UNDRAINED SHEAR STRENGTH PREDICTION AND STABILITY ANALYSIS OF GEOCOMPOSITE REINFORCED EMBANKMENT WITH CLAYEY BACKFILL

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1 Proceeding of the 4 th Asian Regional Conference on Geosynthetics June 7 -, 8 Shanghai, China 585 UNDRAINED SHEAR STRENGTH PREDICTION AND STABILITY ANALYSIS OF GEOCOMPOSITE REINFORCED EMBANKMENT WITH CLAYEY BACKFILL J.-C. Chai, T. Hino, Y. Igaya and A. Miyazaki 4 ABSTRUCT: A method of predicting undrained shear strength (S u ) within a dual function geocomposite reinforced embankment with clayey backfill is described, in which the effects of discharge capacity (Q w ) of the geocomposite, spacing (B) between geocomposite layers, construction speed (V), and the coefficient of consolidation (C v ) of the backfill are considered. Then Q w values of three geocomposites were measured by laboratory tests under the confinement of clayey soils. The test results indicate that to maintain a higher long-term (more than month) confined in clayey soil Q w value, a geocomposite must have a drainage core or tube. Then by referring the measured Q w values, the effects of Q w, B, as well as V on S u distribution within an assumed 5 m high embankment were investigated by the proposed method. With the predicted S u values, the factor of safety (FS) of the assumed embankment has been investigated by Bishop s slip circle method, and the results demonstrate that a 5 m high embankment with clayey backfill can be successfully constructed using dual function geocomposite. Keywords: geocomposite, drainage, reinforcement, embankment, consolidation, factor of safety. INTRODUCTION How to effectively treat the waste clayey soils generated from construction site or dredged from ports is one of the geoenvironmental problems. On the other hand, there is a shortage of granular materials for embankment construction in Japan. Therefore, it is desirable to use waste clayey soils as embankment fills. Since the strength of waste clayey soil is low, in many cases, it can t be directly used as a construction material. A commonly used method is to mix cement or lime into the waste clayey soil to improve its engineering properties first, and then used in engineering constructions. There are reports of using this kind of method for airport construction (Tsuchida and Kang, ). One of the problems of this kind of method is the high ph values of the treated soil, which may have a negative geoenvironmental impact. Another method is to use dual function (reinforcement and drainage) geocomposites. The drainage effect of geocomposite accelerates the self-weight induced consolidation and increases the undrained shear strength (S u ) of the clayey backfill, and the mobilized tensile force in the geocomposite further increases the stability of the embankment. There are reported case histories of the method (e.g. Tatsuoka and Yamauchi 986). However, regarding design a dual function geocomposite reinforced embankment with clayey backfill, there are still issues need to be resolved, such as how to predict the distribution of S u values within an embankment. In this paper, firstly, a method of predicting S u values within a dual function geocomposite reinforced embankment with clayey backfill is described. Then the results of confined in clayey soil discharge capacity tests of three geocomposites are presented. Finally, the factors influencing S u values within an assumed 5 m high embankment and therefore the factor of safety (FS) of the embankment have been investigated. UNDRAINED SHEAR STRENGTH (S u ) To design an embankment with clayey backfill and reinforced by a dual function geocomposite, one of the tasks is to predict S u values within the embankment during the embankment construction. The value of S u of a soil is a function of effective stress, stress history and the mechanical properties of the soil. Ladd (99) proposed an empirical equation to calculate S u value in a ground as follows: S ) ' m u = S σ v ( OCR () Professor, Department of Architecture and Civil Engineering, Saga University, Japan, chai@cc.saga-u.ac.jp Lecturer, Institute of Lowland technology, Saga University, Japan, hino@ilt.saga-u.ac.jp Chief Engineer, Department of Civil Engineering, Saga Prefecture, Japan, igaya-yutaka@pref.saga.lg.jp 4 Chief Engineer, Department of Civil Engineering, Saga Prefecture, Japan, miyazaki-atsushi@pref.saga.lg.jp

