Daiki Hirakawa

Residual Earth Pressure on a Retaining Wall with Sand Backfill Subjected to Forced Cyclic Lateral Displacements

A pair of about 11 m-high soil retaining walls of an U-shaped underground reinforced concrete (RC) structure in Tokyo exhibited a large residual inward (i.e., toward the active side) displacement with potential structural damage, which became 18 cm between the tops of the two walls about three years after its completion. Noticeable settlements of the backfill were observed behind the walls. A series of small-scale model tests was performed in the laboratory to understand this field behaviour. The results from in-situ investigation and model tests showed that this wall behaviour can be attributed to a gradual increase in the residual lateral earth pressure, resulting from cyclic lateral displacements of the walls caused by a small number of relatively large seasonal thermal cyclic displacement of the RC wall facing and bottom slab of the structure, not by a great number of relatively small daily displacement. Three factors for the mechanism of this wall behaviour (i.e., ratcheting, cyclic hardening and cyclic loading-induced residual deformation of the backfill) were identified and analyzed based the model test results. The settlement in the backfill observed in the model tests is consistent with the field behaviour.

Importance of Strong Connection Between Geosynthetic Reinforcement and Facing for GRS Integral Bridge

A new bridge system, called the GRS integral bridge, is proposed. This consists of an integral bridge, which integrates a pair of RC abutment and a continuous girder without using girder supports, and a pair of geosynthetic-reinforced soil (GRS) retaining walls having full-height rigid (FHR) facings constructed after the completion of reinforced backfill walls. A pair of FHR facing function as the abutments to which a girder is integrated. Results from model tests indicate that, when reinforcement layers are firmly connected to the FHR facing, GRS integral bridges become much more cost-effective in construction, long-term maintenance and stability than conventional type bridges, integral bridges with unreinforced backfill and bridges using GRS retaining walls as abutments.

Small Strain Properties and Cyclic Resistance of Clean Sand Improved by Silicate-Based Permeation Grouting

Permeation grouting has been developed as one of countermeasures against soil liquefaction during earthquakes. This study aims at examining the small strain properties and cyclic resistance of jellied sand improved by silicate-based permeation grouting. In the first part of the present study, the small strain properties and cyclic resistance of jellied sand were examined based on the measurements of propagation velocities of longitudinal and shear waves travelling through triaxial specimens and the results from subsequently conducted undrained cyclic triaxial tests. In the second part, straincontrolled small-amplitude cyclic triaxial tests are conducted using non-contact gap displacement transducers to examine the initial elastic moduli of silicate gel itself and intact sand as well as jellied sand. The roles of the stiffness at small and large strains of silicate gel in increasing the undrained cyclic resistance of jellied sand are discussed in detail.

Simple Dynamic Hammer for Evaluation of Physical Conditions of Pavement Structures

Even though low-volume roads usually serve as secondary roads on the entire road network, they are of great importance for the distribution of goods to a number of rural areas in Thailand. However, the budgets for evaluation of the physical condition of the pavement structure of roads in this category and for their subsequent maintenance are surprisingly limited. In addition, the number of specialists required to perform visual inspections is small, and simple static tests on road surfaces are time-consuming and thus expensive. Consequently, it is difficult, if not impossible, to evaluate the physical conditions of pavement structures at a sufficient number of locations. To mitigate this, it becomes necessary to develop a simple test device and a simple means of test analysis to evaluate the physical condition of the pavement structure. This paper introduces a dynamic hammer and discusses a procedure for analysis of the test data. Experiments were performed on both asphalt-paved and unpaved surfaces, which are usually used for low-volume roads. In addition, plate load tests were performed in parallel to compare the test results and validate the analytical framework. It was found that the test device and the framework for analysis of the test data presented in this paper are relevant for use, in particular because of the simplicity of device production, the ease of device use, the time-effectiveness of test performance, and the procedures used for data analysis. Therefore, maintenance can be performed at sufficient numbers of locations of low-volume roads to keep them in good physical condition for serving the entire road network.

