Super-absorbent swellable polymer as grouting material for treatment of karst water inrush

Shui Li,Chenyng M,Renti Liu,b,*,Mengjun Chen,Ji Yn,Zhenjun Wng,Sholong Dun,Husheng Zhng

a School of Civil Engineering,Shandong University,Jinan 250061,China

b Geotechnical and Structural Engineering Research Centre,Shandong University,Jinan 250061,China

c School of Qilu Transportation,Shandong University,Jinan 250002,China

Keywords:Karst Water inrush Model test Grouting material Superabsorbent polymer

ABSTRACT Grouting is the most commonly used method to control water inrush in underground engineering.Traditional cement-based materials are easy to dilute and hard to coagulate under the influence of large flow and high-velocity water inrush.To address these deficiencies,a new type of polymer grouting material with an excellent expansion ratio was synthesised.The material quickly absorbs water and has an expansion ratio of 1:300.The material is composed of a superabsorbent polymer (SAP),glycerol,and ethanol.The effects of water quality on the expansion ratio and expansion rate of the material were examined,and the best solid–liquid ratio for the slurry was determined by fluidity measurements.A karst specially designed pipeline water inrush test device showed that 800 g of SAP can achieve 0.6 m/s water flow blockage in the smooth pipeline,demonstrating that the ability of the SAP slurry to block water inrush is superior to those of other materials.This study provides a reference for water inrush plugging,and has important implications for the reduction and control of karst pipeline-type water inrush disasters,ensuring the safety of construction sites and preventing loss of life and damage to property.

1.Introduction

With the rapid growth of the Chinese national economy,particularly in the mining,water conservation,hydropower generation,and highway and railway development industries,the need for underground construction projects is increasing [1–3].Karst areas have mineral-rich groundwater,as well as well-developed fractures,pipelines,and other hazardous features [4,5].The hydraulic connections of these features significantly affect the safety of mines in karst areas and can cause secondary disasters in the surrounding environment,such as the water inrush that occurred in the limestone mine of Guangxi Huarun Cement Co.,Ltd.,which had a maximum water inflow of 326000 m3/d [6].The continuous drainage of groundwater caused secondary disasters,such as surface subsidence,farmland damage,and water source depletion.Therefore,the control of water inrush is critical for ensuring safe underground construction work.The different forms of groundwater in karst areas are distinguished by their origin,and include karst fissures,pipe veins,pipelines,karst caves,and underground rivers.Among these,karst fissures are the most common.The water inflow from karst fissures is typically small due to their small hydraulic openings,limiting the risk of a sudden major disaster.Pipelines (usually with large cross-sections) are widely distributed throughout karst areas,and are often connected to water-rich strata or surface water sources,which increases the risk of sudden and sustained water inrush disasters[7,8].This is one of the most dangerous karst water inrush types,which has a significant impact on engineering safety and the surrounding ecological environment.

In recent years,a large number of grouting materials have been developed to alleviate the increasingly severe problem of water inrush during underground engineering projects.At present,grouting materials can be classified into inorganic materials,such as cement-sodium silicate,and organic materials,including epoxy resin,polyurethane,acrylate,lignin,asphalt,and other modified grouting materials [9–14].However,existing grouting materials are unsuitable for the treatment of karst pipeline water.Among the inorganic grouting materials,traditional Portland cement has a long setting time (6–8 h) [15],which fails to meet the rapid setting and early strength requirements of dynamic water grouting.Cement-sodium silicate is widely used because of its much shorter setting time;however,the slurry consolidated body has the following disadvantages:high solubility in water,high retrograde strength,and volume in the later stages of solidification,high variation in the ratio between the initial and final setting times,and a short interval between the initial and final setting times[16].Thus,the slurry is unable to effectively diffuse,causing blockages in the drilling and grouting pipelines.Organic grouting materials primarily consist of emulsions and generally satisfy the requirement of dynamic grouting;however,these materials have their limitations.Polyurethane grouting materials are expensive and slightly toxic,while water-soluble polyurethanes are easily diluted and washed away by water,thus losing their function [17].Acrylic grouting materials are weak and highly toxic,and thus,have been banned in many countries [18].Lignin is primarily made of sulphite pulp waste liquor,with sodium dichromate or ammonium sulphate added as a curing agent,which are environmental pollutants [19].

