THE EFFECT OF LOG PILING ON LIQUEFACTION

Transcription

1 Journal of JSCE, Vol. 2, , 214 THE EFFECT OF LOG PILING ON LIQUEFACTION Saima RIAZ 1, Atsunori NUMATA 2, Kaori MIMURA 3, Hiroaki IKEDA 4 an Toshikazu HORI 1 Stuent Member of JSCE, Department of Civil Engineering, Wasea University (3-4-1, Okubo, Shinjuku-ku, Tokyo 169-8, Japan) saimariaz4@gmail.com 2 Member of JSCE, Chief Research Engineer, Research Institute of Technology, Tobishima Corporation (472, Kimagase, Noa-shi, Chiba, , Japan) atsunori_numata@tobishima.co.jp 3 Member of JSCE, Junior Engineer, Engineering Department, Kanematsu-NNK Corporation (3-2 Kojimachi, Chiyoa-ku, Tokyo, 12-83, Japan) k-mimura@knn.co.jp 4 Member of JSCE, Engineer, Showa Material Corporation (2-4, Kita 2 Chome Honoori, Shiroisi-ku, Sapporo city, Hokkaio) ikea@showamaterial.co.jp Laboratory Chief, Department of Geotechnical & Hyraulic Engineering, National Institute of Rural Engineering (2-1-6 Kannonai, Tsukuba-shi, Ibaraki 3-869, Japan) thori@affrc.go.jp This paper introuces a liquefaction mitigation metho that uses log piles as environmentally frienly an practical solution for strengthening civil engineering structures. The liquefaction mitigation measure explore in this paper can be use to increase the earthquake resistance of loose sans by improving the ensity of soil. During the Tohoku Pacific earthquake in 211, liquefaction was pervasive in large portions of the region, especially in Tokyo Bay an the city of Urayasu. Extensive liquefaction cause extensive amage to resiential properties, electricity, water, sewage networks, an briges. The mitigation of global warming is an important issue that requires immeiate attention. Because the use of woo can be effective for preventing global warming, the authors have consiere it to mitigate liquefaction amage. A series of large-scale shaking table tests was performe to investigate the effect of liquefaction mitigation by log piling into sany groun. The results inicate that the metho of log piling is an effective liquefaction mitigation compare with methos for increasing ensity, such as the ensification metho. Portable ynamic cone penetration (PDCP), Sweish weight souning (SWS), automatic ram souning (ARS), piezo rive cone (PDC), an flat ilatometer (FDM) tests, as well as fiel tests, were performe in the city of Urayasu. These tests were performe to confirm the effectiveness of log piling on liquefaction mitigation. Key Wors: liquefaction, shaking table, global warming, log piles, souning 1. INTRODUCTION In 211, an earthquake occurre along the Pacific coast of the Tohoku an Kanto regions of Japan. The earthquake cause extensive amage to life, property an nuclear power plants ue to a tsunami an intensive earthquake motion. Although Tokyo is locate approximately 38 km from the epicenter, the groun motions etecte uring the earthquake were strong enough to cause significant liquefaction to loose reclaime soils in Tokyo Bay an the city of Urayasu 1)-4). Many resiential an commercial builings an lifeline facilities in Urayasu experience extensive amage ue to soil liquefaction. Soil liquefaction cause severe amage to founations, lifelines, an waterfront structures. Excessive settlement an lateral spreaing of the groun an lanslies were inuce by liquefaction. Many stuies on soil liquefaction have been performe to unerstan the mechanism of liquefaction an the ynamic responses of founations in a liquefiable soil )-11). The results of these stuies provie the basis for the evaluation of mitigation methos for liquefaction hazars 12)-16). 144

