山崩

岩盤、風化層、或其混合物因重力的吸引，往下坡方向的運動即為山崩

1. 山崩與其衝擊- 當人口增加，都市與道路開始

擴 張時，自然環境遭受破壞，山

崩 對人類的威脅亦逐漸增加。- 增進對山崩與地質環境的瞭解

， 有助於紓解其災害。

2. 山崩的形式- 山坡地的組成、含水量、邊坡的傾角 等因素影響山崩活動的形式與運動的 快慢。

(1) 涉及山坡地瞬間的破壞，導致黏滯 度高的塊體以崩移、掉落，或滑動 方式運動。(2) 沈積物混合著水份與空氣的流動。

兩種形式：

(1) 崩移：質體沿著弧形上凹的滑動面滑動。公路兩側的山坡地常因人為的開挖調整，破壞了環境的自然平衡狀態，導致崩移作用的發生。

2.1 山坡地的破壞

崩移

(2) 掉落：落石自懸崖上方垂直向下的瞬時掉落，岩塊大小可能超過數百公尺，當其撞擊地面時，可裂解成諸多小塊體。

落石

岩屑掉落

(3) 滑動：質體沿著平面狀滑動面的下坡運動，一般以滑動方式進行。若有斷層、葉理面，或層理面存在時，容易產生滑動。

岩板滑動

岩屑滑動

流動

(1) 泥漿流：泛指飽和沈積物的流動。岩屑流為比砂更為粗大的岩屑流動，流速介於 1m/year 至 100m/hour 之間。若岩屑流之含水量過高，導致整體岩屑流呈現流體的特性時，則稱之為泥流。

2.2 流動

泥漿流

岩屑流

泥流

(2) 粒狀流：未飽和的沈積物、空氣和水份的流動，流動沈積物的重量由顆粒彼此之間的接觸與碰撞來支撐。潛移乃流速極慢，幾乎無法察覺的粒狀流。土流比潛移流動迅速，且可延續數天、數月，甚至於數年。

潛移creep

土流

顆粒流

土流特徵

(3) 顆粒流：相對的較為乾燥或半乾燥的沈積物流動，空氣充滿沈積物顆粒間的空隙。

岩屑崩進

The Blackhawk slide in southern California is a good example of a long rock avalanche.

- 高緯度或高山地區，解凍的冰水飽和了風化層，造成地表舉升。年復一年的冰凍、解凍作用造成地表反覆的上升和下降，風化層如潛移般的往下坡方向移動。

2.3 寒冷氣候的山崩

- 寒冷地區淺層的冰在夏季溶解時，冰水可飽和地表淺層的沈積物，若坡度夠陡，可因重力吸引而形成解凍土流。

石冰川

- 海底山崩作用與陸域相似，海盤常因海底崩移、滑動或流動而斷裂。- 大型之海底沖積扇常見山崩作用之特徵。山崩區域顯示三個明顯的分區：形成區 - 地層下陷與崩移活動區；中央溝渠區 - 沈積物沿此區搬運；堆積區 - 崩落物質重疊堆積於此區。

2.3 海底山崩

海底山崩

3 影響邊坡穩定的因子

重力→坡角水份→邊坡之水文特性地質材料之組成觸發事件之發生與否

剪力強度大於剪應力，邊坡處於穩定狀態

Fs (安全係數 ) =剪力強度 /剪應力

邊坡穩定的條件

3.1 重力與坡角-剪應力與剪力強度為影響邊坡穩定之兩大力量。重力為剪應力之主要因子，其大小與坡角密切相關。剪力強度則為抵抗物質滑動之內聚力，與摩擦力、黏滯度與植生有關。Fs (安全係數 ) =剪力強度 /剪應力-安全係數小於 1 時，則山崩明顯。

3.2 水份- 乾燥的砂傾倒於平坦的桌面時，堆高成圓錐狀，其傾角稱為休止角。若含水量逐漸升高，則可見砂堆轉化成泥漿狀，容易流失。- 沈積物如粉砂和黏土，具有足夠的黏滯力，但被水飽和之後，內部水壓升高，細粒沈積物變得不穩定，甚至開始流動。

3.3 困擾的地質材料(1) 土壤液化：因含水量的升高，使得土壤由固態轉化為液態的現象。土壤液化時的含水量稱為液性限度，其大小因土壤性質而異。黏土含量高的土壤液性限度極大，不易液化，呈現高度可塑狀態。

(2) 膨脹土壤：亦稱為收縮 -膨脹土壤，土體的體積可因飽和或乾燥而膨脹或收縮。(3) 敏感性土壤：此類土壤可因輕度的擾動或再塑作用而喪失其剪力強度。土壤結構受擾動呈現液化或壓密現象之作用，即為再塑作用。

3.4 觸發事件 (trigger events)

(1) 地震和其他震動：突發的震動如地震、爆炸、熱雷雨，或甚至於大卡車路過，都可能升高剪應力，觸發山崩。(2) 火山噴發：大規模層狀火山坡角大，內部包含不穩定的熔岩流與火山碎屑物交替出現，潛在威脅大。

(3) 山坡地開發：河流在坡腳的底切作用，人為不正常的開挖，以及坡頂荷重的增加，都可能引起山坡地的滑動。(4) 水文特性的變化：不尋常或連續性的降雨導致荷重增加，實為許多流動的主因。

道路開挖前

道路開挖後

道路開挖前

道路開挖後

-for engineer to distinguish and recognize the different phenomena for purposes of design and also to take the remedial or safety measures.-for geologist to interpret the past and predict the present trends of topography.