Some scholars have proposed“aggregate grouting”,which involves injecting aggregates of soybean,kelp,and other materials,with favourable expansion performance,into the karst water passage to reduce water cross-section and water inflow velocity followed by the injection of cement-based materials to block water inflow[15,20].The feasibility and effectiveness of this method has been proven by its practical applications.However,a material with high expansion performance is needed to replace soybean because of its low expansion ratio and the difficulties it causes during the grouting operation.Superabsorbent polymer (SAP) is a general term for a class of high-molecular-weight polymers containing strongly hydrophilic groups,and three-dimensional (3D) networks that can absorb large volumes of water[21,22].Since SAPs do not release water even under high pressure [23,24],they are considered suitable candidates to replace soybean.SAPs are utilized in increasing number of applications due to their excellent water absorption and retention.For example,Yang et al.[25] made full use of the water absorption characteristics of SAPs to improve water retention and heat preservation in soil,thereby reducing the volume of water required for irrigation water and improving the survival rate of crops.Cao et al.[26]developed medical bandages and cotton balls that utilise SAPs to rapidly absorb blood from surgical incisions and prevent suppuration.Inspired by these novel applications of SAP,the authors decided to inject SAP into a leaky geological formation.The SAP will form a hydrogel after absorbing a high volume of water,and thus block the cracks in the water channels between the strata,prevent the inrush of groundwater,and improve the safety of underground construction.

In this study,the authors investigated the feasibility of using SAP to plug karst pipeline-type water inrush.An SAP with a swelling ratio of 300 was synthesized and made into a slurry with glycerol and ethylene glycol.The fluidity and water swelling properties of the slurry were studied.A karst pipeline water inrush plugging model with real-time information monitoring was constructed and used to verify the water inrush plugging effect of the SAP slurry.The influence of particle size and water absorption rate on the load-bearing capacity was investigated.A new method of water inrush control was proposed,and the theoretical model of dynamic water grouting technology was further developed,which has great significance for the understanding of water inrush and grouting technology and practical applications in engineering and construction.

2.Experimental

2.1.Advantages of SAP in karst pipeline type water inrush plugging

Existing materials used to block inrush water are based on the gelation of the material components.This process is easily interrupted by high pressure,high water flow,and forced replenishment of karst pipe-type water inrush,resulting in material failure.The cross-linked polymer network of SAP allows it to achieve rapid water absorption due to capillary action and permeation effects.Therefore,SAP can be used to block water inrush,with no detrimental effects on the curing reaction of the material due to water inrush.In addition,the material expands after it comes into contact with water,and it is not necessary to complete the grouting within the setting time of the material,therefore,the influence of setting time on the construction schedule could be neglected.

At present,owing to the lack of effective plugging materials for water inrush,the principle of “drain first and block later” is often adopted in karst pipeline water inrush treatment [27,28].This involves draining groundwater to slow the flow of water,followed by grouting to strengthen the stratum,thus ensuring construction safety.However,the groundwater and the geological environment are interlinked.Water diversion and drainage during the construction process can destabilise the groundwater circulation system.Altering the natural groundwater system and the water inflow path can cause the water level to drop downstream of the water inflow channel,resulting in a series of secondary geological disasters [29,30].These can include sliding of steep slopes,cracking of building foundations,reduced pond water levels,adverse effects on aquaculture industries,and reduced crop production [31,32],all of which can severely affect the lives of the local population.The injection of SAPs into water inrush pipelines in karst areas will result in the conversion of SAPs into hydrogels by the absorption of large volumes of water,which will then expand to block cracks in underground waterways.Ordinary cement grout can then be poured into the cracks to reinforce the karst geology.SAPs can improve slurry retention,and the injected cement slurry further increases the stability and durability of the plugged section,resulting in the permanent plugging of water inflow.This process is depicted in Fig.1.This technology will help reduce groundwater drainage and thereby reduce the ecological damage caused by deliberate discharge of groundwater and generate environmental benefits.

