A Case Study of Thaumasite Sulfate Attack in Tunnel Engineering

(ere existed ever-increasing structural diseases in the Dugongling tunnel after the completion of the construction. Systematic research was carried out based on the information of tunnel engineering geology, disease development, mineral composition of surrounding rock and concrete, and laboratory tests. Results showed that (1) the concrete structure from the Dugongling tunnel suffered from a typical thaumasite sulfate attack (TSA); (2) tunnel diseases developed under the influence of multiple geological factors such as groundwater, karst, gypsum bearing stratum, and geological tectonics. Groundwater aggravated the appearance of engineering defects of various geological factors and accelerated the softening and swelling of surrounding rock. Moreover, the additional load of supporting structure increased, and the concrete structure was prone to sulfate attack; (3) the development of tunnel diseases showed long-cycle and large-scale characteristics, and the types of diseases are heterogeneous. (e tunnel diseases develop rapidly in the third year after the construction. (e structure collapse disease, one of the most serious diseases of tunnel lining, occurred in the fifth year after construction; and (4) according to the conditions of TSA, technical treatment measures were proposed. (ese measures included blocking the flow of CO2− 3 , improving the impermeability of concrete structures, and decreasing the diffusion of external ions and groundwater. (is work provides a solution for the treatment of similar tunnel disease and mechanism analysis.


Introduction
e gypsum rock in the carbonate rock area has multiple influences on the stability of the tunnel. e gypsum rock can not only cause dissolution combined with the co-deposited carbonate minerals [1], but also soften and swell under the action of groundwater [2][3][4]. Moreover, gypsum rock can lead to sulfate attacks in the lining concrete structures [5][6][7][8][9], whereas the thaumasite sulfate attack (TSA), elusive, is relatively late understood by researchers. e formation mechanism of TSA is unclear yet [10,11]. TSAs in tunnels have occurred in Switzerland, Germany, Norway, and Australia [12][13][14][15], while these attacks are more common in water conservancy dam projects [16][17][18] in China. e reports on TSA in Chinese tunnel projects are extremely rare. TSA may dissolve the main hydration products of Portland cement, CSH gel, in concrete tunnel lining. e decomposition of CSH gel leads to the decrease in the bearing capacity of the tunnel lining structure and the reduction in structural safety and reliability [19,20], which is one of the most harmful types of tunnel diseases.
Hazardous engineering conditions caused extremely serious diseases and TSAs in the tunnel supporting and lining structure after construction. Based on the analyses of sulfate erosion conditions and softening and swelling characteristics of surrounding rock, comprehensive tests of the mineral composition of surrounding rock and lining concrete and systematic analyses of the types and development rules of tunnel diseases were carried out to figure out the formation process of diseases in the Dugongling tunnel. Finally, a targeted engineering treatment plan was put forward to effectively treat the diseases. In addition, this kind of treatment provides a reference plan for the tunnel with TSAs.

Project Overview
e Dugongling tunnel [21] is a separated two-way four-lane highway tunnel. e starting and ending pile number of left line is from ZK33 + 226 to ZK35 + 700 with a length of 2474 m and that of right line is from YK33 + 227 to YK35 + 742 with a length of 2515 m; the overall direction of tunnel axis is 264°, and the maximum depth of tunnel is 231 m. e construction clearance of the tunnel has a net width of 10.25 m and a height limit of 5 m (see Figure 1). e Dugongling tunnel was completed in November 2012. Tunnel disease was first detected in the later stage of its construction. As of 2016, the total length of the tunnel structure that needed to be demolished and replaced due to diseases is 1,515 m. e main range of tunnel diseases occurred in sections from K34 + 500 to K34 + 800. Diseases in the Dugongling tunnel were mostly lining concrete cracking, and deformation at early stage of the diseases, no corrosion, and argillization phenomena of concrete were found. e concrete argillization phenomenon had not been discovered until the firebox was removed, as shown in Figure 2. Concrete argillization showed a feature of integrity in the dry environment， while seemed sludge-like under grasping in the wet.
In the supplementary survey of tunnel diseases in 2016, 223 samples from 119 cross-sectional primary support concrete were picked. e mineral compositions of the concrete were mainly calcite, dolomite, quartz, potash feldspar, plagioclase, ettringite, gypsum, and thaumasite. 60 sections of the 119 sections were detected with TSA, accounting for 50.4% of total samples; 84 of 223 samples of primary supporting concrete were detected with thaumasite ( Figure 3), accounting for 37.7% of total samples. It can be seen that the disease in the Dugongling tunnel was a typical case of TSA [22].  Table 1, show an increasing trend in previous years. In 2012, the main kinds of tunnel diseases were lining cracking, pavement uplifting, and cracking. In 2013, the overturning of the cable trench was detected; in 2014, the staff found partial cracks at arch footing and intrusion of the clearance in some sections, as shown in Figure 4; and in 2016 and 2017, the diseases of the Dugongling tunnel deteriorated, leading to the collapse and fall of the secondary lining concrete ( Figure 5) and the extrusion and damage of the inner wall of the cable trench ( Figure 6).

