Geological Exploration and Route Selection in Karst Area of - Plateau of - Railway

Zhang Yu-jun and Yue Zhi-qin

China Railway ErYuan Engineering Group Co.ltd, Chengdu, [email protected]

ABSTRACT City to Guizhou Bijie City section of Chengdu- Guiyang Railway lies in the transitional zone from the southern margin of the Sichuan Basin to the northwestern of Yunnan- Guizhou Plateau. In the transitional zone, karst and karst water are generally controlled by a variety of factors, including lithology, structure, geomorphology, meteorological and water network. With the variation of natural geological conditions, the development degree of karst, the development pattern of karst , karst water distribution and enrichment degree of the karst water change accordingly. Through comprehensive analysis of the factors affecting karst, the study area can be divided into three karst hydrogeological units including the fold zone in the edge of Sichuan Basin, Sichuan Basin - transitional Yunnan-Guizhou Plateau, Yunnan-Guizhou Plateau.Since the end of the Cretaceous, the crust of the study area has experienced two big uplift and three leveled, three ancient karst erosion surface formed. Since the Pleistocene, the earth crust has experienced a number of intermittent rises that result in one to six terraces in the valley and the formation of corresponding multi-cave. The lithologic is the important factor that controls the development of karst; the Qixia Formation and Maokou Formation of Permian are most developed karst in fifteen karst horizons, and the underground river system are in the largest scale, then follows the Triassic; in alternating or interbedded layers of the non-karst and karst, the features of Karst is bedded, and hydraulic connection between layers is weak; The accumulation of karst water is mainly affected by the karst development degree, landform, lithology and geological structures. The results of this study may provide guidance for geology selection of railway and highway in complicated karst mountainous.

KEYWORDS: Karst pattern, Hydrogeological unit, Water abundance, Karst partition

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OVERVIEWS ON PHYSICAL GEOGRAPHY

Geographical location.The section in Chengdu-Guiyang Railway, from Yibin city (Sichuan Province) to Bijie City (Guizhou province), goes through Yibin city, , , and ends in provincial boundaries between Guizhou province and Yunnan province. This railway section, about 140km long, goes across Nanguang River located in the junction of Sichuan and Yunnan province, Chishui River watershed, Chishui River which lies in the junction of Yunnan and Guizhou province and Wujiang River watershed. The study area is about 12710km2.

Geomorphology and evolution. Study area is located in the transitional zone of Sichuan Basin and Yunnan-Guizhou Plateau. The geomorphic conditions in this area from north to south include River valley, hills, low mountains, low-medium mountain. The topographical features show significant stratification which has five planation surfaces from a macro perspective.

Since the end of the Cretaceous to Oligocene, the crust in the study area is in a relatively stable stage, and landform contour have taken shape. During that time, crustal uplift constituted today's Ⅰ ，Ⅱ planation surface. In Oligocene - Miocene, the Himalayan orogeny occurred; the crust rose sharply, prompted active erosion; gradeⅠ,Ⅱplanation surface suffered severe damage and split; so far, the first large cycle of erosion in the study area completed.

Pliocene to Early Pleistocene, the crust which were razed for the second time got into a relatively stable stage. During that time, The second phase of the Karst effect was strong, and non-karst area was subjected to strong chip flow erosion, resulting in early planation height decreasing and its breadth reducing. During the second raze period, the crust uplifted for a time to form Ⅲ，Ⅳ planation surface. In early Pleistocene, the crust again rose strongly; surface erosion revived; canyon began to form; the Karst effect turned to vertical; near the valley karst depressions, dissolved network disintegration generated valley or natural bridge, underground stream. So far, the second largest cycle of erosion in the study area came to an end.

