1. Deformation of argillaceous rock and surrounding rock of tunnel
1. General engineering geological characteristics of argillaceous rocks
Argillaceous rocks are all kinds of mudstone, shale, claystone and argillaceous siltstone. From the point of view of engineering geology and rock mechanics, most argillaceous rocks belong to weak and changeable complex rocks, or to say, most argillaceous rocks belong to soft rocks. Many engineering accidents are related to the poor engineering characteristics of argillaceous rocks. It is worth mentioning that in engineering practice, people often associate argillaceous rocks with expansive rocks. In fact, swelling rock is the worst kind of argillaceous rock, and not all argillaceous rocks are swelling rocks. A large number of engineering practices and studies show that the engineering characteristics of argillaceous rocks and their property changes under the influence of engineering depend on diagenetic cementation and engineering activation of argillaceous rocks. Although the types and degrees of diagenetic cementation of argillaceous rocks in different regions and times in China are extremely complicated, they still have certain regularity (Qu Yongxin et al., 199 1), namely: ① Paleozoic argillaceous rocks are strongly cemented and strongly cemented non-expansive argillaceous rocks, but there are a certain number of weak-medium cemented micro-expansive and weak expansive argillaceous rocks in the upper part of Upper Permian (Upper Shihezi Formation and Shiqianfeng Formation). ② The Mesozoic and Cenozoic argillaceous rocks are dominated by weak cementation and medium cementation. The main regional weakly cemented expansive rocks in China are: upper Jurassic-lower Cretaceous argillaceous rocks, Paleogene argillaceous rocks and Neogene argillaceous rocks, which are the worst regional weak rocks in China. ③ Diagenetic cementation of argillaceous rocks not only controls and affects the swelling potential of rocks, but also controls and affects the strength and weathering durability of rocks, that is, with the increase of cementation degree, the strength and durability increase. ④ The cementation of argillaceous rocks affected by weathering (including archaization and modern weathering) tends to weaken, its strength decreases and its physical and chemical activity increases.
The argillaceous soft rocks along the Yunnan-Tibet railway are mainly distributed in the Yangtze platform area in northwest Yunnan. Triassic and Paleozoic argillaceous rocks in Sanjiang orogenic belt and Himalayan orogenic belt have undergone different degrees of metamorphism, becoming mudstone, phyllite and schist, and their strength and disintegration durability have been improved. Jurassic and Cretaceous in northwest Yunnan are mainly composed of interbedded sandstone and argillaceous rocks, among which argillaceous rocks (including mudstone, silty mudstone and argillaceous siltstone) account for a large proportion, mainly maroon or purplish red and gray or grayish brown, with uniform structure and block or layered structure.
2. Engineering geological problems related to argillaceous rocks
As mentioned above, argillaceous rocks are often easily weathered, which is a kind of rock with poor weathering durability and a sliding layer on the slope. When argillaceous rock is used as slope rock mass of engineering site, landslides will occur in different degrees. According to the analysis of previous engineering examples, tunnel engineering geological problems related to argillaceous rocks are mainly manifested in the following aspects:
(1) Under the condition of deep-buried tunnel excavation, due to the low strength of mudstone, the deformation of tunnel surrounding rock is often large, which affects the safety of tunnel construction and the choice of supporting methods.
(2) The maximum single layer thickness of weakly cemented argillaceous rock in Yanjing area can reach 3 m, which will show obvious expansibility under the condition of alternating dry and wet, promote the large deformation of surrounding rock of tunnel, and have a great impact on the safe operation and support of tunnel.
(3) When argillaceous rock appears in the form of thin interlayer, it is easy to form interlayer shear zone, which is extremely unfavorable to the stability of tunnel slope and surrounding rock.
In the process of railway construction from Dali to Lijiang in northwest Yunnan, tunnel deformation caused by poor engineering properties of argillaceous rocks has appeared, which restricts the construction safety of railway tunnels. For example, the collapse accident of Beiya tunnel in June 2006 has a great relationship with jointed argillaceous rocks.
