Nonlinear seismic amplifications and associated nonlinear soil properties are investigated based on the vertical array records obtained during the 1995 Hyogoken-Nambu earthquake at four sites at very different distances from the earthquake fault zone. These accelerograms were recorded in surface soil layering systems of about 100 m thick consisting of fill, Holocene and Pleistocene or rock base layers. In this research, amplifications in acceleration and velocity between the base and the surface for the main shock and aftershocks are first examined in terms of S-wave velocity ratio between the two levels. A correlation can be found between the amplification and the velocity ratio for linear site responses in the aftershocks, while the amplification is evidently lower for nonlinear response in strong seismic motions for the mainshock. The amplification in acceleration or velocity obviously decreases with increasing input at the base level, indicating remarkable nonlinear amplification characteristics. The acceleration amplification becomes less than unity for base acceleration larger than 0.4 G-0.9 G while velocity amplification stays above unity even for a very large base velocity. Based on the mainshock records, variations in the shear modulus and the damping ratio in the surface soil layers are then evaluated by means of an inversion analysis assuming 1-D vertical SH-wave propagation. A clear strain-dependency can be found in the back-calculated modulus and damping, which are consistent with previous laboratory test results for each kind of soils.

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Behavior of the ground during the 1995 Hyogoken-Nambu earthquake is investigated through various analyses on and comparisons with the earthquake records.
Behavior of the Holocene clay, a soft clay layer, as well as the fill, the liquefied layer, is shown to affect the response of the ground significantly through effective stress analysis. Earthquake response analysis carried out on the section passing the Sannomiya station indicates that the existence of the Holocene clay and its thickness controls the response of the ground surface to a great degree. It is also shown that there was no deamplification effect caused by the nonlinear behavior of the soil in the area called the damage belt, where damage to houses was especially severe.
Incident waves on the top of the upper Osaka Group formation are estimated for several sites where earthquake records were obtained by deconvolution analysis. Their waveforms on the base layer are very similar to each other in a narrow region a few kilometers wide, but are different from those at distant sites, although their PGV are nearly the same at about 80 cm/s.
It is shown that the ground shaking observed at the 1995 Hyogoken-Nambu earthquake was characterized mainly by the surface geology, and that it was are controlled by the nonlinear behavior of the soft clay layer and liquefaction of fill.

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The authors have studied the seismic behavior of the alluvial clay layer on Kobe Port Island during the 1995 Hyogoken-Nambu earthquake. First the shear stress-strain relationship of the clay was estimated from the main shock of seismic array data. Next using the stress wave inferred from array records we conducted a cyclic simple shear test. The samples used in this study were the clay sampled from Port Island and Rokko Island. The clay sampled from the two man-made islands are from the same clay layer, but the clay from Rokko Island was not as consolidated as the clay at Port Island. After consolidation with the confining pressure at the array site, we applied a dynamic shear stress at the same stress rate as the actual earthquake. The shear strain time histories inferred from array records were compared with those from the simple shear test. The strain time history obtained from the simple shear test gave good comparison to that obtained from array records. Finally the settlement due to dynamic shearing was studied through the simple shear test. It was found that the difference of the degree of consolidation affects the seismic behavior of soft clay.

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Studies on the undrained behaviour of Masado soil, a fill material which was chiefly responsible for the massive liquefaction in the reclaimed areas of Kobe city during the Hyogoken-Nambu earthquake, are presented. Results from multiple series of triaxial tests are used as a basis for behavioural characterization within the framework of the state concept. Reconstituted samples of three Masado soils with different gradation curves were used together with undisturbed samples recovered from Kobe Port Island to investigate the effects of the grain-size distribution, soil fabric, initial e-p' state (void ratio-confining stress state) and mode of loading on the undrained behaviour. The location of the steady state line in the e-p' diagram was found to be related chiefly to the gradation of the Masado soils. However, a unique pair of zero residual strength line and steady state line was found to exist in a modified e-p' diagram regardless of the gradation curve, fabric of the soil and mode of loading. It is shown that the state index I_s can be used to quantify the undrained behaviour when using these characteristic states as frames of reference. A procedure is proposed which enables an assessment to be made of the in-situ void ratio and undrained behaviour throughout the depth of a reclaimed deposit if the gradation curve of the fill material is known.