2 Yasua an Ogasawara 17) evaluate a countermeasure for liquefaction that involve the installation of steel pipes, which they etermine to be effective against liquefaction. Numata et al. 18) analyze wooen piles as a remeial measure against liquefaction using the 1964 Niigata earthquake as an example. They consiere a countermeasure that involve riving wooen piles into the groun, which they etermine to be effective against groun liquefaction. They iscusse the urability of woo an misunerstanings concerning the utilization of woo as a structural element in the fiel of civil engineering. They etermine that woo coul be use as a countermeasure against liquefaction. Yoshia et al. 19) conucte an experimental stuy of a liquefaction countermeasure using log piling for resiential houses. Small-scale shaking table tests in a 1-g gravity fiel were performe using a moel groun. They iscovere that wooen piles coul increase the resistance of the groun to liquefaction by increasing the groun ensity by piling an the issipation of excess pore water pressure along the surfaces of the piles. As a result, the magnitue of the settlement of the house, which was set on the improve groun with piling logs, was minimize. Global warming is a significant issue in this century, for which all persons shoul be responsible. It is primarily cause by the iniscriminate eforestation an prouction of carbon ioxie by burning fossil fuels. Woo is instrumental in climate change because trees absorb carbon ioxie from the atmosphere as they grow. Therefore, the expansion of forests is a completely natural way to offset global warming. The amount of carbon ioxie that trees absorb from the atmosphere woul increase if woo was harveste an use as material for improving founations instea of steel an concrete, which require a substantially greater amount of fossil fuels an prouce carbon ioxie uring manufacturing 2)-24). Currently, Japan is a forest-rich country; therefore, a vast amount of woo coul be harveste. Because the use of woo is effective for mitigating global warming, the authors have consiere it as a mitigation measure for liquefaction amage. Logs are rarely use in structural founations. However, uring the construction of the Niigata station in 198, logs were pile into the groun to construct the founation. As a result, no amage occurre uring the 1964 Niigata earthquake in Japan 2) ; however, a builing next to the station, which ha a concrete pile founation, incurre some structural amage. Pile logs continue to support the existing Niigata station; these logs may not ecay. These finings are evience that wooen piles can be use as a liquefaction countermeasure. Wooen piles have been use to remey liquefaction; this environmentally frienly an economical technique is important, specifically, for eveloping countries, such as Pakistan, where cost an economic consierations are critical. Liquefaction-inuce flow cause significant amage to structures uring previous earthquakes, such as the 1964 Niigata earthquake, the 1983 Nihonkai-chubu earthquake, an the 211 Tohoku Pacific earthquake in Japan. Stuies on liquefaction began immeiately after the Niigata earthquake. However, the 199 Hyogoken-nambu earthquake an the 211 Tohoku earthquake accelerate these stuies ue to significant amage to builings, roas, an briges. After the occurrence of these earthquakes, many stuies base on shaking table tests an analyses were performe. Several preictions an countermeasures have been propose an introuce in several esign coes. The evelope countermeasures have been applie to existing structures. Among these countermeasures, the installation of log piles as structural members is a relatively new countermeasure that is effective against liquefaction. The effectiveness of this metho was emonstrate by conucting shaking table tests in the laboratory an in the fiel. The main objectives of the stuy are as follows: 1. To examine the liquefaction behavior of the improve founation using wooen piles. 2. To obtain a cost-effective, practical, an environmentally frienly solution for liquefaction mitigation. 3. To reveal the effect of log piling on liquefaction. 4. To increase the resistance of the groun to liquefaction by increasing groun ensity an confining pressure.. To compare the effects of the ensification metho an the log piling metho on liquefaction. 2. THE SHAKING TABLE TEST (1) Experimental setup A series of large-scale shaking table tests was conucte in a 1-g gravity fiel to evaluate an emonstrate this technique for liquefiable san uring an earthquake. The moel groun was set up in a rigi steel container 3,6 mm long,,7 mm wie, an 1,8 mm high. The container was ivie into two parts; each part ha a with of 2,3 mm with a 1,1 mm cavity between each part (as shown in Fig. 2). The loose liquefiable moel groun was compose of Kasumigaura san with a relative ensity of approximately 48%. The moel groun was prepare using two methos: the ensification metho an the log piling metho. 14