Since then, the earth crust has been in a relatively quiet period , and the razing period started again. This district began to form a large number of red hills, karstic erosion effects transferred to horizontal action. Between the valley, many karst depressions, funnel, karst valleys and many vertical slope caves formed. In the meantime, the northern grade Ⅰ，Ⅱ planation surface has been eroded away; the south has become increasingly narrow, only few isolated peak residual surface left. Grade Ⅲ, Ⅳ planation sueface also suffered splitting, destruction and disintegration, but it is still good in the south. In fact, since the Pleistocene, the crust has been rising intermittently on several occasions, resulting in the formation of one to six terraces in the valley and the corresponding multilayered caves. Major canyon developed in deep direction; rivers were subjected to southward traceability erosion; slope gradient increased; the number of splitting points increased, leading to the formation of underground rivers. Vol. 24 [2019], Bund. 01 103

Table 1: The classification system of geomorphic units Microgeomorphology Altitude Relative Slope Geomorphic type Remark unit (m) elevation (m) (°) Accumulation of Terrace river erosion Low mountains valleys ＜1500 200-500 Erosion structure Ridge low-medium ＜1500 500-1000 mountain Hill low mountains ＜1000 ＜200 ＞25 Erosion Trapezoidal medium- Threshold＜ denudation ＜1500 low mountain 200,Mesa＜20 Trough valley ＜50 ＜25 Gentle hill depression ＜200 ＜25 Medium-low mountain 1000-1500 ＞200 series of peaks Series of peaks valley ＞25 Canyon low-medium V-shaped 1000-1500 ＞200 ＞25 mountain valley Dissolution Honey-comb tectonic Series of peaks ＜100 texture Medium-low mountain of Parallel ridge Valley Parallel 1000-1500 500-1000 ,Parallel with river ridge valley valley Parallel ridge valley Parallel 1500-2500 >1000 medium-high mountain ridge valley Low mountain ＜1000 ＜200 Erosion Valley medium-high V-shaped 1500-2500 ＞200 mountain valley Meteorology and hydrology. Research area belongs to subtropical humid monsoon climate with abundant rainfall. The seasonal rainfall is uneven in a year, with average annual rainfall of 800 ~ 1300mm, the maximum of 1341.9mm in , the minimum of 800mm in Weison County. There is a decreasing trend from north to south. The rainfall mostly concentrates in May to September, accounting for about 75% of the annual rainfall.

The rivers in the study area belong to Yangtze River region, including Nanguang River, Changning River, Yongning River, Chishui River, Niujie River and Liuchong River. Overall, this rivers flow into the Yangtze River from south to north . These rivers originate in the mountains with mountainous features of steep, narrow channel, deep cutting, big drop, cutting depth, many shoals and rapids. The average total amount of runoff is 178.9╳108 m3,and the average runoff depth is 768mm. Vol. 24 [2019], Bund. 01 104

Figure 1: Schematic of railway plan and distribution of karst in Yibin to Bijie section of Chengdu-Guiyang Railway

Stratum and structure

Formation lithology.The study area belongs to Yangzi stratigraphic area, partition of the South China stratigraphic area. The exposed strata include Cambrian ( ) dolomite, limestone, sandstone, shale; Ordovician (O) shale, dolomite, limestone; Silurian (S) sandstone, shale interbedded with ∈ limestone; Permian (P) limestone, sandstone, shale; Triassic (T) dolomite, limestone, sandstone, shale; Jurassic (J) sandstone, mudstone; not- well-developed Quaternary (Q) . Where in there are 15 groups of karst (see table 2) and 14 groups non-soluble rock. The cumulative total length of karst is 53.342km, accounting for 38% of the total line length. Of those, the karst In Jurassic, Permian and Triassic developed most well and distributed most widely.

Table 2: Karst formations Statistics Erathem System Series Junlian、Xuyong area Zhengxiong area Weixin area Middle T l T f、T g T sh、T s Mesozoic Triassic 2 2 2 2 2 Lower T1j T1y T1m

Permian Lower P1m、P1q P1m、P1q P1m、P1q

Silurian Lower S1s S1h S1s

Upper O3w、O3l O3g、O3w、O3j O3w、O3j

Paleozoic Ordovician Middle O2b、O2s O2b、O2s O2b、O2s

Lower O1h O1h O1h、O1t

Middle-upper ∈2-3ls ∈2-3ls ∈2-3ls

Cambrian Middle ∈2g ∈2g ∈2g

Lower 1q 1q Geological structure.The study area is located between the∈ Sichuan-Yunnan∈ longitudinal tectonic belt and Sichuan-Guizhou latitudinal tectonic belt. The study area connect with Guizhou Vol. 24 [2019], Bund. 01 105

Central Uplift which lies on the northern edge of Nanling east-west complex structure belt in the south, and with Sichuan Basin which is a primary sedimentation belt of new China structural system in western China in the north. It is the edge of a settlement belt controlled by fold. Since the yanshan orogeny, the earth crust has changed for three times. Repeated stress action caused tectonic deformation. They not only retained their own features, but also interfered, interspersed, used or modified with each other, leading to the formation of various forms of joint and complex relationship, making structural features of the study area tend to be complicated. According to the mechanical properties of the main structural features of the region and generated links, the study area can be divided into three types including meridional tectonic system, latitudinal tectonic systems and shear tectonic system. Most of the railway line pass through the structural line in big intersection angle.