Post-Construction Time History of Tensile Force in Geogrid Arranged in a Full-Scale High Wall

To confirm a very low possibility of tensile rupture until the end of specified life time of polymer geogrids arranged in a full-scale reinforced soil retaining wall (21 m-high with a slope of 1:0.3 in V:H) constructed to support a taxi way of an airport, time histories of tensile force in the reinforcement were estimated based on those of measured tensile strain in the reinforcements in the wall. To this end, tensile tests were performed on the geogrids and their elasto-viscoplastic properties were evaluated. A constitutive model was developed based on the test results. The model was validated by that the model can accurately predict the tensile load-strain-time behaviour of the geogrids when subjected to arbitrary loading histories. The time histories of tensile force in the geogrids in the wall estimated from the measured tensile strains based on the model indicate that, even in the most severe case among those analysed, the tensile force will increase with time to a very limited extent, towards a value significantly lower than the tensile rupture strength at the end of life time. This result indicates that eventual creep rupture of the geogrids in the wall is not likely unless significant degradation takes place.

EFFECTS OF STRUCTUAL CONDITION FOR STATIC AND SEISMIC STABILITY ON GRS INTEGRAL BRIDGE

剛な一体壁面工に定着したジオシンセティックス補強材で盛土を補強した補強土擁壁は、費用便益比と安定性が高いことが確認されている。この技術では、壁面工を盛土完成後に建設することが重要である。筆者らは一対のジオシンセティックス補強土(GRS)擁壁の壁面工（竪壁）の上端に連続桁を定着した新形式の橋梁（GRS一体橋梁）を提案し、室内模型実験により常時と地震時の安定性を確認してきた。盛土・竪壁・橋桁の一体化により、気温の季節変動による盛土主働崩壊と受働土圧増加による被害を防ぐことができ、地震時安定性が向上する。常時と地震時安定性は、竪壁の杭基礎と盛土のセメント改良によって更に向上する。本論文では、これら模型実験の結果を総括し、常時と地震時の挙動に及ぼす各種構造条件の影響を考察する。

INFLUENCE OF CONSOLIDATION CONDITION ON GEOGRID REINFORCING EFFECT FOR STABILIZED SOIL

固化土の脆性的な力学特性をジオグリッドで補強する技術について検討した．固化処理地盤では養生と圧密が同時に生じているという推察に基づき，水中養生が短く圧密時間が長いケースと，水中養生が長く圧密時間が短いケースで，補強供試体と無補強供試体の強度を比較した．実験の結果，補強効果は，圧密条件の違いに関わらずみかけの粘着力として現れ，水中養生が短く圧密時間が長いケースの強度が大きくなることが明らかになった．

Cement-stabilization of geosynthetic-reinforced backfill for integral bridge

インテグラルブリッジの橋台背面盛土を橋台背面に剛結した多数のジオグリッド層で補強した新しい橋梁形式であるGRS 一体橋梁（Geosynthetic-Reinforced Soil Integral bridge）の地震時及び常時安定性は、盛土が無補強であるインテグラルブリッジと比較して非常に高い。GRS 一体橋梁の地震時崩壊が生じる場合は、橋桁とRC 橋台構造物の慣性力によって生じる盛土上部の受働破壊が引き金となり、最終的には橋台下端が主働方向に変位し破壊に至る。したがって、橋台の背面盛土の受働抵抗を増加させると地震時安定性が向上する可能性が高い。本研究では地震時におけるGRS 一体橋梁の背面盛土セメント固化の影響を振動台実験により、インテグラルブリッジの欠点である気温変動による橋桁の熱膨張収縮が橋台背面のセメント改良盛土に与える影響を繰返し載荷模型実験により検証した。その結果、背面盛土をセメント改良することにより盛土は常時・地震時共に安定性が向上した。