3 Percentage finer by weight P (%) To evaluate structural amage cause by liquefaction, a loa of concrete with a mass of 1.1 t (prototype contact pressure of 1.1 t/m 2 ) was place on the groun surface, which was assume equivalent to a 2-level wooen house. Logs with iameters of 8 cm an lengths of 1 cm were use. Piles were riven by statically pushing them into the moel groun with an oil jack. The tree species of the logs was Japanese cear. Kishia, et al. 26) performe 1-g an centrifuge tests an reveale that the scale of the moel groun i not affect the relationship between the log length an the thickness of the liquefaction layer, an the relationship between the settlement of the improve groun an the settlement of the unimprove groun. Large-, meium- an small-scale shaking table tests were conucte in this stuy. The tests i not yiel iffering results, which inicate that the scale of the moel groun oes not affect the settlement results obtaine from the shaking table tests. (2) Case stuies Four cases (NIP, PD, P4D, an ) were examine in the large-scale shaking table test. The symbol NIP enotes no improvement; PD enotes log piling with an interval of five times the iameter of the pile; P4D enotes log piling with an interval of four times the iameter of the pile an enotes the ensification metho. The large-scale shaking table tests are summarize in Table 1. Two cases were simultaneously teste on the same shaking table, an the rigi container was ivie into two parts as shown in Fig. 2. NIP an PD were teste in container No. 1 an P4D an were teste in container No. 2. For ensification of the moel groun, a vibrator was use to compact the soil in layers. The groun water table was set to GL-.1 m; the surface soil thickness of.1 m was a nonliquefaction layer. A bag of coarse cloth fille with crushe stones was place on the hea of each log as a rain. (3) San sample Samples of Kasumigaura san were collecte (ensity of soil particles ρ s = 2.69 g/cm 3, maximum voi ratio e max = 1.67, minimum voi ratio e min =.66, % grain size D =.3 mm, an uniformity coefficient U c = 2.). The physical properties of the san an grain size istribution curve are shown in Fig. 1. Kasumigaura san contains almost no fine fraction an is a nonplastic material. (4) Container an arrangement of sensors Fig. 2 shows the arrangement an placement of sensors. Accelerometers, pore water pressure gauges Table 1 Case stuies. Cases Metho Container No. NIP No improvement D r = 48%, D c = 92.8% 1 PD Log piling, Interval = D a s = 3.1%, D r = 64% D ro = 49%, D c = 97.3% Densification D r = 91%, D c = 1.6% P4D Log piling, Interval = 4D a s = 4.9%, D r = 7% D ro = 4%, D c = 99.1% D: Diameter of the log at the top a s : Improvement ratio D r : Relative ensity of groun between the log piles D ro : Initial relative ensity D c : Degree of compaction (%) Grain size D (mm) Fig.1. Particle size istribution curve for san sample. an isplacement gauges were installe at ifferent locations in the container. An accelerometer (Ax, Ay, Az, A1x A4x, Ax, Ay, Az, A6x A9x, A1x, A1y, A1z), a pore water pressure gauge (P P12) an a isplacement gauge (D1 D8) were use. In aition, the accelerometer (A6x A9x) an pore water pressure gauge (P7, P8, P11, P12) serve as embee sensors in each log, whereas the remaining accelerometers an pore water pressure gauges were burie in the groun. Sensors Ay, Ay, an A1y were set in the y-axis irection an Az, Az, an A1z were set in the z-axis irection. The remaining accelerometers were set in the x-axis irection. The pore water pressure gauges were installe horizontally in the groun. Accelerometer A was installe irectly on the container. Laser sensors were mounte on a frame above the weight, an targets were fixe on the weight that was place on the groun surface to measure the vertical isplacement of the weight. 2 Kasumigaura san s = 2.69g/cm 3 D 1 =.14mm D 3 =.26mm D =.3mm D 6 =.38mm U c = 2. I p = NP 146

4 D 8=2.6m) Direction of Shaking D (@.4m 7=2.8m) Direction of Shaking No Improvement (NIP) Log piling (PD) Y Case 2 A3x,A4x, Ax,Ay,Az A1x,A1y,A1 z X D2 D4 Cavity Ax,Ay,Az D6 D8 A8x,A9x D1 D3 D D7 A1x,A2x P3,P4 P1,P2 P9,P1 P7,P8 P11,P12 P,P6 A6x,A7x Weight (1m 1m.m, 1.1t) D1,D2 D3,D4 Ax,Ay,Az Log (D.8m L 1.m, Cear) D,D6 D7,D8 A1x,A1y,A1z A2x P2 A1x P1 A4x P4 A3x P3 (a) NIP & PD A7x P6 Ax,Ay,Az P8 P1 A6x A8x PP7 P9 P A9x P12 Displacement Gauge Accelerometer Pore water pressure gauge Cavity: Partition/ Division/ Hollow space Densification () Log piling (P4D) A8x,A9x A1x,A1y,A1z A3x,A4x, Ax,Ay,Az D2 D4 Cavity Ax,Ay,Az D6 D8 D1 D3 A1x,A2x P3,P4 P1,P2 D D7 P9,P1 P7,P8 P11,P12 P,P6 A6x,A7x D1,D2 D3,D4 Ax,Ay,Az Weight (1m 1m.m, 1.1t) Log (D.8m L1.m, Cear) D,D6 D7,D8 A1x,A1y,A1z A2x P2 A1x P1 A4x P4 A3x P A7x P6 Ax,Ay,Az P8 P1 A6x A8x P P7P9 P A9x P12 (b) & P4D Fig.2 Sensor location in large-scale shaking table test. Displacement Gauge Accelerometer Pore water pressure gauge Cavity: Partition/ Division/ Hollow space 147

