Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/34355
標題: Numerical Simulation of The Mechanism of Rainfall-Infiltration-Seepage and Stability Analyses of Collapsed Land
崩塌地降雨-入滲-滲流機制之數值模擬及穩定性分析
作者: 鄭順隆
Chein, Shun-Long
關鍵字: Collapsed land
崩塌地
Finite element method
Limit equilibrium method
numerical simulation of seepage
slope stability
有限元素法
極限平衡法
滲流數值模擬
邊坡穩定
出版社: 水土保持學系所
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摘要: To investigate the landslide mechanism and the associated influence factors on the stability of slope-land, this study performed a systematic rainfall induced seepage and slope stability analyses using two dimensional finite element method and limit equilibrium method. After the establishment of numerical model, the rainfall induced seepage analysis was carried out on the Y4 profile of Li-San landslide area. The numerical results were compared with those from measurements to verify the validity of numerical simulation. Subsequently, several types of fictitious slope were designed for the parametric studies to detect the influence scale and the sensitivity of various influence factors on the slope stability. In parametric studies, the influence factors encompass: Topography factors (slope inclination and slope height), Geological factors (effective cohesion, effective friction angle, unit weight of soil stratum, unsaturated suction friction angle and hydraulic conductivity function), Hydrology (rainfall patterns and initial groundwater table) and Geological structure (thickness of colluviums). Incorporating the dispersal of soaking line, seepage of infiltrated rainwater, variation of groundwater table and change of pore water pressure with the slope stability analyses, it is possible to interpret the landslide mechanism of slope-land in a rational manner. In accordance with the calculation results, several conclusions can be drawn from this study: Rainfall induced seepage analyses were performed on the Y4 profile of Li-San landslide area using the Hyetographs of Typhoon Toraji, Bilis, Aere and Mindulle. It was indicated that Typhoon Toraji can give the most coincidence of the calculated groundwater variation with that from the measurement. In addition, the predicted tendency from other three Typhoons also shows a good agreement to a certain extent. To survey the significance of boundary condition on numerical calculations, this study performed a series of rainfall induced seepage analyses using 4 types of seepage boundary at the lateral side of numerical model. It was found that the influence of boundary condition type is minor if the geometry boundary was located at a distance of 3 times of slope height away from the main slope. As a consequence, it can be deduced that for the identical condition, the influence of boundary condition type on the calculation of factor of safety is also negligible. For a slope with steep inclination or large slope height, the wave front of soaking line beneath the slope toe is capable of infiltrating and dispersing into soil stratum from ground surface with a greater depth during the