10th Asian Regional Conference of IAEG (2015)

Model test of Pile-plank structure working as Roadbed of High Speed Railway overlying
Goafs
Qian-gong CHENG(1,*), Yan LI(1), Jian-lei ZHANG(1), Yu-feng WANG(1), and Xin LIANG(1,2)(1) Department of Geological Engineering, Southwest Jiaotong University, Chengdu, China;
Correspondence to Qian-gong CHENG,
E-mail: chengqiangong@swjtu.edu.cn(2) Faculty of Civil Engineering, Guangxi University of Science and Technology, Liuzhou, China
Abstract
Based on abundant data obtained by the physical model tests of pile-plank structure used
as composite foundation of Wufushan station which overlies goafs in Hefei-Fuzhou high speed
railway, the distributions of pile axial force, earth pressure and settlements were obtained. The
results suggest: (1) the axial force at pile head was maximum, while the value at the depth where
the goafs locate remained constant as the pile was subjected to loads. And the value of axial
force decreased as the depth of pile increased. (2) With the increase of depth of pile, the value ofskin friction of pile also decreased, and there was no skin friction along the sections where thepile crosses the goafs. Besides, no negative skin friction appeared. (3) The change of pile-soil
stress ratio was consistent with the pile-soil load share ratio. The load shared by piles grew
gradually as loads increased, and eventually reached a stable value. (4) The bearing plank andsoils between piles settled uniformly, with a small value. The settlements of roadbeds overlying
goafs and non-goafs were almost same, and there was no deformation at the bottom of goaf
roofs. (5) The pile-plank structure had a good effect on the roadbed remedy.
Keywords: high-speed railway; goaf; pile-plank roadbed; model test; stress; settlement
1.

Introduction

It is compulsory for a high speed railway tocontrol settlement of its roadbed. If there are great areasof goafs, the railway will bypass those goafs, otherwise,
grouting will be implemented. Sometimes, thefoundation overlying the goafs will be reinforced.
Hefei-Fuzhou high speed railway, design speed of
350km, overlies goafs which locate in Shangrao of
Jiangxi Province. In order to control the deformation of
the superstructure, a pile-plank structure was used toreinforce the foundation. As a new remedial measure of
weak foundations, the pile-plank structure has been
applied to many high speed railways in China, such as
the Beijing-Tianjin intercity railway (Shen, et al., 2009;
Jing, 2006), the Beijing-Shanghai high speed railway(Zhang, et al., 2011; Xu, et al., 2012), the WuhanGuangzhou high speed railway (Zhan, et al., 2007), theZhengzhou-Xi’an high speed railway (Su, et al., 2012),
and the Suining-Chongqing high speed railway (Zhan,
et al., 2008). The effectiveness of the pile-plank
structure in such high speed railways is pretty well,
however, there are few cases where pile-plank
structures are used as a reinforced foundation when therailway overlies goafs, so it is necessary to understandthis remedial measure. Wufushan station in the HefeiFuzhou high speed railway locates in the Sishiba town

1of Shangrao of Jiangxi Province. The goafs underlying
the roadbed were produced by coal mining before 1949and during 1980s. This paper studies the settlement
characteristic of the foundation, the mechanism of theload transmission and the interaction between pile andplank by a physical model test.
2.

Project profile

The prototype cross section of the model test
locates at DK499+940 of the Hefei-Fuzhou high speed
railway, and the strata under the station are as follows:layer① is completely weathering sandstone; layer② is
intensely weathering sandstone; and layer ③ ismoderately weathering limestone. And there are two
goafs: No.1 goaf, 2.0 m in height and 2.5 m in width,
locates on the left of the cross section at the depth of
10.5~12.5 m, and the No.2 goaf, 2.8 m in height and2.5 m in width, locates in the middle of the crosssection at the depth of 19.8~22.6 m. Besides, 3.0meter-high embankment fill materials, which arecomposed of 3% cement and grade crushed gravels, lieson the top of a 1.2-meter-thick bearing plank, and theslope rate is 1:1.5. The locations of strata, goafs andpiles are shown in Fig.1(a), where A~D are thenumbers of rails, while I～VII are the numbers of piles

10th Asian Regional Conference of IAEG (2015)

Table 1 Pile length and traversing strata at prototype section
PilenumberPile
length/m

Depth /m
0.00~5.87

I

12

5.87~8.76
8.76~12.00
0.00~2.46

20
2.46~20.00
0.00~2.31
25
2.31~25.00
0.00~4.23
24

4.23~19.58
19.58~24.00
0.00~2.58

~ VII

25
2.58~25.00

Bearing layer

Goafs depth/mmoderately weathering
limestone

10.5~12.5

Stratacompletely weathering
sandstone
intensely weathering
sandstone
moderately weathering
limestone
completely weathering
sandstone
intensely weathering
sandstone
completely weathering
sandstone
intensely weathering
sandstone
completely weathering
sandstone
intensely weathering
sandstone
moderately weathering
limestone
completely weathering
sandstone
intensely weathering
sandstone

which are 12, 20, 25, 24, 25, 25, 25 m in length
respectively. The pile diameter is 1.0 m, and the
distance of pile-to-pile is 5.0 m. The configuration of
the pile-plank structure is shown in Fig.1(b). The
information of piles and the strata piles pass through isshown in Table 1.