Characteristics of karst water

Supply source.The karst water supply mainly come from atmospheric precipitation, followed by surface water, and then bedrock fracture water and mine water. Different karst landforms have different supply sources and characteristics. In karst Valley, because karst terrain is negative terrain, so the two sides of mound mostly belong to non-karst stratum, in which the main supply source is fissure water. In karst mountains and karst canyons, because underground erosion is more fast than surface erosion, so a lot of lateral rivers, streams in the karst top and bottom go directly into the underground as underground stream, which especially have become important sources of karst water supply in the dry seasons. This kind of perennial surface water not only plays an important role in the development process of karst, but also makes the low flow modulus of karst water in this area is a little large.

Runoff and discharge conditions.In the study area, the main karst structure is fold-type with rich water; karst water runoff is alternately in the form of ground-flow or underground-flow; runoff direction is generally longitudinal and lateral runoff is rare; circular runoff can also be seen in the synclinal basin in good restrictive hydraulic condition.

The main discharge patterns include springs, underground river, and so on; discharge ports mainly locate in rivers, the middle or the side of creek; in low-lying areas, discharge ports can also be seen at the junction parts with changing slope, and the contact parts between karst rock and non-karst rock. some karst water flow along the tectonic cracks or ancient corrosion fissures to the deep underground. Centralized and decentralized discharge are the main forms of discharge pattern of regional Karst water; the centralized discharge zone have one to several underground river exit points and springs, and the decentralized discharge has not yet formed a large and long underground river from the watershed to the river valley, and it has been cut by many gullies and creeks, resulting in many springs scattered exposed. The elevation of different discharge ports has big differences, which is generally less than 20m high above the water surface. In a non-karst region, some of the drain port depend on the karst floor elevation, others on the underlying non-karst layer, and the drain ports are Vol. 24 [2019], Bund. 01 106 hundreds of meters high above the water surface. Migration of groundwater reservoir can be categorized into two different types, karst spring systems and underground river system; for the first type, groundwater with laminar flow dispersed among the karst fissures and small piping systems; the second type is the underground river system (centralized discharge); groundwater concentrated in underground pipes with turbulent motion features; the groundwater sometimes has river features of rapid fluctuation. The former is relatively stable with respect to dynamic changes.

Physical and chemical characteristics of karst water.Physical and chemical characteristics of karst water are mainly controlled by lithology and geomorphology, then by groundwater recharge, runoff and discharge conditions. Strong karst mountain folds suffered intense cutting; groundwater circulation alternate rapidly. Karst water is light mineralized water; its total hardness (as CaCO3) is generally less than 300mg / l, and PH value is generally 6.5 to 8.0.

Characteristics of rock formation with karst water

Leikoupo and Jialingjiang of Triassic.Lithology include thin to medium thick-bedded limestone and dolomitic limestone, breccia folder salt and salt-solution breccia. Karst is well developed, and karst topography are primarily depression , funnel, sinkholes, caves and other; karst water is rich with both large pipe flow and lots of karst springs discharged along karst crack. The underground river flow is about 200 ~ 1500 l / s ; the spring flow is more than 1 ~ 50 l / s; groundwater runoff modulus is 3 ~ 21 l / s • km2; gushing water of single well is about 100 ~ 1000m3 / d.

Maokou,Qixia,Liangshan of Permian.This kind of rock group is the main water-bearing formation, made up of thick, pure quality limestone; karst is developed intensely, and form series of peaks, depressions, valleys and other karst landforms. In the underground it forms large multi-caves, and constitutes dendritic and radial piping system. Karst water is abundant; springs and river flow are mostly 150 ~ 700l / s. Groundwater runoff modulus is of 7 ~ 19 l / s • km2; single well inflow is 100 ~ 2400 m3 / d.