Scales in Previous Years.
e disease scales over the years are shown in Figure 7. e disease scale of the tunnel reached 2321 m at the beginning of 2017, which accounted for 47% of the length of the tunnel. It can be seen from Figure 7 that the fastest development period of tunnel diseases took place in 2013 and 2014, 1765 m, which accounted for 76% of total length of diseases; there was a gradual increase in new diseases from 2015 to 2017. e length of new diseases was only 8 m and 58 m in 2016 and 2017, respectively. us, the scale of tunnel disease developed rapidly in the early stage of development of disease, and the growth rate of the total diseases slowed down in the later stage.

Degree of Disease Development.
According to the classification of disease levels from technical specification of maintenance for highway tunnel (JTG H12-2015), the variations of disease scales over the years at all levels are shown in Figure 8.

Typical Cross-Sectional
Cases. e design basis of YK34 + 482～YK34 + 524 section is grade IV surrounding rock. e primary supporting of the tunnel is I16 I steel with a spacing of 1.0 m and C20 concrete with a thickness of 20 cm, and the secondary lining of the tunnel is C25 plain concrete with a thickness of 40 cm; a fire chamber is set on the right side of YK34 + 500 section. e development process of disease is as follows: (1) e process of collapse. During the construction safety inspection in March 2017, cracks around the fire chamber were discovered; the monitoring data in April showed that the length and width of the diagonal cracks on the upper left side of the fire chamber increased, extending upward from the spandrel; cracks continued to develop in May and June, and a small area of concrete surface peeling occurred; in July, the concrete collapsed suddenly ( Figure 9). e results showed that the surrounding rock was gypsum rock with gypsum content of 97%-98% ( Figure 11 and Table 2); the thaumasite content in the primary supporting concrete was 19.3%, and the contents of gypsum and ettringite were 17.1% and 3.8%, respectively.

Geological Cause Analysis of Tunnel Diseases
aumasite sulfate attack is a special type of sulfate attack. e formation of TSA needs slow temperature (below 15°C), water, SO 2− 4 , CO 2− 3 , Si 4+ , SiO 2− 3 , Ca 2+ , etc. e following analyses of the causes of the diseases in the Dugongling tunnel are based on the formation conditions of thaumasite sulfate attack.

Low Temperature.
e tunnel sites in a temperate semiarid continental climate area. e temperature of this area is between −5.9°C and 23°C, and the average temperature of this area is 9.2°C. e temperature in the five months from November to March of the following year is below 10°C; the temperature inside the tunnel ranges from −5°C to 20°C throughout the year, with an average temperature of 8.8°C. erefore, the environment of the tunnel lining concrete structure provides low-temperature conditions for TSA.     e types of groundwater in the tunnel site area are carbonate fissure karst water and perched water, of which the water level of carbonate fissure karst water is lower than the design elevation of the tunnel. erefore, this type of groundwater has little effect on the diseases of the Dugongling tunnel. e perched water, mainly replenished by atmospheric precipitation, has a large seasonal variation.

Supplement of Groundwater.
e annual average precipitation in the tunnel site area is 592.33 mm and its main precipitation period is from July to September, which accounts for 56% to 70% of the annual precipitation. Affected by topography, surface precipitation collects and infiltrates along the mountain to low-lying places. In section K34 + 500∼K34 + 800 of the tunnel, more than 100 iron ore pits distribute on the surface and its cross sections of the pits vary in size, and the shapes are mostly bands and ellipses ( Figure 12) according to the surface survey. e group of surface mine pits provides conditions for surface water collection and irrigation. It is estimated that the surface water infiltration within the mine pits reaches 31 m 3 /d.
After the surface water infiltrates, it circulates along the internal cracks and pores in the rock mass. e dissolution and development conditions of the tunnel rock mass provide a path for this circulation. According to engineering survey data and tunnel construction information, there are many joints, cracks, and dissolution in the surrounding rock of the tunnel, and the dissolution of the tunnel site shows a typical characteristic of combined carbonate and sulfate erosions [24,25]. Most of the dissolution is dissolution pores, channels (Figure 13), and small karst caves (Table 3). Moreover, due to the      Advances in Civil Engineering interconnection between the dissolution pores the air blow in the tunnel drilling is observed (Figure 14).

