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What are the Effects and Consequences of Landslides?

Landslide effects occur in two basic environments: the built environment and the natural environment. Sometimes there is intersection between the two; for example agricultural lands and forest lands that are logged.

Effects of Landslides on the Built Environment

Landslides affect manmade structures whether they are directly on or near a landslide. Residential dwellings built on unstable slopes may experience partial damage to complete destruction as landslides destabilize or destroy foundations, walls, surrounding property, and above-ground and underground utilities. Landslides can affect residential areas either on a large regional basis (in which many dwellings are affected) or on an individual site basis (where only one structure or part of a structure is affected). Also, landslide damage to one individual property’s lifelines (such as trunk sewer, water, or electrical lines and common-use roads) can affect the lifelines and access routes of other surrounding properties. Commercial structures are affected by landslides in much the same way residential structures are affected. In such a case, consequences may be great if the commercial structure is a common-use structure, such as a food market, which may experience an interruption in business due to landslide damage to the actual structure and (or) damage to its access roadways.

Fast-moving landslides such as debris flows are the most destructive type of landslide to structures, as they often occur without precursors or warnings, move too quickly for any mitigation measures to be enacted, and due to velocity and material are often very powerful and destructive. Fast-moving landslides can completely destroy a structure, whereas a slower moving landslide may only slightly damage it, and its slow pace may allow mitigation measures to be enacted. However, left unchecked, even slow landslides can completely destroy structures over time. Debris avalanches and lahars in steep areas can quickly destroy or damage the structures and lifelines of cities, towns, and (or) neighborhoods due to the fact that they are an extremely fast-moving, powerful force.

The nature of landslide movement and the fact that they may continue moving after days, weeks, or months preclude rebuilding on the affected area, unless mitigative measures are taken; even then, such efforts are not always a guarantee of stability.

One of the greatest potential consequences from landslides is to the transportation industry, and this commonly affects large numbers of people around the world. Cut and fill failures along roadways and railways, as well as collapse of roads from underlying weak and slide-prone soils and fill, are common problems. Rockfalls may injure or kill motorists and pedestrians and damage structures. All types of landslides can lead to temporary or long-term closing of crucial routes for commerce, tourism, and emergency activities due to road or rail blockage by dirt, debris, and (or) rocks . Even slow creep can affect linear infrastructure, creating maintenance problems.

Figure 1. A landslide on the Pan American Highway in El Salvador, Central America, near the town of San Vicente, in 2001. (Photograph by Ed Harp, U.S. Geological Survey.)

Figure 1 shows a landslide blocking a major highway. Blockages of highways by landslides occur very commonly around the world, and many can simply be bulldozed or shoveled away. Others, such as the one shown in figure 1, will require major excavation and at least temporary diversion of traffic or even closure of the road.

As world populations continue to expand, they are increasingly vulnerable to landslide hazards. People tend to move on to new lands that might have been deemed too hazardous in the past but are now the only areas that remain for a growing population. Poor or nonexistent land-use policies allow building and other construction to take place on land that might better be left to agriculture, open-space parks, or uses other than for dwellings or other buildings and structures. Communities often are not prepared to regulate unsafe building practices and may not have the legitimate political means or the expertise to do so.

Effects of Landslides on the Natural Environment

Landslides have effects on the natural environment:

The morphology of the Earth’s surface—mountain and valley systems, both • on the continents and beneath the oceans; mountain and valley morphologies are most significantly affected by downslope movement of large landslide masses;
The forests and grasslands that cover much of the continents; and
The native wildlife that exists on the Earth’s surface and in its rivers, lakes, • and seas.
Figures 2, 3, and 4 show the very large areal extent of some landslides and how they may change the face of the terrain, affecting rivers, farmland, and forests.

Figure 2. The active volcano, Mount Shasta in California, USA. Note the landforms in the foreground, caused by a debris avalanche that occurred about 300,000 years ago. The debris avalanche traveled great distances from the volcano and produced lasting landform effects that can still be seen today. (Photograph by R. Crandall, U.S. Geological Survey.)