Loushanguan,Gaotai of Cambrian.Lithology is mainly dolomite, siliceous dolomite; karst is slightly developed; the ground surface mainly forms dissolve ditch and melting tank; underground forms corrosion cracks; karst water quantity is relatively poor;dissolution fissure spring is the main form, with spring flow less than 10 l/ s.

Feixianguan of Triassic, Changxin and Longtan of Permian.Lithology is mainly sandstone, mudstone limestone and argillaceous limestone. Karst is relatively developed in the east portion; spring flow is generally 1 ~ 10 l / s; groundwater runoff modulus is 1.1 ~ 1.5 l / s • km2.

Silurian Middle-lower and Ordovician.Lithology is mainly various clastic rocks, shale rock; karst development is limited due to the small thickness of soluble rock; the main type include dissolved gap or small cave. The spring flow is generally 1 ~ 10 l / s; groundwater runoff modulus is 2.7 ~ 7.0 l / s • km2. Vol. 24 [2019], Bund. 01 107

Characteristics of burial, occurrence and accumulation

for karst water

Characteristics of burial and occurrence for karst water.For the karst water occurrence (storage) recharged mainly by rainfall infiltration, there are two main forms as the following. One is the underground river system in the pipe-shaped karst area, which can be seen in Maokou (P1m), Qixia group (P1q), Leikoupo group (T2l), Jialingjiang group (T1j), Maocaopu (T1m) and other pure carbonate formation; the size of pipeline system is related to karst landform; most karst mountains and karst gorge have relatively big pipeline and many branch forks; underground river runoff length is about 4000 ~ 10000m; in karst valleys, karst pipes are small and less, and underground river runoff length is only about 4m long. The second form is the dissolution pores and fissures, which formed in dolomite based Loushanguan Group ( 2-3ls) and Ordovician (O), Silurian (S), Permian Longtan Formation (P l) and Changxing group (P c) formation. In karst mountain, when karst is not very 2 ∈ 2 developed in the local area, part of Leikoupo (T2l), Jialingjiang Group (T1j) formation have intensive corrosion crack as water channel, and more in the form of spring.

Karst water can be divided into bare karst phreatic water and buried karst confined water according to their burying condition; in this area, exposed karst phreatic water are widely distributed in the area where carbonate rocks outcrop; the buried depth of the water level are greatly affected by topography changing. In karst valley, karst vertical leakage belt (vadose zone) is thin with the water depth generally less than 20m. In karst mountains near the watershed, vertical karst are well developed, where vadose zone is thick with water buried depth up to 100 ~ 200m or more; while the water buried depth gradually become thin in the direction of close to river valley, generally around 20 ~ 100m. In karst gorge area, karst water have to adapt to the erosion base surface as soon as possible, resulting in violent dissolution in vertical direction; karst water is close to the base level of erosion, and the vadose zone is thick with water depth of generally 50 ~ 200m. Buried karst confined water mainly occur in the part of the fold structure and plunging syncline, such as Muai, Luobiao and other regions, where some table aquifer roof depths are less than 80m, but others are more than 20 m above the ground surface.

Occurrence characteristics of karst water controls the distribution of the karst water which is extremely uneven; this phenomenon is especially apparent where underground rivers exist. For example, in Jianzhuping region of Gulin adjacent to the study area, the drilling results reveal that in 3 Maokou (P1m), Qixia group (P1q) karst water gushing volume is less than 140m / d, but in Yudong and Yandiwan, underground river outlet flow is 527.36 l / s, 338.772 l / s, respectively. But in some wide gentle karst valley, karst water occurring in fractures and dissolution pores is more evenly; the exploration tests confirmed that there may be some hydraulic relationship between drill holes, drill hole and karst springs, drill hole and underground rivers. Vol. 24 [2019], Bund. 01 108

Characteristics of accumulation for karst water

The enriching feature of karst water are mainly affected by the development degree of karst, geomorphology, lithology and geological structure. The main features are as follows.

In strongly developed karst area, large-scale underground pipeline system forms easily, which is a good place for karst water gathering; in the area where Maokou (P1m), Qixia group (P1q) and other pure carbonate rocks distribute, karst is most developed, and karst water enrich easily.

