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Polymer grid reinforced pavement on soft clay grounds
Geotextiles and Geomerabranes 9 (1990) 99-123
Polymer Grid Reinforced Pavement on Soft Clay Grounds N. Miura, A. Sakai, Y. Taesiri Department of Civil Engineering, Saga University, Saga, 840, Japan
T. Yamanouchi Department of Civil Engineering, Kyushu Sangyo University, Fukuoka, 813, Japan
&
K. Yasuhara Department of Civil Engineering, Nishinippon Institute of Technology, Kanda, 800-03, Japan
ABSTRACT This paper deals with model and field tests for investigating the mechanism of reinforcement by a polymer grid in suppressing non-uniform settlement of pavements constructed on soft clay ground. A series of laboratory tests on reinforced and unreinforced model pavements in a soil tank indicates that the polymer grid is useful for suppressing non-uniform settlement of pavement under cyclic loading. Deformation analysis by FEM is carried out to make clear the reinforcement effect of a polymer grid in a model pavement. To investigate the performance of a polymer grid in practice, a test road of 300 m length with six sections of different kinds of pavement is constructed on soft clay ground. The function of a polymer grid is discussed by comparing the pavements made by conventional and reinforced methods.
1 INTRODUCTION The Saga plain is located around the shores of Ariake bay in northern Kyushu, Japan (Fig. 1). Here a problematic marine clay, Ariake clay, is deposited. The thickness of the clay deposit varies between 15 and 30 m with an average of 20 m. The clay is very soft and one of the most sensitive 99 Geotextiles and Geomembranes 0266-1144/90/$03.50 (~) 1990 Elsevier Science Publishers
Ltd, England. Printed in Great Britain
lO0
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
N~
Kyushu
0
20 40
60
80 i00 km
Fig. 1. Location of Saga Plain.
clays in Japan. The natural water content ranges from 100 to 170%, the liquid limit from 54 to 139%, plastic limit from 28 to 59% and the sensitivity ratio (the ratio of the strength in the undisturbed state to that in the remolded state) usually exceeds 16.1 Since it is very soft and has high sensitivity properties, the Ariake clay often causes geotechnical problems. 2 One of the problems is non-uniform settlement of roadway which includes rutting in the pavement. This can result from soft spots in the clay, insufficient compaction of base materials, consolidation of subsoil caused by traffic loads and also by land subsidence due to the pumping of groundwater. The influence of traffic loads on the settlement must be especially emphasized. For example, route 219 in the Saga plain showed a settlement of 200 cm after opening to traffic. 3 One-third of the settlement was considered to be a result of traffic loading. To suppress non-uniform settlement of the pavement, such techniques as replacement of the soft clay with a good soil, subgrade stabilization, or replacement with a mixture of good material and lime have been widely used in this district. The last
Polymer grid reinforced pavement on soft clay grounds
101
method mentioned, the replacement of the clay with a mixture, has been considered to be one of the most effective means of mitigating the problem. However, this method is very expensive and also causes some problems to engineering and environmental aspects during excavation and transportation of the clay. In the method of replacing the soft clay with a good soil, the total depth of the pavement becomes very thick, and this leads to a problem of high settlement. The thicker--and hence the heavier--the pavement is, the more settlement predominantly due to consolidation occurs. For these reasons the conventional methods are of no use and engineers have been investigating alternative methods. Recently, Giroud & Bonaparte 4 have noticed and studied reinforcement techniques for suppressing non-uniform settlement of roads constructed on soft ground, especially reinfoi-cement in the base course by polymer grids. They presented a design procedure of unpaved roads with a polymer grid, taking into consideration the confinement and load distribution effects of the polymer grid. Miura 5 suggested in preliminary laboratory tests to show that the mechanism of the polymer grid for the reinforcement of a pavement base was mainly the interlocking effect rather than the membrane tension effect. To investigate the applicability of the polymer grid as a reinforcement in a pavement on soft clay ground, a series of laboratory tests were carded out on model pavements in a soil tank, and deformation analysis by FEM was made to study the settlement characteristics of reinforced pavement. Based on the laboratory experiments, a test road of 300 m length was constructed on the Takeo-Fukudomi route in Saga Prefecture. The results obtained are discussed in this paper.
2 C L A Y B E H A V I O R U N D E R CYCLIC L O A D I N G Behavior of pavement on soft ground such as Ariake deposit is greatly influenced by traffic-induced cyclic loading. The low embankment highway on soft subgrades sometimes suffers from the abnormal settlement which is induced by this type of cyclic loading. 3 The characteristic of this cyclic loading is prolonged one-way loading without stress reversal, lasting over a long period of time. Yamanouchi 6 carded out laboratory model tests and indicated that the consolidation of soft clay under cyclic loading is greater than under static loading. This ~tendency was confirmed by cyclic oedometer and drained triaxial tests on Ariake clay. 7 Generally, clay deformation under longterm cyclic loading with inclusion of drainage consists of: (1) shear deformation under undrained cyclic loading, and (2) volume change due
102
N. Miura, A. Sakai, 1I. Taesiri, T. Yamanouchi, K. Yasuhara
to dissipation of cyclically induced pore pressure. Settlements of soft grounds under long-term cyclic loading are influenced by these two components simultaneously, since the clay under traffic-induced cyclic loading is generally in a partially drained condition. Aiming at reducing the traffic-induced settlements of clay subgrades, reinforcing effects of geonets were investigated in the laboratory 6 and also in the field, 8 but the effectiveness of the geonet against an abnormal settlement under cyclic loading were not conclusively proved in these experiments. Since polymer grids of high strength type, such as 'Tensar', have been developed recently, we have been investigating the function of the polymer grid as a reinforcement in the base, by carrying out model tests of unpaved and paved roads on soft clay. 9 The results obtained from the model tests indicated that a polymer grid is much better than a geonet in suppressing non-uniform settlement of a pavement. However, further laboratory tests and analytical study are required before the utilizing of polymer grids as a pavement reinforcement.
3 C Y C L I C L O A D I N G TESTS ON M O D E L P A V E M E N T S 3.1 Materials and testing method Model pavements were constructed in a test box made of concrete (150 cm x 150 cm in area and 100 cm in depth) as shown in Figs 2(a) and (b). The clay used as a subgrade material was Ariake clay, excavated and transported from a site in Saga City. The basic properties of the clay are as follows: specific gravity =2.625; natural water content = 129%; liquid limit = 117%; plastic limit = 39%. As indicated above, the natural water content of the clay is higher than its liquid limit. This tendency is generally recognized as a property of Ariake clay. 1.2 The clay was reconstituted by applying a pressure of 5 kN/m 2 for 2 months into 60 cm thick subgrade. A subbase and a base with or without a polymer grid were compacted and then a 5 cm thick asphaltic concrete layer was added. This test series used three types of biaxially oriented polymer grid, SSl, SS2 and SS3, installed with strain gages. A cyclic load of 200 kN/m 2 and frequency 0-18 Hz (4 seconds loading and 2 seconds unloading) was applied through a steel loading plate of 20 cm in diameter. Vertical settlements of the surface and also those of each layer were measured by using dial gages and differential transducers. Two types of model pavement were tested as shown in Fig. 3. Test-I series are models reinforced with a one-layer polymer grid, and the Test-II series are models with a two-layer polymer grid system. The detailed description of these models is summarized in Table 1.
Polymer grid reinforced pavement on soft clay grounds
103
'Bellofram'cylinder
~~
Pressuregage
Asphalticc o n c r e t e ~ ~'.-
......'~ :.:......1.: . . ,,,
".'.'.:','.:'~'.'":i':
Base...." [..:.-:'.-.';':-,
.....
~ : .'-%4
N ,
180era
Air compressor •
(a)
(b) Fig. 2. (a) Cyclic loading test for a model pavement; and (b) view of experiment using a test box.
104
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
0-~ Dial gage G~
~: I '
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~sphaltic
I'"
6h
e
}
!
, L_
.--0--~
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Test-I. 2 !a) Test-I.4
? ??