Figure 3. View looking downstream at the confluence of the Río Malo (flowing from lower left) and the Río Coca, northeastern Ecuador, in South America. Both river channels have been filled with sediment left behind by debris flows triggered by the 1987 Reventador earthquakes. Slopes in the area had been saturated by heavy rains in recent days before the earthquake. Debris/earth slides, debris avalanches, debris/mudflows, and resulting floods destroyed about 40 kilometers of the Trans-Ecuadorian oil pipeline and the only highway from Quito. (Photograph by R.L. Schuster, U.S. Geological Survey; information from Reference 32.)

Figure 4. The Slumgullion landslide, Colorado, USA. This landslide (formally referred to also as an earthflow) dammed the Lake Fork of the Gunnison River, which flooded the valley and formed Lake Cristobal. (Photograph by Jeff Coe, U.S. Geological Survey.)

Forest, grasslands, and wildlife often are negatively affected by landslides, with forest and fish habitats being most easily damaged, temporarily or even rarely, destroyed. However, because landslides are relatively local events, flora and fauna can recover with time. In addition, recent ecological studies have shown that, under certain conditions, in the medium-to-long term, landslides can actually benefit fish and wildlife habitats, either directly or by improving the habitat for organisms that the fish and wildlife rely on for food.

The following list identifies some examples of landslides that commonly occur in the natural environment:

Submarine landslide is a general term used to describe the downslope mass movement of geologic materials from shallower to deeper regions of the ocean. Such events may produce major effects to the depth of shorelines, ultimately affecting boat dockings and navigation. These types of landslides can occur in rivers, lakes, and oceans. Large submarine landslides triggered by earthquakes have caused deadly tsunamis, such as the 1929 Grand Banks (off the coast of Newfoundland, Canada) tsunamis.
Coastal cliff retreat , or cliff erosion, is another common effect of landslides on the natural environment. Rock-and-soil falls, slides, and avalanches are the common types of landslides affecting coastal areas; however, topples and flows also are known to occur. Falling rocks from eroding cliffs can be especially dangerous to anyone occupying areas at the base of cliffs, or on the beaches near the cliffs. Large amounts of landslide material can also be destructive to aquatic life, such as fish and kelp, and the rapid deposition of sediments in water bodies often changes the water quality around vulnerable shorelines.
Landslide dams can naturally occur when a large landslide blocks the flow of a river, causing a lake to form behind the blockage. Most of these dams are short-lived as the water will eventually erode the dam. If the landslide dam is not destroyed by natural erosional processes or modified by humans, it creates a new landform—a lake. Lakes created by landslide dams can last a long time, or they may suddenly be released and cause massive flooding downstream.

There are many ways that people can lessen the potential dangers of landslide dams, and some of these methods are discussed in the safety and mitigation sections of this volume. Figure 32 shows the Slumgullion landslide one of the largest landslides in the world—the landslide dam it has formed is so large and wide, that it has lasted 700 years.

Source : The Landslide Handbook—A Guide to Understanding Landslides

DIAPHRAGM WALLS, CUT-OFF WALLS AND SLURRY WALLS

Diaphragm Walls

Diaphragm walls are concrete or reinforced concrete wallsconstructed in slurry-supported, open trenches below existingground. Concrete is placed using the Tremie installation method orby installing pre-cast concrete panels (known as a pre-castdiaphragm wall).

1 : Diaphragm wall excavation

2: Excavation pit retained by tiedback diaphragm wall and partiallybraced by cast-in-place buildingfloor slab.

3: Excavation pit retained bydiaphragm wall and bracing

Diaphragm walls can be constructed to depths of100 meters and to widths of 0.45 to 1.50 meters.Diaphragm wall construction methods are relatively quiet and causelittle or no vibration. Therefore, they are especially suitable for civilengineering projects in densely-populated inner city areas.Due to their ability to keep deformation low and provide low waterpermeability, diaphragm walls are also used to retain excavation pits in the direct vicinity of existing structures.

If there is a deep excavation pit at the edge of an existing structureand groundwater is present, diaphragm walls are often used as themost technically and economically favorable option. They can beused for temporary support or as load-bearing elements of the final building. Diaphragm walls can be combined with any anchor and bracing system.Diaphragm wall panels are also used in deep, load-bearing soillayers as foundation elements to carry concentrated structural loadin the same way as large drilled piles do. These foundation elements are known as “Barrettes”.

