An Experimental Investigation of Soil Arching under Dynamic Loads Gao-xiao HAN 1, Quan-mei GONG 2, and Shun-hua ZHOU 3 1 Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji
网页翻译在线University, 4800 Caoan Road, Shanghai, China 201804; PH (86) 21-69583862;
FAX (86) 21-69583662; email:
2 School of Transportation Engineering, Tongji University, 4800 Caoan Road,
Shanghai, China 201804; PH (86) 21-69583862; FAX (86) 21-69583662; email:
qongqm@tongji.edu
3 School of Transportation Engineering, Tongji University, 4800 Caoan Road,
Shanghai, China 201804; PH (86) 21-69583862; FAX (86) 21-69583662; email:
zhoushh@tongji.edu
ABSTRACT
Soil arching is a common phenomenon in pile supported embankments
resting on soft soil. Due to soil arching, stress acting on soft soil or geosynthetic
reinforcement decreas and stress on piles increas. This can reduce the ttlement
difference along the subgrade cross-ction. Now most of the high-speed railway
subgrade in soft soil area in China has been designed considering the effect of soil
arching. However, during the rvice life of the railway line, the subgrade will carry
on high-cycle dynamic loads and when the subgrade is not high enough, the soil
arching might be influenced and the permanent deformation will increa. In this
resisterpaper the properties of soil arching under dynamic loads have been investigated by
performing model experiments. It was found that the failure of soil arching occurred
at arching foot firstly and then spread upward. Both the thickness of the covering
soil and the density of soil have influences on the soil arching under dynamic load.
With the increa in sand height and density, the required time of failure of soil
arching also increas.
INTRODUCTION
Piled embankments are increasingly ud to construct high-speed railway and
highway on soft soils due to small total and differential ttlements compared to the
traditional soft soils improvement methods. The interactions among embankment fill,
geosynthetic reinforcement, piles, and foundation soil are complex. Sincetake me to your heart歌词
compression stiffness of the pile is greater than that of the foundation soil, the
embankment fill mass directly above the foundation soil has a tendency to move
downward. This movement is partially restrained by shear stress from the
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embankment fill mass directly above the pile cap. The shear stress increas the
pressure acting on the pile cap but reduces the pressure on the foundation soil. This
load transfer mechanism is termed the “soil arching effect”. The inclusion of
geosynthetic reinforcements complicates the load transfer mechanism. The soil
arching has a significant influence on the behavior of piled embankments.
Conquently, in order to get thorough understandings of soil arching
mechanism within piled embankments a number of rearch studies associated with this subject have been performed in the past two decades. Hewlett and Randolph
(1988) conducted 3D model tests and prented a mi-spherical model to describe
soil arching. Low et al. (1994) undertook 2D model tests to evaluate soil arching.
However, pile –subsoil relative displacement was not taken into account. Han and
Gabr (2002) conducted a numerical analysis on geosynthetic-reinforced and
pile-supported earth platform over soft soil and studied the effects of pile material
elastic modulus and geosynthetic stiffness on the degree of soil arching. They
obrved that the soil arching in the embankment soil incread with an increa in
the height of embankment fill and elastic modulus of the pile material. However, soil
arching decread as the tensile stiffness of geosynthetic reinforcement was
incread. Bad on 2-D physical and numerical modeling of pile-supported earth
platform over soft soil, Jenck et al. (2007) obrved that the load transfer onto piles
due to soil arching was effectively controlled by shearing mechanisms (development
of shearing in the granular fill due to differential ttlements at the platform ba
between the soft soil and the rigid piles, leading to arching which partially transfers
the loads onto the piles) and shear strength of the platform or embankment material.
Chen et al. (2008) conducted experimental investigation on reinforced and
unreinforced piled embankments to study the effects of pile –subsoil relative
displacement, embankment height, cap beam width and clear spacing and
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geosynthetics (with different tensile strength) on soil arching. It was obrved that
soil arching and ttlements are influenced significantly by embankment height, cap
beam width and clear spacing, and reinforcement tensile strength. Moreover, soil
arching is strongly dependent on the pile –subsoil relative displacement.
