條形載重於夯實回填土上造成之靜止土壓力

Abstract

本論文探討於靜止擋土牆後方之夯實回填土上施加條形地表荷重造成之土壓力增量。試驗中以氣乾之渥太華砂填入實驗土槽，並以震動式夯實機使砂土之相對密度達到75%。國立交通大學研發地表荷重加載系統以施加0.1公尺寬之條形載重於夯實回填土上。條形載重之中心分別距離檔土牆面0.15公尺、0.3公尺、0.6公尺及0.9公尺。以試驗方法獲得夯實回填土之極限承載力qult，於地表加載之試驗中控制地表荷重強度於qult/3以符合工程設計原則。根據實驗結果，獲得以下各項結論。 1. 為提供均勻壓力於基腳上，使用氣墊夾置於上蓋與剛性基腳之間，是一個可行的方法。 2. 當地表載重施加於靠近檔土牆面（m≦0.1），會降低作用於靜止擋土牆之土壓力。這可能是由於夯實載重造成靠近地表土體之側向壓縮，趨近被動狀態。靠近牆面之地表荷重造成土體之軸向壓縮，同時引致側向伸張變形，改變夯實土之被動狀態而趨於較不被動的狀態(less passive)，因而降低側向土壓力及側向合力□Ph。這個現象會隨著地表加載位置遠離擋土牆面而消失。 3. 對於不同的加載位置，地表載重所引致的側向合力□Ph明顯低於理論的預估值。因此，既有的理論評估方式，對於地表載重於夯實回填上方造成之側向土壓力，提供了一個安全但不經濟的設計。

Dry Ottawa sand was used as backfill material. The horizontal pressure acting on a non-yielding wall due to a strip loading is studied in this paper. The Ottawa sand was placed and compacted the soil bin to achieve the required relative density of 75%. A surcharge loading system was developed at National Chiao Tung University to apply a 0.1m-wide strip surcharge of compacted backfill. The centerline of footing was applied at 0.15 m, 0.30 m, 0.60 m, and 0.9 m from the surface of the wall. Experiments were conducted to determine the ultimate bearing capacity qult of the compacted backfill. From a practical point of view, the intensity of the surcharge applied is controlled to less than qult/3. Based on the experimental results, the following conclusions are made. 1. To provide a uniform normal stress on the footing, the use of an air cushion sandwiched between the lid and steel footing appears to be a practical method. 1. If the surcharge is near the face of the wall (m ≦ 0.1), the surcharge loading will induced a decrease of lateral pressure in the upper part of the compacted backfill. It may be deduced that the surcharge applied on the top of the soil element near the wall (m = 0.1) will compress the soil element vertically. The induced lateral deformation will make the compacted soil “less passive” and the lateral soil thrust □Ph to decrease. This phenomenon would disappear as the surcharge loading move away from the wall. 2. The experimental lateral soil thrust induced by surcharge loading ΔPh is apparently smaller than the theoretical predictions for all surcharge loading positions. All theoretical estimations provide a safe but uneconomical design for the horizontal pressure increasing due to surcharge on the compacted backfill.