.~,?.?? ??
?
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:::--.:J;:
(b)
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(C)
:t::.--z:::::: ::E:.'.::"...........
Test-II.
3
(d )
Fig. 3. Schematicdiagramof model pavementsreinforcedby polymergrid in one-layer and two-layer systems. 3.2 Reinforcement effect of a polymer grid
To evaluate the reinforcement effect of a polymer grid in a model pavement, we measured moduli of subgrade reactions K20 using the steel loading plate of 20 cm diameter before and after the cyclic loading and from which the equivalent value of K30 was calculated for each layer, as shown in Fig. 4 (Test-I.2).
Polymer grid reinforced pavement on soft clay grounds
105
TABLE 1 Description of Model Pavements
Number of polymer grid layer
Test no.
Location of polymer grid
Consolidation Polymer (kN/m2) grid
Test-I. 1 1.2 1.3 1.4
SS2 SS2 SS3
10
Test-II. 1
11.2
SS1
11.3
SS1
Unreinforced On top of the subgrade On top of the subbase On top of the subgrade Unreinforced On top of the subgrade and the subbase On top of the subbase and in the base
Modulus of subgrade reaction, AspnalEiC concrete
J ,i 11111
2'111
i I
Subgrade
1111
Test-I.2
Base
Subbase
K30 (xlO ~) (MN/mS)
7-rrrrn 3
I
fl H A
j
Before cyclic loadin E After cyclic loading
Fig. 4. Modulus of subgrade reaction of each layer in Test-I.2. Table 2 shows the ratios of K values of each layer to that of clay layer K¢, m e a s u r e d after cyclic loading tests. The results indicate that the K/Kc value of reinforced p a v e m e n t is 30% higher than that of the unreinforced one. Surface settlement of a model pavement reinforced by a one-layer polymer grid is c o m p a r e d with that of the unreinforced model p a v e m e n t in Fig. 5, illustrating that the polymer grid is very effective in suppressing the settlement due to cyclic loads. If we set the critical settlement for asphaltic concrete at 5 m m , the n u m b e r of cyclic loadings resulting in critical settlement will be increased from 2500 to 7500 in the case of Test-I.4, 10 500 in Test-I.2 and 20 000 in Test-I.3. These test results clearly reveal that the reinforcement SS2 is m o r e effective than the others. The comparative experiments on the effect of SS1 in the two-layer polymer grid system are shown in Fig. 6, which also shows that the polymer grid can effectively suppress the settlement of the
106
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara TABLE 2 Ratio of Moduli of Subgrade Reactions of Model Tests
Unreinforced (A) Test-L1
Reinforced (B) Test-L2
B/A
2.8 6.4 19-3
3.7 8.1 25.3
1.32 1.27 1.31
K3o(Subbase ) / K3o(Subgrade ) Ka0(Base)/K30(Subgrade)
Kao(Asphalt) / K3o(Subgrade )
Number of cycles i0' I
0 I0 2 4
~
-
T
e
ost- .2
s
t
i0' I
-
I
.
10 4 I
3
i0'
(ss2: On top of
z
6
8 ~
o
Test-I.4 p
/ ",. ~ of the subgrade) ~ , ~
:
%
IO
Test-I.1 (Unreinforced) ~,
12
At the center of loading plate 14
Fig. 5. Reinforcement effect of one-layer polymer grid system.
pavement. In the case of Test-II.2 in Fig. 6, the accumulated settlement was generally less than 50% of that of the unreinforced pavement. This suggests that the polymer grid functions more effectively in suppressing the settlement when it is placed on the subgrade rather than the subbase. Studies using geonets have shown similar behavior. 3 Figure 7 shows the strain distribution of a polymer grid and its change with an increasing number of cyclic loadings. The magnitude of the tensile strain of the polymer grid is maximum at the point beneath the center of the loading plate. This figure also shows the maximum tensile strain being larger in Test-I.2 than in Test-I.3. This tendency is consistent with the results in Figs 5 and 6. It can therefore be said that if the settlement of clay subgrade is to be suppressed the polymer grid will work better when it is placed on the subgrade than on the subbase. Tensile forces acting on the polymer grids are shown in Fig. 8. The stress was calculated from the product of strain and Young's modulus. Since the
107
Polymer grid reinforced pavement on soft clay grounds
Number of cycles
oiO
102 I
103 "
10"
I
l0 s
I-
E
T e s t - l l[. 2 (Reinforced)
4-) r( 0 E
T e s t - I I . 3 (Reinforced)
,-4 4-) 4-) 0
"Test-II. i (Unreinforced)7 % t
10
~
At the c e n t e r of loading plate
15 Fig. 6. Reinforcement effect of two-layer polymer grid system.
Polymer g r i d ~ (i layer) ~/Z~2~
800
IL-~
A
? O
v .°
N=lON ]
•
N=IO~
i
/w ~ ;~
-800
Polymer g r i d (2 layers)
,
the subbas,
.H
Test-I.3(ss2) ~%
O
-1600
• est-
.2(+s2) - , -
-2400 -3200 75
I
/
2. . . . . .
?
~ _ \g Q I
I
60 45
I
I
30
15
:~,~
0
1-es~-i1.L J
J
I
15
30
45
Distance from the center
I
60 75
(cm)
Fig. 7. Strain distribution of polymer grid in Test-II.2.
108
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara Distance 15 0 Z
f r o m the c e n t e r 30
(cm)
45 l
60
1
v
2 U
o
3
%4
4 tQ
(9
Test-I.2
(ss2)
O Test-I.4
(ss3)
5 6
Fig. 8. Calculated tensile force in polymer grids.
Young's modulus of SS2 is larger than that of SS3, the tensile force of the former becomes larger than the latter, even though the magnitudes of strain of SS2 are smaller than those of SS3 (see Fig. 7). The distribution of tensile force by the polymer grid is closely related to the distribution of the settlement of the corresponding layer, suggesting that the suppression of the settlement of a pavement by a polymer grid is due substantially to the membrane tension effect. It should be noted that the membrane tension effect of the reinforcement in a base will act effectively only when the reinforcement is placed at a slightly tensioned state and in a concave upward shape. Figures 9(a) and (b) show the results obtained in a preliminary test, where the polymer grid SS1 was placed in a slightly convex shape. Compressive strains were developed in every part of the polymer grid when a vertical load was applied to the surface of the model pavement. However, the polymer grid still worked as a reinforcement, as shown in Fig. 9(a). In this case, the membrane tension effect does not contribute to reinforcement but the interlocking effect does.
3.3 Deformation analysis by FEM To clarify the function of reinforcement in the pavement, a deformation analysis was carried out. In the analysis of a reinforced soil structure with a heterogeneous material such as the polymer grid, an analytical method capable of expressing the behavior of a discontinuous plane should be used. The method used was one where the joint element, considering the mechanism of shear resistance of the polymer grids in soils, is combined with the truss element transmitting the axial force only. 9 The joint element
Polymer grid reinforced pavement on soft clay grounds
(Au r =2.? kN/iII’) 0
Unreinforced
0
Reinforced
Number of cycles (a)
B s -1600 10
ld
u?
104
lo5
Number of cycles
(b) Fig. 9. (a)Resultin a preliminary
test in a model pavement; and (b) strain developed in polymer grid placed in a convex shape in the base.
representing the property of a discontinuous plane has two unit stiffnesses, a normal stiffness, K,, and a shear stiffness, KS. The former expresses a transmission of compressive force only, and the latter a sliding against a shear displacement. The polymer grid is modeled by the truss element whose ends are connected by the pin joints. Figure 10 shows the finite element model for polymer grid reinforced soil. Also, Fig. 11 depicts a finite element model of the reinforced pavement. The analysis for a typical model of Test-I.2 (see Fig. 3(a)), where a one-layer polymer grid is used, was carried out. In this analysis all the
110
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara •
•
.
•
•
°
.
Soil ................ •
°
1
o
_u~0 0 ~ c _~
,
°
9
" *, . t
T
o
.~ °
•
,
o
.