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The reclaimed lands in port and harbour area of Kobe city developed extensive liquefaction during the intensive shaking of the 1995 Hyogoken-Nambu earthquake. Among these islands, the northern portion of Port Island was reclaimed with soil materials consisting of decomposed granite. Masado, a soil containing about 50% gravel had been considered to be less liquefiable than clean sand. This catastrophic event, however, created the need to re-examine the susceptibility of such gravelly soils to liquefaction. To examine the undrained behaviour of the Masado soil, a multiple series of triaxial compression and extension tests was carried out on reconstituted soil samples as well as on intact soil samples. Two different preparation methods were employed to produce samples with different fabrics. The different soil fabrics prepared by the different methods were shown to exhibit different undrained deformation behaviour during monotonic straining. However, the largely deformed steady states of specimens having the same grain size characteristics were shown to be determined uniquely independent of the method of sample preparation, and also whether the deformation mode is triaxial compression or extension. It was also found that the steady state lines for soils of decomposed granite with different particle gradations are parallel to each other in the plot of void ratio against logarithm of effective confining stress. The steady state characteristics of the Masado soil based on the laboratory tests are described in this paper.

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Mechanisms regarding the earthquake-induced settlements of saturated sandy deposits with level or nearly level ground surface are revealed based on new post-earthquake stress-strain constitutive analysis and experimental evidence. It was found that : 1) the residual volumetric strain of saturated sand ε_vr, induced by drained consolidation following cyclic undrained loading, depends on an irreversible dilatancy component ε_vd.ir and a residual shear strain during the consolidation γ_r only; 2) change in ε_vd.ir can universally be measured using an index “relative compression, R_c” for different sands over a wide range of density; and 3) a consistantly good correlation exists between R_c and maximum double amplitude shear strain induced by preceding cyclic undrained loading. From these theoretical and experimental findings, new analytical and empirical methods with a high degree of physical rationale were developed for predicting earthquake-induced maximum undrained shear strain and the resulting residual volumetric strains. As a consequence, the seismic ground surface settlement potential for level sandy deposits can be evaluated only by integrating the relative compression along ground depth. The proposed methods were confirmed to be effective by comparing the predictions with observations made during the 1995 Hyogoken-Nambu earthquake.

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During the 1995 Hyogoken-Nambu earthquake, large vertical residual ground settlements and horizontal displacements extensively occurred in liquefied soils behind caisson-type quay walls where lateral movements of the walls towards sea were led by strong seismic shaking. Case studies further show that both significant vertical residual ground settlements and horizontal displacements were also induced by this earthquake in liquefied level sandy ground with a sufficiently large lateral extent where no significant initial shear stresses were present. The mechanism regarding the occurrence of the above phenomena is revealed through new post-liquefaction stress-strain constitutive analysis. Consequently, the residual post-liquefaction ground settlement and horizontal displacement are found to be interdependent of each other, so that neither cannot be determined separately in principle. A new methodology with pragmatic charts is developed for concurrently evaluating the residual post-liquefaction ground settlement and horizontal displacement in liquefiable ground. The prediction results using the proposed method agree with actual observations during the 1995 Hyogoken-Nambu earthquake.

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Liquefaction-induced large ground displacement triggered by quaywall movement is investigated by case studies on the 1995 Hyogoken-Nambu and the 1964 Niigata earthquakes, and by a shaking table experiment on model ground. Permanent ground strain is dicussed, as is its relationship with the damage rate to buried water and sewage pipes. The followings are obtained by summarizing the results of the case studies and the experiment : (1) the magnitude of the ground displacement 200-300 m away from the quaywall is still governed by the quaywall movement; (2) the damage rate of buried pipes for water and sewage has a close correlation with the tensile ground strain in the axial direction; (3) the behavior of the liquefied soil due to a boundary movement varied greatly depending on the magnitude of the boundary movement. The liquefied soil behaves as a solid body when the boundary movement is large. On the contrary, it behaves as a liquid when the boundary movement is small.

