10th Asian Regional Conference of IAEG (2015)

Structure Characteristics of Neogene and Engineering Response to Deep Excavation

Qinghong DONG, Hao YANG, Qi LU and Qiang ZHANG
School of Resources and Geosciences, China University of Mining and Technology, China
E-mail: dongqh@cumt.edu.cn
Abstract
Due to the impact of lithology, composition, groundwater and other factors, theformation of Neogene appear more differences in structure in different sections and depth in
Xuchang coalfield, China. After the hydrogeological structure, broken and unconsolidated
layers discussions and deformation simulations, the structural characteristics of Neogene
and its effect on deep excavation are analyzed. The main understandings are list as follows:
1) The Neogene system in this areas consists of sandstone and mudstone, which are thebasis of aquifers and aquifuges in macroscopic view. 2) The structure of the thick Neogene
controlled by weak structural segments, soft stratum and bedding faces. And this changes
the hydrogeological structure. 3) Model tests reveal that the soft-hard interlaying andstructure network control the deformation of Neogene during mining. The failure zones
appear in coal roof and the bottoms of Neogene system have some similar characteristics.

Keywords: Structural Characteristics, Neogene, Deep Excavation1.

the argillaceous sediment has good water-resisting
properties to cut off the hydraulic connection between
water body(s) and aquifer(s). But the discontinuity
and weakness of Neogene strata are still obstacles tojudge its engineering ability of water-resisting.
We take a coal field in Yuzhou, Henan Province
as an example to reveal the distribution and structureof Neogene, and its effects on deformation.

Introduction

Neogene system is composed by Miocene andPliocen, which was confirmed by the International
Commission on Stratigraphy in 2009 (Berggren et al.
1995; GuiSen et al. 2005; Yupeng et al. 2010; Guisen
et al. 2009). There are many coal basins are coveredby thick layers of Neogene and Quaternary in
Shandong and Henan Province, China. These
overburden layers are simply regarded as
unconsolidated layers in a large amount of research
papers about coal mining. Xinjing et al. (2011) pointout that the Neogene strata in the area around the
Xiaolangdi Reservoir, China is a kind of incomplete
consolidated argillaceous sediment and its
mechanical properties range between that of soil androck. Yongshuang and Yongxin (2000) find that thiskind of geo-material is very easily to cause
engineering geological problems after comparison theuniaxial compressive strength, which ranges from
0.3-1.5 MPa, of some Neogene strata.
The directly strategies of mining flood prevention
under water body and aquifers is keeping mining
deformation within the permitted limits. The
experience way is to limit the mining thickness or theratio of mining depth / mining thickness does notexceed a critical value. Another accurate
measurement is to control the limit tensile straincaused by mining (Baohong, 1995). Due to the highcontent of clay in some Neogene layers in this area,

2.

Geology, hydrogeology conditions

Yuzhou coal field locates in the mid-west ofHenan, China, where the II-1 coal seam of lower
Shanxi formation in Permian is the main mining seam.
Fig.1 and 2 give the location of Quandian Coalmine
and the bottom elevation of Neogene.
The formations in this area include Cambrian,
Carboniferous, Permian, Neogene and Quaternary.
The Quandian Coalmine lies in a sedimentary basincovered the western Henan, the eastern Shanxi, thenorthern Hubei and the southern Shaanxi, which is alandlocked lake basin and has a huge thickness of
Neogene and a thin layer of Quaternary system. The
colors of this Neogene are usually gray andbrown-red. Conglomerate, pebbly sandstone,
argillaceous siltstone, mudstone and shale are themain compositions of the Neogene. It is over
300-900m’ thick for this Neogene in Quandian
Coalmine and no more than 40-80m’s thick for the
Quaternary on the top.

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10th Asian Regional Conference of IAEG (2015)
Beneath the Cenozoic is the Shanxi group ofPermian, which is the main coal-bearing stratum in
the Coalfield. Under the Shanxi group is Taiyuangroup of Carboniferous, and it is not the focus of this
discussion. The top boundary of the Shanxi group is
the erosion surface, and that is also the bottom of the
Neogene system. Above the coal seam II-1, it is a setof light gray to dark gray sandy mudstone, fine tomedium grained sandstone and siltstone. These layers
are the overburden above the coal seam II-1, and theresidual thickness of this is only 30-60m.The fossil
erosion surface above the Shanxi formation plunges
from north to south as Fig.2 shows and the maximum
inclination is 20-24°. The coal measures have thesame dip direction with the fossil erosion surface
while the dip angle is 15° to 20°. Fortunately, all thefaults did not cut through the Neogene. The
overburden rock mass above coal seam II-1 is no
more than 60 m’s height, and the aquifers in Neogene
may bring troubles to mining.