2/D continuum element . . ~
, "0 •
"--!
' /
Polymer . . . . . ..grid o
element \ I / J o i n t element
Fig. 10. Finite e l e m e n t m o d e l for p o l y m e r grid reinforced soil.
Fig. 11. Analytical m o d e l of a reinforced p a v e m e n t .
TABLE 3 M a t e r i a l C o n s t a n t s used in Analysis
E(kN/m 2) Asphaltic concrete Base Subbase Clay
5.0 2.5 1.0 2-0
× 104 × 104 X 104 X 103
v 0.38 0.43 0.43 0.47
Polymer grid reinforcedpavement on soft clay grounds
111
Distance from the center (cm) 15
"~0.2
~
0
4J
~O.ll e-t 4~
to 0.6<' ~
30
45
Calculated
Calculated
O
60
(reinforced)
(unreinforced) Observed (Test-I.2)
i
75
0 •
N=IO ~
Unreinforced Reinforced
0.81
Fig. 12. Comparison between calculated and observed settlements in model pavements (Test-I.2).
Distance from the center (cm) 15 30 45 60 i
o
_
75
Q ~ O
v
2ooo
-
~
~Caleulated
o
= 4000
°~
Q Observed N=10'
6000 Fig. 13. Comparison between calculated and observed strains in polymer grid in Test-I.2. materials composing the pavement were assumed elastic. Young's modulus (E) and Poisson's ratio (v) of the materials, described in Table 3, were used in the numerical analysis. Also, stiffnesses of the joint element were estimated as Ks = 10 x 103 MN/m 3 and Kn = 10 X 102 MN/m 3, respectively. First, we calculated initial stresses resulting from the grid weight. Loading was then applied at the nodal point through the loading plate. Details of this analysis have appeared elsewhere. 1°,11 Figures 12 and 13 show comparisons of the analytical and experimental results of surface settlement and strain of the polymer grid at 10 000 cycles,
112
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
respectively. It can be seen in Fig. 12 that the analytical settlement curve of the reinforced pavement is not so different from that of the unreinforced pavement. This is because the analysis only takes into consideration the membrane tension effect but not the interlocking effect of a polymer grid. On the other hand, the experimental evidence in Fig. 12 clearly indicates that the polymer grid definitely suppressed the settlement of the model pavement under cyclic loading. These facts suggest that the interlocking effect of a polymer grid plays an important role in suppressing surface settlement of the pavement. In order to take into consideration the interlocking effect in the deformation analysis, further investigation is required.
4 P R A C T I C A L INVESTIGATION IN A TEST R O A D 4.1 Outlines of a test road
Since the laboratory tests on model pavements have proved that a polymer grid is useful for reinforcing a pavement on soft ground, it was suggested to the Saga Prefectural Office to carry out a road test of this new method. The main purpose of the test road is to check the function of the polymer grid as a reinforcement of pavement on a very soft ground and also to explore the practical aspects in placing polymer grids in road construction. In November 1987, the construction of test sections started on a public road--the Takeo-Fukudomi route--in Shiroishi town, Saga, and it was completed in February 1988. The thickness of Ariake clay in this area is about 15 m. Pavements of this district have experienced settlement caused not only by consolidation (in both static and cyclic loadings) but also by land subsidence, about 36 mm a year in 1987. Hence the development of a new technique for suppressing non-uniform settlement of pavements is strongly desirable. Figure 14 illustrates the schematic diagram of the 300 m long test road which was divided into six sections of 50 m each. In the subsoil of Ariake clay, tailings from coal mines had been laid to a thickness of about 60--80 cm. Some parts of these tailings were excavated and compacted into a flat surface of approximately the same CBR (California Bearing Ratio) as at other sections. According to preliminary CBR tests of the subgrades, the average values were CBR = 6 in sections 1, 4, 5, and CBR = 4 in sections 2, 3, 6. As stated above, the basic conditions of subgrade of this route were not necessarily preferable to the field test, however it was expected to obtain useful information from this project. Sections 1 and 3 were constructed by conventional methods; the former
Polymer grid reinforced pavement on soft clay grounds Section i
. 2
. 3
5000
5000
5000
3000
. 4
-5
-6
5000
5000
5000
113
Asphalt 15
Base Subbase
stabilization
Polymer grid (SS3)
Earth pressure Polymer { gauge (I] g r i d (SS3)
oi Io.
Earth pressure gauge
(21
.IU
I
Polymer grid
Iol
(SS2)
o Unit
(cm)
Fig. 14. Plan and cross section of the test road. by 40 cm thick lime stabilization and the latter by laying a subbase of 25 cm and a base course of 20 cm, each of them 5 cm thicker than that of the reinforcement sections. In sections 2 and 4, SS3 polymer grids were placed at the interfaces as shown in Fig. 14. In sections 5 and 6, SS2 polymer grids were placed in the same manner. The total depth of the base and subbase in the reinforced sections 2, 4, 5 and 6 was reduced by 10 cm compared with the unreinforced section 3. To investigate the effect of a polymer grid on the function of stress distribution, earth pressure cells were installed in each section as indicated in Fig. 14. The construction costs of sections 2-6 including section 3 were almost the same, i.e. 3600-3800 yen/m E. Section I cost 5600 yen/m E. As far as the function of the pavement itself is concerned, the conventional method of section 1 may be the most preferable one in suppressing non-uniform settlement. However, it is disadvantageous in the respect that the cost is very high and it also requires additional excavation and transportation of the clay in replacing it with a mixture of soil and lime. Due to these problems, engineers have been investigating other methods suitable for pavements on soft ground. These should also be of low cost. Therefore, the following discussion will mainly compare the performances of section 3 and the reinforced sections. To evaluate the bearing capacity of every layer of each section, plate loading tests of 30 cm in diameter were carried out. Also, Benkelman beam tests were performed, and at the same time vertical stresses were
114
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
(a)
(b) Fig. 15. (a) Placing polymer grid on subbasc; (b) damping and compaction of base material; (c) profilometcr test immediately after completion.
Polymer grid reinforced pavement on soft clay grounds
115
(c) Fig. 15.--contd.
measured through earth pressure cells. Figures 15(a), (b) and (c) show the construction works of the test road. During the construction, it was found to be difficult to place the polymer grid in a concave shape under the slight tension required. The polymer grid was therefore simply placed flat on the crusher-run layer without any tensioning. 4.2 Observational results
Moduli of subgrade reactions, K30, obtained by loading tests immediately after the completion of the pavement are shown in Fig. 16. The K values of the subgrades of sections 1 and 6 are low. Those of the other four sections are almost the same. The values of the modulus K30 on the pavement surface indicate that the reinforced sections (2, 4, 5 and 6) are weaker than the conventional method, section 3. Relations of vertical pressures in the five sections, shown in Fig. 17, are similar to that of the moduli K30 in Fig. 16. These results are rather unexpected and disagree with the model test results in the laboratory. Such an unexpected response might be due to an insufficient compaction of the base owing to the elastic rebound of the polymer grid after the roller has passed. The performances of the test sections changed with time after completion, as described below. Figure 18 shows the change in vertical pressure during six months after completion of the test road. At 110 cm below the pavement, the vertical pressure
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
116
e
50
i ~o
Immediately
0
:40
after c o n s t r u c t i o n
0
=30
04-- Base
0
0
0
~
O
/k/
A
in.,__ Subgrade
0 -20-
~
10 o
t
[] D I 1
I 2
I I 3 4 Section
I 5
Subbase
I 6
Fig. 16. Comparison of modulus of subgrade reaction of test sections.
i00 _
•
Immediately
after c o n s t r u c t i o n
Z
•
• •
Earth pressure gage ( I )
~ 50 I.l
A t.