Low-lying areas have good catchment conditions conducive to the karst water accumulation. For example, The basin in mountains close to the Daba town, 450m elevation, are surrounded by debris rocks with elevation of 900 ~ 1400m. Karst water of Jialingjiang Group (T1j) and Leikoupo (T2l) group all converge into the basin bottom; there are seven underground river outlets with different size and karst springs, and the total water flow is about 2850 l / s; the famous Grand Yudong, Xiaoyudong underground river are all here, and areas of Gongxian County Xunchang, Xunsi etc. are also similar.

Karst water discharge is generally controlled by Lateral river banks, which are karst water collection sites.

The karst water accumulate easily in the contact portion between carbonate rocks and debris rocks, because karst is highly developed in the vicinity of such a contact surface, where holes and tubes form easily and karst water gather. In the study area, this situation can also be seen on the top and bottom of carbonate rocks, which belong to Jialingjiang group (T1j), Leikoupo group (T2l),

Cambrian ( ) to Permian Changxing group (P2c).

Anticline∈ core portion is broken and fractured, so karst water is conducive to accumulate. YiLe anticline at Zhenxiong county which the railway go through, is a close linear inter-fold, developing compression-twist fracture in the core and a lot of vertical and layered cracks; the terrain is gentle hill valley, and the impermeable debris rocks in two sides are conductive to gather karst water; natural spring water flow is 52 l / s; in addition, some of karst are strongly developed in nose type anticline, such as Xunsi nose anticline, Junlian nose anticline; watershed conditions on the landscape are good, and karst water is highly concentrated.

Anticline core, plunge end, syncline tilted end, junction and composite parts of fold structure belong to stress concentration zone; fractures and cracks are well developed, often forming rich water zone of fracture. For example, Yudong crack band is a intensive fracture development zone in the turning portion of a fold; Maokou (P1m), Qixia group (P1q) limestone with gentle inclination, forming Karst Series of peaks and depression landforms; karst water appears piped enrichment, and river flow rate is 5m3 / s.

Karst water runoff has a strong selective process; karst water gather easily along the fault, resulting in rich water fault. For example, it can be seen in a branch fault of the Zhuchaojin fault. Vol. 24 [2019], Bund. 01 109

When the hanging wall and footwall of a fault are carbonate rocks and debris rocks respectively, carbonate rocks are more likely to gather water. This is similar with the aforementioned contact area of debris and carbonate rocks water. For example, the northwest wall of Yinquanwan Tuaokou fault is Lower Permian limestone and southeast wall is Silurian rocks; debris rocks tend to be impermeable, but limestone rocks tend to gather groundwater easily; the maximum flow of spring is 49.68 l / s , and the total flow of four broken springs is 62.2 l / s.

Table 3: The statistics of the groundwater property of the aquifer Water classification index Runoff modulus of Groundwater Water Distribution 2 Water inflow of drilling ( Area Aquifer 2 Spring or groundwater l/s ◌ּ km Type yield area (km ) groundwater Yearly Dry Rainy (flow (l/s) mean Q(T/d) q(T/d◌ ּ m season season runoff Kast-fissure and Rich P1 100~1000 101.03 cavern water of Moderate T2l、T1j 10~100 24.84 Carbonate rocks Poor ∈ <10 19.64 Junlian Kast-fissure and cavern water of Clastic and Poor O2＋3、S1s、S1l <10 17.16 carbonate rocks interbedded

Rich T2l、T1j、P1 100~1000 44.68 Kast-fissure and Moderate T l、T j 10~100 31.78 cavern water of 2 1 T l、T j、∈、 Carbonate rocks Poor 2 1 <10 1.62 O1t、O1h Xuyong Kast-fissure and Moderate O2＋3、S1s、S1l 10~100 18.24 cavern water of Clastic and 、 carbonate rocks Poor T1f P2l <10 8.53 interbedded 5.83～ Rich P 510 100~1000 7.85 1 11.2 Kast-fissure and T2sh、T1m、P1 1.22～ 625.96～ 27.39～ cavern water of Moderate 2297.19 10~100 3.56 、∈ Carbonate rocks -O1h 7.96 641.8 45.78 1.17～ 258.6～ Poor T m、∈ -O h 590.9 <10 2.42 6.48～11.92 Weixin 1 2-3 1 3.24 385.3 Kast-fissure and cavern water of T s、T s-T sh、 Clastic and Poor 2 2 2 1982.08 <10 1.57 146.88 4.2 、 carbonate rocks P2- T1y O1m- S1s interbedded