2. Weak rock mass and ductile shear zone in metamorphic rock series and large deformation of tunnel surrounding rock.
Schist (chlorite schist, mica schist, talc schist, mica quartz schist, amphibole schist, etc. ), phyllite, thin layer, etc., metamorphic rock series is rich in flaky minerals, which are not only weak in lithology, but also widely developed in foliation and foliation with obvious anisotropy, and usually belong to weak rock groups in engineering. In shallow tunnel, due to the high degree of weathering, the rock properties become extremely weak, and the tunnel is extremely difficult to support; Under the condition of deep-buried tunnel excavation, the surrounding rock may be peeled off, buckled, floor heave and other large deformation phenomena, which seriously affects the tunnel construction safety and the choice of support methods. According to the field investigation and comprehensive analysis of regional geological data, this kind of weak rock mass is mainly distributed in the following sections along the project: ① Hutiaoxia Town-Xiaozhongdian; ② Benzilan-Deqin North; ③ Tongmai-Lulang; ④ Milin-Gacha and other areas. It should also be noted that there are numerous large-scale fault zones along the Yunnan-Tibet railway, and the combination of fault gouge and cataclastic rocks may cause large-scale deformation. Because the influence of fault zone on engineering is well known, I won't repeat it here.
A special structural type, ductile shear zone, is widely developed in northwest Yunnan-Sanjiang area, which belongs to the product of orogenic belt together with the vast metamorphic rock series in southeast Tibet and Himalayan structural zone. Under the condition of medium temperature and high pressure, the faults formed by strong tectonic deformation of orogenic belt are mostly ductile shear zones in the form of schistosity and cleavage, and a few are brittle faults, so typical fault gouges and cataclastic rocks are rarely seen. Because the distribution range and extension direction of ductile shear zones in Sanjiang area are basically consistent with the overall extension direction of planned Yunnan-Tibet railway lines, railway lines will inevitably pass through these ductile shear zones, which will have a certain impact on the planning and construction of Yunnan-Tibet railway.
1. Structural characteristics of ductile shear zone in northwest Yunnan-Sanjiang area
The development of ductile shear zone in northwest Yunnan is mainly controlled by regional geological structure. Influenced by many tectonic movements in geological history, the ductile shear zone reflecting compressive shear is very developed in northwest Yunnan, and its spatial distribution and occurrence are mostly consistent with regional active faults, and it has locally become an important part of the fault zone. According to its concentrated distribution area and related regional fault zones, from Lijiang, Yunnan to Yanjing, Tibet, from south to north, it can be divided into three areas with dense cleavage zones: Zhongdian fault ductile shear zone, Jinshajiang fault ductile shear zone and Lancangjiang fault ductile shear zone (figure 1 1-20). The spatial distribution direction of the three concentrated distribution areas changed from near north-south direction to northwest-southeast direction, which was in a positive order. The ductile shear zones in each distribution area are concentrated, large in scale (hundreds to hundreds of meters) and staggered in space.
(1) Lancang River fracture ductile shear zone
The ductile shear zone starts from Yanjing Town, Mangkang County, Xizang Autonomous Region in the north, passes through Foshan and Shui Gu in the south, and reaches Shengping Town, Deqin County, Yunnan Province, with a length exceeding 130 km. The ductile shear zone is widely distributed in this area, and the Lancang River fault meanders along the Lancang River valley, which is consistent with the ductile shear zone, and sometimes cuts the ductile shear zone. The shear zone mainly exists in the form of cleavage, with a strike of 330 ~ 340 and a large dip angle greater than 80. Cleavage passes through strata of different ages, and the latest stratum is Paleogene lacustrine light yellow metamorphic feldspathic sandstone mixed with white limestone, indicating Cenozoic ductile shear zone activity. Cleavage cuts rocks with different lithology, and the plate cleavage in slate is the most neat, which is often consistent with bedding; The cleavage distribution in limestone is uneven, and there are often extruded lenses, and some are large extruded lenses. Cleavage in sandstone and basalt is between slate and limestone (Figure 1 1-2 1). There are dikes interspersed in the cleavage zone, and scratches, steps and pinnate structures develop locally along the cleavage plane, indicating that there are multi-stage displacements along the cleavage plane.
Figure 1 1-20 Distribution area of compressive cleavage zone
Figure 1 1-2 1 section of the ductile shear zone in the north of Luwacun on the right bank of Lancang River.