I
i
I
I ....
i
2
3
4
5
Earth p r e s s u r e gage (2)
Section
Fig. 17. Comparison of vertical stresses of test sections.
of all sections did not vary much after completion of construction. However, at 55 cm below the pavement surface the vertical pressure at section 4 increased while those at other sections remain nearly constant. At section 3 the vertical pressure at subgrade was less than others. An elevation survey of surface settlement is indicated in Fig. 19. The change of surface elevation depends mainly on land subsidence, consolidation and deformation of the subgrade. The amount of land subsidence
Polymer grid reinforced pavement on soft clay grounds '
'
I
!
i
117
!
Section
150 z
10o
_.._.....~~ J
~
5
5O
4 2 6 3 5
4~
0 I
I
!
I
I
I
I
0
1
2
3
4
5
6
1
Month
Fig. 18. Change of vertical pressure with time.
!
|
!
i
r
i
I
1
I
I
I
I
I
0
1
2
3 Month
4
5
6
0 A
i 2O
3O
Fig. 19. Change of surface elevation with time.
around this area is estimated to be about 12-16 mm during the last six months, hence the excessive settlements of sections 3 and 6 are considered to be caused by the consolidation and/or deformation of the subsoil. As far as settlement is concerned, sections 2, 4 and 5 appear better than the others. Figure 20 shows the change in flatness of the pavement six months after completion. The changes in most of the reinforced sections were similar to each other. However, section 3 deteriorated quicker than the others, a tendency consistent with the data of Fig. 19. Further measurement of the
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
118
3.0
,
!
,
|
!
,
Section
v
2.0
1.0
I
i
I
0
1
2
I
I
I
I
3 Month
4
5
6
Fig. 20. Change of flatness of pavement surface.
!
t
i
i
,
,
Section
4
A
6
I
I
I
0
1
2
,I
3
i
I
I
4
5
6
Month
Fig. 21. Development of crack percentage with time.
i0
.
.
.
.
.
.
Section
8 6 4
I
I
I
I
!
I
I
0
1
2
3
4
5
6
Month
Fig. 22. Change in rut depth during 6 months.
Polymer grid reinforced pavement on soft clay grounds
Section
- -
~ 2.0
0
119
6
I
I
I
0
1
2
I
3 Month
I
I
I
4
5
6
Fig. 23. Deflection m e a s u r e d by B e n k e l m a n b e a m test.
flatness is required to compare a long-term performance of each section. The crack percentage of the pavement for the last six months is shown in Fig. 21. The high value of crack percentage of section 4 is caused by longitudinal cracks near the shoulder resulting from an irrelevant cause. There are few cracks in sections 1, 3 and 5 compared with sections 2 and 4. Shown in Fig. 22 are ruttings of all sections. The ruts of the surfaces increased with time on all sections. In general, the differences in rut are not large, but after six months sections 3 and 6 are a little worse while sections 4 and 5 are better. Deflections from Benkelman beam tests plotted in Fig. 23 indicate that section 3 is much better than the others. Deflections of all sections have been decreased in a similar rate during the last six months. Shown in Figs 24(a) and (b) are details of the Benkelman beam tests carried out at different points from those of Fig. 23, six months after the construction. Figures 24(a) and (b) show those in sections of C B R = 6% and 4% in the subgrade, respectively. By comparing the deflection behavior of sections 4 and 5, it can be seen that the latter deflection is smaller than the former. This result, taking into consideration the fact that the K value of the subbase of section 5 was the lowest (as shown in Fig. 16), indicates that SS2 is much better than SS3 as reinforcement in the subbase. The same fact becomes apparent by comparing the deflections of sections 2(SS3) and 6(SS2) in Figs 23 and 24(b), referring to the K values of the subgrades of the two sections (see Fig. 16). The above-mentioned results are comparable with those obtained in laboratory tests shown in Fig. 5, as seen in the difference between Test-I.2 and Test-I.4. On comparing the effect of depth of the reinforcement, it was expected that some information would be obtained from the comparison between sections 2 and 4, and also between sections 5 and 6. However, no clear tendency among them has been found so far, as shown in Figs 23 and 24.
120
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
-i.0
40
1.0
i~ /
¢~,,,,,,
SectiOn
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~,'
--
~'~ 5 - -
4
I
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I
0 Distance
from
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5 the
wheel
load
(m)
(a)
-i.0
I
I
I
t
40 1.0 40 40 2.0
3.0 -5
0 Distance
from
5 the
wheel
load
i0 (m)
(b)
Fig. 24. (a) Deflection curve with movement of wheel in Benkeiman beam for sections 1,4, 5; and (b) deflection curve with movement of wheel in Benkelman beam for sections 2, 3, 6.
Polymer grid reinforced pavement on soft clay grounds
121
To summarize the observational results mentioned above: among the reinforced sections with polymer grid, the performance of section 5, where SS2 is placed on subbase, is better than the others. Section 3, by a conventional method, shows disadvantages in settlement and flatness, but is advantageous in crack percentage and deflection, compared with reinforced sections. Reinforcement by a one-layer polymer grid is comparable to the bearing function of a 10 cm thickness of base material. Final evaluation on the function of reinforced pavements will be made after continuous measurement for one year.
5 CONCLUSIONS To clarify the mechanism of polymer grid reinforcement in pavements on soft grounds, laboratory tests on model pavements in a soil tank were carried out and the results were analyzed by FEM. On the basis of the laboratory tests, a test road of 300 m length was constructed with six sections, each made by different methods. These included conventional methods and reinforced methods. The measurements were compared and discussed. From these investigations, the following conclusions can be drawn: 1. Laboratory tests on model pavements clearly show that a polymer grid in base material works effectively in suppressing a non-uniform settlement due to cyclic loading of pavement on soft ground. Of the three kinds of polymer grid tested, SS2 is better than the others when placed on a subbase. 2. Laboratory tests indicate that the magnitude of surface settlement closely relates to the tensile force induced in the polymer grid. This implies that one of the mechanisms of reinforcement by a polymer grid is the membrane tension effect. The polymer grid should be placed in a concave shape to develop the membrane tension. 3. A laboratory test also shows that a polymer grid suppresses the non-uniform settlement of pavements even when it is placed in a slightly convex shape where only compressive strain is induced in the polymer grid. This implies that the second mechanism of reinforcement by a polymer grid might be the interlocking effect. 4. Road tests indicate that the performance of section 5, where polymer grid SS2 is placed on the subbase, is better than those of other reinforced sections. Section 3, by conventional method, has disadvantages in settlement and flatness aspects, but advantages on the crack percentage and deflection, compared with reinforced sections.
122
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
5. From the results obtained by the six-month measurement, it can be said that the long-term function of the one-layer polymer grid as a reinforcement in pavements is comparable to the function of a base material about 10 cm thick. 6. From the practical viewpoint, concerning placement of the polymer grid in road material, the interlocking effect can be expected but much of the working of the membrane tension effect in polymer grids cannot, especially immediately after the completion of the pavement. Therefore, laying of the polymer grid must be carried out so as to induce as much interlocking effect as possible.
ACKNOWLEDGMENTS The first author deeply thanks Messrs Mouri and Otsubo of the Saga Prefectural Office for their continuous support to his research work at Saga University. The authors acknowledge Nippon Hodo Co., Ltd, Mitsui Petrochemical Industries Ltd and Mitsui Petrochemical Industry Products Co., Ltd for their support in carrying out the laboratory tests, the field work and measurements.
REFERENCES 1. Nakamura, R., Onitsuka, K., Aramaki, G. & Miura, N. Geotechnical properties of the very sensitive Ariake clay in Saga plain. Proc. Syrup. Environmental Geotechnics and Problematic Soils and Rocks, Bangkok, 1988, 533-44. 2. Miura, N., Bergado, D. T., Sakai, A. & Nakamura, R. Improvements of soft marine clays by special admixtures using dry and wet jet mixing methods. 9th Southeast Asian Geotechnical Conference. 1, Bangkok, 1987, 8.35-8.46. 3. Yamanouchi, T. & Yasuhara, K. Settlement of clay subgrades after opening to traffic. Proc. 2nd Australia and New Zealand Conf. Geomechanics. 1, Brisbane, 1975, 115-20. 4. Giroud, J. P. & Bonaparte, R. Design and unpaved roads and trafficed areas with geogrids. Proc. Symp. Polymer Grid Reinforcement, London, 1984, 116-27. 5. Miura, N., Use of polymer grid against settlement under cyclic loading. Research Report, Saga Prefectural Office, 1988 (in Japanese). 6. Yamanouchi, T. Experimental study on the improvement of the bearing capacity of soft ground by laying resinous net. Proc. Symp. Foundations on Interbedded Sands, Perth, Australia, 1972, 102-8. 7. Yasuhara, K., Yamanouchi, T., Fujiwara, H., Aoto, H. & Hirao, K. Approximate prediction of soil deformation under drained-repeated loading. Soils and Foundations, 23(2), (1983) 13-25.