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Effective stress analyses are performed on the seismic response of port structures during the 1995 Hyogoken-Nambu earthquake. The structures analyzed include : caisson type quay walls which moved up to 5 m maximum toward the sea; pneumatic caisson type bridge foundations which moved toward the sea, reducing the distance between the bridge foundations across a sea channel by a total of 0.8 m; and composite breakwaters which settled 2 m. Although these structures were all a rigid concrete type and shaken by the same earthquake, the mode and extent of deformation/failure were different depending on the backfill and embedment conditions of the structures.
The constitutive model of soil used for the seismic analyses is a multiple mechanism model defined in strain space. The model can take the effect of principal stress axis rotation into account, which is known to play an important role in the cyclic behavior of sand under anisotropic stress conditions. The model parameters are calibrated based on insitu and laboratory investigations, including in-situ freeze sampling. The results of the effective stress analyses are consistent with those measured, including the mode and extent of deformation/failure, suggesting that the cyclic behavior of soil, idealized through the effective stress model, can explain the variety of seismic response of the port structures in a unified manner.

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Seismic damage to gravity quay walls in Port Island and other reclaimed islands during the Hyogoken-Nambu earthquake in 1995 consisted of the tremendous lateral displacement of caisson boxes. The field investigation immediately after the earthquake found that there was no sand boiling in the vicinity of damaged quay walls. This finding was in contrast with the fact that a large amount of sand was boiled in most other parts of the island. It was disputable, therefore, whether or not any role was played by subsoil liquefaction in the damage of quay walls. Shaking table tests were carried out in the present study in order to understand the mechanism of the wall displacement. The test results indicate that the significant deformation of walls was a consequence of the combined effects of shaking and the development of pore water pressure in sand underlying the wall. The wall displacement increased as the intensity of shaking became greater and the underlying sand was made looser. The effects of shaking frequency were important. The stress-strain and stress-path behaviors of the foundation sand were reproduced by using the experimentally recorded time histories of acceleration and pore water pressure.

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A detailed investigation was made of a landing pier of steel pipe piles damaged by the 1995 Hyogoken-Nambu earthquake; the external force which acted thereon and the mechanism of failure were inferred. The pier was of an open type supported by vertical and batter piles and was constructed in front of the sea wall caisson. The investigation covered the displacement and inclination of the sea wall and the pier, and the buckling and deformation of the steel pipe piles were examined after the piles were extracted. In the vicinity of the area near the investigated pier, a horizontal acceleration of 327 gal had been recorded, so a large inertial force may have acted on the sea wall caisson and the pier. In addition, the replacement sand below the caisson may have undergone liquefaction, causing the sand and the rubble to shift, and imposing a horizontal force directly upon the piles, thus causing their breakage. Hence, in the present analysis a total of eight cases were studied under various conditions including the presence/absence of horizontal force due to ground movement, the loading direction and the magnitude of the horizontal load, in addition to the inertial force of the structure generated by the earthquake. As a result, it was found that the case wherein the inertial force of the structure was in the direction opposite to that of the ground deformation load most correctly explained the condition of the damage which actually occurred.

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A pile inclinometer survey is conducted for the steel pipe piles supporting a four-story steel structure that survived the Hyogoken-Nambu earthquake, together with some other measurements, in order to clarify the degree of integrity of the building foundation. The building is located on a wharf surrounded by the sea on three sides. At the site, fill deposits about 20 m thick have been compacted by the vibro-rod compaction method. Subsurface investigations are also conducted for the compacted fill and underlying soil deposits.
The investigations reveal the following : (1) ground subsidence due to the earthquake is estimated to be 30 to 100 mm; (2) the compacted fill has 60 to 80 percent greater resistance to liquefaction than untreated fill; (3) piles about 40 m long have been deformed not only above the interface between the reclaimed fill and the underlying alluvial clay layer (Ma13 layer), but also at the lower interface of the Mal3 layer; (4) the pile heads are judged to have been moved by at most 344 mm in the north-northeast to northeast direction.
The moved distance and direction of the pile heads are explained on the basis of the compiled data regarding the relationships among the moved distance of pile-supported buildings, the distance between the buildings and quay walls, the lateral displacement of the quay wall crests and the thickness of the reclaimed fill.