(2) Jinshajiang fault ductile shear zone
The ductile shear zone related to the Jinsha River fault is mainly distributed on both sides of the fault and within its shear zone. The ductile shear zone of Jinsha River in the investigation area is typical in the bridge section of Benzilan Town-He Long in Deqin County, where the cleavage distance is 0.5 ~ 20 cm, which is distributed in the direction of 330 ~ 350, and the dip angle is steep, generally around 70 ~ 80, and it is partially upright (Fig.65438 Fault triangle landform develops along the ductile shear zone. In soft and hard rocks, the weak layer is strongly flaked, and the hard layer is crushed and squeezed to form a lens. The lens in limestone often forms a reef island protruding from the river. When the lithology changes along the cleavage direction, folds and bends will occur. The occurrence of schistosity or cleavage is unstable, and there is interpenetration between planes, which leads to rock fragmentation.
(3) Zhongdian fault ductile shear zone
The ductile shear zone mainly extends along the Zhongdian fault and is distributed in a certain range on both sides of the fault. Cleavage is densely distributed in ductile shear zone, and local ductile shear zone becomes the main component of fault zone. At the mouth of Shanghutiaoxia, the occurrence of ductile shear zone is closely related to Zhongdian fault, with strike of 365,438+00 ~ 330 and dip angle of 75 ~ 85. The profile of cleavage (schist) physical and chemical rock mass extends over 400 m, which is developed in gray-black slate, gray cleavage (schist) physical and chemical basalt and dark gray cleavage limestone, and is cut by east-west fracture. The general cleavage structural plane spacing is 1 ~ 20 cm, and the structural plane is flat and fine, with calcite and timely veinlets interspersed in the rock mass. Physicochemical development in some areas, with a thickness of about 0.2~5 cm, is shriveled and broken, and is easy to deform under the action of gravity. Within 400 m, there are many geological disasters such as collapse, falling rocks and landslides.
Figure 1 1-22 section of ductile shear zone in Benzilan area, Deqin County
Figure 1 1-23 Profile of the ductile shear zone at Hutiaoxiakou, right bank of Jinsha River, Yulong County, Lijiang.
2. Analysis of the influence of ductile shear zone on tunnel engineering
The ductile shear zone in northwest Yunnan is widely distributed, which is often characterized by intensive cleavage and rock mass fragmentation. The rock properties in this area are complex and the engineering geological properties are changeable. It often intersects with faults, cleavage zones and jointed dikes in other directions, and is further reformed in the later tectonic movement and external dynamic action, and some folds are strong, which makes the cleavage in the zone more dense and complicated, and the rock mass properties further deteriorate. Besides the slope stability, the ductile shear zone also has a significant influence on the tunnel stability.
Figure 1 1-24 Bending and buckling mechanism of fractured rock mass and simplified mechanical model
When the angle of cleavage plane or schist plane in ductile shear zone is steep and the strength of rock mass (such as slate and phyllite) is weak, the surrounding rock of tunnel can bend and buckle. At the roof and shoulder of the tunnel, the combination of cleavage plane and other secondary joint planes is easy to form unstable blocks, which will cause the roof of the tunnel to fall off or even roof fall. If the design tunnel strike is parallel to the cleavage plane strike, as a group of dominant rock mass structural planes in the surrounding rock of the tunnel, the rock mass near the side wall is easy to peel off or collapse along the cleavage plane; In the area with high physicochemical degree and developed folds, the rock mass is relatively broken, which is unfavorable to the overall stability of the tunnel. When it is combined with other structural planes in space, a large-scale landslide may occur. Especially when the local stress level is high, the buried depth of the tunnel is large, and the energy storage of the fractured (foliated) rock mass is strong (foliated limestone, foliated basalt, etc.). ), in the case of one side confining pressure unloading, it is easy to appear the phenomenon of rapid bending and buckling. The mechanism is that the tunnel excavation causes the rapid unloading of rock mass, and under the strong geostress, cracks are further developed along the cleavage (foliation) plane of the parallel tunnel wall, which divides the surrounding rock into thin plates. Strain energy is stored and displaced towards the tunnel. When the displacement exceeds a certain critical value, the cracks spread rapidly, causing the surrounding rock of the tunnel to deform rapidly and even rock burst (Figure 1 1-24).