e Collection of Groundwater around the Tunnel.
In the process of groundwater circulation in the rock mass, the groundwater accumulates in some sections due to the influence of the geological structure. e compound folded structure is in the K34 + 900-K35 + 200 section of the cave body, consisting of two synclines (S1, S3) and one anticline (S2) and striking nearly the north-south direction: the S1 syncline crosses the tunnel at an angle of 84°, the east wing occurrence is 200°∠5°, and the west wing occurrence is 122°∠25°; S2 syncline crosses the tunnel at 88°obliquely, the east wing occurrence is 122°˂ 25°, and the west wing occurrence is 280°˂ 20°; and S3 syncline crosses the tunnel at 72°, the east wing occurrence is 280°˂ 20°, and the west wing occurrence is 80°˂ 4°. In the composite fold structure section of the tunnel, the surrounding rock below the sidewall is mainly marl, which has a strong water barrier effect. e Microscopic observation e rock has lump tectonics and a granular structure. Gypsum is tabular and colorless with a particle size distribution of 0.01-2.0 mm and has a negative protrusion. e highest interference color is first-order yellow and white, and cleavage in one direction is observed. Dolomite has a semi-automorphic rhombohedral shape with a particle size distribution of 0.01-0.03 mm and is unevenly distributed. It has a prominent flash protrusion and high-order white interference color.  Advances in Civil Engineering coexistence of the two causes the concentration and occurrence of groundwater around the tunnel of the S1 and S3 syncline structure.
Based on the above analyses, surface precipitation flows along the surface and forms a pool at the mine pit, and flows down into the rock mass. Meanwhile, dissolved pores, caves, and small caves are the main paths for groundwater to circulate in the rock mass; groundwater converges around the tunnel under the influence of compound folded structure and marl, which provides the water for the formation of thaumasite sulfate attack.  (Figure 15). e color of gypsum rock, similar to that of marlstone, is grey-white in general and grey-brown in parts (Figure 16), and the maximum content of gypsum rock in some rock samples is more than 90% (Table 2).

Other Reasons.
In addition to the TSA, the softening and swelling characteristics of the surrounding rock contributed to the terrible disease in the Dugongling tunnel. e formation of marl has the properties of softening and swelling, and marl mechanical test report is listed in Table 4. Identification results of core samples of the surrounding rock are shown in Table 5. e softening coefficient of marlstone is 0.72～0.818, the free expansion rate is 0.001-0.094, the expansion force Pp varies from 0.99 to 20.6 kPa, and the saturated water absorption rate is 2.9～6.0%. It shows that the marlstone from the Dugongling tunnel has the characteristics of water absorption softening and water loss disintegration. e irregular distribution of gypsum in the limestone formation aggravated the softening and swelling features of gypsum. According to the indoor expansion tests of lateral confinement on anhydrite in the Dugongling tunnel [21], the expansion forces of the specimens with anhydrite content are greater than 90% reaching 1.94-3.1 MPa, as shown in Figure 17. e existence of anhydrite made the swelling characteristics of the surrounding rock more significant.

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Marlstone and gypsum rock softened and swelled under the action of groundwater. On the one hand, the softening of the surrounding rock of the tunnel reduced the bearing capacity of the surrounding rock. On the other hand, the swelling of the surrounding rock produced lateral expansion forces, both of which deteriorated the stress condition and load of supporting and lining structures. e variations of marlstone and gypsum rock along the longitudinal direction of the tunnel were superimposed on the unfavorable conditions of surrounding rock fragmentation, crack development, and karst, which caused the inhomogeneous distribution of loads on the supporting and lining structures longitudinally and laterally, resulting in the longitudinal cracking of the tunnel sidewall, the overturning of cable trenches, and uplifting of pavement.

Results of the Analysis of Tunnel Disease Causes.
Surface pits provided favorable objective conditions for groundwater replenishment. Joint fissures and karst bodies (dissolved pores, caverns, and caves) developed in the rock mass and then provided paths for the circulation of groundwater. e composite fold structure and the water barrier formed by marl made the groundwater converge behind the tunnel section lining and kept accumulation. It dissolved and carried a large amount of sulfate ions and carbonate ions when the water flowed through, which was prone to thaumasite sulfate attack in the low temperature. In summary of various factors, the formation of tunnel diseases can be described in Figure 18 as follows.

Engineering Treatment Plan
According to the causes of tunnel diseases, the treatment of tunnel diseases can be done in the following aspects: groundwater treatment, surrounding rock reinforcement, and lining structure optimization.