Fig. 1 Geological conditions and the bottom of Neogene around the Quandian Coalmine

K5

1602
100

Quaternary

K6
0.5

K7
0.5

1

1

0.5

1

Neogene0-100

Elevation(m)

-200

-300

II -3
eam
s
l
Coa
II- 1
seam
l
a
Co

-400

-500

-600

-700

0

ne
og e ane
N rm i
Pe

K5

100

200m

Drill hole

Aquifer

Mass content
of d<0.075mm

Aquifuge

Fig. 2 Granularity variation and hydrogeological structure of section A-A’

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10th Asian Regional Conference of IAEG (2015)
The surface water has a very close hydraulicconnection with Quaternary in this area. Ying River,
one of the tributary of the Huaihe River, is flowing
through the western of this area. The river valley isabout 300-500 m’s wide, and transport 177.6 million
m3 water annually. A crisscrossed artificial irrigation
network connects with the Ying River and its
upstream reservoir. It can be seen from Fig.1, there isa small reservoir locates hereabout the outcrop of coalseam II-1 in the northwest of Quandian Coalmine,
which has a storage capacity of 325000m3. The
reservoir basin is composed of sand and gravel layersof Quaternary, and they are porous aquifers. It is only
40 m’ thick of this Quaternary layers and all havewater contact with the surface water bodies. Hence,
the mining engineering in this area has to face theproblems of shallow water resource protection andmining water inrush prevention in deep level.
According to the exploration works mainly bydrill holes, the Neogene system in this area can bedivided to several interbed aquifers and aquifuges that
illustrated in Fig.2. The aquifuges are always
composed by weak cemented clay, sand and gravel
layers. Its structure has obvious relevance with its
particle size distribution, especially with the degree ofclay content. Pumping tests have been arranged in theupper, middle and bottom of Neogene and the resultsof these pumping tests are listed in Table 1. The
sequence of these aquifers in Table 1 is consistent
with the drill K6 in Fig.2 and these results assure themain aquifers are all of weak water abundance. It iseasy to see that the three terminal water level fromaquifer Top, Mid and Bot have obvious differences.
The units-inflows are relative uniformity in aquifer
Top and Mid. But it is not so uniformity like these
two aquifers, there is a large variation scope ofunits-inflow appears in the Bot aquifer because of theheterogeneous of gravel layers in the bottom Neogene
system. These three aquifers also have different
permeability and it is clearly that the aquifer Top andBot have a more well conductivity than that of the
Mid aquifer. Such situation shows there is a goodimpermeability layer in the middle of Neogene. Thatis the Top, Mid and Bot aquifer have not direct
hydraulic connection with each other. As the aquifer
Bot has weak water abundance, water inrush
prevention is not a very serious problem to mining
safety. But when there appears drastic excavationengineering in underground, the deformation or
failure happened in the aquifuges and weak wateraquifers would disturb its ability of cutting off thewater contact with the upper water body.
3.