rainfall. Consequently, the soaking line may join the groundwater table together within short rainfall duration. In general, for P1 instrumentation point located at the depth of 2 m below the slope toe, the initial pressure head is approaching to zero from a negative value during rainfall and eventually turns into positive value due to the overlap of soaking line and groundwater table. This also implies the area adjacent to P1 instrumentation point has been converted from unsaturated to saturated state and the suction of unsaturated zone,vanishes entirely and the equivalent shear strength originated from suction, , becomes valid. At the mean time, for P2 and P3 instrumentation points which located at the depth of 2.5 m below the mid-slope surface and at the depth of 3 m below the slope top respectively, the pressure head remain negative and this indicates both points are still situated at the unsaturated zone in between the soaking line and groundwater table. Moreover, based on the aforementioned discussions, it can be deduced that the potential sliding failure surface may initiate at the area adjacent to slope toe instead of the other areas of slope. For the situation which possesses a Fs >1 at the last time step with rainfall duration of tf-1, the overlap zone of soaking line and groundwater table is merely restricted to a relatively small extent nearby the slope toe. Accordingly, the minimum factor of safety, Fsmin(>1), was calculated based on the potential surface sliding through the entire slope. On the contrary, for the situation which possesses a Fs <1 at the first time step with rainfall duration of tf, the overlap zone of soaking line and groundwater table may expand to a certain extent along the up-slope direction and form a saturated zone at the down-slope. Therefore, the failure factor of safety, Fsfailure(<1), was calculated merely for the potential surface sliding through the particular saturated zone. Since the shear strength of unsaturated soil can be expresses by the modified Mohr-Coulomb equation as, it is apparent the increase of relevant strength parameters such as:can promote the stability of slope during rainfall. However, the increase of unit weight of soil stratum,may decrease the stability. In addition, it was found that the variation of hydraulic conductivity function simply shows very tiny effect on the groundwater table during rainfall. The pressure head of soil stratum below the slope surface is mainly affected by the scatter of soaking line whereas the slope stability is dominated by the development of overlap zone of soaking line and groundwater table along the slope surface. According to the distribution profile of volumetric water content of specific instrumentation points, it was observed that the promotion of entire hydraulic conduction function expands the unsaturated zone while the demotion contracts during rainfall. For both rainfall patterns, Typhoon-Rain and Plum-Rain, higher average rainfall intensity leads to shorter rainfall duration required for the formation of soaking line and earlier occurrence of slope