ABSTRAKUji shaking table dilakukan pada model struktur yang kaku ditempatkan di atas pasir jenuh untuk mempelajari perkembangan tekanan pori di pasir dekat struktur dan untuk mempelajari faktor-faktor yang mempengaruhi penurunan struktur. Catatan penurunan bangunan beton bertulang selama gempa niigata 1964 ditinjau untuk perbandingan dengan hasil uji model.Studi-studi menunjukkan: 1. Tekanan pori yang berlebih yang berkembang di bawah pusat dari struktur model lebih kecil daripada yang jauh dari struktur. 2. Rasio tekanan pori berlebih terhadap tegangan efektif awal di bawah pusat dari struktur model menurun dengan struktur menjadi lebih berat.PENINGKATAN TEKANAN PORITipe sejarah waktu dari tekanan pori berlebih dan percepatan dari shaking table ditunjukkan dalam Gbr.6. Kurva tekanan pori menunjukkan bahwa sejarah saat tekanan pori pada level yang sama hampir sama, meskipun tekanan pori pada tingkat dangkal, P1 ke P3, menunjukkan fluktuasi yang lebih besar daripada di tingkat yang lebih dalam. Bisa dijelaskan, karena, bahwa kehadiran dinding ujung tampaknya tidak menyebabkan kelainan yang signifikan dalam distribusi tekanan pori di dalam bagian tengah kotak dimana tranducers dipasang. Rasio tekanan pori berlebih terhadap tegangan awal yang efektif vertikal, u / zo, ditampilkan di sisi kanan Gambar. 6 mencapai hampir 100%, menunjukkan bahwa pencairan itu terjadi.Gambar. 7 menunjukkan tipe riwayat waktu tekanan pori berlebih akibat getaran, percepatan table (sekitar 100 gal pada 3 Hz), dan penurunan struktur. Skala sisi kanan dari catatan tekanan pori memberikan rasio tekanan pori berlebih, u, tegangan vertikal efektif awal yang, zo, ditinjau dari teori elastisitas, dengan asumsi bahwa lapisan pasir diam di dasar kaku homogen, isotropik dan bahan elastis linear. Kontur zo yang seperti yang ditunjukkan pada setengah kiri Gambar. 8.Dapat dilihat pada Gambar. 7 bahwa tekanan pori berlebih di bawah pusat struktur, di P1 dan P4, lebih kecil dari yang di luar struktur, dan segera di luar dan di bawah struktur, di P2 dan P5, peningkatan tekanan pori lebih cepat dengan lebih getaran tajam daripada di tempat lain.Gambar 9 menunjukkan tekanan maksimum kelebihan pori, umax, dan rasionya terhadap stres yang efektif, zo dan z1. Definisi zo telah diberikan sebelumnya. Agar rasio tekanan pori untuk menunjukkan terjadinya pencairan saat mencapai 100 persen, tekanan pori harus dibagi dengan tegangan efektif yang kompatibel dengan distribusi tegangan saat pencairan segera terjadi. Meskipun sulit untuk memperkirakan distribusi tegangan tersebut akurat, versi ideal sederhana dapat diberikan dengan asumsi kasus fiktif seperti yang ditunjukkan pada bagian kanan pada gambar. 8, di mana batas gesekan, MN, ada di tanah di luar struktur. Hal ini terkait dengan situasi di mana struktur dilakukan semata-mata oleh tanah langsung di bawah struktur. Nilai z1 didefinisikan oleh

Effect of pore pressure buildupThe effect of pore pressure development on the settlement of the structure is illustrated in Fig. 13 in which contours of the excess pore pressure ratio, u/zo, are shown for four different ratios of the settlement of the structure to the depth of the sand, S/D. The settlement is small for (a) and (b) for which the pore pressure ratio in the sand directly below the structure stays within 60 percent for the entire depth. On the other hand, the settlement becomes considerably greater for (c) and (d) for which the zone of pore pressure ratio exceeding 60 percent spreads beneath the structure.It is interesting to note that the excess pore pressure ratio of about 60 percent seems to correspond to a critical condition where liquefaction failure is imminent, i.e., both the shear strain and pore water pressure begins to increase abruptly during undrained cyclic shear test. This is illustrated in Fig. 14 in which the shear stress ratio is plotted against the ratio of the effective stress at the critical condition to the initial effective stress (the lower scale), or against the excess pore pressure ratio at the critical condition (the upper scale). The test result on the toyoura sand were obtained with a ring torsion apparatus (yoshimi and oh-ka, 1973). It can be seen in Fig. 14 that at a critical pore pressure ratio of 60 percent as pointed by an arrow, the dynamic shear stress ratio is 0.18 which roughly corresponds to the horizontal acceleration during the shaking table tests.To show more directly the effect of pore pressure buildup on the settlement of the structure, the ratio of the relative settlement