After railway and highway constructed with piled embankments are put into
u, the soil arching existing in piled embankments will probably carry on cyclic
dynamic traffic loads, and then the features of soil arching under dynamic load will
have direct influences on the behavior of railway and highway. Therefore, it is
necessary to investigate the features of soil arching under dynamic load. However,
most of studies performed before only focud on the static properties of soil arching,
while very limited studies have been done on it under dynamic loads.
This paper prents the results of a model test of soil arching under dynamic
load. In the model test, effects of different thickness of the covering soil and the
different density of soil on soil arching under dynamic load were investigated.
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MODEL TEST Model test t-up Model test system consisted of a ba made of iron, a cylinder, and a vibration exciter. There was a hole （with the diameter D of100mm ）which could be clod and opened as necessary in the ba. Toughened glass was ud for the wall of the cylinder to allow obrvation. The diameter and height of the cylinder was
150mm and 300mm respectively .As shown in Figure 1, the signal acquisition
system was DH-5922 dynamic signal measurement and analysis system who
frequency range was 0 to 2000Hz and the sampling frequency ud in the test was
200Hz. The stress transducer adopted is DYB-1 resistance strain and the range,
resolution, and nsitivity coefficients of the transducer were 0-50KPa, 0.13%, and
2.1 respectively.
Material properties
Figure 2 shows grain-size distribution of sand. The coefficient of uniformity
Cu is 3.33, the coefficient of curvature Cc is 0.97, and the angle of shearing
Figure 2. Grain-size distribution of sand
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Program of experiments The experimental program is detailed in Table 1. In order to verify whether a soil arching could be formed according to the experimental method ud in this experiment, Test 1 was performed. After determining the validity of the method, a preliminary investigation of the influences of different thickness of covering soil and different density of soil on bearing property of soil arching was bad on Test 2-4 and Test 3, 5, and 6 respectively.
Table 1.Experiment Details
Model test procedure
After installation of the test t-up, sand was filled one layer after another and
compacted, during which soil stress transducers (SSTs) were placed at certain depth
(exception to Test 1) .There were six transducers in Test 2, 3, 5, and 6 and three
transducers (at lower layer) in Test 4. The transducers were numbered SST1-6 as
shown in Figure 1. After reaching required height of sand we opened the hole which
was clod before to form a soil arching under lf weight. Then, for Test 1, the
cylinder was removed and the sand was divided into two parts to investigate the
formed soil arching (As shown in Figure 3). For the rest of the tests, a dynamic load
film是什么意思generated by the exciter and located in the center of the top of sand was applied to an
area with a diameter of 200 mm until the soil collapd. The magnitude and
frequency of the dynamic load were 140N and 50Hz. At the same time the signal
acquisition system began to work to collect data until the sand collapd.
Figure 3. Soil arching under lf weight
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TEST RESULTS Soil arching under lf weight An approximately circular arching was formed above the hole in Test 1 and the height of the soil arching was 60mm (as shown in Figure 4).
Figure 4. Profile of Test 1 (unit: mm ）
Stress in the sand
The variation of stress showed nearly identical characteristics in Test 2-5.
Figure 5 was the curve of stress versus time of Test 2. The results showed in Figure
6-10 were obtained by curve fitting of original data for convenience of analysis. It
showed that when h/D was 2 and 2.5, the stress measured by SST1 was smaller than
that measured by SST2 and SST3, while the stress measured by SST4～6 were
just the opposite. When h/D was 1.5, the stress measured by SST1 was not smaller
than that measured by SST2 and SST3 at first. This may be ud to conclude that a
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complete soil arching was formed when the h/D was 2 and 2.5 and an incomplete
soil arching was formed when h/D was 1.5.
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Figure 5. Results of test 2
Except Test 4, after the dynamic load was applied, the stress measured by
SST1～3 remained at stable values for a short time at first and then began to
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