2 586 l u z u z= = z=l z B = Drain x Direction of water flow Deformation Deformation z u x= = Deformation Drain x k s k (a) Plan strain vertical drain (b) Plan strain horizontal drain Fig. Plane strain unit cells where σ v is vertical effective stress, OCR is overconsolidation ratio, and S and m are constants. Ladd (99) proposed that the range for S is.6 to.5 and for m is.75 to.. For an embankment construction, OCR =., and to predict S u, the value of σ v has to be evaluated first. Chai and Miura () proposed a simple method for calculating the degree of self-weight induced consolidation of embankment backfill and it is summarized as follows. Hird et al. (99) extended Hansbo s solution (98) for vertical drain consolidation under axisymmetric condition to plane strain condition. A vertical plane strain unit cell adopted by Hird et al. (99) is shown in Fig. (a). In the case of embankment construction, the geocomposite serves as a drain is placed horizontally. Figure (b) shows a horizontal plane strain unit cell. Although the deformation patterns in Figs. (a) and (b) are different, both of them satisfy equal strain assumption, a basic assumption of Hansbo s solution. Therefore, equations derived for Fig. (a) case can be used for Fig. (b) case by using the corresponding notations adopted. With the notations in Fig. (b), the average degree of consolidation at a distance of x from drainage surface (x = ) is as follows: u x 8T U = exp( ) () μ The expressions for T and μ are as follows: C t T = () 4B k k bs (4) μ = + ( l x x ) + ( )( bs bs + ) B Q k w where: C is the coefficient of consolidation of clayey soil, t is time, B is the half width of a plane strain unit cell, k and k s are the hydraulic conductivities of clayey soil and smear zone around a drain, respectively, Q w is discharge capacity of a geocomposite per unit width, l is drainage length, x is the distance from drainage surface, and b s =B/B (B is the half width of the smear zone). s B x=l B = During embankment construction, backfill is placed layer by layer. To predict pore pressure variation during embankment construction by Equations () to (4), following assumptions are made. () Approximate the construction process by stepwise loads. () Take total load at i step as p i, degree of consolidation at time t i as U i. At t i, incremental load of j step Δp j is applied, then for total load p j =p i + Δp j, the degree of consolidation (U jo ) at t i can be calculated as: U U p i i jo = (5) p + Δp i j An imaginable time corresponding to U jo (under load p j ) is: B t = μ ln( U ) jo jo (6) C Using the moment (t i ) of applying Δp j as a new origin for time, if the time from the new origin is t j, then, time for calculating degree of consolidation at time t j will be (t jo +t j ). When the average degree of consolidation of each layer of an embankment at a given time is known, σ v and therefore S u can be calculated. For embankment construction, it can be considered that there is no smear zone (k/k s =, and B l is not needed). Then with the method presented above, there are totally six parameters needed: Q w ; C; k; B; construction speed (V), and the constant S in Eq. () (OCR =, and m is not needed). Further V and B can be specified by design. Then remaining four parameters are, Q w, C, k, and S. S is an empirical parameter and can be determined based on local experience. Finally, Q w, C, and k must be measured by laboratory or field tests. Some test results on Q w values of three geocomposites will be described in next section. LABORATORY TEST RESULTS ON Q w Test Equipment Q w values provided by manufacturers are normally under the condition that the geocomposites are confined by rubber membrane or between two parallel plates (ASTM ). However, if using geocomposite for constructing an embankment with clayey backfill, there is a possibility that clayey particles enter the drainage channel through filter and influence Q w value. The flow rates of three geocomposites were investigated under the confinement of clayey soil using a triaxial type discharge capacity test device (Fig. ). During the tests, tap water was used and re-circulated by a micro-pump. The test