2.2.Criteria that SAP must fulfil for use as a grouting material

(1) The SAP must have high expansion performance,such that a small volume of slurry can be injected,which will then rapidly expand to block the water inrush channel by absorbing a large volume of water.

(2) Controlling setting time of the material is critical to the success or failure of the project.If the setting time is too short,it is difficult since the slurry may coagulate too soon and block the grouting hole.If the setting time is too long,the coagulation process is easily interrupted and the material washed away by water,resulting in blocking failure.Therefore,the expansion rate of the SAP slurry should be adjustable to meet the needs of a particular construction project.

(3) The development of materials must make full use of existing equipment and technology.The SAP must be a slurry that can be pumped into the stratum by a grouting machine,thus enabling efficient and continuous grouting.

(4) Pipeline water inrush in karst areas often flows rapidly.Various anti-dynamic water dispersive grouting materials have been developed,but these have high viscosity,which makes pumping the materials difficult.This increases not only energy consumption but also the load on the grouting machine,which can damage the machine.To avoid such problems,the SAP slurry must have relatively low viscosity,allowing it to flow easily and spread throughout the pipeline.

Fig.1.Method for plugging karst pipeline water inrush.

(5) Most importantly,the grouting material should be safe and non-polluting to ensure the safety of construction personnel and the surrounding environment.The optimal SAP performance is summarized in Fig.2.

2.2.1.SAP carrying fluid

Because SAP is granular,it cannot be effectively transported to underground karst pipelines.It is necessary to prepare a slurry with SAPs and other liquids,which can then be transported to the karst water inrush channel by a grouting pump.The liquid added to prepare the slurry is called a carrying liquid.Since polymers absorb water,the carrier solution cannot be an aqueous salt solution.The particles in the suspension should well-distributed throughout the carrier fluid,and should be environmentally friendly,with no harmful effects on groundwater.Glycerol is a viscous,colourless,transparent,safe,and non-toxic hygroscopic liquid that does not react with SAP at room temperature (0–40 ℃)[33].The relative density of glycerol is 1.263,which is close to that of water;thus,it is fully miscible in water.Ethanol is a colourless and transparent liquid [34],which is miscible with glycerol and water,and has a relative density of 0.816.The solution of glycerol and ethanol readily forms a suspension and it has the ability to carry and does not react with polymer particles.Therefore,a mixture of glycerol and ethanol was used as the carrying liquid.

2.2.2.Swelling ratio

To measure the swelling ratio of the polymer particles,multiple samples (SAP 1 g),each composed of particles of a uniform size,were placed into tea bags,which were immersed in deionised water and brine of different concentrations (0.5%,1.0% sodium chloride,calcium chloride,magnesium chloride solution,four concentrations are used for each salt,for a total of eight saline solutions).After the material finished swelling,residual water was removed in a centrifuge (at a speed of 1000 r/min for 1 min),to determine the change in mass.The swelling ratio was calculated using Eq.(1).

whereSWis the swelling ratio;M0the mass of dried SAP of water absorption;ithe order of weighing andi=1,2,...,and the interval between the two tests is 30 s;andMithe mass of SAP after water absorption forith test.

Fig.2.Optimal characteristics of SAP grouting materials.

2.2.3.Water absorption rate measurement

To measure the expansion rate of SAP,1 g of SAP with uniform size was put into tea bags and immersed in the brine solutions described above,and aliquots were extracted every 30 s.Residual water was removed in a centrifuge (at a speed of 1000 r/min for 1 min),and the change in water absorption capacity of the SAPs was measured.The water absorption rate was calculated with the mass increment of the gel according to Eq.(2).

whereQiis the increase in water absorption of two adjacent tests.

