10th Asian Regional Conference of IAEG (2015)

An analysis of the geological conditions and the structural characteristics of the Changyu
Caverns
Dan YANG(1), Xiewen HU(1), Ying WANG(1), Wei Li(2)(1) Faculty of Geosciences and Environmental Engineering of Southwest Jiaotong University, China
E-mail:happytime827@163.com(2) China Research Institute of China Railway Engineering Company Limited, China
Abstract
The Changyu abandoned mining zone is a system of underground quarries that havebeen excavated for more than 1500 years. There are hundreds of caverns of different sizes
and the distribution form is complex. The caverns were mined for a long time without asystemic and scientific plan. Under the complex geological environment geological disasters
have resulted. Hydro-geological conditions, engineering geological conditions, structuralcharacteristics of the caverns and the rock mass characteristics at the Changyu abandoned
mining zone have been analyzed based on field geological investigation, theoretical analysis
and the finite difference method. The advantageous and disadvantageous factors wereanalyzed and summarized for the structural stability of caves. The structural stability,
mechanical characteristics and stress fields within the surrounding rock are calculated with
the FLAC in this paper. The geological conditions and the structural characteristics of the
Changyu abandoned mining zone are summarized in this paper, as a basis for the study ofthe caverns’ stability.

Key words：The Changyu caverns；Rock mass characteristics；Numerical analysis

1.

Introduction

underground caverns and the Longmen Grottos(LILi-hui et al., 2005; LI Li-hui et al.,2004; ZHANG
Chengyu, 2003; DING Wu-xiu et al., 2003).
The Changyu caverns are artificial and large
underground cavern groups formed by mining. Thereis an intimate relationship between the distribution andstructural characteristics of the caverns and geologicalenvironmental factors. But, it cannot be sufficient toreveal the mechanical characteristics in Changyu
caverns from the existing research results. For further
protection and development of Changyu caverns,
according to the site survey, the influencing factors ofcavern structure stability are analyzed and summarized,on the basis of the geological environment
surrounding the caverns and the distribution andstructure characteristics of the caverns.

Changyu Caverns, which was more than 1.55km2
of quarry area, were excavated for more than 1500
years until 1998 when the government determined tostop mining there because of long-term exploitation. In
recent years, due to the weathering of rock mass andother factors, the geological disasters of the Changyu
caverns are increasing year by year. In 1997 August,rock falls occurred in the Biyutan areas, which werethe most important events in recent years, ofcontinuous rock fall and other geological disasters.
There is a lot of research on the mechanicalproblems of underground cavern groups at home andabroad(Pan Bietong, 1992; Wang Zhongfen,2002;
Kawasaki S et al., 2001; He Yan et al., 2000; Wang
Dongyun, 1994). But most of these undergroundcavern groups are of small and medium size, while
research about large cavern groups is rare. Recent
research about large underground caverns mainly
concentrated on the famous Israeli Bet-Guvrin ancient
quarries(Hatzor Y H et al., 2002) and the world’s
natural and cultural heritage of the ancient salt
Wieliczka caverns in Poland(Zuber A et al., 2000). In
China studies focus on the Longyou ancient2.

Geological backgroundThe caverns researched is located in the southwest
and center of the Changyu, Zhejiang province, china .
the caverns is Huaxuandong cavern, Huaju cavern
and the Biyutan cavern, as Fig. 2.
2.1 The lithologic characteristics of the area1

10th Asian Regional Conference of IAEG (2015)

around the Changyu caverns
Changyu cavern is located in the bell-shaped
Mesozoic volcanic activities area of East Zhejiang,
and is in the middle of the Wenhuang plain. The strata
are mainly composed of Jurassic and Quaternary
gravel layers, and there are andesites distributed in thelocal mountain.
2.2 Geological structure
The geological structure of the caverns is simple. It
has few faults and the structural features are mostly
joints. The joints are compression-shear, and parts of
the joint combinations are conjugate. According to thestatistical results of measurements of the joints on theoutcrops, such as the bedrock of the slope and cavern,
the extended length of each single group generally
ranges from 5~50m, which is micro tension, is 2~10mm wide, and the density of joint rock mass isdifferent. According to the rose diagram of joint dip
orientation (Fig. 1), most of the joint dip orientation isnearly north (N) and SSW, and SE takes third place.