Efek tekanan kontakThe solid curves in fig. 16 show the effect of the contact pressure of the structure on the settlement for test series A conducted at relative density of approximately 50 percent. The curves shift toward left as the contact pressure increase. As the curves become nearly vertical, the contact pressure can be said to approach the ultimate bearing capacity. It is interesting to note that the average effective stress ratios below the structure under the bearing capacity failure conditions are nearly proportional to the contact pressure, i.e., less than 25 percent for q=0,2t/m2, and 70 percent for q=0,6t/m2.The solid symbols in fig. 16 connected by a dashed line are for the tests in which shaking was stopped as soon as the sand away from the structure had liquefied. The data from this type of tests have already been shown in fig.9. Fig. 16 and the settlement data in fig. 9 show that the heavier structure settled less than the lighter one when intensity and duration of vibration were not excessive. For stronger shaking, however, the trend may be reversed and the heavier structure may suffer greater settlement.

Fig 9 shows the maximum excess pore pressure, umax, and its ratio to the effective stress, zo and z1. The definition of zo has been given previously. In order for the pore pressure ratio to indicate the occurrence of liquefaction when it reaches 100 percent, the pore pressure must be divided by the effective stress that is compatible with the stress distribution when liquefaction is imminent. Althougt it is difficult to estimate such stress distribution accurately, a simple idealized version may be given by assuming a fictitious case as shown in the right half on Fig. 8, in which a frictionless boundary, MN, exists in the soil outside the structure. This corresponds to a situation in which of the structure is carried solely by the soil directly below the structure. The value of z1 is defined by

shaking table test were conducted on models of rigid structures placed on saturated sand in order to study the pore pressure development in the sand near the structure and to study the factors which influenced the settlement of the structure. The settlement records of reinforced concrete buildings during the niigata eatrhquake of 1964 were reviewed for comparison with the model test results.The studies showed : 1. The excess pore pressure developed below the center of the model structure was smaller than that away from the structure. 2. The ratio of the excess pore pressure to the initial effective stress below the center of the model structure decreased as the structure became heavier. 3. For both the model and prototype the settlement of the structure decreased as the width of the structure increased for a given depth of liquefaction. 4. A pair of rigid walls embedded on both sides of the model structure had a considerable effect on reducing the excess pore pressure below the structure and a marked effect on reducing the settlement of the structure.PORE PRESSURE DEVELOPMENTTypical time histories of the excess pore pressure and the acceleration of the shaking table are shown in Fig.6. The pore pressure curves show that the time histories of the pore pressure at the same level are nearly the same, although the pore pressure at the shallower level, P1 to P3, show greater fluctuation than those at the deeper level. It may be stated, therefore, that the presences of the end walls did not seem to cause significant anomalies in the pore pressure distribution within the central portion of the box where the tranducers were installed. The ratio of the excess pore pressure to the initial effective vertical stress, u/zo, shown on the rigth side of Fig. 6 reached nearly 100%, indicating that liquefaction did occur.Fig. 7 shows typical time histories of excess pore pressure due to vibration, the table acceleration (approximately 100 gal at 3 Hz), and of the settlement of the structure. The right-hand side scale of the pore pressure record gives the ratio of the excess pore pressure, u, to the initial effective vertical stress, zo, evaluated from the theory of elasticity, assuming that the sand layer resting on a rigid base is homogeneous, isotropic and linear elastic material. The contours of zo are as shown in the left half of Fig. 8.It may be seen in Fig. 7 that the excess pore pressure below the center of the structure, at P1 and P4, is smaller than that outside the structure, and that immediately outside and below the structure, at P2 and P5, the pore pressure increases more rapidly with more marked pulsation than in other places.Fig. 9 shows the maximum excess the maximum excess