加勁護岸之側向承載力之數值研究

Numerical Study for Geosynthetic Reinforced Revetments on Lateral Bearing Capacity

吳政諺(Zheng-Yan Wu)

指導教授 ： 陳榮河

國立臺灣大學/工學院/土木工程學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201602050

摘要

加勁擋土結構的設計理念可分為Mechanically Stabilized Earth Wall (MSEW)與Geosynthetics Reinforced Soil Composite (GRSC)。前者之設計理念是將加勁材視為背拉構件(tie-back wall)。而後者將加勁材與土壤視為複合材料進行設計。GRSC系統相較於MSEW，加勁材不須具高強度，牆體自力性佳，有利於對抗牆面撞擊及牆基受淘刷。因此，本研究採用PLAXIS 3D數值軟體，以數值模擬之方式，研究GRSC壁壘在承受以擬靜態方式加載之土石流衝擊力時，牆體變形、加勁材受力狀況與牆體破壞模式。內容共包含：(1)加勁壁壘受土石流衝擊之影響；(2)加勁壁壘之側向承載力與破壞型式；(3)土壤或加勁材參數對側向承載力之影響；(4)側向荷載型式與破壞型式之關係；(5)牆趾淘刷對側向承載力之影響；(6)壁壘牆面受土石流剪力之影響。
由研究結果發現，壁壘之側向承載力主要由壁壘種類(灌凝式或縫接式牆面)與壁壘寬高比W/H控制。灌凝式壁壘之側向承載力會高於縫接式壁壘，且壁壘之W/H越大，側向承載能力越高。對於灌凝式加勁壁壘，當W/H < 1.0時，破壞型式屬於傾倒破壞。當1.0 < W/H < 3.0時，破壞型式屬於底部滑動破壞。當W/H > 3.0時，破壞型式屬於內填土之被動擠壓破壞。對於縫接式加勁壁壘，當W/H < 1.6時，破壞型式係延地表附近加勁層之滑動破壞，當W/H > 1.6時，破壞型式是在壁壘頂部的內填土發生局部擠壓破壞。參數分析之結果顯示，壁壘寬高比是主要影響側向承載力的因子，其次是內填土之凝聚力、摩擦角與單位重。而內填土之剪脹角、加勁材垂直間距、加勁材勁度對壁壘之側向極限承載力影響不大。改變施加側向力的範圍(70%牆高)會影響縫接式壁壘之破壞型式，當W/H < 0.8時與原本相同屬於地表上加勁層的滑動破壞。當 0.8 < W/H < 2.4時，破壞模式控制在牆體之界面滑動與背力側之主動破壞。當 W/H > 2.4時，破壞模式為內填土被動破壞。而不論是何種類型的壁壘，牆趾淘刷都會明顯降低壁壘的側向承載力。，若考慮土石流於壁壘牆面造成之剪切力(其值為側推力的1/5)，相較於牆面無施加剪力之狀況，灌凝式與縫接式壁壘都會使牆體變形增加許多。

關鍵字

加勁壁壘 ； 土石流 ； 側向承載力 ； 破壞模式 ； 數值分析

並列摘要

Geosynthetic-reinforced soil walls can be classified into two types according to the design concept: Mechanically Stabilized Earth Walls (MSEW) and Geosynthetics Reinforced Soil Composites (GRSC). The former is designed and constructed in the same manner as a tied-back wall; while the latter is treated and designed as a composite material. Compared to MSEW, GRSC is more deformable and can withstand higher impact force. In this study, the finite element software, PLAXIS 3D, was used to analyze the behaviors of the GRS barriers under quasi-static lateral pressures induced by debris flows. The influencing factors investigated were as follows: (1) grouted-facing and wrap-around facing barriers; (2) geometry of the barriers; (3) parameters of backfill and reinforcement; (4) load pattern; (5) scouring; (6) debris flow induced shear stress on the facing of the barriers.
The results of finite element analysis indicated that the lateral bearing capacity depended on the aspect ratio (W/H: ratio of wall width to wall height), and that the failure mode depended on both facing type and aspect ratio. For grouted facing barriers with W/H < 1.0, they would fail in overturning. For 1.0 < W/H < 3.0, a sliding failure occurred along the backfill-foundation interface. For W/H > 3.0, passive soil failure occurred within the barrier and near the loading side.
As for the wrap-around barrier, when W/H < 1.6, sliding failure occurred along the bottom layers of reinforcement. When W/H > 1.6, the ultimate lateral bearing capacity was reached due to local passive soil failure occurred on the top of loading side.
The parametric study showed that the W/H ratio, backfill cohesion c, soil unit weight, and friction angle φ were the major influencing factor to the lateral bearing capacity.
When the foundation of the barrier was scoured by debris flows, its lateral bearing capacity would decrease 30 to 50%.
Considering the shear stress in addition to lateral pressures induced by debris flows, the lateral displacement would be much increased for both types of barrier.

並列關鍵字

GRS barrier ； debris flow ； lateral bearing capacity ； failure mode ； numerical analysis

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