instability. For an identical average rainfall intensity of 20 mm/hr, the slope instability occurs earlier in Plum-Rain pattern (tf=5~6 hr) than that in Typhoon-Rain pattern (tf=7~8 hr). This is due to the fact that the difference of input value of rainfall intensity in the initial stage. The rainfall duration required for the overlap of soaking line and groundwater table is shortened as the initial groundwater table is high (hwo value is low). For 9th hr rainfall duration, the overlap zone of soaking line and groundwater table is formed for hwo=1.5 m, the soaking line and groundwater table is joined together for hwo=3.5 m and no overlap occurs for hwo=7 m. The effect of colluviums thickness on the rainfall duration required for the initiation and the infiltration depth of soaking line is insignificant. However, a thick colluviums layer may retard the combination of soaking line and groundwater table. In conclusion, the most influential factor to the slope stability is slope inclination (β) and slope height (H) is succeeded in sequence.
為探討坡地之崩塌機制及其穩定性影響因子參數研究,本研究採用二維有限元素法及極限平衡法來進行滲流數值模擬及邊坡穩定分析。在建立一套完整的降雨滲流數值分析模式後,對梨山地滑區Y4剖面進行降雨入滲數值分析,並將數值分析結果與現場監測資料進行比對,以確認數值模擬之有效性。 隨之,設計多組虛擬邊坡,針對各項邊坡穩定影響因子進行參數研究,以檢核各種因子在邊坡穩定性上所具有之影響性及靈敏度。進行參數研究之影響因子包括:地形因子(坡度及坡高);地質因子(有效凝聚力、有效內摩擦角、土層單位重、未飽和土層之當量吸力摩擦角及水力傳導係數函數);水文影響因子(颱風雨型、梅雨雨型及初始地下水水位)及地質構造因子(崩積層厚度)。將降雨期間滲浸潤線擴散、入滲雨水滲流、地下水水位變動及地層孔隙水壓變化之分析成果運用於邊坡穩定分析計算,可對坡地之崩塌機制進行合理的詮釋。 綜合本文各項研究成果,可得下列結論: 棃山地滑區Y4剖面採用桃芝、碧利斯、艾莉及敏督利四個颱風之降雨條件進行降雨入滲分析,結果顯示桃芝颱風之數值模擬地下水水位變化趨勢與監測結果最為吻合,其餘颱風之分析趨勢亦有相當之吻合度。 由於邊界條件對滲流數值分析之結果有重大的影響。針對四種不同類型之側向滲流邊界條件進行檢核得知,當邊界遠離邊坡主要滲流區達3倍坡高(3H)時,不同側向滲流邊界條件之設定對滲流分析之數值解影響將非常微小。因此,在同樣的條件下,採用四種不同邊界條件之滲流分析結果來進行邊坡穩定計算,其穩定性安全係數計算值將非常接近而無太大差異。 邊坡坡度愈陡或坡高愈大時,降雨期間其坡趾下方的浸潤線前端波(Wave Front)會愈深入地表,因此在較短之降雨延時內浸潤線即可能和地下水水位線結合。一般而言,在降雨期間P1監測點(坡趾下方2 m)周圍區域由於浸潤線與地下水水位線的結合,將導致該監測點之壓力水頭由負值趨近於零後再轉為正值,亦即由未飽和狀態轉為飽和狀態,未飽和區土層之吸力完全消失且其當量吸力抗剪強度轉為無效。在此同時,P2監測點(坡面中點下方2.5 m) 及P3監測點(坡頂下方3 m)之壓力水頭則仍維持負值,此顯示兩監測點周圍區域仍處於浸潤線與地下水水位線間之未飽和區。再者,由上述結果可進一步推估潛在滑動破壞面將首先發生於坡趾區域。 對於降雨期間之安全係數計算,在安全係數仍大於1的最後一個時階,邊坡之浸潤線與地下水水位線之結合區只侷限於坡趾附近小範圍區域,因此最小安全係數Fsmin(>1)大體上仍以滑動面通過整個坡面來進行計算。反之,在安全係數己小於1的最前一個時階,浸潤線與地下水水位線之結合區將會往上邊坡方向延伸一個範圍,並在下邊坡形成一個飽和區,因此破壞安全係數Fsfailure(<1)將以滑動面通過整個飽和區來進行計算。 未飽和土壤之Mohr-Coulomb剪力強度公式,因此邊坡之有效凝聚力、有效內摩擦角、未飽和土層之吸力及其當量吸力摩擦角之增加皆會提升邊坡之穩定性安全係數Fs值。再者,土層的單位重在穩定性分析中主要用來計算滑動面上之下滑驅動力及正向作用力。土層單位重之增加將會降低穩定性安全係數Fs值。 整體水力傳導係數函數值向上調升或向下調降,在降雨期間對既有之地下水水位影響不大。邊坡下方土層之壓力水頭主要受降雨浸潤線擴散之影響,而邊坡穩定性則受浸潤線與地下水水位線結合區在坡面上之延伸範圍所控制。由特定監測點之體積含水量深度分佈剖面來判釋,整體水力傳導係數函數值向上調升將促使地表未飽和區之範圍擴大。反之,向下調降將會縮小未飽和區之範圍。 對颱風及梅雨兩種雨型而言,平均降雨強度愈大,降雨浸潤線形成所需之降雨延時愈短,而邊坡也愈早發生不穩定。在平均降雨強度同為20 mm/hr時,邊坡在梅雨雨型條件下(tf=5~6 hr)較颱風雨型條件下(tf=7~8 hr)提前發生不穩定。其主要原因為輸入之梅雨雨型初期降雨強度呈波動式之遞升,此與颱風雨型者呈均勻線性式之遞升不同所致。 邊坡之初始地下水水位線愈高(hwo愈小),其在降雨時與浸潤線發生結合之降雨延時也愈短。以降雨延時第9 hr而言,在hwo=1.5 m之情況,將形成浸潤線與地下水水位線結合區域。而在hwo=3.5 m之情況,兩線正好發生結合。最後,在hwo=7 m之情況,兩線仍無結合之跡象。 崩積層之厚度對浸潤線形成所需之降雨延時及向下之擴散深度影響不大,但對浸潤線與地下水水位線之結合有遲滯之作用。因此,當崩積層之厚度增加時,其浸潤線與地下水水位線發生結合所需之降雨延時亦將隨著增加。 由參數研究靈敏度分析中可發現,各項邊坡穩定性影響因子中,邊坡坡度β之改變對邊坡穩定性的影響度最高,而邊坡坡高H次之。
URI: http://hdl.handle.net/11455/34355
其他識別: U0005-1108200609231000
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