Polymer grid reinforced pavement on soft clay grounds
123
8. Yamanouchi, T., Gotoh, K., Yasuhara, K. & Yonemura, N. A new technique of lime stabilization of soft clay. Proc. Symp. Soil reinforcing and Stabilization techniques, Sydney, Australia, 1978, 531-41. 9. Yasuhara, K., Hirao, K., Miura, N., Yamanouchi, T. & Ryokai, K. The use of geotextile against settlement of soft clay under cyclic loading. Proc. 3rd Int. Conf. Geotextiles, Vienna, Austria. 1, 1986, 193-8. 10. Kutara, K., Gomado, M., Takeuchi, T. & Maeda, Y. Use of geotextiles as a countermeasure for differential settlement in road embankments. JSSMFE, 33(5) (1985) 27-32. 11. Sakai, A., Miura, N. & Mouri, K. Model test and analysis on the reinforced base of pavement on soft ground. Reports of the Faculty of Science and Engineering, Saga University, 16(2), (1988) 133-40.
Polymer Grid Reinforced Pavement on Soft Clay Grounds N. Miura, A. Sakai, Y. Taesiri Department of Civil Engineering, Saga University, Saga, 840, Japan
T. Yamanouchi Department of Civil Engineering, Kyushu Sangyo University, Fukuoka, 813, Japan
&
K. Yasuhara Department of Civil Engineering, Nishinippon Institute of Technology, Kanda, 800-03, Japan
ABSTRACT This paper deals with model and field tests for investigating the mechanism of reinforcement by a polymer grid in suppressing non-uniform settlement of pavements constructed on soft clay ground. A series of laboratory tests on reinforced and unreinforced model pavements in a soil tank indicates that the polymer grid is useful for suppressing non-uniform settlement of pavement under cyclic loading. Deformation analysis by FEM is carried out to make clear the reinforcement effect of a polymer grid in a model pavement. To investigate the performance of a polymer grid in practice, a test road of 300 m length with six sections of different kinds of pavement is constructed on soft clay ground. The function of a polymer grid is discussed by comparing the pavements made by conventional and reinforced methods.
1 INTRODUCTION The Saga plain is located around the shores of Ariake bay in northern Kyushu, Japan (Fig. 1). Here a problematic marine clay, Ariake clay, is deposited. The thickness of the clay deposit varies between 15 and 30 m with an average of 20 m. The clay is very soft and one of the most sensitive 99 Geotextiles and Geomembranes 0266-1144/90/$03.50 (~) 1990 Elsevier Science Publishers
Ltd, England. Printed in Great Britain
lO0
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
N~
Kyushu
0
20 40
60
80 i00 km
Fig. 1. Location of Saga Plain.
clays in Japan. The natural water content ranges from 100 to 170%, the liquid limit from 54 to 139%, plastic limit from 28 to 59% and the sensitivity ratio (the ratio of the strength in the undisturbed state to that in the remolded state) usually exceeds 16.1 Since it is very soft and has high sensitivity properties, the Ariake clay often causes geotechnical problems. 2 One of the problems is non-uniform settlement of roadway which includes rutting in the pavement. This can result from soft spots in the clay, insufficient compaction of base materials, consolidation of subsoil caused by traffic loads and also by land subsidence due to the pumping of groundwater. The influence of traffic loads on the settlement must be especially emphasized. For example, route 219 in the Saga plain showed a settlement of 200 cm after opening to traffic. 3 One-third of the settlement was considered to be a result of traffic loading. To suppress non-uniform settlement of the pavement, such techniques as replacement of the soft clay with a good soil, subgrade stabilization, or replacement with a mixture of good material and lime have been widely used in this district. The last
Polymer grid reinforced pavement on soft clay grounds
101
method mentioned, the replacement of the clay with a mixture, has been considered to be one of the most effective means of mitigating the problem. However, this method is very expensive and also causes some problems to engineering and environmental aspects during excavation and transportation of the clay. In the method of replacing the soft clay with a good soil, the total depth of the pavement becomes very thick, and this leads to a problem of high settlement. The thicker--and hence the heavier--the pavement is, the more settlement predominantly due to consolidation occurs. For these reasons the conventional methods are of no use and engineers have been investigating alternative methods. Recently, Giroud & Bonaparte 4 have noticed and studied reinforcement techniques for suppressing non-uniform settlement of roads constructed on soft ground, especially reinfoi-cement in the base course by polymer grids. They presented a design procedure of unpaved roads with a polymer grid, taking into consideration the confinement and load distribution effects of the polymer grid. Miura 5 suggested in preliminary laboratory tests to show that the mechanism of the polymer grid for the reinforcement of a pavement base was mainly the interlocking effect rather than the membrane tension effect. To investigate the applicability of the polymer grid as a reinforcement in a pavement on soft clay ground, a series of laboratory tests were carded out on model pavements in a soil tank, and deformation analysis by FEM was made to study the settlement characteristics of reinforced pavement. Based on the laboratory experiments, a test road of 300 m length was constructed on the Takeo-Fukudomi route in Saga Prefecture. The results obtained are discussed in this paper.
2 C L A Y B E H A V I O R U N D E R CYCLIC L O A D I N G Behavior of pavement on soft ground such as Ariake deposit is greatly influenced by traffic-induced cyclic loading. The low embankment highway on soft subgrades sometimes suffers from the abnormal settlement which is induced by this type of cyclic loading. 3 The characteristic of this cyclic loading is prolonged one-way loading without stress reversal, lasting over a long period of time. Yamanouchi 6 carded out laboratory model tests and indicated that the consolidation of soft clay under cyclic loading is greater than under static loading. This ~tendency was confirmed by cyclic oedometer and drained triaxial tests on Ariake clay. 7 Generally, clay deformation under longterm cyclic loading with inclusion of drainage consists of: (1) shear deformation under undrained cyclic loading, and (2) volume change due
102
N. Miura, A. Sakai, 1I. Taesiri, T. Yamanouchi, K. Yasuhara
to dissipation of cyclically induced pore pressure. Settlements of soft grounds under long-term cyclic loading are influenced by these two components simultaneously, since the clay under traffic-induced cyclic loading is generally in a partially drained condition. Aiming at reducing the traffic-induced settlements of clay subgrades, reinforcing effects of geonets were investigated in the laboratory 6 and also in the field, 8 but the effectiveness of the geonet against an abnormal settlement under cyclic loading were not conclusively proved in these experiments. Since polymer grids of high strength type, such as 'Tensar', have been developed recently, we have been investigating the function of the polymer grid as a reinforcement in the base, by carrying out model tests of unpaved and paved roads on soft clay. 9 The results obtained from the model tests indicated that a polymer grid is much better than a geonet in suppressing non-uniform settlement of a pavement. However, further laboratory tests and analytical study are required before the utilizing of polymer grids as a pavement reinforcement.