Rich P1 100~1000 1～2.6 32 190 30412 Kast-fissure and T f、T g、P 、 cavern water of Moderate 2 2 1 10~100 36～92 9.6 1～2 1123～3974 Carbonate rocks O1h Poor T1m、∈2-3-O1h <10 Zhenxiong Kast-fissure and cavern water of 、 、 T2g T2g-T2f ～ Clastic and Poor 、 <10 0.74 0.1 0.74 carbonate rocks P2- T1f O1m- S1h interbedded Vol. 24 [2019], Bund. 01 110

INTRODUCTION TO KARST GEOLOGY

Distribution characteristics of karst development.The karst in the study area can be divided into exposed type, buried type and covered type according to its burial and distribution conditions. From karst development point of view, exposed karst is the most developed one, buried karst and covered karst follows. The three karst types all have the hidden karst form. Of Which, exposed karst distribution area is 7060km2, accounting for 55.5% of the total area of the whole region, and the total amount of karst groundwater recharge is 1,650,300,000 m3 / y. And different combinations and different spatial lithological composition also lead to great differences in the degree of development regarding exposed karst, but buried and covered karst type have little difference. Depending on the development of karst, the study area can be divided into four types including strong growth, moderate growth, weak growth, less-weak development.

lithology factors influencing karst development.The primary factors affecting the karst rocks are the chemical composition in carbonate rocks; the content of soluble calcium, magnesium, etc., and the proportion of acid insoluble content directly controls the speed and intensity of karst. The former can improve the level of karst development, and the latter may reduce the porosity of carbonate rocks with its increasing content in the rock, thereby affecting the intensity of karst. According to carbonate rocks lithology, chemical composition and the combinations of non-karst, the karst formations of carbonate rocks has been divided as shown in Table 4.

Table 4: Karst development degree relationship with lithology statistics Karst The groundwate Area Proportion Proportion degree r recharge Lithological Rock group of karst Exposed strata resources character develop Hundred 2 ment (km ) （﹪） million （﹪） stere/ year Limestone, Purecarbonate T l、T j、P m、 Strong 2 1 1 Dolomitic 2970 42 11.239 68 rock P q 1 limestone T f、T sh、S s、 Impure 2 2 1 Clastic rocks Medium O、 ls、∈ g 1460 21 2.763 17 carbonate rock 2-3 2 with Carbonate 、 1q、 Arbonate ∈ P c、P l 、∈、 Clastic rocks rockwith clastic Weak 2 ∈1 1345 19 1.482 9 S l、Om+t with Carbonate rock 1 Non T f、S h、∈ m- Mudstone,Shale, Clastic rock karst 1 2-3 l 1285 18 1.019 6 j、J、K Sandstone areas Total 7060 100 16.503 100

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District characteristics of karst water

Karst features of fold zones on the edge of Sichuan basin:This study area mainly locate in parts of the area of Xingwen County, Gongxian County and Junlian County, as well as in northern Yunnan Weixin County.Landforms mainly include erosion low mountains and karst erosion troughs; a small amount of the geomorphy are series of peaks low mountains by karst erosion, gentle hill depressions by karst erosion, low mountains valley by erosion, mesa-like low- medium mountains by erosion, ridge-like low-medium mountains by erosion. The study area locates in the Changning duplex anticline and are adjacent to fold belt; the southern is Dayuanzi anticline; there are Yinyangbei fault, Xingwen fault and Zhuchaojin fault. The geological conditions lead to the broken rock formation. Karst aquifer rock group and the lithology are mainly Triassic Leikoupo and Jialingjiang Group, Feixianguan Formation, Permian, Ordovician and Cambrian limestone, and there are three ancient karst erosion surface. In view of river basin, Changning River, Yongning River and Nanguang River are included; These rivers flow from south to north into the Yangtze River, water system in a dense network. Surface water become the local base level of erosion area, elevation 300 ~ 500m, belonging to the planation surface in basin stage. Study area is generally at an elevation of less than 1000m; some individual ridges are in elevation 1200 ~ 1300m, belonging to the planation surface in three basin stage, and the highest point of the terrain is Wudingpeng, elevation 1497m.The study area has all types of Karst forms with distinctive characteristics. The mountain is a typical exposed karst regions; karst is well developed; karst groundwater has obvious unevenness; karst forms mainly include underground river, water caves, karst shaft, collapse, water-filled karst funnel, karst springs and others. karst trough is a typical covered karst, covering layer thickness ranging from a few meters to 10 meters; there are a relatively uniform karst; underground river, karst water filling funnel are well developed; many underground river outlets are present in the trough area, such as Xingwen County Nanba, Gaoxian County suburbs and other exports, reflecting the karst river water discharge controlled by the modern river base level of erosion.The study area has large scale karst, rich in karst water. Such as the Xiaoyanwan sinkhole in Xingwen (650 meters in long diameter, 480 meters in short diameter, 208 m in depth), Yuanjia-dong (the widest width up to 162m, the maximum difference between roof and floor elevation 70 meters), and this kind of scale is rare in the world.