legend(N)

Fig. 1 The rose diagram of the joint rend of the
exploration area

Fig. 2 distribution of the caverns
Table 1 the physical and mechanical properties of rockDry
Dry
Compressive Softening
Strength of Elasticity Poisson
Dry specificcoefficient strength of extension modulus ratio
strengthdensity gravity compressive
Gs
Rw
KR
σtw
E
μ
extension
strength
Lithology ρd
g/cm3

Rd
MPa

MPa

σtd
MPa

MPa

GPa

volcanic
tuff

2.31

2.57

66.0

47.3

0.71

10.4

6.98

26.2

0.25

andesite

2.88

2.88

84.8

66.2

0.78

10.2

5.86

35.2

0.24

2

10th Asian Regional Conference of IAEG (2015)

2.3 Hydro-geological conditions
According to the field survey, the groundwater ofthe caverns can be divided into loose rock pore water,
bedrock fissure water and accumulation water in the
caverns.
The pore water in loose rock mainly occurs inloose pores of the mountain surface and slope foot, andthe aquifers are gravel soil with clay, and cohesive soil
with gravel. It is 2 ~6m depths, and the local depth is
12m. Because of the geomorphologic characteristics ofthe terrain, it cannot form a unified water table, thepore water in loose rock is downward slope runoff, andpart of it infiltrates the bedrock fissures.
Bedrock fissure water: Groundwater mainly occurs
in the rock mass structural fissure and the rock mass is
brecciated tuff with blocky structure. Joints andfissures are highly developed with poor connectivity,
so the bedrock fissure water is extremely poor quality.
Accumulation water in caverns: most of the part ofcaverns under the surface have been not backfilled,
and have accumulated groundwater, but the depth of
the accumulated water is not consistent, varying in therange of 10m~20m. Groundwater dynamics arecontrolled by seasonal changes, and are mainly
recharged by atmospheric rain.

and Huaju caverns, compared with the channel, thecaverns are larger, and most of these caverns are
small clear spaces. So the size of cavern controls thestability of the caverns. To this end, the structure andsurrounding rock of these caverns were analyzed.

Photo. 1 A cavern excavated by quarrying

2.4 The physical and mechanical properties ofrockRock mass in the caverns is mainly tuff. Part of itis volcanic clastic sedimentary rock and ground.
There is clay in the volcanic tuff, while the size of therock fabric is varied, from large conglomerate andbreccias to fine volcanic tuff and a few clay minerals.
The combination of these different components anddifferent particle sizes is arranged with no obvious
stratification, mostly in a blocky structure. Due to thecharacteristics of the rocks, it is not easy to failureload along the layers or split joints under vibration.
Through experiment, the rock mechanical parameters
are shown in table 1.
3.

Photo. 2 A cavern formed by rock block
stacking

Analysis of the characteristics ofthe cavern group

3.1 Analysis of the characteristics of the cavern
structure
During the long period of mining, the complex
structure of the caverns in the mining area of
Changyudongtian was formed, mainly including
Huaxuan caverns, Huaju caverns and Biyutan caverns.
The Huaxuan and Huaju caverns were well protected
because the surrounding rock of these caverns isrelatively stable. These caverns are very complex, andconnected by channels. Due to excessive mining, theBiyutan caverns were damaged completely by rockfalls. According to field investigation in the Huaxuan

Photo. 3 The cavern with water3

10th Asian Regional Conference of IAEG (2015)(1) According to the formation mechanism of thecaverns, this can be divided into two categories: One isformed by quarrying (see Photo. 1), and the other isformed by giant rock falls and accumulation (seePhoto. 2).These caverns were mainly formed byquarrying and connected by channels, and these form atunnel network. There are a few caverns formed by
giant rock falls and accumulation, and only threecaverns. The stability of these caverns controlled by
giant rock falls has less affect on the stability of thecaverns. So the stability of the caverns formed byquarrying is the subject of this analysis.
(2) The large cavern spans over 70 m, its height
more than 53 m. The diameter of the small cavern is 5
~ 8 m, and its height is more than 7 m, as shown in
Photo. 4. According to field investigation, the rockmass in the caverns is mainly tuff, mostly in block
structure. Joints and fissures are highly developed, so
the large-span cavern can be formed and be stable. The
cavern scale largely depends on the surrounding rock,and the better the integrity of the surrounding rock, thelarger the cavern can be.