Groundwater Treatment.
According to the geological and hydrogeological conditions of the engineering project and the influence of groundwater in tunnel diseases, the treatment of groundwater mainly included programs as follows:

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(1) Surface mine pits were backfilled and sealed to prevent the surface water collection and irrigation through the pits (2) Grouting in the surrounding rock was adopted in the fracture zone and section of the tunnel to block the collection of groundwater behind the tunnel lining structure (3) e section with all-inclusive waterproofing secondary lining where tunnel lining was replaced cut off the contact between the secondary lining of the tunnel and the groundwater (4) e tunnel waterproof and drainage systems were improved and restored to avoid long-term collection and retention of groundwater in the tunnel

Surrounding Rock Reinforcement in the Tunnel.
According to the softening and swelling characteristics of the surrounding rock of the tunnel and the development of karst, the reinforcement design of the tunnel surrounding rock was carried out, including the following: (1) Long and short anchor rods were conducted in the tunnel to enhance the self-bearing capacity of the tunnel and control the expansion and deformation of the surrounding rock  Identification basis GB/T17412.1 7 Essential mineral 90% gypsum 8 Secondary minerals 4-6% dolomite and 4-6% anhydrite 9 Accessory mineral Zircon 10 Microscopic observation e rock has a bedded tectonics, a fibrous structure, and a metasomatic residual structure. Gypsum is fibrous. e protrusion is low and negative, and the highest interference color is first-order yellow and white, and is arranged in a directional arrangement. Anhydrite is columnar, and most of it is converted into gypsum, and a small amount of residue can be seen in the center of the crystal. e protrusion is middle and positive, the highest interference color reaches third-order green, and it exhibits parallel extinction and polysynthetic twin development with a directional arrangement. Dolomite has an allotriomorphic granular structure with a particle size distribution of 0.005-0.02 mm, and it has a prominent twinkling and high-order white interference color. 10 Advances in Civil Engineering

Treatment of the Tunnel Lining Structure.
According to the characteristics of the tunnel lining structure disease and the swelling and softening of the surrounding rock, the lining structure of the diseased section of the tunnel was treated as follows: (1) e primary supporting and secondary lining replacement programs were adopted for the tunnel concrete under sulfate attacks. To improve the strength and load-bearing capacity of the tunnel, the supporting and lining structures were enclosed into a ring, of which the initial supporting was reinforced by I20 steel framework with a spacing of 60 cm and C30 shotcrete with a thickness of 28 cm, and the secondary lining adopted a C35 reinforced concrete structure with a thickness of 50 cm ( Figure 19). (2) Design for sulfate attack resistance of the lining structure concrete: the occurrence of TSA needs many factors and complex mechanisms [27][28][29][30]. From the perspective of engineering applications, measures including waterproofing and reducing the source of sulfate ions were taken to prevent this disease. e measures can be summarized as "blocking the flow of CO 2− 3 , improving the impermeability of concrete structures, and decreasing the diffusion of external ions and groundwater." e detailed programs are as follows: (i) e cement, coarse and fine aggregates, and external additives in the primary supporting concrete of the tunnel contained no limestone materials. e basalt was used as gravel by replacing limestone.
(ii) Admixtures in concrete (including preservatives, water-reducing agents and other additives, fly ash, and other admixtures) contained or produced no carbonate ions and sulfate ions. (iii) e impermeability grade of the initial supporting concrete was greater than P6, preferably greater than P8, and the impermeability grade of the secondary lining concrete was greater than P8.

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(iv) e sulfate-resistant concrete was used as the primary supporting and secondary lining concrete.
At present, the Dugongling tunnel has been opened to traffic for three years after the disease treatment, and the tunnel lining structure is in good technical condition ( Figure 20).

Conclusions
(1) e disease of the Dugongling tunnel, mainly thaumasite sulfate attack, was a long cycle with multiple types and large scale. It took 6 years from the first discovery of the disease to the treatment of tunnel diseases. During this period, the diseases continued to develop, the number of disease types increased from the first 3 to 8, and the outbreak of diseases occurred in the third year after construction (i.e., in 2014). e most serious disease, the lining structure collapse, took place in the fifth year after construction (i.e., in 2016). (2) e formation process of the diseases: surface water was collected from surface mine pits and infiltrated underground, and then, it circulated to marlstone formation via karst caves, dissolution cavities, and joint fissures to form the perched water; under the influence of compound folded structure and the drainage system of the tunnel, groundwater was collected and infiltrated around the tunnel; the lowtemperature environment of the groundwater and the sulfate ions and carbonate ions in it caused TSAs to the tunnel lining structure, which deteriorated the  strength and induced structural diseases of the tunnel lining structure; under the influence of groundwater, the surrounding rock of the tunnel softened and expanded, the load of the tunnel lining structure increased, and finally the lining cracked. (3) Considering the formation conditions of TSA and the characteristics of the surrounding rock of the tunnel, the treatment measures of the tunnel diseases can [31][32][33] be proposed as "blocking the flow of CO 2− 3 , improving the impermeability of concrete structures, and decreasing the diffusion of external ions and groundwater."

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare no conflicts of interest.