1: Diaphragm wall excavation usinghydraulic cutter disk and grabexcavation

2: Guide wal

3: Diaphragm wall chisel

If diaphragm are socketed into impermeable soil layers of sufficientthickness or if they are combined with seal slabs (grout injection ortremie concrete slabs) almost waterproof excavation pits are created.

After reducing the initial groundwater level within theexcavation, only small amounts of residual water will penetrate.

Diaphragm Wall construction using grab excavation and removable Stop-End Pipes

• Preliminary excavation to 1.0 – 1.5 meters below groundelevation to install guide walls

• Prior to diaphragm wall excavation, cast-in-place or pre-cast concrete guide walls are placed.

These braced guide wallsstabilize the soil in the upper diaphragm level and provide a stableguide-way for the grab. In addition, they also support thediaphragm wall reinforcement and provide sufficient bearing forthe hydraulic jacking system to remove the Stop-End Pipes.

Thespace between both guide walls serves as a storage space for thestabilizing fluid.

• Lamella excavation Using hydraulic grabs with clam shell sizesof 2.8, 3.4 or 4.2 meters, single diaphragm panels can beexcavated down to tip elevation. To avoid collapsing ground,trixotropic stabilizing fluids (Bentonite slurry = clay/water slurry)are pumped into the excavated panel. Depending on groundcondition and geotechnical design, several single panels can becombined to one large lamella. Hard soil, rock or obstructions canbe removed using chisels in addition.

1: Diaphragm wall reinforcing cagewith block-out

2: Stop-end pipe with hydraulic pipeextractors

3: Flat steel joint element

• Stop End Pipe Installation. To separate the single concretingphases, stop-end pipes are installed at both panel fronts. Thesehave the same diameter as the panel’s wall thickness and areremoved after initial concrete setting. The remaining semicircularjoint provides a very good interlock between the individualconcrete panels.

• Slurry Refreshing.

• Placing of Rebar Cage

• Concrete Placing by Tremie Method. Simultaneously withplacing concrete, slurry is pumped from the panel to be refreshedand re-used in the next panel excavation. Since the slurry isreplaced by concrete, this method is called “Double-PhaseMethod”.

• Removal of Stop -End Pipes after concrete setting usinghydraulic pipe extractors.Diaphragm Wall Joint Design• There are three (3) types of joint design used for diaphragm wallsconstructed by the grab excavation method• Steel stop-end pipes, which are removed before the concrete hasset completely (as mentioned before). Concrete seeped aroundthe stop-end pipes can be safely removed by the use of chisels.

• Pre-cast reinforced concrete panels, which remain in the panelsas permanent stop-ends (high weight, twice the number of joints).Seeping concrete can not be removed safely.• Steel joint element. This flat steel panel element contains one ortwo elastic joint tapes, which remain in the setting concrete afterthe joint element has been removed. Removal of the element canonly take place after the adjacent panel was completelyexcavated

If diaphragm walls are constructed using the hydro-cuttertechnique, stop- end pipes do not need to be installed. After theprimary panel set sufficiently, the secondary panel excavation willslightly cut into the fresh concrete to ensure a tight overlap duringthe concrete placement.

Excavation

Double rope grabs as well as grabs guided by telescoping Kellybars are commonly used for excavation in soils. Hydro-cutters canbe employed for rock as well as soft soil excavations.

They continuously cut into one panel by sucking the soil-bentonite slurryat the cutter head while replacing it with fresh bentonite at thepanel’s top.

Construction sequence

Two different construction techniques, the alternating method (orPilgrim) and the continuous method can be distinguished forexcavation. During the alternating method, only primary panels willbe placed leaving out the following secondary panels. Following thefirst primary panels, gaps will be closed by the adjacent secondarypanels. Primary and secondary panels will have different sizes dueto the use of stop end pipes.During the continuous excavation method (Endless Panel), all thepanels are excavated in one continuous process. Therefore they allhave the same size.

Cut-off Walls

1: Cut-off wall applications

2: Cut-off wall Rostock,Warnowquerung

3: Cut-off wall site set up

Cut-off walls are vertical slurry walls with very low waterpermeability to minimize the ground water flow.In contrast to the known load-bearing, impermeable retaining wallssuch as:

• Concrete secant pile walls

• Reinforced two-phase diaphragm walls

• Sheet pile wallsare cut-off walls mostly without any load-bearing function.