5 thickness. Fig.3 Improvement ratio. Fig.4 (a) Schematic of unimprove moel groun. The initial relative ensity D ro, the relative ensity after improvement D r, an the improvement ratio are shown in Table 1. The improvement ratio a s is efine as the percentage improvement of the improve founation over its unimprove state, which is calculate by the interval between the logs (4D, D) an the cross section of the log at the top as follows (shown in Fig. 3): A a s (1) 2 B In the log piling metho, the relative ensity D r was calculate by the ensity between the log piles using the iameter at the top of the log piles multiplie by the groun thickness. In the ensification metho, the relative ensity D r was calculate by the groun () Moel groun construction a) Natural moel groun (NIP) The natural moel groun was constructe using the following proceure an as shown in Fig. 4 (a): 1) Sensors were installe at each layer level, i.e., heights of 3, 6, 9, an 1,1 mm. 2) Wet san was poure through a perforate mesh into the water to a maximum thickness of 3 mm. 3) This proceure was one twice to make san layers reach heights of 6 mm an 1,1 mm. After each layer was create, the groun was levele. 4) Water level was set at a height of 1, mm. ) A layer of surface soil with a thickness of 1 mm was place on the groun an the groun was levele. This step complete the initial construction of the groun. b) Compacte moel groun () The compacte moel groun was constructe as follows: 1) Sensors were installe at the same epth as the initial groun. 2) Wet san was poure into the water to achieve a maximum thickness of 3 mm. 3) The soil was compacte with a vibrator to check the ensity of the soil. 4) This proceure was one twice to construct san layers with heights of 6 mm an 1,1 mm. After each layer was create, the groun was levele. ) The water level was establishe at a height of 1, mm. 6) Surface soil with a thickness of 1 mm was place on the groun an then the groun was levele. This step complete the construction of the compacte moel groun. c) Log piling moel groun (PD, P4D) The log piling groun was constructe as follows an as shown in Fig. 4 (b): 1) The initial groun was constructe using the same steps for the NIP moel groun. 2) Piles were statically pushe into the groun at pre-etermine intervals an alternate positions. 3) Piles were inserte into the groun between the previously inserte piles. 4) After installing the piles, rains were place above the heas of the piles at a epth of 1 cm. Weight was place on the surface of the groun after completion of the moel groun. (6) Input motion The moel groun was shaken in a horizontal irection with a sinusoial wave with peak amplitue of Gal, frequency of 4 Hz an uration of 8 sec, as shown in Fig.. The pore water pressures an response accelerations were simultaneously recore 148

6 3. RESULTS OF SHAKING TABLE TEST Fig.4 (b) Moel groun preparation for PD. Fig. Input motion of large scale shaking table. on the ata recorer. After the excess pore water pressure ha completely issipate, the vertical isplacements of the logs an the groun surface were measure by a point gauge. Constant amplitue of twenty cycles was recore with five waves increasing at the beginning an five waves ecreasing at the en. The process was repeate eight times with ifferent amplitues, which range from -4 Gal with an interval of Gal. (1) Time history uring shaking Fig. 6 shows an example of the acceleration time history with the same target input acceleration of 1 Gal. The value σ v ' is the initial effective overburen pressure that was calculate from the ensity, as mentione in Fig. 6. An equivalent input acceleration yiels a ifferent structural response when logs are use. The unimprove groun experiences a large settlement, whereas the settlement ecreases an is almost negligible in the log piling metho. In the case of NIP, the settlement of the structure began when the pore water pressure achieve an initial effective overburen pressure at P2, but the increase in pore water pressure for P4D was not significant. This performance is almost ientical to the performance for ; however, in the case of PD, the performance of shaking table varies between the performances of NIP an P4D. Therefore, the pore water pressure substantially influences the settlement of the structure. In the case of NIP, the response acceleration at A2 an A iffers espite an equivalent relative ensity an input acceleration, which may be attribute to the overburen pressure conition. A was mounte on the moel structure an A2 was burie insie the groun. The response accelerations at A an A1 iffer, which may cause a significant shear stress uner the weight. (2) Settlement of moel structure an excess pore water pressure Fig. 7 shows the relationship between the excess pore water pressure ratio an the settlement uner the weight. The excess pore water pressure ratio is calculate as the excess pore water pressure ivie by the effective overburen pressure ( u/ v ). Large settlement occurre when an excess pore water pressure of approximately 1. was achieve. It is the point where the increase in pore water pressure ( u) is equivalent to the initial vertical effective overburen stress ( u/ vo =1.). The water pressure becomes sufficiently high to counteract the gravitational pull on the soil particles an effectively float or suspen the soil particles. The pore water pressure an the settlement of the structure were obtaine uring the shaking table test. The initial effective overburen pressure was calculate by multiplying the thickness of the san layer by the unit weight of san (γh). The excess pore water pressure ratio was calculate by iviing by the effective overburen pressure. An excess pore water pressure of 1. correspons to approximately 149