2.2.4.Fluidity of the SAP slurry

Fluidity is a measure of the diffusion of the grouting material,and high fluidity ensures the stability and pumpability of the slurry[35,36].A fluidity test can thus provide a reference for the selection of grouting pressure in the field.The slurry flow performance was tested at 18 ℃using slurries with different mixture ratios (see Table 1).

Table 1Fluidity of slurry with different mix proportions.

2.3.Karst pipe-type water inrush plugging and pressure tests

2.3.1.Model of water inrush plugging

To study the diffusion mechanism of the SAP slurry in a waterbearing structure and the mechanism of water plugging,a visual karst pipeline water inrush test device was invented for this study(Fig.3).The device consisted of four parts:a water supply system,an SAP slurry injection system,a simulated karst pipeline,and a real-time data acquisition and processing system.

Photographs of the device are shown in Fig.4.The water supply system consisted of a 3-m3pressurised water tank and an air compressor unit with a range of 0–2 MPa (provided by Shanghai Qianbo Machinery Co.,Ltd).The air compressor provided constant pressure to the confined water tank to ensure stable water flow.

The SAP slurry injection system was located at the front end of the simulated karst pipeline and consisted of a nitrogen bottle and feeder.The SAP slurry was stored in the feeder,and the slurry was pressed into the pipeline by nitrogen to study the ability of the slurry to plug water inrush.

The karst pipeline simulation system was 20 m long,and comprised 2-m-long dismountable acrylic pipelines tied together with loose threads.A collecting tank was used at the outlet of the acrylic pipe to collect and treat the wastewater and slurry produced by the test.

At the entrance of the pipeline,a high-pressure hose was connected to the water supply system and equipped with a water valve and flow sensor,which provided real-time monitoring and recording of the flow rate during the test.The inner wall and bottom of each acrylic pipe were equipped with water flow pressure sensors,which enables real-time monitoring and recording of the wall pressure generated by material expansion during the experiment.

2.3.2.Testing of pressure capacity of the model for plugging

The hydraulic bearing capacity of the SAPs with different particle diameters and different water absorption ratios was tested by the hydraulic bearing capacity testing device (Fig.5).The device includes a water pressure supply device,data monitoring device,and pressure pipeline.

The water supply system was the same as that described in Section 2.3.1.The karst pipeline simulation system was 1 m long,with an inner diameter of 40 mm.The water pressure supply device was connected with the pipeline.The front end of the pressure pipeline was provided with a water outlet valve (discharging the air in the pipeline in time) and a pressure sensor.

The experimental steps were as follows.

(1) An amount of polymer slurry (see Table 2) was poured into the pressure pipeline from the inlet,followed a certain amount of water was poured in,which was absorbed by the slurry causing it to expand.

(2) The pipeline was connected to the water pressure supply device,and water pressure was allowed to gradually build up at the entrance.When the material had a whole slip,pressurisation was stopped,and the bearing pressure was recorded.

(3) The device was cleaned,and the data were analysed.

Fig.3.Schematic of karst pipeline water inrush test device.

Fig.4.Karst pipeline water inrush test device.

3.Results and discussion

3.1.Physical and chemical properties of the SAP slurry

3.1.1.SAP expansion mechanism

The expansion process of SAP is relatively complex.Dry polymer particles have a solid 3D network structure and contain a large number of hydrophilic groups (Fig.6).On contact with water,the water molecules form hydrogen bonds with hydrophilic groups in the SAP,resulting in the release of sodium cations from the hydrophilic groups.As the concentration of free sodium ions increases,the osmotic pressure inside the solid 3D network structure becomes higher than that of the solution,which causes the water molecules to move into the SAP,resulting in rapid expansion of the polymer particles [21,37].Simultaneously,the 3D network structure produces an inward elastic contraction force.As the water molecules enter the interior of the particles,the osmotic pressure decreases,and the elastic contraction force increases.With the expansion of the particle volume,the force generated by the internal and external osmotic pressure is balanced with the force produced by the shrinkage of the 3D network structure,and the water absorption of the polymer particles reaches an equilibrium.