3.

intensely weathering
sandstone
intensely weathering
sandstone

moderately weathering
limestone

19.8~22.6

intensely weathering
sandstone

Model test

3.1 Model size and boundary conditions
Scale model was used to study the behavior of thepile-plank structure roadbed, and the geometricalresemblance constant Cl was 25. The geometricalparameters are shown in Table 2.
Table 2 Geometry parameters of model
Model elements
Width of roadbed
Thickness of bearing
plank
Length of pile
Pile-to-pile distance
Height of No.1 goaf
Width of bearing plank
Height of fill materials
Diameter of pile
Width of goaf bottom
Height of No.2 goaf

Prototypesize/m26.0

Modelsize/m
1.040

1.2

0.048

25
5.0
2.0
35.0
3.0
1.0
2.5
2.8

1.000
0.200
0.080
1.400
0.120
0.040
0.100
0.112

Compared with prototype, the model is affected
remarkably by boundary conditions. As to the boundary
conditions, Kishida made many tests in sand box tofind an appropriate ratio of width to pile diameter (B/D)
and that of distance between pile end and the bottom of

Fig.1 Location of strata, goafs and pile-plankarrangement (unit: m)2

10th Asian Regional Conference of IAEG (2015)

Table 3 Size of prototype and model
Boundary
Alongrailway
Acrossrailway
HeightPrototypesize/mModelsize/mRemarks25

1.0

Considering the additional size which eliminate boundary effect

70

2.8

Considering the additional size which eliminate boundary effect

45

1.8

Height of padding +thickness of bearing plank +length of piles, andconsidering the additional size which eliminate boundary effectthe sand box to pile diameter (Z/D), and he believedthat when B/D was more than 10 and Z/D was morethan 6, the effect of boundary conditions could be
neglected (Zhang, et al., 1990). Xie et al. (2005)
proposed that the effect area of boundary was 3 times
as large as the size of bearing plank, and the effectdepth was 15 times of pile diameter by summarizing onsite data of buried depth of pile group foundation, thesize of bearing plank, and the range of effect of earthpressure. During the process of model test ofdiaphragm walls, Wen et al. (2007) assumed the effectarea of boundary was 3 times larger than the size ofbearing plank, and the effect depth was the largest
value in the settlement calculation consideringboundary conditions.
The sizes of model box in this paper are as follows:
the ratio of width of the box to pile diameter B/D was
10, the distance between the pile end and the bottom of
the model box was 15D, the length along railway wasthat of 5 row piles plus effect area, and the width acrossrailway was that of roadbed and effect area. Afterconsidering boundary conditions, the sizes of model are
listed in Table 3.
3.2 Materials
The deformations of piles are almost elastic, so theelastic modulus is firstly considered in choosing theirsimilar materials. After a comparison with many othermaterials, PPR (pentatrico peptide repeats) was finallyselected as the substitution of piles with 32 mm outer
diameter, 23 mm inner diameter, and 0.962GPa elastic
modulus. The bearing plank was constructed by using
cast-in-site concrete to connect with piles.
There are 3 types soil in site, i.e. completely
weathering sandstone, intensely weathering sandstone,
and moderately weathering limestone. Uniformly
graded fine and coarse sands substituted for completely
weathering sandstone and intensely weathering
sandstone respectively. Fine sands were also the similarmaterials of roadbed.
The moderately weathering limestone layer which
spreads under the piles ends is the bearing layer, so thecompressive strength and elastic modulus are main
influential factors to consider when choosing its similarmaterial. In the model test, composite materials consistof 1.00kg medium-coarse sands, 0.10kg gypsum and0.04kg cements were used to build the bearing layer.

3

3.3 Model construction
After the construction of model box, the strata
where piles and goafs were placed were cast. Samematerials that substituted the moderately weathering
limestone were used to build the goafs roofs. Next, theprefabricated roofs were connected with the goafs.
Finally, the entire goafs (Fig.2) were completed. The
bearing plank was constructed by using cast-in-site
concrete as the last step of the model construction.