7 Acceleration Settlement S (mm) A (cm/s2) A 4 8 A2 P2 P1 NIP P4 P3 1 Response Acc A2x) Response ACC Ax) - 1 P2 1-1 P1 1 - Input Acc Ax 1 Free Fiel A7 P6 P9 P PD 1 P6 P - Ave: 17gal Input Acc Ay Input Acc Az - Free Fiel Input Acc Ax 1 Acceleration Settlement S (mm) A (cm/s2) A2 P2 P1 1 Response Acc A2x) P4 P3 Response Acc (Ax) Input Acceleration Excess pore Excess pore A (cm/s2 ) water pressure water ressure - 1 σvo ' P2 1 1 A1 2 4 Observe settlement 6 8 P1 P9 Response Acc A1x Response Acc A7x) σvo ' P6 1 A7 P6 P P4D 1-1 P4 Input Acceleration Excess pore Excess pore A (cm/s2 ) water pressure water pressure Acceleration Settlement S (mm) A (cm/s2) A 8 Uner Structure (b) PD 2 Observe settlement P9 (a) NIP 6 σvo ' P1 Uner Structure 4 P1 Response Acc (A1x) Response Acc A7x) Ave: 17gal Input Acc Ay Input Acc Az Observe settlement 6-1 P3 4 1 A1-1 P4 2 Excess pore Input Acceleration Excess pore water pressure water pressure A (cm/s2 ) Observe settlement 6 Excess pore Input Acceleration Excess pore water pressure water pressure A (cm/s2) Acceleration A (cm/s2) Settlement S (mm) 2 σvo ' P P3 P1 P9 P 1 σvo ' σvo ' - - Ave: 16gal Input Acc Ay Input Acc Az Input Acc Ax - Free Fiel Ave: 16gal 1 1 Input Acc Ay Input Acc Az Free Fiel Uner Structure Input Acc Ax - 1 Uner Structure (c) () P4D Fig.6 An example of time history of shaking table test (target input motion 1Gal). 1 1

8 Settlement S(mm) Excess pore water pressure ratio 4mm Δu/ vo ' NIP 1 NIP 2 NIP 3 NIP 4 PD PD 6 PD 9 PD 1 P4D P4D 6 P4D 9 P4D Fig.7 Relationship between excess pore water pressure ratio an settlement. 4 mm of settlement. Therefore, the point at which a settlement of 4 mm is obtaine efines the point that the groun begins to liquefy; the acceleration at this point is efine in this paper as the cyclic resistance. (3) Input acceleration an cumulative settlement Fig. 8 shows the relationship between the input acceleration (A i ) an the cumulative amount of settlement base on the results improve by increasing ensity an log piling. A large settlement of approximately mm occurre for 1 Gal in the case of NIP. For PD, a small settlement occurre for 1 Gal. In the cases of P4D an, a small settlement occurre for more than 2 Gal. P4D (D r = 7%) performe well an similar to the ensification metho (D r = 91%). Fig. 8 shows that the relationship between settlement an relative ensity is inversely proportional, i.e., the settlement of the soil ecreases with increasing relative ensity. Thus, increasing ensity can effectively improve a liquefiable groun. Settlement ecreases with a ecreasing interval of piles. In Fig. 8, the shape of the curve for the log piling metho (P4D, PD) iffers from the shape of the curves for NIP an. The curve for log piling is concave own an becomes constant at the en, whereas the curve is similar to the NIP curve, which exhibits a ecreasing tren. The curves behave ifferently, whereas physical parameters such as the shaking table, the container size, the groun water table location, the soil type, an the thickness of the soil are ientical in both cases, with the exception of the log piles. In the case of log piling, the soil assume the loa an resiste against earthquake motion when the moel groun was shaken. However, when the input motion was excessive an continually increase, the soil liquefie an the loa was transferre to the logs. Because the log cannot liquefy absolutely, the logs are capable of supporting weight if the groun improve by the log piling metho experiences unexpecte large motion. Therefore, this metho is fail-safe against liquefaction amage. (4) Liquefaction assessment Fig. 9 shows the relationship between relative ensity (D rmm ) an cyclic resistance, in which D rmm is the relative ensity calculate by the minimum metho 27). Input motions for four cases are etermine for a settlement of 4 mm, as shown in Fig. 8. These input accelerations are use as cyclic resistances to assess the liquefaction potential, as shown in Fig. 9. R 2 is the cyclic resistance ratio at 2 cycles ( / vo ) where vo is the initial effective overburen pressure an is the shear stress. In the case of these experiments, the cyclic resistance ratio is the cyclic resistance ivie by the acceleration ue to gravity (98 Gal). ha (2a) t i ' Gh (2b) vo t ' vo A i G (2c) Because the main input motion consists of 2 waves, this cyclic resistance ratio is nearly equivalent to the cyclic resistance R 2, which is efine by elemental tests. The results for Tonegawa san etermine by the cyclic unraine triaxial test 28) are also plotte in Fig. 9. Piles with smaller pile intervals result in greater soil compaction between the piles ue to the ecreasing volume of the san groun. The liquefaction resistance of soil increases with increasing soil ensity. Cyclic resistance for ense specimens with relative ensities 91%, 7%, an 64% is increase three times as compare to the no-improvement groun with relative ensity 48%. The reason for the R 2 of Tonegawa being larger than the R 2 of may be attribute to the fining 11