3 587 procedure has been described elsewhere (e.g. Chai and Miura ). Materials Type 断面構造 Sketch Non woven geotextile 不織布 Woven A geotextile 織布 4.5 mm 断面写真 Photo 材料 Material 不織布 : ポリプロピレン polypropylene 織布 : ポリプロピレン単位面積重量 Unit weight 678g/m Geocomposite Three geocomposites (A, B and C) were used and their structures and some of index properties are given in Fig.. Clayey soils Two types of clayey soils were used as confinement materials. One was remolded Ariake clay. Its liquid limit was 5% and plastic limit was 54%. The clay content (< μm) was about 48%. Another one was a mixture of the Ariake clay and decomposed granite passing through. mm sieve with a ratio of : by dry weight. It will be called mixed soil later. The liquid limit and plastic limit of the mixed soil were 6.6% and.7% respectively. Test Results For geocomposite A and B, the tests were conducted under the confinement of both the Ariake clay and the mixed soil, and for geocomposite C, the tests were conducted only under the mixed soil confinement. All the tests were conducted with a hydraulic gradient of.. The confining pressures (σ) adopted were, 5, and kpa. The test results are given in Figs 4 to 6 for geocomposite A, B and C respectively. Generally, the flow rates reduced with elapsed time and increase of σ. It is considered that the reduction on the flow rate with elapsed time is mainly due to the clogging of the drainage paths caused by the soil particles entered the openings of the geotextiles. Geocomposite A, which does not have a drainage tube or core, had the lowest flow rate and C had the highest flow rate. For geocomposite A (Fig. 4), when σ = kpa, the flow rate was practically zero. Under σ = and 5 kpa, at one month elapsed time, it became to m /year/m. The test with the mixed soil confinement showed a Head difference ΔH Confine pressure Sample (i) Set-up Fig. Illustrations of test set-up Fixed plate Clayey soil mm Rubber membrane Geocomposite (ii) Cross-section for geocomposite sample B C Non woven geotextile 不織布 PETモノフィラメント PET tube Non woven geotextile 不織布 Core コア 4. mm 7. mm Non-woven geotextile: 不織布 polypropylene : ポリプロピレン 8g/m Tube: PET filament チューブ ;PETモノフィラメント spacing: mm, inside diameter: mm Non-woven geotextile: 不織布 polypropylene : ポリプロピレン Core: high density コア polyethylene : 高密度ポリエチレン g/m Fig. Structures and index properties of geocomposites Rate of water flow, Q (m /year/m) σ=kpa Ariake σ=kpa} clay σ=kpa} Mixed soil Geocomposite A, i =. 4 Elapsed time, t (days) Fig. 4 Rate of flow of geocomposite A Rate for water flow, Q (m /year/m) σ=kpa σ=kpa σ=kpa } Ariake clay } Mixed soil Geocomposite B, i =. 4 5 Elapsed time, t (days) Fig. 5 Rate of flow of geocomposite B Rate of water flow, Q (m /year/m) Geocomposite C, i =. Mixed soil σ=kpa 4 Elapsed time, t (days) Fig. 6 Rate of flow of geocomposite C

4 588 higher long-term (more than one month) flow rate. Geocomposite B has one drainage tube per. m width and had higher flow rate than A. At one month of elapsed time, the flow rate is to m /year/m under σ = to kpa (Fig. 5). Geocomposite C has a drainage core and the reduction on flow rate with the elapsed time is less significant than A and B, and under σ = kpa, within one month, the flow rate almost not reduced (Fig. 6). Also, the initial flow rates for σ = 5 and kpa are almost the same (Fig. 6). Short-term (lasted for about hours) test results indicate that for geocomposite C, when the confining pressure increased to more than 5 kpa, obvious reduction of the flow rate was observed and the flow rate of σ = kpa was about 7% of that of σ = kpa. This result indicates that the filter of geocomposite C is strong. Above discussions clearly indicate that to maintain a higher long-term (more than one month) flow rate under clayey soil confinement with a σ of more than 5 kpa, a geocomposite must has a drainage core and a strong filter. The discharge capacity (Q w ) is defined as the flow rate under a hydraulic gradient of.. If linearly converting the results given in Figs 4 to 6 to i =., at one month elapsed time, the range of Q w will be to, m /year/m. STABILITY ANALYSIS Conditions Assumed For Analysis The geometry of a 5 m high embankment assumed for stability analysis is shown in Fig. 7. It is further assumed that the failure surface will not pass through the foundation. As shown in Fig. 7 the geocomposite is discontinued at the middle of the embankment. Although whether the geocomposite will be continued through the whole width of an embankment depends on the construction procedure, discontinuous assumption is in the safe side in term of stability analysis, in which the possible pullout failure of the geocomposite can be considered. Other assumed conditions are listed in Table. The Q w values are selected referring the laboratory test results presented above. The basic value of m /year/m is about the average values of geocomposite B at one month elapsed time for σ = to kpa. Regarding the allowable tensile force (T a ), the ultimate tensile