3 C Y C L I C L O A D I N G TESTS ON M O D E L P A V E M E N T S 3.1 Materials and testing method Model pavements were constructed in a test box made of concrete (150 cm x 150 cm in area and 100 cm in depth) as shown in Figs 2(a) and (b). The clay used as a subgrade material was Ariake clay, excavated and transported from a site in Saga City. The basic properties of the clay are as follows: specific gravity =2.625; natural water content = 129%; liquid limit = 117%; plastic limit = 39%. As indicated above, the natural water content of the clay is higher than its liquid limit. This tendency is generally recognized as a property of Ariake clay. 1.2 The clay was reconstituted by applying a pressure of 5 kN/m 2 for 2 months into 60 cm thick subgrade. A subbase and a base with or without a polymer grid were compacted and then a 5 cm thick asphaltic concrete layer was added. This test series used three types of biaxially oriented polymer grid, SSl, SS2 and SS3, installed with strain gages. A cyclic load of 200 kN/m 2 and frequency 0-18 Hz (4 seconds loading and 2 seconds unloading) was applied through a steel loading plate of 20 cm in diameter. Vertical settlements of the surface and also those of each layer were measured by using dial gages and differential transducers. Two types of model pavement were tested as shown in Fig. 3. Test-I series are models reinforced with a one-layer polymer grid, and the Test-II series are models with a two-layer polymer grid system. The detailed description of these models is summarized in Table 1.
Polymer grid reinforced pavement on soft clay grounds
103
'Bellofram'cylinder
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(b) Fig. 2. (a) Cyclic loading test for a model pavement; and (b) view of experiment using a test box.
104
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
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~: I '
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.--0--~
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Fig. 3. Schematicdiagramof model pavementsreinforcedby polymergrid in one-layer and two-layer systems. 3.2 Reinforcement effect of a polymer grid
To evaluate the reinforcement effect of a polymer grid in a model pavement, we measured moduli of subgrade reactions K20 using the steel loading plate of 20 cm diameter before and after the cyclic loading and from which the equivalent value of K30 was calculated for each layer, as shown in Fig. 4 (Test-I.2).
Polymer grid reinforced pavement on soft clay grounds
105
TABLE 1 Description of Model Pavements
Number of polymer grid layer
Test no.
Location of polymer grid
Consolidation Polymer (kN/m2) grid
Test-I. 1 1.2 1.3 1.4
SS2 SS2 SS3
10
Test-II. 1
11.2
SS1
11.3
SS1
Unreinforced On top of the subgrade On top of the subbase On top of the subgrade Unreinforced On top of the subgrade and the subbase On top of the subbase and in the base
Modulus of subgrade reaction, AspnalEiC concrete
J ,i 11111
2'111
i I
Subgrade
1111
Test-I.2
Base
Subbase
K30 (xlO ~) (MN/mS)
7-rrrrn 3
I
fl H A
j
Before cyclic loadin E After cyclic loading
Fig. 4. Modulus of subgrade reaction of each layer in Test-I.2. Table 2 shows the ratios of K values of each layer to that of clay layer K¢, m e a s u r e d after cyclic loading tests. The results indicate that the K/Kc value of reinforced p a v e m e n t is 30% higher than that of the unreinforced one. Surface settlement of a model pavement reinforced by a one-layer polymer grid is c o m p a r e d with that of the unreinforced model p a v e m e n t in Fig. 5, illustrating that the polymer grid is very effective in suppressing the settlement due to cyclic loads. If we set the critical settlement for asphaltic concrete at 5 m m , the n u m b e r of cyclic loadings resulting in critical settlement will be increased from 2500 to 7500 in the case of Test-I.4, 10 500 in Test-I.2 and 20 000 in Test-I.3. These test results clearly reveal that the reinforcement SS2 is m o r e effective than the others. The comparative experiments on the effect of SS1 in the two-layer polymer grid system are shown in Fig. 6, which also shows that the polymer grid can effectively suppress the settlement of the
106
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara TABLE 2 Ratio of Moduli of Subgrade Reactions of Model Tests
Unreinforced (A) Test-L1
Reinforced (B) Test-L2
B/A
2.8 6.4 19-3
3.7 8.1 25.3
1.32 1.27 1.31
K3o(Subbase ) / K3o(Subgrade ) Ka0(Base)/K30(Subgrade)
Kao(Asphalt) / K3o(Subgrade )
Number of cycles i0' I
0 I0 2 4
~
-
T
e
ost- .2
s
t
i0' I
-
I
.
10 4 I
3
i0'
(ss2: On top of
z
6
8 ~
o
Test-I.4 p
/ ",. ~ of the subgrade) ~ , ~
:
%
IO
Test-I.1 (Unreinforced) ~,
12
At the center of loading plate 14
Fig. 5. Reinforcement effect of one-layer polymer grid system.
pavement. In the case of Test-II.2 in Fig. 6, the accumulated settlement was generally less than 50% of that of the unreinforced pavement. This suggests that the polymer grid functions more effectively in suppressing the settlement when it is placed on the subgrade rather than the subbase. Studies using geonets have shown similar behavior. 3 Figure 7 shows the strain distribution of a polymer grid and its change with an increasing number of cyclic loadings. The magnitude of the tensile strain of the polymer grid is maximum at the point beneath the center of the loading plate. This figure also shows the maximum tensile strain being larger in Test-I.2 than in Test-I.3. This tendency is consistent with the results in Figs 5 and 6. It can therefore be said that if the settlement of clay subgrade is to be suppressed the polymer grid will work better when it is placed on the subgrade than on the subbase. Tensile forces acting on the polymer grids are shown in Fig. 8. The stress was calculated from the product of strain and Young's modulus. Since the
107
Polymer grid reinforced pavement on soft clay grounds
Number of cycles
oiO
102 I
103 "
10"
I
l0 s
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T e s t - l l[. 2 (Reinforced)
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,-4 4-) 4-) 0
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10
~
At the c e n t e r of loading plate
15 Fig. 6. Reinforcement effect of two-layer polymer grid system.
Polymer g r i d ~ (i layer) ~/Z~2~
800
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A
? O
v .°
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•
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i
/w ~ ;~
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,
the subbas,
.H
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O
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.2(+s2) - , -
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I
/
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?
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I
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I
I
30
15
:~,~
0
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J
I
15
30
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Distance from the center
I
60 75
(cm)
Fig. 7. Strain distribution of polymer grid in Test-II.2.
108
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara Distance 15 0 Z
f r o m the c e n t e r 30
(cm)
45 l
60
1
v
2 U
o
3
%4
4 tQ
(9
Test-I.2
(ss2)
O Test-I.4
(ss3)
5 6
Fig. 8. Calculated tensile force in polymer grids.
Young's modulus of SS2 is larger than that of SS3, the tensile force of the former becomes larger than the latter, even though the magnitudes of strain of SS2 are smaller than those of SS3 (see Fig. 7). The distribution of tensile force by the polymer grid is closely related to the distribution of the settlement of the corresponding layer, suggesting that the suppression of the settlement of a pavement by a polymer grid is due substantially to the membrane tension effect. It should be noted that the membrane tension effect of the reinforcement in a base will act effectively only when the reinforcement is placed at a slightly tensioned state and in a concave upward shape. Figures 9(a) and (b) show the results obtained in a preliminary test, where the polymer grid SS1 was placed in a slightly convex shape. Compressive strains were developed in every part of the polymer grid when a vertical load was applied to the surface of the model pavement. However, the polymer grid still worked as a reinforcement, as shown in Fig. 9(a). In this case, the membrane tension effect does not contribute to reinforcement but the interlocking effect does.
3.3 Deformation analysis by FEM To clarify the function of reinforcement in the pavement, a deformation analysis was carried out. In the analysis of a reinforced soil structure with a heterogeneous material such as the polymer grid, an analytical method capable of expressing the behavior of a discontinuous plane should be used. The method used was one where the joint element, considering the mechanism of shear resistance of the polymer grids in soils, is combined with the truss element transmitting the axial force only. 9 The joint element
Polymer grid reinforced pavement on soft clay grounds
(Au r =2.? kN/iII’) 0
Unreinforced
0
Reinforced
Number of cycles (a)
B s -1600 10
ld
u?
