply tube and a return pipe.According to the needs of the project，two kinds of brine circulation system can be used，and the positive circulation can be used to supply liquid under normal conditions.During the positive freezing period，the temperature difference between the inlet and outlet of the freezer is generally 3-7℃，and during the negative freezing period，the temperature difference between the inlet and export of the freezer is 1-3℃.The evaporation temperature of ammonia in evaporator is 5-7℃different from that of brine around it.The endothermic rate of the surface of the freezer is unit time，and the heat absorption per unit area is 263-292 W/m2.In order to observe the leakage of the brine in the freezing pipe，the meter shall be installed at the inlet and outlet of the demineralized water main pipe and the freezing unit.

The brine cycle described above is referred to as a closed-water circulation system.A kind of open circuit return brine circulation is also used in foreign countries，the main characteristic point of which is no liquid collecting ring，and each return pipe returns liquid separately，which is convenient to observe whether each frozen pipe salt water is leaking or not.In this way，the amount of brine passing through the loop is large，and the closed circuit cycle is more complex.The brine pipeline should be treated strictly with heat preservation.In general，the heat loss of the brine pipeline accounts for about 25％of the total refrigerating capacity of the freezing station.

（3）Cooling water circulation.

The function of cooling water cycle in the refrigeration process is to cool the superheated steam of the compressor into liquid ammonia in order to re-evaporate into the evaporator.Cooling water releases heat from ammonia vapor to the atmosphere.The lower the cooling water temperature is，the higher the refrigeration coefficient is，but the cooling water temperature is generally 5-10℃lower than that of ammonia.The cooling water is driven by the pump，heat exchange is carried out through the condenser，and then flows into the cooling tower pool.The circulating water after cooling should be replenished by groundwater at any time.

3）Construction characteristics of freezing method

Freezing method is mainly used in winter construction.In addition to freezing method，admixture method（salt mortar method）should be preferred in winter construction.When using freezing construction，the thickness of frozen soil wall，the spacing of frozen pipe group and the length of frozen pipe，freezing sequence and thawing sequence should be determined according to the construction progress，the strength requirement of frozen soil wall and the excavation sequence，etc.，according to which the appropriate freezing equipment should be selected.In addition，it is necessary to measure the temperature of the representative parts in the construction，and according to the measured results，it is determined whether to maintain the freezing of the frozen soil wall in the form of continuous or intermittent cooling；At the same time，the expansion of formation freezing and the subsidence of thawing should be observed in time and the corresponding treatment measures should be taken.

Install refrigeration equipment on the ground，use ammonia（NH3）as refrigerant，cool the solution of calcium chloride（CaCl2）of refrigerant（commonly called brine）to-20 to-30℃，and send the brine into the frozen pipe by circulating pump and PVC liquid supply pipe inserted into the depth of the frozen pipe.The heat outside the tube is continuously absorbed by low temperature brine for a long time to freeze the surrounding strata.After the brine absorbs the heat of the formation，the temperature rises，and under the action of the circulating pump，it returns to the refrigeration equipment and recools through the loop tube.The original liquid ammonia，after evaporating under the condition of decompression，absorbs the heat of brine，compresses and condenses it and liquidates it，circulates and flows in the pipeline，and reuses it.The diameter of frozen soil cylinders formed around each frozen pipe increases with time.These cylinders intersect into dense and closed frozen soil walls，which can withstand water，earth pressure and block groundwater，excavate strata and build lining under their protection.

Freezing construction has the following characteristics：

（1）Sealing water.There is free water（in general，the water content should be more than 10％，otherwise the auxiliary work method to increase the humidity of soil layer）can freeze into frozen soil and form frozen soil wall.Whether it is permeable layer or water insulation layer，frozen soil wall can block groundwater invasion and form a dry construction environment.