Third, argillaceous altered soft rock and large deformation tunnel surrounding rock.
The geological structure evolution of Sanjiang and southeast Tibet where Yunnan-Tibet railway passes is extremely complicated, and it has experienced many periods of intense tectonic magmatism, accompanied by magmatic hydrothermal mineralization and hydrothermal alteration. In addition, since Neogene, the strong uplift of the Qinghai-Tibet Plateau is characterized by different neotectonic activities and the landform of high mountains and valleys, which makes the construction of Yunnan-Tibet railway face extremely complicated engineering geological environment and various complicated engineering geological problems. Among them, various types of igneous intrusions widely distributed along the railway line often form clay (montmorillonite, illite, kaolinite) altered rock zones under hydrothermal alteration, especially montmorillonite altered rock, which is both extremely weak soft rock and typical swelling rock, and the deformation and failure of tunnels and slopes caused by it is more prominent (photo11-/kloc- The regional development law of orogenic belt, tectonic magmatic belt and hydrothermal metallogenic belt determines its regional distribution, development characteristics and engineering characteristics. For example, the northwest section of Yunnan-Tibet railway is mostly montmorillonite, and the Tibet section is mostly kaolinite and illite. Therefore, it is necessary to analyze the types, formation mechanism and development law of altered rocks according to the tectonic-hydrothermal process and mineralization law along the railway, and study the bad engineering geological characteristics of altered rocks, so as to guide the engineering geological prediction and prevention of engineering problems in the distribution area of altered rocks along the Yunnan-Tibet railway.
Altered rocks along the Yunnan-Tibet Railway are an important part of the Himalayan-Sanjiang metallogenic domain, which spans the Hengduan Mountain area in eastern Qinghai-Tibet and western Sichuan-Yunnan, with an area of 50× 104 km2. Located in the eastern part of the Tethys-Himalayan tectonic belt, this belt turns sharply to the south, and it is one of the most important magmatic hydrothermal metallogenic domains in China. The distribution law of altered rocks is mainly manifested in the obvious zoning of altered rock types and development degree. According to the field investigation and test analysis, the Yunnan-Tibet Railway can be roughly divided into three areas, namely, the distribution area of montmorillonite basic ultrabasic rocks in northwest Yunnan, the distribution area of Deqin-Basu altered rocks and the distribution area of altered rocks in southern Tibet.
1. Distribution area of montmorillonite altered rocks in northwest Yunnan
Most of the exposed rocks in this distribution area are montmorillonite altered rocks, which have the characteristics of small monomer scale, high alteration degree and poor engineering properties. The clay mineral composition of altered rocks is mostly montmorillonite, and the minerals are single, which leads to that the altered rocks in this area are both soft rocks with extremely low strength and typical swelling rocks. The altered rocks in this distribution area are mainly characterized by parent rock montmorillonite. It mainly belongs to post-magmatic hydrothermal alteration rocks, with a small amount of volcanic hydrothermal alteration. According to the distribution characteristics of montmorillonite basic ultrabasic rocks in northwest Yunnan, three altered rock zones can be roughly divided (Figure 1 1-25).
(1) Dali-Jinping basic ultrabasic rock belt is located at the eastern edge of Jinsha River-Ailaoshan suture belt and the western edge of Yangtze paraplatform. Small and medium-sized igneous rocks are densely distributed in Haidong area of Dali and Jinping area at the southern end, and the extension arrangement of individual rocks is consistent with the structural orientation. The rock assemblage can be mainly divided into two types: ① The annular basic ultrabasic rocks are mostly distributed in NW-SE strip, and the single rock mass is layered, lenticular and oblate. Rock mass is usually ultra-basic inside and ultra-basic outside, most of which are in approximate integrated contact with surrounding rock, and a few of them run through surrounding rock. Generally, the length of rock mass is less than 1.0 km, the width is several hundred meters, and the MgO content is 28.8% ~ 39.2%. (2) Layered basic ultrabasic rocks are mainly many widely distributed but small-scale rock walls, with a thickness of several meters to tens of meters, and a few occur in the form of rock plants. Their rock types include diabase, diabase or diabase porphyrite. Most of these dikes are distributed along the secondary faults and their intersections in this area.