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The field performance of various pile foundations that experienced soil liquefaction and lateral spreading in the 1995 Hyogoken-Nambu earthquake are summarized. It is found that (1) damage tends to occur in non-ductile piles and at the interface between liquefied and nonliquefied layers; (2) the piles of a building near the waterfront show different failure modes in the direction perpendicular to the waterfront, while those away from the waterfront show similar deformation patterns. Cyclic and permanent ground displacements during and after earthquakes are estimated from field and aerial photographic surveys as well as analyses of strong motion records, and a simplified method for evaluating those displacements is presented. Pseudo-static analyses using p-y curves are conducted for well-documented case histories of pile foundations on which ground displacements estimated from the proposed method are imposed together with inertial forces from superstructures. The computed results agree reasonably well with the field performance of various pile foundations subjected to different ground movements. These findings confirm that the kinematic forces from cyclic and permanent ground displacements together with their spatial variation could have significant effects on pile damage. The proposed model could be effectively used to estimate deformation and stresses in piles subjected to ground movements.

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During the 1995 Hyogoken-Nambu earthquake, extensive liquefaction occurred on reclaimed lands causing serious damage to pile foundations. This paper reports the investigation of the damage to a pile foundation located in a liquefied soil as well as the results of its static and dynamic response analyses. Three piles of the building were examined by inserting a remote controlled television camera and an inclinometer into the hollow spaces of the piles. Serious damage was observed at the bottom of the fill where the piles were bent toward southwest. No serious damage was observed near the pile heads except for a shear crack found on one pile. Dynamic effective stress response analyses were conducted on a soil column and a soil-pile-building system and the process of the damage to piles was analyzed. The effect of the non-liquefied crust overlying liquefied soils on pile stresses due to inertial force is analyzed in detail, and the soil stiffness to be used in analyses is proposed. Pseudo-static analyses were conducted using a model consisting of a beam on nonlinear Winkler type springs. The effects of input parameters and the applicability of the pseudo-static analysis are examined by comparing the numerical results with the observed damage and with the results of the dynamic analyses.

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The 1995 Hyogoken-Nambu earthquake caused widespread liquefaction on the reclaimed islands in Kobe. The surrounding quay walls moved significantly towards the sea leading to horizontal movement of the soil behind the quay walls. Investigation of the piles installed in the reclaimed islands clearly showed the need for piles in such grounds to be designed taking into account the effects of lateral spreading of liquefied soils. The present study is aimed at establishing a simple design method for pile foundations subjected to such lateral spreading. A series of centrifuge tests were conducted to this end. Pile-to-wall distance, the thickness of non-liquefied layer, and the input earthquake motion were chosen as experimental variables, and their effects on pile behavior are discussed. The results show that the presence of the non-liquefied layer at soil surface and pile-to-wall distance play a very important role in the pile response. The non-liquefied layer seems to move as one unit towards the sea. A subsequent simple elastic analysis shows that the reduction in soil modulus in the non-liquefied layer near the quay wall may be an important aspect in estimating the pile response.

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A modified and pseudo-static limit-equilibrium approach to evaluate active earth pressure at high seismic load levels is proposed. Although it is similar to the Mononobe-Okabe method, the proposed method considers the effects of strain localization in the backfill soil and associated post-peak reduction in the shear resistance from peak to residual values along a previously formed failure plane. The proposed method can reflect differences in the peak shear resistance of the backfill soil with different degrees of compaction; yields a realistic active earth pressure coefficient which is smaller than that predicted by the Mononobe-Okabe method using a residual shear resistance; can be adapted to analyses with a large horizontal seismic coefficient where the Mononobe-Okabe method using the residual shear resistance is not applicable; and renders a reduced and more realistic size of active failure zone in the backfill soil at high seismic load levels compared to that predicted by the Mononobe-Okabe method.

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