104
lo5
Number of cycles
(b) Fig. 9. (a)Resultin a preliminary
test in a model pavement; and (b) strain developed in polymer grid placed in a convex shape in the base.
representing the property of a discontinuous plane has two unit stiffnesses, a normal stiffness, K,, and a shear stiffness, KS. The former expresses a transmission of compressive force only, and the latter a sliding against a shear displacement. The polymer grid is modeled by the truss element whose ends are connected by the pin joints. Figure 10 shows the finite element model for polymer grid reinforced soil. Also, Fig. 11 depicts a finite element model of the reinforced pavement. The analysis for a typical model of Test-I.2 (see Fig. 3(a)), where a one-layer polymer grid is used, was carried out. In this analysis all the
110
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara •
•
.
•
•
°
.
Soil ................ •
°
1
o
_u~0 0 ~ c _~
,
°
9
" *, . t
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o
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,
o
.
2/D continuum element . . ~
, "0 •
"--!
' /
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element \ I / J o i n t element
Fig. 10. Finite e l e m e n t m o d e l for p o l y m e r grid reinforced soil.
Fig. 11. Analytical m o d e l of a reinforced p a v e m e n t .
TABLE 3 M a t e r i a l C o n s t a n t s used in Analysis
E(kN/m 2) Asphaltic concrete Base Subbase Clay
5.0 2.5 1.0 2-0
× 104 × 104 X 104 X 103
v 0.38 0.43 0.43 0.47
Polymer grid reinforcedpavement on soft clay grounds
111
Distance from the center (cm) 15
"~0.2
~
0
4J
~O.ll e-t 4~
to 0.6<' ~
30
45
Calculated
Calculated
O
60
(reinforced)
(unreinforced) Observed (Test-I.2)
i
75
0 •
N=IO ~
Unreinforced Reinforced
0.81
Fig. 12. Comparison between calculated and observed settlements in model pavements (Test-I.2).
Distance from the center (cm) 15 30 45 60 i
o
_
75
Q ~ O
v
2ooo
-
~
~Caleulated
o
= 4000
°~
Q Observed N=10'
6000 Fig. 13. Comparison between calculated and observed strains in polymer grid in Test-I.2. materials composing the pavement were assumed elastic. Young's modulus (E) and Poisson's ratio (v) of the materials, described in Table 3, were used in the numerical analysis. Also, stiffnesses of the joint element were estimated as Ks = 10 x 103 MN/m 3 and Kn = 10 X 102 MN/m 3, respectively. First, we calculated initial stresses resulting from the grid weight. Loading was then applied at the nodal point through the loading plate. Details of this analysis have appeared elsewhere. 1°,11 Figures 12 and 13 show comparisons of the analytical and experimental results of surface settlement and strain of the polymer grid at 10 000 cycles,
112
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
respectively. It can be seen in Fig. 12 that the analytical settlement curve of the reinforced pavement is not so different from that of the unreinforced pavement. This is because the analysis only takes into consideration the membrane tension effect but not the interlocking effect of a polymer grid. On the other hand, the experimental evidence in Fig. 12 clearly indicates that the polymer grid definitely suppressed the settlement of the model pavement under cyclic loading. These facts suggest that the interlocking effect of a polymer grid plays an important role in suppressing surface settlement of the pavement. In order to take into consideration the interlocking effect in the deformation analysis, further investigation is required.
4 P R A C T I C A L INVESTIGATION IN A TEST R O A D 4.1 Outlines of a test road
Since the laboratory tests on model pavements have proved that a polymer grid is useful for reinforcing a pavement on soft ground, it was suggested to the Saga Prefectural Office to carry out a road test of this new method. The main purpose of the test road is to check the function of the polymer grid as a reinforcement of pavement on a very soft ground and also to explore the practical aspects in placing polymer grids in road construction. In November 1987, the construction of test sections started on a public road--the Takeo-Fukudomi route--in Shiroishi town, Saga, and it was completed in February 1988. The thickness of Ariake clay in this area is about 15 m. Pavements of this district have experienced settlement caused not only by consolidation (in both static and cyclic loadings) but also by land subsidence, about 36 mm a year in 1987. Hence the development of a new technique for suppressing non-uniform settlement of pavements is strongly desirable. Figure 14 illustrates the schematic diagram of the 300 m long test road which was divided into six sections of 50 m each. In the subsoil of Ariake clay, tailings from coal mines had been laid to a thickness of about 60--80 cm. Some parts of these tailings were excavated and compacted into a flat surface of approximately the same CBR (California Bearing Ratio) as at other sections. According to preliminary CBR tests of the subgrades, the average values were CBR = 6 in sections 1, 4, 5, and CBR = 4 in sections 2, 3, 6. As stated above, the basic conditions of subgrade of this route were not necessarily preferable to the field test, however it was expected to obtain useful information from this project. Sections 1 and 3 were constructed by conventional methods; the former
Polymer grid reinforced pavement on soft clay grounds Section i
. 2
. 3
5000
5000
5000
3000
. 4
-5
-6
5000
5000
5000
113
Asphalt 15
Base Subbase
stabilization
Polymer grid (SS3)
Earth pressure Polymer { gauge (I] g r i d (SS3)
oi Io.
Earth pressure gauge
(21
.IU
I
Polymer grid
Iol
(SS2)
o Unit
(cm)
Fig. 14. Plan and cross section of the test road. by 40 cm thick lime stabilization and the latter by laying a subbase of 25 cm and a base course of 20 cm, each of them 5 cm thicker than that of the reinforcement sections. In sections 2 and 4, SS3 polymer grids were placed at the interfaces as shown in Fig. 14. In sections 5 and 6, SS2 polymer grids were placed in the same manner. The total depth of the base and subbase in the reinforced sections 2, 4, 5 and 6 was reduced by 10 cm compared with the unreinforced section 3. To investigate the effect of a polymer grid on the function of stress distribution, earth pressure cells were installed in each section as indicated in Fig. 14. The construction costs of sections 2-6 including section 3 were almost the same, i.e. 3600-3800 yen/m E. Section I cost 5600 yen/m E. As far as the function of the pavement itself is concerned, the conventional method of section 1 may be the most preferable one in suppressing non-uniform settlement. However, it is disadvantageous in the respect that the cost is very high and it also requires additional excavation and transportation of the clay in replacing it with a mixture of soil and lime. Due to these problems, engineers have been investigating other methods suitable for pavements on soft ground. These should also be of low cost. Therefore, the following discussion will mainly compare the performances of section 3 and the reinforced sections. To evaluate the bearing capacity of every layer of each section, plate loading tests of 30 cm in diameter were carried out. Also, Benkelman beam tests were performed, and at the same time vertical stresses were
114
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
(a)
(b) Fig. 15. (a) Placing polymer grid on subbasc; (b) damping and compaction of base material; (c) profilometcr test immediately after completion.
Polymer grid reinforced pavement on soft clay grounds
115
(c) Fig. 15.--contd.