The following cut-off wall types can be distinguished:

• Cut-off walls constructed using diaphragm wall techniques

• Secant pile walls from concrete of slurry

• Thin slurry walls• Injection walls• Jet-Grouting walls

• Freezing walls

They can be used as:

• Cut-off walls underneath water dams with core seals in areas ofpermeable soils to socket into lower impermeable layers toprevent undercurrent

• Cut-off walls for “watertight” excavation pits outside of theload-bearing retaining structure to minimize water inflow into thepit

• Cut-off walls to enclose brown fields and contaminated areas withpenetration into lower impermeable soil layers

Cut-off walls constructed using diaphragm wall techniques

If slurry walls are intended to act as cut-off walls without any loadbearing function, a mixture of water, bentonite, cement and maybefiller can be used.This slurry remains in the excavated panel and hydrates.

It alsoremains as a plastic seal, so that the wall can follow smalldisplacements in the soil without cracking. Since the slurry remainsin the panel, this is called the Single-Phase technique. After completion of the guide wall, the excavation proceeds with:

• One long-boom excavator (max. depth of 10m)

• Or by the use of slurry wall grabs or hydro-cutters Using the single-phase technique, panel depth is limited due to therelatively short time from placing and setting of the suspension.In deeper panels, the Two-Phase technique is used to construct aslurry wall. Construction is similar to the cast in-situ diaphragm wallinstallation. After completion of the panel’s excavation, the actualsealant slurry will replace the stabilizing bentonite fluid.

This sealanthas to be placed using the slurry-displacement or tremie methodand needs to have a 0.75 to/m³ higher unit weight than the bentonite slurry to replace it.To improve permeability and contaminant resistance, combinationcut-off walls can be installed using the Single-Phase system. Sheetpiles or plastic liner sheets can be installed within abentonite-cement-slurry wall.

Cut-off walls with embedded sheet piles or structural beams beingconstructed using the single phase method can also be used as”water- tight” excavation pits.In the process the sheet piles or beams act as load transferringelements and will extend to required depth below the excavation pit.

The Cut-off wall as sealing element will only penetrate the artificialseal slab or reach down to the natural impermeable soil layer.Sheet pile or beam walls can be tied back

Thin Slurry Walls

1: Construction of a vibrated thinslurry wall, Doemitz

2: Vibro-beam

3: Thin vibro slurry wall

Thin slurry walls also can act as vertical cut-off walls to retainhorizontal groundwater flow.In contrast to cut-off walls constructed using the diaphragm walltechnique (replacing the soil by slurry sealant), thin slurry wallsdisplace the soils using a vibrated steel profile.

During the extraction process, sealants are injected into the created cavities.Drivable soils, such as sands and gravels, are required for thisinstallation method. The created slurry wall thickness depends onthe shape of the steel profile used and the soil conditions.

Thickness varies between 5 cm in sands and 20 cm in gravel. Incombination with high-pressure jet grouting, wall thickness of up to30 cm can be achieved.A continuous wall is created by overlapping single penetrationelements installed one after another by the vibrated steel profile.

A guide plate attached at the beam`s flange is running down thealready completed web of the previous panel. This ensures thecorrect overlap to the previous panel.

Source : spezialtiefbau.implenia.com

What are stone columns ?

Stone columns are constructed using down-hole vibratory probe methods similar to those used in vibro-compaction. The main difference is that instead of using coarsegrained soil to simply fill the void created by the vibro-compaction operation, stone or other clean, coarse grained materials are placed, and compacted, to form a narrow structural element (i.e. a column) which functions as one or more of the following:

1. enhance the average shear strength and bearing capacity of a weak soil mass,
2. transfer a surface load to deeper competent materials, or
3. provide easy drainage of temporarily high pore water pressures.

Stone columns are ideally suited for improving soft silts and clays and loose silty sands. Stone columns under suitable conditions will:

• increase a soil’s bearing capacity and shear resistance
• reduce settlements,
• increase the time-rate of consolidation,
• reduce liquefaction potential, and
• stabilize existing slopes affected by low shear strength soils .