(2) The Jinshajiang basic and ultrabasic rock belts are mainly located between Jinshajiang fault and Deqin fault. The rock belt is about 240 kilometers long and 25 kilometers wide from north to south. There are more than 200 known rock masses, which constitute more than 20 rock groups, such as Baimangxueshanyakou and Dongzhulin in Deqin, and each rock group consists of several to ten rock groups. The distribution of ultrabasic rocks is usually consistent with faults. The length of a single rock mass is tens to hundreds of meters, and the aspect ratio is generally 5: 1 ~ 20: 1. Most of the rocks are monoclinic layered or lenticular, which is almost consistent with the occurrence of surrounding rocks. Basic rocks are generally exposed in long strips, tens to hundreds of meters long and tens to tens of meters wide. Rock types mainly include diabase porphyrite, diabase and gabbro. Neutral and acidic intrusions are also distributed. A large number of montmorillonite minerals in the moraine at the northern foot of Baimang Snow Mountain in Deqin come from this altered rock zone.
(3) The single rock mass in Zhongdian-Lijiang intermediate acid porphyry belt is complex in shape and small in scale. The main rock types are diorite porphyry, granodiorite porphyry, syenite adamellite porphyry, adamellite porphyry and granite porphyry, which are mostly distributed on both sides of regional faults.
2. Deqin-Basu altered rock distribution area
This area can be divided into two zones, one is the basic and ultrabasic rock zone of Lancang River, and the other is the upper reaches of Sanjiang River. The former is mainly located on the east side of Lancangjiang fault, and there are nearly 100 known rock masses, the distribution of which is consistent with the structural line, and most of them are NNW- near SN direction. The latter is dispersed and usually related to sulfate deposits in the oxidation zone of sulfide deposits in dry valleys. The altered rock belt in this distribution area has the following development characteristics:
Photo 1 1- 1 Features of Typical Altered Soft Rock in Yunnan-Tibet Railway
Figure 1 1-25 Geological background and distribution map of main altered rock belts in Sanjiang area
(1) Altered rocks are mostly located in structural faults or areas broken by structures.
(2) The color characteristics of a large number of supergene sulfate deposits and oxidation zones, such as brown, reddish brown and orange, and the distribution of pyrite, indicate that the formation of many altered rocks is related to hydrothermal alteration and supergene oxidation of sulfide deposits (veins).
(3) The alteration degree and altered rock properties are extremely uneven.
(4) According to the field investigation and original rock XRD test results, the altered igneous rocks are mainly intermediate-acid magmatic rocks, and the rock alteration is mainly plagioclase montmorillonite; Chloritization of biotite and amphibole.
(5) The altered rocks located in the oxidation zone of sulfide ore are obviously kaolinized due to the action of water (the relative content of kaolinite is 19% ~ 25%).
3. Distribution area of altered rocks in southern Tibet
According to the types of altered rocks, this area can be further divided into three zones (subtypes), namely, Himalayan high-temperature hydrothermal altered rock zone, Bomi regional large-fault granite medium-low temperature hydrothermal altered rock zone and basic ultrabasic rock montmorillonite altered rock zone.
(1) Himalayan suture zone of high-temperature hydrothermal altered rock belt belongs to China and even the world, with 43 hot springs and 42 boiling springs with temperatures higher than 80℃. Groundwater is rich in elements such as S, Cl and Na, and the hydrochemical types of hot water are: SO4 HCO3-Na, HCO3 SO4-Na and SO4 Cl-Na. Because the groundwater is rich in Cl-, kaolinite generally occurs in altered rocks, especially in andesite altered rocks in Riduo hot spring area, and the content of kaolinite reaches 28% ~ 55%. It should be pointed out that the action of hot fluid in the altered rock zone is uneven, and it is plane alteration along the fault, and the altered rock has poor properties (Figure 1 1-26, Figure 1 1-27).
(2) The quantitative XRD test results of hydrothermal altered rock zone in Bomi regional fractured granite show that the clay minerals in this altered rock zone are illite/montmorillonite mixed-layer minerals with high mixed-layer ratio, containing a small amount of illite and chlorite. Illite mainly comes from the alteration of potash feldspar, and chlorite is the alteration product of amphibole and biotite. Under the action of medium-low temperature groundwater, secondary CaCO3 is often distributed in fault zones. Because the rocks in the fault zone are relatively broken, and the minerals in the high mixed layer are larger than those in the I/S mixed layer, this kind of broken altered rocks usually have high physical and chemical activity and extremely low strength, which is easy to cause geological disasters in tunnel or slope excavation.