9 Settelment S(mm) Settlement S(mm) Cummulative Settlement ΣS(mm) Settlement S (mm) Settlement S (mm) Input Acceleration Ai (gal) Dr= 91% Dr= 6% 1 Dr= 71% Relative ensity Dr (%) Relative ensity Dr (%) Log piling 1 Density increase Small scale shaking table 1gal Toyoura san D ro =% Log piling Density increase Smallscale shaking table 1gal Tonegawa san D ro =4% 1 Dr= 48% NIP PD P4D Fig.8 Relationship between input acceleration an cumulative settlement. Cyclic resistance R Tonegawa san Kasumigaura (Densification) Kasumigaura (Log piling) Tonegawa CS = 4mm Nc = 2 Log piling Relative Density D rmm (%) Fig.9 Relationship between relative ensity an cyclic resistance. that the response acceleration at the groun surface is larger than the response acceleration of the input motion. If response accelerations are use for, then the curve will be higher an closer to the Tonegawa curve (elemental test line). The R 2 of the log piling is larger than the R 2 of the for the same relative ensity an the same input motion. () Relationship between relative ensity an settlement Fig. 1 shows the relationship between relative ensity an the amount of settlement at the 1 Gal target input acceleration to compare the ensification metho with the log piling metho. Fig. 1 was evelope using the settlement value at 1 Gal from Fig. 8 for four ifferent cases (NIP, PD, P4D, an ). The relative ensity in the groun between the log piles was calculate consiering the change in groun surface height. Settlements of the groun that were improve by log piling were less than the settlements obtaine by the ensification metho, which inicate that log 3 (a) Toyoura san Relative ensity Dr (%) Log piling Density increase Meium scale shaking table 1gal Silica san D ro =6% (b) Tonegawa san (c) Silica san () Kasumigaura san Fig.1 Relationship between relative ensity an settlement. piling is more effective than the ensification metho for the same relative ensity. In Fig. 1, four types of sans (Kasumigaura, Silica, Toyoura, an Tonegawa) are use to verify the performance of other sans an the scales of the shaking table. Toyoura san is a special san with no fine particles (%). Silica san is artificial san with 11% fine particles, an Tonegawa san is natural san with 8% fine particles. A small-scale container with internal imensions of 76 mm 4 mm 28 mm was use to perform the tests on Toyoura san an Tonegawa san. For the small-scale test, an input motion of Hz with waves in the rising part, 22 main motion waves an waves in the ening part was observe. The input motion was uniaxial in the longituinal irection of the shaking table. The input motion increase the amplitue of this wave at every Gal stage 29). A weight of 11. kg was place on the moel groun to measure the settlement uner the weight. A meium-scale container with imensions of 1, mm 2, mm 1, mm was use to perform tests on the silica san. An input motion of 1. Hz with 22 waves in the rising part, 3 main motion waves an 7 waves in the ening part was observe. In the meium-scale container, no weight was place on the surface of the moel groun an the settlement was etermine by measuring the settlement of the groun surface 3). 1 Relative ensity Dr (%) Log piling Density increase Large scale shaking table 1gal Kasumigaura san D ro =48% 12

10 D 8=2.6m) D (@.4m 3.6 In each case, the log piling line is always above the ensity increase line, which inicates that log piling is more effective than the ensification metho. No Improvement (NIP) Log piling (PD) GROUND INVESTIGATION BETWEEN LOGS Numerous factors affect liquefaction strength, e.g., relative ensity, coefficient of cohesion Cc, grain size istribution, soil type, confining pressure, permeability of soil, prior stress-strain history an overconsoliation ratio. In this paper, the relative ensity an confining pressure are consiere to be the key factors for improving liquefaction resistance. The moel grouns were strengthene by the log pile an ensification methos. The groun improve by the metho is strengthene by ensification. Numerous types of sounings were performe to explain the increase in liquefaction resistance in the case of the groun improve by log piling. PDCPT Cavity Densification () Log piling (P4D) Cavity SWS PDC ARS DMT Unit :m Fig.11 Positions of souning tests. (1) Souning tests The following tests were performe on the moel groun: a) Portable ynamic cone penetration test (PDCPT) b) Sweish weight souning (SWS) test c) Automatic ram souning (ARS) test ) Piezo rive cone (PDC) test The results of these tests an the flat ilatometer (DMT) test will be separately iscusse. a) Portable ynamic cone penetration test The PDCPT is performe by ropping a kg hammer from a height of cm to measure the penetration epth per blow for each teste epth 31). The penetration epth ranges from 1 to 12 cm an the cone iameter is 2. cm with a 6 angle with the bottom ege. The following formulae 32) are use to calculate the correcte N value for PDCPT: If N 4 If N 4 N N (3a) N. 66N (3b) where N is the number of blow counts to a maximum epth of 1 mm (number). b) Sweish weight souning test The equipment for this test consists of a screw point at the tip, a steel ro with a length of 1 m, a ea weight of 1, N an a top hanle. The equipment is rotate an the number of half rotations (18 o rotations) require for 1 m of penetration is counte (N sw ) 33). If the soil is har, N sw is significant. The following formula 34) is use to calculate the N value obtaine from the SWS test: N.2W sw. 67N sw (4) where W sw : ea weight of Sweish souning (N) N sw : number of half rotations (18 rotations) for a 1 m penetration. c) Automatic ram souning test In the automatic ram souning test, a ro with a length of 1 m, a iameter of 32 mm an a weight of 63. kg is use. The cone has an outsie iameter of 4 mm, length of 9 mm an weight of kg. The height of fall for the ARS is mm an the epth of penetration is 2 mm 3). For ARS If N If N N N N (a) N N m (b) N N (c) where N : number of blow counts (number) N m : mean number of blow counts to a maximum m m 13