3.2 Analysis of the characteristics of the rocksurrounding the caverns
As a stone mine, the rock mass structure andmechanical properties of Changyudongtian should bebetter. But in order to further evaluate the stability of
the caverns, the characteristics of the rocksurrounding the caverns are summarized andanalyzed.
(1) The surrounding rock of the caverns is
primarily volcanic tuff, with massive structure, andtuff structure.
(2) There are penetrating joints in the inner walls,
and most of these are closed joints, without
groundwater seepage. But when there are too many
joints, due to the joints cutting each other, the integrityof the rock mass has deteriorated. There is a lot ofwater in most of the caverns (Photo. 3).According to
the field investigation and analysis the water should be
mainly supplied by surface rainfall, so this can provethat the joints of the rock mass should be closed joints
and have poor connectivity.
(3) The walls of the caverns are approximately
vertical, due to limitations of the mining process. The
structure of the rock walls is irregular, and form steps.
Depressions usually appear (as shown in Photo. 1), andsometimes there are unstable rocks.
4.

Analysisofmechanics
characteristics
ofrock
surrounding cavern
Based on the analysis of the Changyudongtian
caverns, the geological environment conditions andthe structural features of the caverns, it has been shownthat the geological conditions are good and thestructural characteristics of the caverns are stable. But
due to other factors which impact the caverns, such as
excavation, weathering of rock mass and geologicaldisasters, the regional stress fields have changed, andthe structural stability of the caverns group havecertainly been impacted. Because of the collapsing of
the Biyutan, the rock mass close to the collapse area iswidely distributed, and a free face is formed. The
regional stress in the rock mass has changed, andstresses in the face have disappeared, so the stability
has decreased.
So, the cavern with a wall which is a free face, isthe most important cavern affecting the stability of thecaverns. The cavern near the collapse area with a freeface as a wall is being analyzing and calculation donewith FLAC(Fig. 3), a continuum model framework.
The cavern is 24m wide. The model is 168m wide
and 175m high, and the grid space is 1m. Because thecavern near the surface, we only considered thegravity stress. The surround wall of the cavern is
volcanic tuff, and the parameters of the model are
determined according to the table 1.

Photo. 4 The structure of bell-shaped(3) Most of the caverns are bell shaped, and thecavern cross-sectional size reduces gradually frombottom to top. As a structure of rock and soil in a mine,
the bell-type is more stable when affected by force. It
can remain stable without any supporting measures.
(4) Cavern distribution in the mine is dense with
only thin walls between some small caverns: thethicknesses of the walls are from 2m to dozens ofmeters. From the analysis of rock mass mechanics, due
to the cavern distribution in the mine, the internal
stress fields of the rock mass have changed greatly. In
some areas there will be inevitable cross plastic zones,
which is adverse to the stability of the overall group of
caverns. According to the field investigation, the rockmass is mainly hard with the saturated averagecompressive strength of 47.3 MPa, so the overall
caverns group should remain stable.

410th Asian Regional Conference of IAEG (2015)

4.1 Stress field of surrounding rockBy calculation, the natural state of thesurrounding rocks under the self-weight stress field isshown in Fig. 4,Fig. 5 and Fig. 6. The maximum andminimum principal stress distributions increase
gradually from the surface to the deeper levels of thecaverns. The characteristic of the major principal
stress on the boundary is approximately parallel to theboundary (as show in Fig. 6), and the minor principal
stress is close to zero. The stress concentration at thefoot and the top of the cavern is clear. In the range of
0.5 times of diameter of the caverns, the major
principal stress direction is close to the verticaldirection, and the minor principal stress is in ahorizontal direction. It was showed that in the naturalstate, due to the good mechanical properties ofsurrounding rock, the rock stress distribution
surrounding the cavern under the influence of cavern
formation is not significant.