measured through earth pressure cells. Figures 15(a), (b) and (c) show the construction works of the test road. During the construction, it was found to be difficult to place the polymer grid in a concave shape under the slight tension required. The polymer grid was therefore simply placed flat on the crusher-run layer without any tensioning. 4.2 Observational results
Moduli of subgrade reactions, K30, obtained by loading tests immediately after the completion of the pavement are shown in Fig. 16. The K values of the subgrades of sections 1 and 6 are low. Those of the other four sections are almost the same. The values of the modulus K30 on the pavement surface indicate that the reinforced sections (2, 4, 5 and 6) are weaker than the conventional method, section 3. Relations of vertical pressures in the five sections, shown in Fig. 17, are similar to that of the moduli K30 in Fig. 16. These results are rather unexpected and disagree with the model test results in the laboratory. Such an unexpected response might be due to an insufficient compaction of the base owing to the elastic rebound of the polymer grid after the roller has passed. The performances of the test sections changed with time after completion, as described below. Figure 18 shows the change in vertical pressure during six months after completion of the test road. At 110 cm below the pavement, the vertical pressure
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
116
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of all sections did not vary much after completion of construction. However, at 55 cm below the pavement surface the vertical pressure at section 4 increased while those at other sections remain nearly constant. At section 3 the vertical pressure at subgrade was less than others. An elevation survey of surface settlement is indicated in Fig. 19. The change of surface elevation depends mainly on land subsidence, consolidation and deformation of the subgrade. The amount of land subsidence
Polymer grid reinforced pavement on soft clay grounds '
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around this area is estimated to be about 12-16 mm during the last six months, hence the excessive settlements of sections 3 and 6 are considered to be caused by the consolidation and/or deformation of the subsoil. As far as settlement is concerned, sections 2, 4 and 5 appear better than the others. Figure 20 shows the change in flatness of the pavement six months after completion. The changes in most of the reinforced sections were similar to each other. However, section 3 deteriorated quicker than the others, a tendency consistent with the data of Fig. 19. Further measurement of the
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
118
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Polymer grid reinforced pavement on soft clay grounds
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119
6
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flatness is required to compare a long-term performance of each section. The crack percentage of the pavement for the last six months is shown in Fig. 21. The high value of crack percentage of section 4 is caused by longitudinal cracks near the shoulder resulting from an irrelevant cause. There are few cracks in sections 1, 3 and 5 compared with sections 2 and 4. Shown in Fig. 22 are ruttings of all sections. The ruts of the surfaces increased with time on all sections. In general, the differences in rut are not large, but after six months sections 3 and 6 are a little worse while sections 4 and 5 are better. Deflections from Benkelman beam tests plotted in Fig. 23 indicate that section 3 is much better than the others. Deflections of all sections have been decreased in a similar rate during the last six months. Shown in Figs 24(a) and (b) are details of the Benkelman beam tests carried out at different points from those of Fig. 23, six months after the construction. Figures 24(a) and (b) show those in sections of C B R = 6% and 4% in the subgrade, respectively. By comparing the deflection behavior of sections 4 and 5, it can be seen that the latter deflection is smaller than the former. This result, taking into consideration the fact that the K value of the subbase of section 5 was the lowest (as shown in Fig. 16), indicates that SS2 is much better than SS3 as reinforcement in the subbase. The same fact becomes apparent by comparing the deflections of sections 2(SS3) and 6(SS2) in Figs 23 and 24(b), referring to the K values of the subgrades of the two sections (see Fig. 16). The above-mentioned results are comparable with those obtained in laboratory tests shown in Fig. 5, as seen in the difference between Test-I.2 and Test-I.4. On comparing the effect of depth of the reinforcement, it was expected that some information would be obtained from the comparison between sections 2 and 4, and also between sections 5 and 6. However, no clear tendency among them has been found so far, as shown in Figs 23 and 24.
120
N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
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Polymer grid reinforced pavement on soft clay grounds
121
To summarize the observational results mentioned above: among the reinforced sections with polymer grid, the performance of section 5, where SS2 is placed on subbase, is better than the others. Section 3, by a conventional method, shows disadvantages in settlement and flatness, but is advantageous in crack percentage and deflection, compared with reinforced sections. Reinforcement by a one-layer polymer grid is comparable to the bearing function of a 10 cm thickness of base material. Final evaluation on the function of reinforced pavements will be made after continuous measurement for one year.
5 CONCLUSIONS To clarify the mechanism of polymer grid reinforcement in pavements on soft grounds, laboratory tests on model pavements in a soil tank were carried out and the results were analyzed by FEM. On the basis of the laboratory tests, a test road of 300 m length was constructed with six sections, each made by different methods. These included conventional methods and reinforced methods. The measurements were compared and discussed. From these investigations, the following conclusions can be drawn: 1. Laboratory tests on model pavements clearly show that a polymer grid in base material works effectively in suppressing a non-uniform settlement due to cyclic loading of pavement on soft ground. Of the three kinds of polymer grid tested, SS2 is better than the others when placed on a subbase. 2. Laboratory tests indicate that the magnitude of surface settlement closely relates to the tensile force induced in the polymer grid. This implies that one of the mechanisms of reinforcement by a polymer grid is the membrane tension effect. The polymer grid should be placed in a concave shape to develop the membrane tension. 3. A laboratory test also shows that a polymer grid suppresses the non-uniform settlement of pavements even when it is placed in a slightly convex shape where only compressive strain is induced in the polymer grid. This implies that the second mechanism of reinforcement by a polymer grid might be the interlocking effect. 4. Road tests indicate that the performance of section 5, where polymer grid SS2 is placed on the subbase, is better than those of other reinforced sections. Section 3, by conventional method, has disadvantages in settlement and flatness aspects, but advantages on the crack percentage and deflection, compared with reinforced sections.
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N. Miura, A. Sakai, Y. Taesiri, T. Yamanouchi, K. Yasuhara
5. From the results obtained by the six-month measurement, it can be said that the long-term function of the one-layer polymer grid as a reinforcement in pavements is comparable to the function of a base material about 10 cm thick. 6. From the practical viewpoint, concerning placement of the polymer grid in road material, the interlocking effect can be expected but much of the working of the membrane tension effect in polymer grids cannot, especially immediately after the completion of the pavement. Therefore, laying of the polymer grid must be carried out so as to induce as much interlocking effect as possible.
ACKNOWLEDGMENTS The first author deeply thanks Messrs Mouri and Otsubo of the Saga Prefectural Office for their continuous support to his research work at Saga University. The authors acknowledge Nippon Hodo Co., Ltd, Mitsui Petrochemical Industries Ltd and Mitsui Petrochemical Industry Products Co., Ltd for their support in carrying out the laboratory tests, the field work and measurements.
REFERENCES 1. Nakamura, R., Onitsuka, K., Aramaki, G. & Miura, N. Geotechnical properties of the very sensitive Ariake clay in Saga plain. Proc. Syrup. Environmental Geotechnics and Problematic Soils and Rocks, Bangkok, 1988, 533-44. 2. Miura, N., Bergado, D. T., Sakai, A. & Nakamura, R. Improvements of soft marine clays by special admixtures using dry and wet jet mixing methods. 9th Southeast Asian Geotechnical Conference. 1, Bangkok, 1987, 8.35-8.46. 3. Yamanouchi, T. & Yasuhara, K. Settlement of clay subgrades after opening to traffic. Proc. 2nd Australia and New Zealand Conf. Geomechanics. 1, Brisbane, 1975, 115-20. 4. Giroud, J. P. & Bonaparte, R. Design and unpaved roads and trafficed areas with geogrids. Proc. Symp. Polymer Grid Reinforcement, London, 1984, 116-27. 5. Miura, N., Use of polymer grid against settlement under cyclic loading. Research Report, Saga Prefectural Office, 1988 (in Japanese). 6. Yamanouchi, T. Experimental study on the improvement of the bearing capacity of soft ground by laying resinous net. Proc. Symp. Foundations on Interbedded Sands, Perth, Australia, 1972, 102-8. 7. Yasuhara, K., Yamanouchi, T., Fujiwara, H., Aoto, H. & Hirao, K. Approximate prediction of soil deformation under drained-repeated loading. Soils and Foundations, 23(2), (1983) 13-25.
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8. Yamanouchi, T., Gotoh, K., Yasuhara, K. & Yonemura, N. A new technique of lime stabilization of soft clay. Proc. Symp. Soil reinforcing and Stabilization techniques, Sydney, Australia, 1978, 531-41. 9. Yasuhara, K., Hirao, K., Miura, N., Yamanouchi, T. & Ryokai, K. The use of geotextile against settlement of soft clay under cyclic loading. Proc. 3rd Int. Conf. Geotextiles, Vienna, Austria. 1, 1986, 193-8. 10. Kutara, K., Gomado, M., Takeuchi, T. & Maeda, Y. Use of geotextiles as a countermeasure for differential settlement in road embankments. JSSMFE, 33(5) (1985) 27-32. 11. Sakai, A., Miura, N. & Mouri, K. Model test and analysis on the reinforced base of pavement on soft ground. Reports of the Faculty of Science and Engineering, Saga University, 16(2), (1988) 133-40.