回填土夯實對被動土壓力之影響

Abstract

本論文探討回填土夯實對作用於擋土牆被動土壓力之影響。本研究利用國立交通大學模型擋土牆設備探討平移模式牆位移所造成土壓力之變化。試驗採用36%、60%與80%相對密度之渥太華砂為回填材料。根據實驗結果，獲得以下各項結論。 1. 對鬆砂而言，當牆開始移動時，土壓力開始增加，最後達到一極限土壓力。土壓力大約呈現三角狀分佈。Coulomb及Terzaghi 理論可以合理估計因牆移動所造成之被動土壓力。 2. 對緊砂而言，土壓力係數Kh隨牆移動而增加，當土壓力係數達到尖峰值後，Kh逐漸下降，最後達到一極限值。Coulomb及Terzaghi 理論利用尖峰摩擦角fpeak估計之土壓力係數會比實驗所獲得之尖峰Kh值及極限Kh值為高。在大量的牆位移（如S/H = 0.12）之下，Kh的極限值可將土壤殘餘強度fr引入Terzaghi 理論加以推估。 3. 無論回填土之初始密度為何，當牆位移量S/H超過0.12後，土壓力係數Kh會到一固定值。這是因為在破壞土楔內之土壤都已達到臨界狀態，其剪力強度應利用土壤的殘餘抗剪角fr估計。 4. 對鬆砂而言，Coulomb及Terzaghi 理論都稍微低估其被動土壓力。對中等緊密砂及緊砂而言，Terzaghi 理論利用尖峰抗剪角fpeak計算出之被動土壓力係數與實驗值相當吻合。若使用直剪試驗所獲得之殘餘抗剪角fr來計算Coulomb及Terzaghi 被動土壓力係數可發現，計算值與在大的牆移動量之下所獲得之土壓力係數與實驗值很接近。 5. 在計算緊密狀態下回填土之被動土壓力時，必須考慮破壞土壤之膨脹及強度折減現象。將土壤之殘餘強度引用在Coulomb及Terzaghi理論可合理估計在牆發生大位移時的被動土壓力。依此保守的設計，擋土牆將可確保維持在安全的狀態。

This paper presents experimental data of earth pressure acting against a vertical wall, which moved toward a mass of dry sand compacted at different densities. Ottawa sand with relative densities of 36%, 60%, and 80% are tested. The instrumented retaining-wall facility at National Chiao Tung University was used to investigate the variation of earth pressure induced by the translational wall movement. Based on this study, the following conclusions can be drawn. 1. For loose sand, as the wall starts to move, the earth pressure increases, and eventually a limiting passive earth pressure is reached. The shape of pressure distribution is approximately a triangular. Coulomb and Terzaghi's theories would provide a good evaluation of passive thrust due to the translational wall movement. 2. For dense sand, the horizontal earth pressure coefficient Kh increases with the increasing wall movement. After reaching a peak value, Kh decreases and finally remained an ultimate value. Coulomb and Terzaghi solutions calculated with a peak f angle are greater than the experimental peak and ultimate passive thrusts. At a large wall movement, the ultimate Kh could be properly estimates by introducing the critical state concept into the Terzaghi theory. 3. When the passive wall movement S/H is greater than 0.12, the passive soil thrust Kh reaches to a constant value regardless of its initial density. It may be deduced that, the soil along the failure surface has reached the "critical state", and the shearing strength on the surface could be estimated with the residual fr angle. 4. For loose backfill, Coulomb and Terzaghi's theories slightly underestimated the passive thrust. For medium dense and dense backfill, the peak experimental results are in good agreement with Terzaghi's solution calculated with fpeak. If the residual fr angle obtained from the direct shear tests is used in the Coulomb and Terzaghi's formula, the theoretical solutions are found to be in good agreement with the experimental passive thrust at large wall movements. 5. When calculate the passive earth pressure in dense backfill, it is recommended to consider the dilation and the strength reduction of soil along the failure surface. The passive earth pressure under a large wall deformation could be successfully approximated by introducing the residual soil strength into Coulomb and Terzaghi's theories. The conservative design will keep the retaining wall always on the safe side.