Sichuan Basin - karst features in Yunnan-Guizhou Plateau transition zone:The area mainly locates in Weixin county and Zhenxiong county of Yunnan province, Xingwen county and the southern region of Junlian county of Sichuan province. Railway elevation ranges from 330m to more than 1700m; elevation difference reaches 1370m; average slope reaches 26 ‰.The landforms in the study area mainly include erosion low mountains, medium-low mountain of dissolution tectonic parallel ridge valley, medium-low mountain series of peaks by dissolution tectonic, parallel ridge valley medium-high mountain by dissolution tectonic, valley medium-high mountain by erosion karst and ridge-like low-medium mountains by erosion. Regional folds include Xinjie anticline and others; the main faults include Weixin fracture, Zhangjiabian fracture, Chayuan fracture, Sanqiutian fracture Vol. 24 [2019], Bund. 01 112 and Dianqi-Lutang fracture, etc. Karst aquifer rock group mainly include Triassic Shizishan group, the Jialingjiang group, Feixianguan Formation, Permian, Silurian, Ordovician, Cambrian strata, etc. There are three ancient karst erosion surface. Watershed basins include Nanguang river, Chishui river and the upper reaches of Niujie river; because of this band locates in the transition zone of the Yunnan-Guizhou Plateau and Sichuan Basin, steep mountains are steep, and hydraulic gradient is large due to the present of groundwater recharge in the runoff area; groundwater is mostly developed in stepwise condition; water system is in dense network, but it is not fully developed; there are generally underground rivers in the upper streams of rivers.The study area almost has all types of Karst forms with obvious characteristics. The medium and high mountains belong to exposed karst regions; some individual troughs are covered karst area; the segments in low mountains through the anticline plunges are buried karst regions. Karst is well developed, and the main forms of karst are rivers, a large number of pipeline-like caves and sinkholes tapered. Numerous river outlet are present in the valley slope, such as in Longli dam in Weixin county.The topography of the study region is quite uneven, but the overall elevation of the ridge line is 1600 ~ 1800m, belonging to the planation of Xinjieping stage forming from end of Daloushan era to the early the Quaternary. In Sichuan Province,it is generally 1600-1800m elevation planation in Maxianbao period; the underground river entrance elevation is up to 1150->1480m, and export elevation is 800 ~ 1480m; pipe-shaped cave entrance elevation is higher than 1700m; conical cave sinkholes elevation is 1290 ~ 1530m.

Karst features of Yunnan-Guizhou plateau:The main area locate in Zhenxiong county and Bijie city, Guizhou province.Geomorphology in this region mainly include depressions Series of peaks by dissolution tectonic and Medium-low mountain Series of peaks by dissolution tectonic. Folds and faults are developed; folds include Dazhuo syncline, Chishui River syncline; faults are mainly Guanchang dam rupture and Guanmen mountain rupture. Karst aquifer rock group are mainly Triassic, Permian, Silurian, Ordovician and Cambrian limestone; there are three ancient karst erosion surface. River basin are composed of the Chishui river basin and Liuchong river basin, and they all flow into the Yangtze river; water system distribute as dense network; surface waters become the base level of erosion in the area.There are almost all types of karst forms with distinctive characteristics in this area. It is a typical exposed karst regions; some sections are buried karst and covered karst. Karst is well developed; karst forms mainly include underground river, karst depression and skylights. Numerous river outlet are present in the valley area, such as Huayudong town in Zhenxiong county, Yile town and other places.The study area is a typical Yunnan-Guizhou plateau with high terrain and relatively small elevation difference of usually 500 ~ 800m; ridgeline altitude is 1600 ~ 1800m; it is a razed plane formed after Yanshan orogeny, early Tertiary. After the action of karst and erosion denudation for a long time, karst is well developed. Since the Yunnan-Guizhou Plateau uplift sharply, the erosion rate is lower than the dissolution rate; the lower part of the region cut are not deep enough, so that the degree of development of the large karst are mainly affected by erosion, leading to the formation of large enclosed karst system, such as Tianbatou anticline, Shikan syncline, and Yile anticline,etc. Karst water often cross the non-karst river in the form of surface river, or they are Vol. 24 [2019], Bund. 01 113 uplifted by the underlying non-karst layer, resulting in surface water outlets are often tens to hundreds of meters above the river surface. The river entrances in this area has the highest elevation of 1800m, and exports elevation is mostly about 1600m; The elevation of karst depression and skylights is about 1500 ~ 1800m.