Landsliding study, why?

A. Processes of LandslidingIt is a continuous series from cause to

effect. Landslides take place when

geologic, topographic and climatic

influences are common.

The process leading to the development of slide has its beginning with the rock formation, when its physical properties are determined, and includes all crustal movement, erosion, weathering, until some action sets a mass of it in motion downhill. All true slides involve the failure of earth material under shear stress.

B. Stability Treatment

-modification of factors contribute to

slope shape : unloading the active

part and loading its toe.-drainage of the sloped area : subsurface and surface drainage system.

-supporting an anchorage of a sliding slope: retaining walls, grouting, cast-in piles, anchorage (rock bolt, monostrand, tendon), etc.“All methods should prevent the water in near-surface layer from freezing.”

C. Solution of Problem

-avoidance of potential slides.

-excavation : economic consideration.

-drainage.

-restraining structures.

邊坡穩定處理

擋土牆

排水

開挖

植生

擋土牆 + 岩錨

植生

擋土牆 + 柵欄

明隧道

Factors That Contribute to High Shear Stress

a. Removal of lateral support a)  erosion by : (1) streams and rivers in the production of most natural slopes.

(2)  glacial ice, many valleys in mountains regions were deeply cut by glaciers, when ice retreated, landslides occurred on a large scale.(3)  waves, and longshore current or tidal currents.(4)  Subaerial weathering, wetting and drying, and frost action.

b)  creation of new slope by previous rockfall, slide, subsidence or large-scale faulting.c)  human agencies(1)  cuts, quarries, pits, and canals.(2)  removal of retaining walls, sheet piling, etc.(3)  draining of lakes or drawdown of reservoirs.

b. Surcharge

a)  natural agencies

(1)  weight of rain, hail, snow, and

water from springs.

(2)  accumulation of talus overriding

landslide material.

b)  human agencies(1)  construction of fill.(2)  stockpiles o fore or rock.(3)  wastepiles from strip mining.(4) weight of buildings and other structures and trains.(5) weight of water from leakage pipelines, sewers, canals, reservoirs, etc.

d. Transitory earth stresses

Earthquakes have triggered a great

many landslides, both small and very

large and disastrous. Their action is

complex, involving both increase in

shear stress, and, in some examples,

decrease in shear strength.

They produce horizontal accelerations that may greatly mod blasting, machinery, and traffic also produce transitory earth stresses.ify the state of stress within slope-forming material. In the case of potential circular-arc failure, horizontal acceleration causes a moment about the center of the arc, which when directed toward the free slope adds to its instability.

d. Regional tilting Progressive increase in slope angle through regional tilting has been suspected as a contributing cause to some landslides. The slope must obviously be on the point of failure for such a small and slow-acting change to be effective.

e. Removal of underlying support

a) undercutting of banks by rivers

and waves

b) subaerial weathering, wetting and

drying, and frost action .

c)  subterranean erosion

(1)   removal of soluble material such as

carbonates, salt, or gypsum, collapse of

caverns

(2) washing out of granular material

beneath firmer material

d)  human agencies, such as mining

e)  loss of strength in underlying material

(1)   large masses limestone over shale

(2)   compact till over clay

failure by lateral spreading

f)  lateral pressure due to

(1)  water in cracks and caverns

(2)  freezing of water in cracks

(3)  swelling – hydration of clay,

hydration of anhydrite

  Factors That Contribute to Low Shear Strength

Factors can be divided into two groups. The first group includes factor deriving from the initial state or inherent characteristics of the material.

The second group includes the changing or variable factors that tend to lower shear strength of the material.

a.  Initial statea)  compositionb)  texture(1) loose arrangement of individual particles in sensitive clays, marl, loess, sands of low density, and porous organic matter.(2) roundness of grains.

b)  gross structure(1) discontinuities such as faults, bedding planes, foliation in schist, cleavage, joints, and brecciated zones.(2) massive beds over weak or plastic materials.(3)  strata inclined toward free face.(4) alternation of permeable beds and weak impermeable beds.

b. Changes due to weathering and other physico-chemical reactionsa)  physical disintegration of granular rocks under frost action, thermal expansion.b)  hydration of clay materials.c)  base exchange in clays (influence on physical properties of clays).

d) drying of clays (results in cracks and

loss of cohesion and allows water to seep

in).

e) drying of shale may create cracks on

bedding and shear planes.

f) removal of cement by solution.

c. Changes in intergranular forces due to pore watera) buoyancy in saturated state decreases effective intergranular pressure and friction.b) intergranular pressure due to capillary tension in moist soil is destroyed upon saturation.c) seepage pressures of percolating ground water result from viscous drag between liquid and solid grains.

d.  Changes in structurea) fissuring of pre-consolidated clays due to release of lateral restraint in a cut.b) effect of disturbance or remolding on sensitive materials such as loess and dry or saturated loose sand.