Stone columns, in general, are most economically attractive for sites requiring column lengths less than 35 ft. deep and preferably about 20 ft. deep below the surface.

Unsuitable soil conditions for stone columns include soils having thick layers of very soft or sensitive clays and organic materials. If the thickness of the unsuitable soil layer is more than the diameter of the stone column, then stone columns may not be appropriate because the very soft soils will not provide adequate lateral support of the stone column. In addition, stone column construction can be hampered by the presence of a thick, dense overburden, or soils with boulders, cobbles or other obstructions that may require pre-drilling prior to installation of the stone column.

Stone columns are constructed using either a vibro-replacement or vibro-displacement installation with the stone aggregate placed using either top or bottom feed methods.

Vibro-Replacement :
Vibro-replacement involves a wet installation method that replaces deep, narrow pockets of the in-situ soil with stone aggregate columns. In this method a high-pressure water jet, located at the tip of the probe, is used to excavate a narrow, open (uncased) hole. Once the hole is progressed to the design depth, the hole is flushed out several times by raising and dropping the probe to remove any loose silt and sand at the bottom of the hole. The vibro-probe is retracted and a limited amount of stone is placed into the hole from the top. The probe’s vibration mode is turned on and it is inserted into the hole to compact the lift of stone. The probe is retracted again and the process repeated until the stone column is formed to the ground surface. During the entire operation, water is continually pumped into the hole to prevent collapse and to keep the aggregate clean. This method is best suited for sites with soft to firm soils with undrained shear strengths of 200 to 1,000 psf and a shallow groundwater table, and where drill wash and spoil
containment and disposal can be practically handled.

Vibro-Displacement :
When a cleaner or lesser environmental impact operation is preferred, stone columns should be constructed using the vibro-displacement method. The operation is a dry installation method where the stone aggregate can be placed into the hole from the top or from aggregate ports at the bottom of the probe. Although the probe’s dead weight and vibration, in lieu of water jetting, is used to excavate the hole, air jetting and/or pre-augering may be used to prevent clogging of the aggregate ports or to assist in advancing or extracting the probe. This method is best suited for
sites where collapse of the hole during the column’s installation is unlikely.

What is vibro-compaction?

Vibro-compaction is a ground improvement method that uses a specialized vibrating probe for in-situ subsurface compaction of loose sandy or gravelly soils at depths beyond which surface compaction efforts are effective.

The vibrating probe densifies loose granular, cohesionless soils by using mechanical vibrations and, in some applications, water saturation to minimize the effective stresses between the soil grains which then allows the oil grains to rearrange under the action of gravity into a denser state.

Vibro compaction to densify loose, silty sands for an interim spent fuel cask storage pad in Braceville, Illinois.

Generally, vibro-compaction can be used to achieve the following enhanced soil performance or
properties:
• Increased soil bearing capacity
• Reduced foundation settlements
• Increased resistance to liquefaction
• Compaction to stabilize pile foundations driven through loose granular materials
• Densification for abutments, piers and approach embankment foundations
• Increased shear strength
• Reduced permeability
• Filling of voids in treated areas

Two rigs completing vibro compaction for liquefaction mitigation and settlement at a casino.

The vibrator is hung from a crane cable or, in some instances; it is mounted to leads in a similar fashion as foundation drilling equipment. The vibrator penetrates under its self weight (or crowd of the machine if mounted in leads) and, at times, with assistance from the action of water jets. The goal is that the vibration and water imparted to the soils ransforms the loose soils to a more dense state.

The Vibro Compaction Process

Advantages, Disadvantages and Limitations

1. Advantages

As an alternative to deep foundations, vibro-compaction is usually more economical and often results in significant time savings. Loads can be spread from the footing elevation, thus minimizing problems from lower, weak layers. Densifying the soils with vibro-compaction can considerably reduce the risk of seismically induced liquefaction. Vibro-compaction can also be cost-effective alternative to removal and replacement of poor load-bearing soils. The use of vibro-compaction allows the maximum improvement of granular soils to depths of up to 165 feet. The vibro-ompaction system is effective both above and below the natural water level.