Structure characteristics of Neogene

According to the observation experiences fromthe around coalfields, the mining cracks can extend
up to 14-17 times mining thickness. Because thethickness of the overburden rock mass above the coalseam II-1 is no more than 60 m and the averagemining thickness is 7 m, it is easy to predict that the
top of the mining cracks in this area may reach 98 to
119m’ height. That is to say the cracks zone may
cover the aquifer or aquifuge at the bottom of
Neogene.
On aim to gathering the structure characteristics,
physical and mechanical properties of the Neogene
and Permian, three drill holes are arranged around thereservoir in the west of Quandian Coalmine. The
section B-B’ as Fig.3 shows include drill K1 and K2
can reveal the stratigraphic sequence of this area.
Density, moisture content, specific gravity, particle
analysis, expansion, uniaxial compressive strength,
shear strength, elastic modulus of the cores from drill
K1, K2 and K3 are tested according to Chinese soil
test standards. The tests illustrate the uniaxial
compressive strength ranges from 0.5 to 2 MPa.
These results are very closely similar with that fromYongshuang and Yongxin (2000). But almost all thegained properties are randomly distributed or discrete
distributions. These data cannot reflect any obviously
differences varying with depth except the internal
friction angle as Fig.4 shows. These superficial
results are difficult to assist analyzing thehydrogeology conversion trend of the Neogene
caused by mining and also may hid some essential
factors such as discontinuities for mine safety. The
main reason may be the test specimen is more
complete.
Basing on the drilling records, it can be foundthere are several crushing parts and aquifers in eachdrill hole. The results of pumping tests for eachaquifer are list in Table 1. These segments can beconnected with each other in a cross-section throughthe drills contrast. From Fig.3, it can be seen thebroken or unconsolidated segments mainly exist in

Table 1 Results of pumping tests
AquiferUnits inflows(L/s∙m)Water level in elevation(m)Permeability coefficient
(m/d)

Top
MiddleBottom

0.0307-0.0545
0.0032-0.0078
0.004-0.219+62.88-+80.16+46.07-+42.58+31.28-+121.96

0.2177-0.5477
0.01075-0.031
0.02050-0.3795

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10th Asian Regional Conference of IAEG (2015)

Fig. 3 The weak segments imbedded in Neogene of section B-B’groups. Among these, the group 6 at the bottom of
Neogene system consists of clay, clayey sand andgravel has been considered to be an aquifer and will
be disturbed by mining inevitably. The group 5 is aweak layer and may probably lead to the separation
between group 6 and 4. While the group 4 is anaquifuge with great thickness and slice aquifer
imbedded in it. It is a stable layer that spread all the
study area and has more clay content. This segment isa mix group of intact and relatively fractured layers.
The group 3 is also a broken or unconsolidated
segment. The cores of this group consist of large
content of illite and most of them disintegrated
rapidly. The group 2 has the similar structure with thegroup 4. The top is the group, which is a combinationof Quaternary and weathered Neogene, and this is anaquifer that has good water contact with the surface
water body. According to the above information, thelayer interfaces and the weak groups have made up
the structure of Neogene system, which is a notuniformity system.
If the mining caused deformation or damage
extends to the bottom of Neogene system, thestructure of Neogene system and properties can bringsome special effects to mining safety.

Cohesion(kPa)

40

30

20

10

0
150

200

250

300

350

400

Internal friction angle（°）

Depth（m）

30

25

20

15

10

150

200

250

300

350

400

Depth（m）

4.

Fig. 4 Mechanical properties changing with depth

Deformation, failure simulation

Laboratory
simulation
experimentsonconsidering three factors and three levels wereconducted in order to analyze the influences ofdifferent mining thickness, overburden thickness andlithological of Neogene (Photo. 1). The geometricallysimilar scale is 1: 200 for all the models. They are all

two main layers. On considering the aquifers andaquifuges, particle distributions and crushing parts,
the Neogene system of this area can be divided into 64