（2）High strength.It is generally considered that frozen soil is a kind of Viscoelastic-plastic material，its strength is related to soil quality，bulk density，moisture content，salt content and temperature，which can reach 2-10 MPa，which is much greater than the strength of molten soil，thus playing the role of structural support wall.

（3）Strong adaptability.It is suitable for all kinds of soil layer and many kinds of underground projects，especially for underground projects with large water content，weak strata and difficult or unable to be constructed by other construction methods.

（4）Reproducibility.At the end of the construction，the soil layer is restored to the original state，and the damage to the soil layer is very little.This is impossible to compare with other methods of work.

（5）Avoid obstacles.It has the ability to circumvent obstacles，freeze reinforcement and seal water.

（6）Pollution-free.Electricity energy in exchange for cold energy，does not pollute the atmospheric environment，no harmful substances discharge，no pollution to groundwater.In the high environmental protection requirements of the project，its advantages are particularly obvious.

（7）Controllability.Freezing duration，frozen wall thickness，frozen wall shape and so on can be controlled.

（8）Applicability.It can be constructed under dense building area and existing engineering building without foundation pit drainage，which can avoid the adverse effect of foundation settlement caused by pumping water on the surrounding buildings.

（9）The construction is convenient，no support，no anchor，open construction can be carried out and the construction area can be expanded，and the construction period can be shortened.

4）Freezing method is more mature technology in shield construction

（1）Selection of refrigerant.Calcium chloride solution with wide temperature range，large heat capacity，abundant sources and cheaper price is usually chosen.It is the most widely used refrigerant in refrigeration engineering.

（2）Selection of the refrigerant.Freon is used as a refrigerant，and liquid nitrogen or dry ice is also used as the refrigerant.Wherein the liquid nitrogen can reduce the temperature of the freezing tube to-190℃，while the liquid of the dry ice and the alcohol is up to-70℃，so that the freezing tube can be formed in a few hours.

（3）Selection of refrigeration equipment.At present，in shield engineering，whether in，out of the hole reinforcement，or side channel construction，refrigeration capacity is easier to meet the requirements.

（4）The arrangement of freezing pipe.Frozen soil walls formed vertically are usually rectangular and plum blossoms.The arrangement of plum blossoms may be more reasonable，but it should be arranged flexibly according to local conditions，depending on the needs of the project.

（5）Calculation of frozen soil strength.The main factors affecting the instantaneous strength of frozen soil are temperature，moisture content，particle composition and freezing speed of soil.

①The effect of temperature：the strength and cementitious ability of ice increase with the decrease of temperature，so the strength of frozen soil also increases with the decrease of temperature.

②The effect of water content：when the water content of soil reaches saturation，the strength of frozen soil increases with the increase of water content；after the water content reaches saturation，the strength of frozen soil decreases with the increase of water content due to the precipitation of ice；when the water content is much higher than the saturated water content，the strength of frozen soil is about the same as that of ice.

③The influence of particle composition of soil：When other conditions are the same，the thicker the soil particles，the higher the strength of frozen soil，and vice versa.This is mainly caused by the difference of bound water in soil caused by different particle composition.Coarse soil such as coarse sand，gravel and gravel，in which there is almost no bound water，once the soil is frozen，there is no unfrozen water，so the strength is high；on the contrary，the particles of clay are fine and the total surface area is very large，which contains more adsorbed water and thin film water，but the adsorbed water is generally not frozen.The film water is frozen under certain negative temperature conditions，which increases the viscosity and activity of frozen soil and decreases the strength.

④The influence of freezing speed：the freezing speed is fast，the fine particle ice in frozen soil is more，the strength of frozen soil is high，on the contrary，it is low.

The durable compressive strength of frozen soil is about 1/2-1/2.5 of instantaneous compressive strength，the durable shear strength of frozen soil is about 1/1.8-1/2.5 of instantaneous shear strength，and the durable tensile strength of frozen soil is about 1/12-1/16 of instantaneous tensile strength.