2. Disadvantages and Limitations

Vibro-compaction is effective only in granular, cohesionless soils. The realignment of the sand grains and, therefore, proper densification generally cannot be achieved when the granular soil contains more than 12 to 15 percent silt or more than 2 percent clay. The maximum depth of treatment is typically limited to 165 feet, but there are very few construction projects that will require densification to a greater depth.

Like all ground improvement techniques, a thorough soils investigation program is required. Yet, a more detailed soils analysis may be required for vibro-compaction than for a deep foundation design because the vibro-compaction process utilizes the permeability and properties of the in-situ soil to the full depth of treatment to achieve the end result. A comprehensive understanding of the total soil profile is therefore necessary which typically requires continuous sampling or in-situ testing.

Equipment access over the site must also be considered. Since the operation requires use of a large crane, a relatively flat work bench with a width of at least 25 ft must be possible near all areas to be treated.

Wet vibro-compaction requires the use of water to jet the vibrator into the ground. The effluent from the jetting process requires at least temporary containment to allow any fine soil particles to settle out and be disposed. Further, this method of ground improvement may not be acceptable if the existing subsurface environment, either soil or water is contaminated. If contamination is present, use of water jetting may cause its dispersion and therefore other ground improvement methods should be considered.

What type of pavement is used for airports runway?

The materials used for airports is generally the same as what is used for roadways, however, the depths, or thicknesses are different, and the tolerances are much tighter at an airport. The material for runways usually needs to meet a much tighter spec.

A typical section for an airport can use asphalt or concrete. Below is a generic look at the structural section for either asphalt or concrete from an FAA Advisory Circular on Aiport Pavement Design and Evaluation.

You will notice that the materials in the middle are thicker and then taper to thinner. This is because the loads on the runway are primarily from the 2 landing wheels, which will be in the middle of the runway. The effective tire width is pictured below.

The surface must be smooth and well bonded, and resistant to the shear stresses of the airplane wheel loads. The non-skid surface must not cause undue wear on the airplane tires . The surface must be free of loose particles that could damage the airplane or people. In order to meet this requirement, there must be good control of the mix. This usually requires a central mixing plant be used for the hot mix asphalt.

The base course is integral to flexible pavement design such as asphalt. The loading in flexible pavements transfers downward and outward. For this reason, the base, subbase, if used, and subgrade contribute to the strength of the pavement section. For concrete pavement, the concrete provides the strength to the structural section.

The base course must be of sufficient quality that it won’t fail, or allow failure in the subgrade. It must be able to withstand the forces from the airplane wheel loading without consolidating which would cause the surface course to deform. The base course uses very select material with very hard and durable aggregate. The requirements for the base course are very strict.

Permeability of Soil: Definition, Darcy’s Law and Tests

Definition of Permeability:

It is defined as the property of a porous material which permits the passage or seepage of water (or other fluids) through its interconnecting voids.

A material having continuous voids is called permeable. Gravels are highly permeable while stiff clay is the least permeable, and hence such a clay may be termed impermeable for all practical purpose.

The study of seepage of water through soil is important for the following engineering problems:

1. Determination of rate of settlement of a saturated compressible soil layer.

2. Calculation of seepage through the body of earth dams and stability of slopes for highways.

3. Calculation of uplift pressure under hydraulic structure and their safety against piping.

4. Groundwater flow towards well and drainage of soil.

Darcy’s Law (1856) of Permeability:

For laminar flow conditions in a saturated soil, the rate of the discharge per unit time is proportional to the hydraulic gradient.

q = kiA

v = q/A = Ki … (7.1)

Where q = discharge per unit time

A = total cross-sectional area of soil mass, perpendicular to the direction of flow

i = hydraulic gradient

k = Darcy’s coefficient of permeability

v = velocity of flow or average discharge velocity

If a soil sample of length L and cross-sectional area A, is subjected to differential head of water h₁– h₂, the hydraulic gradient i will be equal to [(h₁– h₂)/L] and we have q = k. [(h₁– h₂)/L].A.

When hydraulic gradient is unity, k is equal to V. Thus, the coefficient of permeability, or simply permeability is defined as the average velocity of flow that will occur through the total cross-sectional area of soil under unit hydraulic gradient. Dimensions are same as of velocity, cm/sec.