11 epth of 2 mm (number) ΔN m : mean number of blow counts ue to friction or torque (number). ) Piezo rive cone test The PDC is a hanhel evice esigne to penetrate soils to epths of 1 m with a 2 mm iameter cone. The 6 cone is force into the groun by rising an ropping an 8 kg hammer 36). The following formulae 37) are use to calculate the correcte N value for the PDC: N N 1 N N m 2 1 N. 16 M r (6a) (6b) where N m : mean value of blow counts to a maximum epth of 2 cm N : penetration resistance : penetration epth (cm) M r : torque or moment (N-m). (2) Results of souning tests Fig. 11 shows the positions of the souning tests performe before an after the improvement in the groun. Fig. 12 shows the results of the souning tests, which show the relationship between the SPT N-value an the epth. The SPT N-values explaine in the figures were obtaine from each souning test an converte from each result. The ata for equivalent epths were average for each case. The results of the NIP case by ARS was almost zero because the blow energy was too strong for very loose san. The results show that the N-value increases with increasing epth; the N-values of PD, P4D, an are greater than the N-values of NIP; an the N-values of P4D are greater than the N-values of PD, with the exception of PDCP an ARS. The increase in the N-value with increasing epth can be attribute to the increase in confining pressure an improve ensity. Due to the increase in the N-value, the ensity of the groun also increases. These results reveal that the groun between the logs was strengthene by log piling an the ensification metho. Fig. 13 shows the relationship between the relative ensity an the N-values for a epth of.7 m. The relationship obtaine by Meyerhof 38) is also plotte in Fig. 13. Meyerhof's ata significantly iffer from the ata in this paper. Because the overburen pressure in this stuy is consierably low, the absolute value is not important; however, the shape is useful information. is only one atum; thus, the relationship obtaine by the ensification metho was enote by a broken re line, which signifies Meyerhof's relationship. The relationship obtaine by log piling yiels a higher N-value than the relationship obtaine by the ensification metho. Due to an increase in the N-value for the groun between the pile logs, the ensity of the groun also improve. (3) Flat ilatometer test Confining pressure is one of the most important factors affecting liquefaction resistance. DMT was use to estimate the lateral stiffness or horizontal earth pressure of the soil before an after improvement by an log piling. The flat ilatometer is an in situ evice that is use to etermine the in situ soil lateral stress an the soil lateral stiffness, an to estimate other engineering properties of subsurface soils. This test measures the horizontal earth pressure by placing a boar-shape instrument with a thickness of 1 mm an a with of 96 mm with into the groun an pushing the groun horizontally using a circular expanable steel membrane with a iameter of 6 mm on one sie. The test involves riving this steel blae into the groun, inflating the steel membrane an measuring the corresponing pressure an eformation. The penetration of the steel blae is usually achieve using common in situ penetration equipment; for instance, the equipment use in the stanar penetration test. The DMT can be use to test extremely soft soils to extremely stiff soils 39). The primary reason for the increase N-value for the groun between the pile logs is the increase ensity an confining pressure. To clarify the effect of confining pressure, the horizontal earth pressure was measure by the DMT. The horizontal earth pressure was measure at two points for each case of NIP, PD, P4D, an. The ata from the NIP were use as the original groun ata prior to improvement. Fig. 14 isplays the results of the DMT: (a) shows the effective horizontal earth pressure with epth an (b) shows the rate of increase after improvement. The ata obtaine from the two points in each case were average at the same epth. No atum is obtaine using the DMT for epths greater than. m because the NIP groun was too soft for this instrument. The rates of increase excee 1. an range from approximately 1.1 to 1., with the exception of one point. The horizontal earth pressure increases from 1.1 to 1. times as large as the pressure prior to improvement. The increase in the N-value may be ue to the increasing horizontal earth pressure. (4) Verification of remeial measures The reuction in the susceptibility to liquefaction is 14