strength provided by the manufacturers for geocomposite A, B and C are: 49. kn/m, kn/m and 8 kn/m respectively. For geocomposite A, the test was conducted using a mm wide strip sample with a strain rate of %/min. The failure strain was about %. For geocomposite B, the test was conducted using Table Parameters used for stability analysis Parameters Discharge capacity of geocomposite, Q w (m /m/year) Basic Range of value variation Spacing between geocomposite, B (m).5.5. Coefficient of consolidation of. backfill, C v (m /day) Speed of construction, V (m/day).5 Hydraulic conductivity of backfill, k (m/day) Total unit weight of backfill, γ t (kn/m ) Allowable tensile force in geocomposite, T (kn/m).8. Geocomposite and. 5 m Fig. 7 Geometry of an assumed embankment sample of mm wide and mm long (between clamps) with a strain rate of %/min. The failure strain was about 4%. For geocomposite C, it is mentioned that the test was conducted per ISO 9 (ISO 99) and the failure strain was 6%. Referring the above information, T a values of, 5 and kn/m are assumed. It is not intended to use Q w and T a values of a specific geocomposite, rather the analysis is try to provide some general information on the effect of Q w and T a on the stability of a dual function geocomposite reinforced embankment with clayey backfill. Predicted S u Values Within the Embankment With the method presented above and the values of parameters listed in Table, the distribution of S u within the assumed embankment can be predicted using Eq. (). In the calculation, the adopted value of constant S in Eq. () is.5. Effect of Q w and B Figure 8 shows the effect of Q w and B (spacing) on S u distribution at the end of construction. It can be seen that Q w has a significant influence on S u value. S u increases with Q w, but the increase rate reduced with the increase of Q w. Increasing in spacing (B) reduced S u CL

5 589 Embankment height, H (m) 5 4 C =. m /day T =.5 m/day Spacing =.5 m Spacing =. m Q w = m /year Undrained shear strength, S u (kpa) Fig. 8 Effect of Q w on predicted S u values Embankment height, H (m) 5 4 V=. m/day.5 Spacing =.5 m C =. m /day Q w = m /year Undrained shear strength, S u (kpa) Fig. 9 Effect of construction speed on S u values values. However, with the increase of Q w value, the difference between B =.5 m and. m cases gradually reduced, which means that using a geocomposite with a higher Q w value, a relatively larger spacing (e.g. B =. m) can be adopted and it may result in a more economic design. Effect of construction speed (V) Figure 9 shows the distributions of S u at the end of construction with three different V values. Reducing in V increases the time for self-weight induced consolidation and results in higher S u values. Factor Of Safety (FS) With the predicted S u values given in Figs 8 and 9, and T a values in Table, FS of the assumed embankment (not including foundation) was analyzed by using Bishop s slip circle method (Bishop 955). The program used is ReSSA (.) (Leshchinsky ). Failure mechanisms of the reinforcement considered are () rupture and () pullout. In case of pullout failure, the interface shear resistance between a geocomposite and backfill soil was 8% of the corresponding shear strength of the backfill soil. Effect of Q w. and B Figure shows the variation of FS with Q w for B =.5 m (solid line) and. m (dashed line) cases. It can be seen that FS increases with the increase of Q w, but the increment rate is gradually reduced. When Q w > 5 m /year/m, the increase rate is small. If taking FS >. as a basic requirement (JRA 999), for not considering the reinforcement effect (T a = ), B =.5 m and Q w 5 m /year/m, and B =. m and Q w m /year/m cases can satisfy the requirement. Comparing the FS of B =.5 m and. m cases, two points can be made. The first one is that with the increase of Q w, the difference between FS values of B =.5 m and. m becomes smaller (the difference on S u values becomes smaller, Fig. 8). The second one is that for B =. m case, the effect of T a on FS is smaller than that of B =.5 m case because B =. m case has the less number of reinforcement layers. Effect of construction speed (V) The effect of construction speed on FS is shown in Fig. The slower the construction speed, the higher the FS value. For Q w = m /year/m, without reinforcement effect, FS >. requirement can t be satisfied even for V =.5 m/day case. FS >. can be satisfied for V =.5 m/day and T a = 5 kn/m case. Effect of total unit weight (γ t ) of embankment backfill The degree of self-weight induced consolidation is not influenced by γ t (magnitude of load). Therefore, the vertical effective stress (σ v ) within the embankment, and therefore S u values (Eq. ()) in the embankment are linearly proportional to γ t. However, increase on γ t will increase driven force for slip failure. As a result, if not FS.5 V =.5 m/day.5 B =.5 m B =. m T = T = T = 5 kn/m T = 5 kn/m T = kn/m T = kn/m Q w (m /year/m) Fig. Effect of Q w and B on FS FS.5 Q w = m /year/m B =.5 m T = T = 5 kn/m T = kn/m Construction speed, V (m/day) Fig. Effect of V on FS