3.1.2.Effect of water quality on the SAP swelling ratio

The expansion ratio of sap can be affected by the water quality present at different locations [38–41].Table 3 lists the chemical characteristics of karst water in different regions of China.Water quality in karst areas changes with the seasons.For example,when the water source is sufficient in summer,water scour is strengthened and the ion content in the water increases.The ions present in the water depend on the geological conditions and lithology.The effect of water quality on the SAP is significant,and an external solution with high osmotic pressure will negatively affect the expansion ratio of the material.Therefore,it is of great importance to study the effect of ion type and concentration on the polymer particles.

Table 2SAP slurry proportion.

Table 3Chemical characteristics of karst water in June from different areas.

Table 4SAP slurry ratio.

Fig.7a shows the expansion ratio of SAPs with different particle sizes in deionised water;evidently,particle size has little influence on the expansion ratio,and the weight of absorbed deionised water is >300 times that of the original polymer particle.Fig.7b shows the SAP’s water absorption rate with different particle sizes.The particle size of the SAP has a considerable influence on its water absorption rate,with larger particles resulting in a slower absorption rate.

In addition,the concentration of the external solution has an impact on the swelling ratio (Fig.7c).The swelling ratio gradually decreases with increasing salt concentration.This is because when the concentration of the external solution increases,the osmotic pressure of the SAP 3D network is reduced,resulting in a reduction in the water absorption capacity of the SAP.

Fig.7d–f shows that different ions have a significant influence on the SAP swelling ratio,with the order of influence degree from weak to strong being NaCl,MgCl2,and CaCl2.At the same ion concentration,the influence of calcium ions is higher than that of sodium ions due to the coordination between calcium ions and carboxylate ions,which reduces the osmotic pressure and thereby reduces the water absorption capacity of the SAP.Based on the above results,the water quality was found to significantly affect the swelling performance of the SAP slurry.Therefore,the analysis of groundwater quality is essential before grouting.

3.1.3.Influence of water quality on the SAP’s water absorption rate

Fig.8 shows the water absorption rate of the SAP in different salt solutions.The absorption rates initially increase and then rapidly decrease,and are significantly affected by the particle size.The absorption rate of SAPs >100 mesh is generally higher than that of other particle sizes.The smaller the particle,the larger the specific surface area and the higher the absorption rate.Another common phenomenon was observed:the time from contact with water to the completion of the expansion of SAP particles>100 mesh was half that of SAPs with 30–50 mesh.Therefore,the SAP expansion rate can be adjusted according to the engineering requirements.In the process of grouting,SAPs with larger particles are pumped first to ensure that the particles are transported for a long distance through the surging channel,and then SAPs with smaller particles are pumped to realise the proximal plugging of karst water inrush.In addition,the authors found a strange phenomenon in magnesium chloride and calcium chloride solutions—the SAP absorbs water and then releases a certain proportion of it,and finally reaches a stable state.Therefore,the existence of magnesium ions and calcium ions has a negative impact on the plugging of water inrush,which not only significantly reduces the expansion performance of the SAP but may also cause volume shrinkage of the plugging body,thus reducing its hydraulic bearing capacity.The influence of water quality on the SAP expansion performance discussed in this work can provide a reference for engineering applications.

Fig.5.Water pressure bearing capacity test device.

Fig.6.SAP expansion process.

Fig.7.SAP expansion characteristics.

Fig.8.Water absorption rate of SAP with different particle sizes in different concentrations of salt solution.

Fig.9.Fluidity of SAP slurry.

3.1.4.Effect of mixing ratio on flow performance

Fluidity has a substantial influence on pumpability.To obtain an appropriate proportion of grout,and guide the selection of grouting pressure at construction sites,the variation of grout fluidity with different particle sizes was studied.

