鄰近傾斜岩石面對擋土牆主動土壓力之影響(II)

Abstract

本計劃研究鄰近傾斜岩石面對作用於擋土牆主動土壓力的影響。若一座重力式擋土牆頗為接近傾 斜岩石界面，牆後回填土受到岩石面邊界限制，因此牆後之主動土楔可能無法完全的發展出來，作用 於擋土牆之主動土壓力可能與傳統Coulomb 及Rankine 理論算出之的主動土壓力不同。本研究計畫以 試驗室實驗方式探討岩石界面傾斜角、及傾斜界面底部與擋土牆間水平間距b 對牆面主動土壓力之 影響。模型擋土牆高H = 0.5 m，以安裝於模型擋土牆上的土壓力計量測牆面之水平土壓力。本研究運 用一雙向加勁鋼質介面板來模擬鄰近的傾斜岩石界面，本計畫擬進行試驗的界面板位置包含下列狀況 的組合: (1) b = 0, 50 mm, 100 mm, 150 mm, 250 mm , 350 mm, 500 mm, 及2,000 mm； (2) 傾斜角 = 0, 50o, 60 o, 70 o, 80 o 及90 o。所有實驗都採用氣乾渥太華砂為背填土，本研究第一年採用空中霣降法備製 均勻密度的疏鬆(Dr = 35%)砂土試體。為了模擬現地狀況，本研究第二年將採用震動式平板夯實機， 在試驗室夯壓土壤至Dr = 75%。本研究以1g 物理模型獲得之實驗結果，將與理論解、數值分析結果、 及離心機試驗結果相互印證。本研究第一年部份已獲得國科會補助51.1 萬元經費，進行研究並獲得下 列重要發現:(1)當岩石界面距擋土牆距離越近(岩石界面傾斜角增大或水平距離b 減小)，牆面土壓力 受界面板的影響越大，測得之主動土壓力合力Pa,h 越小，且合力作用點位置高於理論值0.333H。(2) 當傾斜角等於90 度時(擋土牆與介面板相互平行)，牆後之背填土只剩下薄薄的一層，因此測得之主 動土壓力明顯低於Coulomb 理論值。

This study investigates the lateral earth pressure acting on a vertical rigid wall, which moves away from a constrained backfill of dry sand. The instrumented retaining-wall facility at National Chiao Tung University is used to investigate the effects of an adjacent inclined rock face on the development of active earth pressure. The inclination angle of the rock face with the horizontal is defined as beta angle, and the horizontal spacing between the base of the inclined rock face and the wall is expressed as b. The rock face inclination angles beta = 0o, 50o, 60o, 70o, 80o, 90o and the horizontal spacing b = 0, 50 mm, 100 mm, 150 mm, 250 mm , 350 mm, 500 mm, and 2,000 mm are tested. For all experiments, the surface of backfill is horizontal and the height of the backfill above the wall base H = 0.5 m. Horizontal earth pressure against the wall is measured with the soil-pressure transducers mounted on the model wall. A steel interface plate is designed and constructed to simulate the inclined rock face near the retaining wall. Air-dry Ottawa sand is used throughout this investigation. For the first year of this study, the backfill is deposited by air-pluviation from the slit of a hopper into the soil bin and the relative density of soil achieved will be 35%. Note that the backfill with the relative density of 35% is a very loose material. In practice, the granular backfill behind a retaining wall is generally recommended to be compacted to achieve a relative density of 70-75%. In the second year of this study, the loose backfill will be compacted to a relative density of 75%. Experimental results obtained with this 1-g physical model tests will be compared with theoretical solutions, numerical findings and centrifuge test results. The first-year part of this study has been carried out under the financial support the National Science Council, and the following major conclusions had been made. (1) As the inclined interface approached the wall (inclination angle beta increased or spacing b decreased), the lateral soil thrust on the wall decreased and its point of application elevated. (2) For an inclination angle of 90° (interface parallel to the vertical wall), only a thin layer of backfill was sandwiched between the wall and the rock face. The measured lateral earth pressure was significantly less than Coulomb’s prediction. (3) The intrusion of an inclined rock face into the active soil wedge would increase the factor of safety against sliding and overturning, and the evaluation of FS with Coulomb’s theory would be on the safe side.