Figure 1 1-26 Profile of development characteristics of altered rocks in Riduo hot spring area, Tibet
Figure 1 1-27 Profile of Altered Rocks in Dezhong Hot Spring Area, Tibet
(3) Montmorillonite altered rock zone of basic ultrabasic rocks. This type of altered rock is represented by Qusong Hongqi chromite, and the surrounding rock of ore body (pyroxenite) has a high degree of montmorillonite alteration and poor engineering properties (photo11).
4. Influence of argillaceous altered soft rock on tunnel construction.
Due to the poor engineering geological characteristics of argillaceous altered rocks, serious rock deformation and failure problems often occur in tunnel excavation and slope excavation. For example, the surrounding rock of Heluoshan tunnel in Dali-Lijiang section of Yunnan-Tibet railway is basalt which has been altered by superheated liquid, and it is a relatively complete combination of basalt and montmorillonite altered rock. During the tunnel construction in 2005, there were five landslides related to montmorillonite altered rocks in the range of about 5 km from DK55+622 to DK6 1+7 10, sometimes even once when it was less than 100 m, and the landslide volume was generally 20 ~ 30m3. Because the strongly clayey altered rock mass is often in the form of soil or mud, field technicians often regard it as the product of tuff and its full strong weathering. In fact, the occurrence of landslide is mainly due to the very weak nature of montmorillonite altered rock, which is easy to expand and deform under dry-wet alternation and relaxation conditions. In addition, the joints of surrounding rock are developed and broken, and the self-stabilization ability after excavation is poor, which leads to the collapse of surrounding rock (Figure 1 1-28). In Tibet, because alteration is often accompanied by salt precipitation in the oxidation zone of sulfide deposits in the dry valley, not only the engineering properties of rock mass are weak, but also the dissolution and salt expansion of sulfate precipitation are important engineering geological problems, which make the problem more complicated.
Fig. 1 1-28 failure characteristics of altered rocks in the tunnel face of Heluoshan (DK6 1+235).
Field investigation and laboratory tests show that the distribution of altered rocks is similar to that of hydrothermal deposits along the railway. In the engineering geological investigation of altered rocks, attention should be paid not only to the distribution of hydrothermal deposits and regional hydrothermal altered rocks, but also to the distribution of hot springs and underground hot water, which can also cause long-term changes in rocks. Tunnel construction should mainly strengthen the prevention and control work from the following aspects:
(1) According to the development characteristics and distribution law of altered rocks in the engineering area, combined with previous geological survey results, the advanced geological prediction of tunnel engineering is made mainly through in-depth and detailed tunnel geological records and comprehensive prediction of horizontal drilling and TSP, so as to grasp the distribution of altered rocks and the changes of engineering properties in time.
(2) When excavating tunnel in clay altered rock area, advance support should be strengthened. According to the thickness and nature of altered rock, the length of advance support conduit should be lengthened, and the circumferential support spacing should be shortened in the exposed part of altered rock. Fully enclosed support should be implemented in time to stabilize the excavation face. For the radial anchor rod of normal support, it should be combined according to the length of surrounding rock. In the weak and broken rock mass area, the anchor rod can be properly enlarged and lengthened.
(3) In the distribution area of argillaceous altered rocks, the construction method should be reasonably selected, and the tunnel excavation should adopt micro-step method, with less blasting and more excavation to reduce disturbance. In addition, the surrounding rock of the tunnel in the altered rock distribution area usually changes greatly, and when TBM is used for construction, the machine may be stuck because of the large and uneven deformation of the weak surrounding rock. Therefore, TBM technology is not suitable for tunnel construction with a large number of such surrounding rocks.
(4) The material composition and structure of argillaceous altered rocks determine that the problems of tunnel collapse and mud leakage are more obvious when there is water. Therefore, in water-rich areas, the combination of blocking and drainage should be adopted, and curtain grouting or over-drilling should be used to divert water to minimize the interaction between tunnel surrounding rock and groundwater.