As shown in Fig.9,as the solid–liquid ratio increases,the fluidity of the slurry gradually decreases,and the smaller the particle size,the more obvious the phenomenon.In addition,under the same solid–liquid ratio,particle size has a significant influence on the fluidity.The smaller the particle size,the lower the fluidity,indicating that particle refinement enhances the agglomeration of slurry particles at the macro level.

Compared with ordinary Portland cement slurry,when the water-cement ratio of common grouting is 0.8:1–1.1:1 and 1.1:1–1.2:1,the fluidity is 29–33 and 33–36 cm.Practice has proved that the fluidity under this condition is more suitable for grouting.Therefore,for the SAP with particle sizes >100 mesh,the ratio of SAP:glycol:ethanol chosen was 1:1:1,resulting in a slurry fluidity of 36 cm,which meets the requirements of the pumpability of the slurry.For the SAP with particle size of 30–50 mesh and 50–100 mesh,the optimal ratio of SAP:glycerol:ethanol was 2:1:1 because on the premise of meeting the liquidity requirements,a carrier liquid of the same quality can deliver more SAP,which can reduce the grouting cost.

3.2.Experimental results of water inrush plugging test

3.2.1.Flow analysis

By studying the physical and chemical properties of the slurry,the proportions were determined as presented in Table 4,and then the blocking effect was studied.During the experiment,the initial velocity of the water flow was adjusted to 0.6 m/s.The SAP slurry was pressed into the karst pipeline simulation system through the slurry injection system.The pressure and velocity changes in the pipeline were recorded.

Three experiments were carried out to study the plugging effect with different particle sizes.To allow direct observation of the sealing effect,a transparent acrylic tube was employed in the simulation device.The blocking effect is shown in Fig.10a.During the test,the slurry moves forward under the action of dynamic water,continuously absorbs water,and expands,thus consuming the kinetic energy of the flow.Simultaneously,it compresses and finally forms the plugging body.The test shows that the slurry has superior karst pipeline-type water inrush blocking ability.

Fig.10.Results of SAP tests.

Fig.10b shows the change in water velocity in the pipeline.The slurry prepared with the SAP with particle size of>100 mesh takes 18 s from the start of grouting to realise plugging,the 50–100 mesh SAP slurry takes 27 s,and finally the 30–50 mesh SAP slurry takes 34 s.These results show that larger particles,lower the water absorption rate,and increase the time taken to form the plugging body.Therefore,the swelling rate of the slurry can be controlled by adjusting the SAP particle size.

3.2.2.Pressure analysis

To monitor and record the change in flow pressure,sensors are installed on the karst pipeline simulation system.Fig.11 shows the pressure at different locations during the three experiments.The pressure near the water supply was the highest,owing to the high density of the SAP particle groups near the water supply end(Fig.11d and e)and the large elastic modulus of the plugging body;therefore,the wall pressure was high when deformation occurred under the action of water pressure.By comparison with Fig.11a–c,when the water is blocked,swelling an SAP with larger particle size produces a lower expansion pressure.Owing to the increase in particle size and the reduction in water absorption rate,the density of the particles decreases with the scouring of water.As a result,the density of the plugging body decreases;thus,the distribution of particle size will affect the expansion pressure.

3.3.Pressure bearing capacity

3.3.1.Influence of particle size on bearing capacity

A bearing capacity test was carried out in the pipeline by pressing the plugging body out of the pipeline.When the plugging body reaches its ultimate bearing capacity,it loses stability,resulting in slippage.The friction coefficient of the test pipeline is low(the static friction coefficient is 0.2),therefore the pressure bearing capacity depends on the frictional resistance between the pipeline surface and the plugging body.To further explore the influence of SAP particle size on the pressure bearing capacity,the ultimate bearing capacity of the SAP with different particle sizes under different swelling ratios was studied.The experimental results are shown in Fig.12.With an increase in the initial size,the bearing capacity decreases at the same swelling ratio.Because the density is identical,the internal friction of the polymer particles decreases with an increase in particle size,thereby reducing the bearing capacity of the water pressure.