Line selection by geology

In view of railway construction, the main engineering geological problems caused by karst are the influx of karst water into tunnel or foundation pit, soaking roadbed. Karst caves often cause roadbed collapse and affect the stability of tunnel surrounding rock and pit slopes. Railway line selection in karst region should follow the following principles. Select less soluble rock shale, marl, sandstone and others to bear railway as far as possible; in karst areas, try the best to choose a site with thick covering, filled karst caves or less developed karst area using the shortest route; avoid possible structure fracture zone, faults, densely fracture zone, intense karst development zone with good conditions of water alternating; avoid the contact zone between karst rocks and non-karst rock.

In the karst hydrogeology area located in the fold zone in Sichuan basin, karst are mainly horizontal, large scale, and have abundant water, such as the water inflow of a single-line railway is more than 500,000 m3. The recommended line scheme, avoiding karst region as far as possible, mainly go through the Jurassic, Cretaceous, Triassic and other non-karst segment, crossing the Permian karst development zone using bridge, embankment, tunnels and other shallow buried short tunnel.

In the karst hydrogeology area located in the transitional zones of Sichuan Basin - Yunnan- Guizhou Plateau Karst, there are mainly vertical karst developed along the strike direction of rock formation and the axial direction of structure; Triassic karst have layer formation; the elevation of railway is mainly depended the relative elevation crossing the underground river.

The karst hydrogeology area in Yunnan-Guizhou Plateau have both horizontal and vertical karst forms; the railway mainly go through the weak developed karst zone; lithology, geological structure and watershed method are combined together to select the railway line; using high line scheme ( more than 220m higher than the original line scheme); avoiding Tianbatou anticline, Shikan syncline, Yile anticline and other closed karst systems as far as possible; avoiding the Tongche river, Yudong river and other karst water discharge passages; decreasing the line elevation so that the line can stretch in the Triassic Feixianguan non-karst rocks. The line almost bypass the Permian karst development zone.

Vol. 24 [2019], Bund. 01 114

CONCLUSIONS

From macroscopic view, Yunnan-Guizhou Plateau slope area along Cheng-Gui railway can be divided into three geological units including the folds on the edge of Sichuan basin, transitional zones of Sichuan basin, Yunnan-Guizhou plateau; each unit containing geological, hydrological, meteorological and other natural conditions have a great difference; this differences regarding the degree of karst development and karst spatial appearance are more obvious.

The degree of Karst development and karst water enrichment in each natural geological unit are influenced by types of karst formations, geological structure, karst site (valley, watershed and valley slope, etc.), karst history; there are also relatively big difference; through the analysis of influence factors on karst development, karst region can be divided into strong, medium, weak, less-weak and other regions, contributing to the decision of a reasonable rail line scheme.

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[14] Rao Jun-ying, Fu He-lin, Liu Yun-si, Yin Quan, and Zhang Hai-ming: “Model Test Study on In-service Karst Highway Tunnel” Electronic Journal of Geotechnical Engineering, 2013(18.A): 135-146. Available at ejge.com.

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Vol. 24 [2019], Bund. 01 116

Editor’s note. This paper may be referred to, in other articles, as:

Zhang Yu-jun and Yue Zhi-qin: “Geological Exploration and Route Selection in Karst Area of Yunnan-Guizhou Plateau of Chengdu-Guiyang Railway” Electronic Journal of Geotechnical Engineering, 2019 (24.01), pp 101-116. Available at ejge.com.