10th Asian Regional Conference of IAEG (2015)
plane models and 2 meter’ long, 0.25 meter’
thickness. The rock materials are prepared according
to its laboratory intensity. The rock mass above coalseam II-1 is a uniform layer of argillaceous sandstone.
The thickness of this argillaceous sandstone is theheight from the bottom of Neogene system to the roofof the coal seam II-1, which is represented by H andcalled overburden height. Among the photos, M is the
thickness of mining slice. Another changing factor islithology of the group 5 to 6 in Neogene system. In
order to examine the impact of intensity level on the
deformation and failure in group 5 or 6 , three typesof material formulation are designed to represent a
soft, a soft-hard interbed and a hard Neogene layer
covered on the surface of Permian. These materials
are made of soft clay, interbedded combination ofsoft clay and hard sandy clay, hard sandy clay.
As seen in Photo.1(a)-(c), under the sameoverburden height, the damage area extended higher
significantly with the increase of mining thickness, so
did the damage area in Neogene system near theerosion surface. Tension fissures, bedding subsidence
and breakage will appear in the layered strata of
Neogene system in the circumstance of mining
thickness increasing.
When the mining thickness is kept 4m, we can see
in Photo.1 (d)-(f) that the damage area of overburdenrock mass decrease with the overburden thickness
decreasing. The damage range of Neogene system isclosed affected by the layered combination. The pure
soft clay combination lead to the minimum damage in
Neogene system and the second, third are thesoft-hard interbed combination, the hard combination
respectively.
When the bottom layer of Neogene consists bysoft-hard combination, as shown in Photo.1 (g)-(i),the thickness of the overlying rock varying from 30 m
to 50 m, the Neogene and overburden rock mass havegood synergistic deformability while the mining
thickness changes from 2 m to 6 m, although different
scales of fractures will appear in Neogene.
Qingxiang (2003) and Qingxiang et al. (2009) had
established a mining caused “Arched beam” model todescribe the deformation of the thick soil layer
covered on the shallow buried thin bedrock. Hanfu(2012) had analyzed the instability criterion of hugeclay layer during mining. The basic precondition of
their researches is the assumption that soil or rock is
uniform and continuous. But the in fact situation of
the Neogene system is a relatively complex network
structure and the deformation, damage of Neogene
are coupled by its native or mining induced structure.
So more microscopic structure analysis needconsidering in the future.

(a) M=2m, soft(b) M=4m, soft-hard interbed(c) M=6m, hard(d) H=30m, soft-hard interbed(e) H=40m, hard(f) H=50m, soft(g) H=30m, M=4m(h) H=40m, M=2m(i) H=50m，M=6m

Photo. 1 Destruction in overburden rock mass and the bottom of Neogene

510th Asian Regional Conference of IAEG (2015)
5.

Conclusions, suggestions

Shackleton N. J.(1995): Late Neogene chronology:
New perspectives in high-resolution stratigraphy,
Geological Society of America Bulletin, Vol.107,
No.11, pp.1272-1287.
GuiSen Z. and Huijun Y. (2005): A brief introduction
to international stratigraphic chart and global
strtotype section and point, Journal of stratigraphy,
Vol.29, No.2, pp.188-204.
Guisen Z., Yunbai Z. and Huijun Y.(2009): A brief
introduction of the international stratigraphic chart,
Journal of stratigraphy, Vol.33, No.1, pp.1-10.
Hanfu H.(2012): Tectonic motion and control of
overburden in thin bedrock fully-mechanizedsublevel caving face, Xuzhou: China University of
Mining and Technology.
Qingxiang H. (2003): Simulating on damage law of
disturbed thick sandy soil layer and transference ofload in shallow seam, Journal of Chang'an
University, Vol.23, No.4, pp.25-27.
Qingxiang H., Pei Z. and Aiju D. (2009):
Mathematical model of “arch beam” of thick sandy
soil layer movement in shallow seam, Rock and soil
mechanics, Vol.30, No.9, pp.2722-2726.
Xinjing L., Houguo F., Zhenling L. and Yi Z. (2011):
Soft rock/hard soil research of strength anddeformation characteristic test method. Zhengzhou:
The yellow river water conservancy press.
Yongshuang Z. and Yongxin Q. (2000): Definition of
hard soil-soft rock and its discrimination andclassification, Geological Science and Technology
Information, Vol.19, No.1, pp.77-80.
Yupeng Y. and Yu L.(2010): An International debate
on the status of the Quaternary as a formal
chronostratigraphical unit and the final ratificationof the Quaternary system/period, Advance in Earth
Sciences, Vol.25, No.7, pp.775-781.

This paper formed the following understandings:
The Neogene of Yuzhou coalfield could be
divided into 6 groups according to grain compositionand core fragmentation degrees, which is a structural
influential factor in determine the characteristics ofdeformation and failure after the deep excavation.
The structure of the thick Neogene layer in thisarea is controlled by weak structural segments, soft
stratum and bedding faces. It was found soft-hardinterlaying and structure network control thedeformation of Neogene during mining.
Experiments considering complex network
structures of Neogene are important direction tofurther analyze and evaluate deformation-seepage
action of Neogene.
Acknowledgements
The authors gratefully acknowledge the supportof the National Natural Science Foundation of China
under Grant No.41272343. We also would like toexpress our acknowledgements to the reviewers fortheir constructive comments.

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