Fig. 5 the minimum principal stressFig. 6 principal total stress tensors

4.2 Potentially unstable areas
The calculation results show that the actual
maximum shear strain distribution of the surroundingrock under natural conditions, as shown in Fig. 7, andthe maximum shear strain in the bottom of feet and top
in the local is relatively large, and is an easily
deformed area. A cavern potential fracture plane,
which are marked by the red line in Fig. 8, is locatedin the left foot at about 60 degrees obliquely upwardand in the ceiling position. It takes the generalcharacteristics of a shallow buried cavern.

Fig. 3 Profile of the cavern

5.

Conclusions(1) Changyudongtian caverns, abandoned after
being mined for many years, are surrounded primarily
by hard rock. Few joints developed, and these aremostly closed joints with poor connectivity.
(2) The regional tectonic conditions are
relatively simple. Faults are not developing, and theFig. 4 the maximum principal stress5

10th Asian Regional Conference of IAEG (2015)

References
DING Wu-xiu, CAI Li-peng, CHEN Jian-ping et al.
Study on Structural Features of Weathered Zone inLongmen Cavern[J]. Journal of Luoyang University.
2003，18(4)：79~82(in Chinese))
Hatzor Y H, Talesnick M, Tsesarsky M. Continuousand discontinuous stability analysis of thebell-shaped caverns at Bet Guvrin,Israe[J].
International Journal of Rock Mechanics and Mining
Sciences, 2002, 39(7):867-886.
He Yan，Li Zhiyi. A study of geological disease
analyses and renovation methods in Lingquan
Temple Grotto of Henan，China[J]. Rock and Soil
Mechanics，2000，21(1)：56–59.(in Chinese))
Kawasaki S，Tanimoto C. Deterioration phenomena
and conservation methods of stone Buddha images inJapan[J]. Frontiers of Rock Mechanics andSustainable Development in the 21st Century，2001，
5：413–416.
LI Li-hui，MU Hui-chong，YANG Zhi-fa et al.
Engineering geological conditions and diseases oflongyou large ancient underground caverns[J].
Journal of Hunan University of Science &Technology(Natural Science Edition)，2004，19(3)：18~22(in Chinese))
LI Li-hui，YANG Zhi-fa，YUE Zhong-qi，et al.
Deformation and reinforcement methods of ancient
cavern group in Longyou county[J]. Chinese Journalof Rock Mechanics and Engineering，2005，24(12)：2018-2027(in Chinese))
Pan Bietong. The corrosion diseases and control
methods of the carbonate rock mass in Longmen
grottoes. Cultural relic protection and environmental
geology. Beijing: China University of Geosciences
Press, 1992)
Wang Zhongfen. The Famous Caves and Grottoes in
China[M]. Beijing：China Travel and Tourism Press，
2002.(in Chinese))
Wang Dongyun. Analyses of factors affecting
destruction of rock mass by weathering in
Baodingshan grotto[J]. Journal of Engineering
Geology，1994，2(2)：54–65.(in Chinese))
Zuber A，Grabczak J，Garlicki A. Catastrophic anddangerous inflows to salt mines in Poland as related
to the origin of water determined by isotope
methods[J]. Environmental Geology ， 2000 ，
39(3/4) ：299–311.
ZHANG Chengyu. The Effect on the Rock Mass
Stability due to Hydro-Occurrence Environment inLongmen Grottoes of Luoyang[J]. Acta Scientiarum
Naturalium Universitatis Pekinensis. 2003，39(6)：829~834 (in Chinese))

Fig. 7 the maximum shear strain

Fig. 8 the maximum shear strain

structural feature is mainly the joint. Due to rock mass
joint development, and poor connectivity, bedrock
fissure water doesn’t develop.
(3) Although the chamber is large, and close toeach other. The cavern structure favors cavern stability,
and the integrity of the surrounding rock is good,
therefore the groundwater seepage effect is smaller.
(4) Because the cavern surrounding rockmechanics performance is good, the influenced rangeof the rock stress field is not more than 0.5 times thecavern span, and even in bad conditions the cavern cankeep stability.
According to the analysis, Changyudongtian large
caverns group under the present condition is stable.
Due to one thousand years of mining and rockweathering, there are unstable rocks mass in the local.
So, it is necessary to rectification.

Acknowledgements
The first author Dan YANG is grateful toprofessional Xiewen Hu and Ying WANG, who give
us suggestion. Also, the authors thank DaGe SONG
and Chris.

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