Fig.2 Fabrication of goafs
3.4 Monitoring instrument
Monitoring points were installed to collect the dataof (1) pile axial force; (2) earth pressure; and (3)settlements. The resistance value of electric resistancestrain gauge is 119.9  0.1  , and the sensitivity
ratio K   2.08  1 % . The measuring range of the
micro earth pressure cells installed under piles andbetween piles is 50kPa, and 400kPa of those installedon the top of piles. Micrometer gauges were used tomonitor settlements, and the measuring range is 5 mm
and precision is 0.001 mm.
The layout of the monitoring points is as follows:(1) micro earth pressure cells were installed between 2
piles and between 4 piles under the bearing plank tocollect the data of earth pressure, and the numbers were10th Asian Regional Conference of IAEG (2015)

T1~T4. (2) The Cj1 and Cj2 settlement monitoring points
were set at the bottoms of No.1 and No.2 goafs roofs
respectively. (3) There were another 4 settlementmonitoring points (Cj3~Cj6) between 2 piles andbetween 4 piles under the bearing plank. (4)No.Cj7~Cj10 settlement monitoring points were set on
the top of the bearing plank. (5) Resistance strain
gauges were pasted along the No. I ~ Ⅳ piles, andmicro earth pressure cells were set on the bottom andthe top of these piles, and the numbers were T5~T8. The
layout is shown in Fig.3.

Where: Pd is dynamic load, and  is dynamic ratio,
which equals 3.0 when the design speed is 300 km/h;
and Pj is static load.
In order to make detailed study of stress anddeformations when the model subjected to loads, theloads were divided into 8 levels, i.e. 3 levels of fillmaterials loads, 3 levels of train loads, and 2 levels ofadditional loads. The details are shown in Table 4.
Table 4 Load on road embankment model
Number
1
2
3
4
5
6
7
8

Load sourceFillmaterials
Fillmaterials
Fillmaterials
One train
Two trains
Four trains
Additional
loads
Additional
loads

3.70
4.03
4.70

Loading
mode
Gravity offill materials
Gravity offill materials
Gravity offill materials
Weights
Weights
Weights

5.70

Weights

6.30

Weights

Load/kN
1.12
2.24
3.36

4、Results and analysis

Fig.3 Monitoring points arrangement
3.5 Loads
The test loads included the gravities of fillmaterials above the bearing plank and ballastless track,
train load and additional load. The dynamic load due tothe running of the train can be determined by Formula(1).

Pd   Pj(1)

4

4.1 Pile axial force
The pile axial forces of No.1~4 are shown in Fig.4.
The axial force of No.1 pile declined as the depth
increased, and remained constant when the pile passed
through goafs. When the pile was subjected to the level
1 and level 2 loads, the axial force was small, evenreached 0 at the base of the pile. By contrast, the axial
force at the base was 0.061 kN, and at the top was
0.114 kN when the pile was subjected to the level 8
load. In that situation, the tip resistance accounted for
53.5% of the entire loads.
The trend of the No.2 pile axial force was similar
to that of No.1 pile. The axial force at the base of thepile was 0.047 kN and that on the top was 0.119 kN
when the level 8 load was imposed, and the tip
resistance represented 39.5%, while the proportion of
the skin friction was 60.5%, indicating that skin friction
borne more loads as the length of piles increased.
No. 3 and No.4 piles do not pass through goafs.
The axial forces decreased faster than those of No.1 andNo.2 piles with the increase of piles depth, because thebearing layers supported No.1 and No.2 piles were
mimic limestone, while that supported No.3 and No.4
were coarse sands which deformed relatively larger , asa result, skin friction was mobilized more.
4.2 Skin friction
Fig.5 shows the skin friction of No.1~4 piles. The
skin friction of No.1 pile fell gradually along the pile,and reached 0 at the goafs. The entire skin friction
accounted for 46.5% when the pile subjected to the10th Asian Regional Conference of IAEG (2015)

Fig.4 Distribution of axial force of piles

Fig.5 Skin friction of piles
level 8 load. The skin friction of No.2 pile made up
60.5% of the load, and the trend was similar to that of
No.1 pile. Because of different pile lengths anddifferent distributions of strata, the ratio of tip

5

resistance to skin friction of No.1 piles distinguishedfrom that of No.2 piles. When the load was small, there
was no skin friction at the lower part of No.2 pile. With
the increase of the loads, the skin friction was

10th Asian Regional Conference of IAEG (2015)

mobilized gradually. The skin frictions of No.3 and 4
piles also experienced a downtrend from the top to thebase of piles. When the load was level 8, the proportionof skin friction of No.3 pile was 88.4%, bearing most ofloads. The skin friction of No.4 pile experienced asimilar trend due to same length and distribution ofstrata.
4.3 Pile-soil stress
The variation of earth pressure between piles isshown in Fig6. As depicted in the figure, there were 4
stages of earth pressure between piles with the growthof loads: when the loads were 0~3.36 kN, earthpressure went up steadily; when the loads were
3.36~4.03 kN, the rate of increase became lager; when
the loads were 4.03~4.70 kN, the rate of increase
dropped; when the loads were 4.70~6.30 kN, the rate ofincrease rose gradually to a constant value. These 4stages suggested that there was a period of adjustmentof earth pressure between piles, after that, the rate ofincrease remained stable at a constant value.

of No.1 and 2 piles were smaller than those of No.3 and4 piles when these 4 piles subjected to same loads.