6 59 FS V =.5 m/day Q w = m /year/m γ t = 5 kn/m γ t = kn/m γ t = 5 kn/m γ t = kn/m B =.5 m B =. m Mobilized tensile force, T (kn/m) Fig. Effect of γ t on FS considering the reinforcement effect, γ t will not influence FS values. In case of considering the reinforcement effect, since the allowable tensile force (T a ) in the geocomposite not increases with increase of γ t, the corresponding FS value will reduce. Figure compares FS values of γ t = 5 and kn/m cases. For T a > cases, FS of γ t = kn/m case is smaller. The larger the T a value considered, the larger the difference. CONCLUSIONS A method of predicting undrained shear strength (S u ) within a dual function geocomposite reinforced embankment with clayey backfill is described. The results of laboratory discharge capacity (Q w ) tests for three geocomposites confined in clayey soils are presented. Referring the test results on Q w, the S u distribution within an assumed 5 m high embankment and therefore the factor of safety (FS) of the embankment have been investigated. The detailed conclusions on the test and analysis results are as follows. () Q w values of geocomposites. The flow rates of the geocomposites reduced with increase of confining pressure and elapsed time. To maintain a higher longterm (more than one month) Q w value, a geocomposite must have a drainage tube or core and a strong filter. The tests were conducted under a hydraulic gradient i =., and if linearly convert to i =., the range of Q w values is to, m /year/m at one month of elapsed time. () Undrained shear strength (S u ). Regarding the effect of Q w and spacing between geocomposite layers, B, on S u, for the conditions investigated, it shows that when Q w m /year/m, the difference on S u values for B =.5 m and. m becomes small, which implies that using a geocomposite with a higher Q w value, a larger spacing (e.g. B =. m) can be adopted. () Factor of safety (FS). The analysis results indicate that with a requirement of FS >., even without considering the reinforcement effect (T a = ), B =.5 m and Q w 5 m /year/m, and B =. m and Q w m /year/m cases can satisfy the requirement. When T a =, the FS value is not influenced by the unit weight of backfill (γ t ), but for T a >, FS reduces with the increase of γ t. REFERENCES ASTM () Standard guide for selection of test methods for prefabricated vertical drains (PVD), ASTM D697-, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA. Bishop A. W. (955). The use of the slip circle in stability analysis of slopes, Geotechnique 5, pp Chai, J.-C. and Miura, N. (). Long-term transmissivity of geotextile confined in clay. Proc. 7 th Inter. Conf. on Geosynthetics, Nice, France (Ed. By Ph. Delmas and J. P. Gourc), Balkema Publishers, : Hansbo, S. 98. Consolidation of fine-grained soils by prefabricated drains, Proc. th Int. Conf. Soil Mech. and Found. Engrg., Stockholm, : Hird, C. C., Pyrah, I. C. & Russell, D. 99. Finite element modeling of vertical drains beneath embankments on soft ground. Geotechnique, 4(): ISO (99). Wide width tensile test of geotextiles and geogrids. ISO 9, International Organization for Standardization, Geneva, Switzerland. Japan Road Association (JRA) (999). Guideline for slope and embankment stability measure, Geotechnical Engineering for Road Construction. Japan Road Association. Ladd, C. C. (99). Stability evaluation during staged construction. J. of Geotech. Eng., ASCE, 7(4): Leshchinsky Dov (). An example of using ReSSA in comples geometry of reinforced slope. ADAMA Engineering Inc., USA. Tatsuoka, F. and Yamauchi, H. (986). A reinforcing method for steep clay slopes using a non-woven geotextile. Geotextiles and Geomembranes, 4: Tsuchida, T. and Kang, M. S. (). Case studies of lightweight treated soil method in seaport and airport construction project. Proc. th Asian Regionla Conf. on Soil Mechanics and Geotechnical Eng., Leung et al. (eds): 49-5.

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