3.3.2.Influence of swelling radio on bearing capacity

The effect of the expansion ratio on the bearing capacity is shown in Fig.13.For particles of the same size,the higher the swelling ratio of the SAP,the weaker the pressure bearing capacity of the plugging body.Because the SAP’s water absorption ratio increases,its elastic modulus decreases.Under the same strain,the ultimate bearing capacity decreased.Therefore,it is necessary to adjust the grouting rate to form the plugging body with appropriate swelling ratio and density to ensure safety.Therefore,this section provides a reference for the theoretical study of plugging karst pipeline water inrush with SAP granular materials,which is of great significance for guiding engineering design and construction.

Fig.11.Pressure results of SAP tests.

Fig.12.Influence of particle size on the pressure bearing capacity.

Fig.13.Ultimate friction of SAP with different particle sizes.

3.4.Application prospect and environmental benefits

With the rapid pace of urbanisation,underground structures also develop rapidly,and underground engineering accidents occur more frequently.Water inrush caused by underground excavation in karst areas is a major concern.Innovative materials can help overcome technical difficulties and prevent disasters.

SAP is first injected to rapidly absorb inrush karst water and convert it to a static form.Ordinary cement slurry is then injected behind the plugging body to supplement the geological strength of the karst water inrush area.The efficient treatment of dynamic water in karst areas will prevent underground engineering disasters caused by inrush water during construction,which lead to engineering delays and induce surface subsidence and other geological disasters.In the past,the large water flow in karst areas was controlled by a scheme involving drainage followed by grouting treatment,which often caused the water level in the surrounding areas to subside,altering the ground stress and resulting in uneven settlement of foundations and cracks in buildings.Random water diversion and drainage can easily destabilise the groundwater circulation system,disturb the natural groundwater system,and damage the environment.The treatment method proposed in this paper can block water inrush in karst areas without drainage or speed reduction.Therefore,SAP grouting materials have excellent potential for application in the field of underground engineering and has significant social benefits.

4.Conclusions

A novel karst water inrush treatment material,SAP,was developed with a swelling ratio of 300.Different cations were found to have a significant influence on the SAP swelling ratio,with the order of influence degree from weak to strong being NaCl,MgCl2,and CaCl2.This allows expansion rate of the SAP to be easily adjusted according to engineering requirements.SAP is safe to use and environmental-friendly.

A mixed solution of glycerol and ethanol was employed as a carrier liquid in the SAP slurry,and the optimal solid–liquid ratio of the slurry is dependent on the particle size.For the SAP with a particle size >100 mesh,the optimal ratio of SAP:glycerol:ethanol was 1:1:1 and for particle sizes in the range of 50–100 and 30–50 mesh,the optimal ratio of SAP:glycerol:ethanol was 2:1:1 in the slurry.

A visual karst pipeline-type water inrush simulation device(pipe length of 20 m,inner diameter of 40 mm) was built.The model test results show that a pipeline water inrush at 0.6 m/s can be blocked with only 800 g SAP,demonstrating that SAP has superior water inrush blocking effect compared to existing materials.

This study proposes an innovative technique for the treatment of water inrush disasters in karst areas.The dynamic water is converted to static water by the SAP slurry,and then ordinary cement slurry is injected to reinforce the stratum in the water inrush area.This method can effectively prevent water inrush,resulting in economic and environmental benefits,as well as improvements in safety.This article provides a reference for the design and construction of water inrush control projects.

Acknowledgements

The authors are grateful for the financial supports from National Key Research and Development Project (No.2019YFC1805402),National Natural Science Foundation of China (Nos.U1906229 and U1706223),and Project supported by the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (No.52021005).

Appendix A.Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijmst.2021.06.004.