Fig.7 comparison of stress on top, bottom and earthpressure
The distinctions between stress on the top of piles
and the earth pressure were pretty large, and the rate ofincrease of stress on the top of piles was larger, because
the elastic modulus of piles was much larger than thatof soils.
The pile-stress ratio is shown in Fig.8. The data
was the mean value of stress on the top of piles and themean value of earth pressure between piles. It is shownthat the ratio increased from 31 when the load was 1.12
kN to 127 when the load was 30 kN, which indicatedthat piles borne more loads.

Fig.6 earth pressure between piles
Although the earth pressure between piles under
different parts of bearing plank was similar, there werestill slight distinctions. The trends of earth pressure
between 2 piles and between 4 piles were same,
however, the earth pressure between piles that were
under the middle and the boundaries of bearing plankwere different, and the middle one was larger than theboundaries.
The comparison of stress on the top and base ofpiles and earth pressure between piles is shown in Fig.7.
At the initial stage of loading, the stress on the top ofpiles increased rapidly with increasing loads, after that,the stress went up gradually with a stable rate,suggesting that the loads borne by piles rose as theloads increased.
When the loads were small, the stress on the baseof piles was 0, when the loads grew gradually, a part ofloading transmitted to base, and the stress on the baseincreased, afterwards, such stress grew steadily. The
stress on the bottoms of No.1 and 2 piles was larger
than those of No.3 and 4 piles, because the skin friction

6

Fig.8 Pile-soil stress ratio
4.4 Pile-soil load share ratio
The pile-soil load share ratio is shown in Fig.9.
The loads borne by piles increased with growth of loads.
When the load was 1.12 kN, the pile load share ratio
was 23.6%, because the deformation was small underthat load, and the soil borne most loads. As the loadincreased, the deformation also increased, the stress ofthe piles increased remarkably due to large elastic
modulus, as a result, the pile load share ratio increased
to 50.6% when the load was 4.03 kN. The soil was10th Asian Regional Conference of IAEG (2015)

mobilized gradually when the loads increased, which
affected the pile-soil load share ratio. Finally, the pileload share ratio was 55.8%. We can also find that when
the load was 4 kN, the two lines intersected because theloads supported by piles and soil both accounted for50%.

to each other except for the settlements of boundary
points (Cj9 and Cj10), and all were smaller than the soil
settlement, indicating that the bearing plank transferred
the contact between roadbed and foundation fromflexible to stiff and control the different settlement well.
5、 Conclusions

Fig.9 Pile-soil load share ratio
4.5 Settlements
The settlements of monitoring points are shown in
Fig.10. The settlements of bearing plank and soils wereboth small, within an acceptable range. When the loadincreased from 1.12 kN to 6.30 kN, the settlements ofbearing plank and soils increased from 0 to more than
0.13 mm. Although there was difference at different
parts, the entire settlement was small, which meant thatthe bearing plank with a large stiffness control thedifferent settlement well. What is more, there was nosettlement at the roofs of No.1 and 2 goafs.

Fig. 10 Monitoring points settlements
4.6 Plank effectFig.4 also shows that the pile axial forces of 4
piles were similar when the piles subjected to samelevel load. For example, when the load was level 8(6.30 kN), the axial forces were 0.11~0.12 kN. Thisphenomenon suggested that the bearing planktransmitted load uniformly to piles due to large stiffness.
Besides, as depicted in Fig.10, the settlements ofmonitoring points on the bearing plank surface closed

7

Based on the physical model test, this paper
studies the behavior of pile-plank structures working as
the composite foundation overlying goafs in a highspeed railway. The main conclusions can be describedas follows.
(1) The axial force on the top of piles was largest,and decreased rapidly at the upper part of piles. The
axial force was constant when passing through goafs.
The axial force on the tops of piles was similar whenpiles subjected to same load.
(2) The skin friction on the upper part of piles waslarger, and was 0 at the goafs. There was no negative
skin friction occurred.
(3) The change of pile-soil stress ratio wasconsistent with the pile-soil load share ratio. As theload increased, the rate of increase of loads borne bypiles decreased gradually to a stable value.
(4) The settlements of bearing plank and soils
increased rapidly at the initial stage, after that, the rateof increase decreased, and reached a stable value.
(5) With same load, the axial force of different piles
closed to each other, because the bearing planktransmitted the load uniformly. Besides, the entire
settlement of bearing plank was small, and thesettlements at goafs and non-goafs were same.
(6) There was no deformation at the bottom of goafs
roof, which meant that the bearing plank control thedeformation well.
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