12 NIP NIP PD N Value P4D N Value PD P4D NIP NIP N Value PD P4D N Value PD P4D Depth D(m) Depth D(m) Depth D(m) Depth D(m) N Value (Number) NIP-SWS PD-SWS P4D-SWS -SWS N Value (Number) NIP-CPT PD-CPT P4D-CPT -CPT N value (Number) NIP-ARS PD-ARS P4D-ARS -ARS N value (Number) NIP-PDC PD-PDC P4D-PDC -PDC (a) SWS 2.2 (b) PDCPT 2.2 (c) ARS 2.2 () PDC Fig.12 Results from the souning tests Large scale shaking table SWS.7m (Log piling).7m (Density increase) Meyerhof Large scale shaking table PDCPT.7m (Log piling).7m (Density increase) Meyerhof Relative Density D r (%) Relative Density D r (%) (a) SWS (b) PDCPT Large scale shaking table ARS.7m (Density increase).7m (Log piling) Meyerhof Large scale shaking table PDC.7m (Log piling).7m (Density increase) Meyerhof Relative Density D r (%) Relative Density Dr (%) (c) ARS () PDC Fig.13 Relationship between relative ensity an N values. 1

13 Depth GL-(m) Depth GL-(m) Effective horizontal earth pressure P ' (kpa) NIP-DMT PD-DMT P4D-DMT -DMT Average DMT Fig.14 Results of the DMT base on the increase in N-values (strength) an confining pressure. Because no irect metho can be use to estimate the increase liquefaction resistance, the effect of groun improvement (egree of compaction) was confirme by N-values obtaine from ifferent souning tests. Another reason for the increase liquefaction resistance is that the shear moulus of the groun-logs complex groun is higher than the shear moulus of the original groun. The shear moulus of woo is approximately 6 MPa an the shear moulus of the groun is approximately 1 MPa. Therefore, the shear moulus of the combine groun-logs composite groun is higher than the shear moulus (61 MPa) of the original groun. The liquefaction resistance is higher ue to the increase in the shear moulus of the improve groun. Souning tests (SWS, PDCP, ARS, an PDC) an flat ilatometer tests were use to confirm the effective improvement on liquefaction. Another reason for the reuce liquefaction may be the issipation of pore water pressure along the periphery of the logs. During riving, the shear stress along the irection of the axis of the pile cause an increase in the total raial stress, which in turn cause an increase in the pore water pressure. The excess pore pressures generate in this process are subsequently assume to issipate by outwar raial flow of pore water along the surface of the pile. Thus, the groun will ensify ue to the increase in effective raial stress. In aition, it is easy to generate a voi an water film between the surface of the woo an the particles of san because the surface of the woo is coarse an wooen piles move laterally by shaking. Therefore, water can issipate along the periphery of the logs. As a result, resistance to liquefaction shoul increase Rate of increase of effective horizontal earth pressure P a '/P b ' PD-DMT P4D-DMT -DMT Average DMT. CONCLUSIONS A series of 1g shaking table tests was carrie out to evaluate the performance of two groun improvement techniques against liquefaction, log piling, an ensification. A test with no improvement was also performe to compare the behaviors of sans. Consiering the effect of installing log piles in ensifying the groun an ensification by compacting the soil, liquefaction analysis is propose that uses pore pressure generation an settlement of structure for the groun treate with log piles. Both the settlement of structure an excess pore water pressure are consiere to be affecte as they ecrease because of ensification. This ensification ecreases with istance from the log pile. Observing the test macroscopic phenomenon an analyzing the ata of pore pressure an settlement, the following conclusions were reache: 1) The effect of liquefaction mitigation by the log piling metho is larger than the effect of liquefaction mitigation by the ensification metho. 2) The egree of compaction is increase by 16% by the log piling metho an 113% by the ensification metho as compare to unimprove groun. 3) If the groun improve by the log piling metho experiences significant earthquake motion, the log is capable of supporting overburen stress because the log cannot liquefy absolutely. Therefore, this metho is fail-safe against liquefaction amage. 4) The effect of liquefaction mitigation by the log piling metho increases with ecreasing intervals between the logs an by piling logs with 4D intervals, which is equivalent to the effect of the ensification metho with 91% relative ensity. ) The primary reason for the increase in liquefaction resistance is that the groun is strengthene by an increase in ensity an confining pressure. 6) Results of tests performe on loose specimens (D r =48%) inicate that higher pore pressure is evelope with ecreasing relative ensity. Loose specimens showe up to three times higher liquefaction potential compare to ense specimens (D r =91%, 7%, 64%). 7) Safe sie esign for log piling metho is possible for liquefaction mitigation by estimating liquefaction resistance from calculate groun ensity between log piles. 8) Groun-logs complex groun has higher shear moulus than original groun; therefore, liquefaction resistance will also increase. ACKNOWLEDGEMENTS: The financial assistance provie by Wasea University, Japan is 16

14 gratefully acknowlege. The authors acknowlege Professor Masanori Hamaa (Wasea University) for his continue support. The authors are grateful for the assistance of Masaho Yoshia (Professor), Takumi Murata (Fukui National College of Technology), Taashi Hara (Associate Prof.), Akiko Sakabe (Kochi University) an Syusei Ogawa (Nagaoka University of Technology) in the shaking table tests. The authors also appreciate the assistance by the staff members of Tobishima Corporation, especially Shigero Miwa. 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