The coefficient of permeability depends on the particle size and various other factors. Some typical values of coefficient of permeability of different soils are given in Table 7.1.

Discharge Velocity and Seepage Velocity:

The total cross-sectional area of the soil mass is composed of sectional area of solids and voids, and since flow cannot occur through the sectional areas of solids, the velocity of flow is merely an imaginary or superficial velocity.

The true and actual velocity with which water percolates through a soil is called the velocity of percolation or seepage velocity. It is the rate of discharge of percolating water per unit of net sectional area of voids perpendicular to the direction of flow.

Validity of Darcy’s Law:

In accordance with the Darcy’s Law, the velocity of flow through soil mass is directly proportion to the hydraulic gradient for laminar flow condition only. It is expected that the flow to be always laminar in case of fine-grained soil deposits because of low permeability and hence low velocity of flow.

However, in case of sands and gravels flow will be laminar upto a certain value of velocity for each deposit and investigations have been carried out to find a limit for application of Darcy’s law.

According to researchers, flow through sands will be laminar and Darcy’s law is valid so long as Reynolds number expressed in the form is less than or equal to unity as shown below –

Where v = velocity of flow in cm/sec

D_a = size of particles (average) in cm.

It is found that the limiting value of Reynolds number taken as 1 is very approximate as its actual value can have wide variation depending partly on the characteristic size of particles used in the equation.

Factors affecting permeability are:

1. Grain size

2. Properties of pore fluid

3. Void ratio of the soil

4. Structural arrangement of the soil particle

5. Entrapped air and foreign matter

6. Adsorbed water in clayey soil

4. Effect of degree of saturation and other foreign matter

k will decrease if air is entrapped in the voids thus reducing its degree of saturation. Percolating water in the field may have some gas content, it may appear more realistic to use the actual field water for testing in the laboratory.

Organic foreign matter also has tendency to move towards critical flow channels and choke them up, thus decreasing permeability.

5. Effect of adsorbed water – The adsorbed water surrounding the fine soil particle is not free to move, and reduces the effective pore space available for the passage of water.

Capillarity-Permeability Test:

The set-up for the test essentially consists of a transparent tube about 40 mm in diameter and 0.35 m to 0.5 m long in which dry soil sample is placed at desired density and water is allowed to flow from one end under a constant head, and the other end is exposed to atmosphere through air vent.

At any time interval t, after the commencement of the test, Let the capillary water travel through a distance x, from point P to Q. At point P, there is a pressure deficiency (i.e., a negative head) equal to h_c of water.

If the coefficient of permeability is designated as k_u at a partial saturation S, the above expression can be rewritten as –

In order to find the two unknowns k and h_c in the above equation, the first set of observations are taken under a head h₁. As the capillary saturation progresses the values of x are recorded at different time intervals t.

The values of x² are plotted against corresponding time intervals t to obtain a straight line whose slope, say m, gives the value of [(x²₂– x²₁)/(t₂ – t₁)] . The second set of observation are taken under an increased head h₂ and values of x² plotted against corresponding values of t to obtain another straight line, whose slope m₂ will give the value [(x²₂– x²₁)/(t₂ – t₁)].

By substitution in Eqn. 7.1, we obtain the following two equations, which are solved simultaneously to get k and h_c.

The porosity n required in the above equation is computed from the known dry weight of soil, its volume and specific gravity of soil particles.

Permeability of Stratified Soil Deposits:

In general, natural soil deposits are stratified. Each layer may be homogeneous and isotropic. When we consider flow through the entire deposit the average permeability of deposit will vary with the direction of flow relative to the bedding plane. The average permeability for flow in horizontal and vertical directions can be readily computed.

Average Permeability Parallel to Bedding Plane:

Figure 7.9 shows several layers of soil with horizontal stratification. Let Z₁, Z₂, ….Z_n be the thickness of layers with permeabilities k₁, k₂, … k_n.

For flow parallel to bedding plane the hydraulic gradient i will be same for all layers. The total discharge through the deposit will be the sum of discharges through individual layers.

Average Permeability Perpendicular to Bedding Plane:

For flow in the vertical direction for the soil layers shown in Fig. 7.10.

In this case the velocity of flow, v will be same for all layers the total head loss will be sum of head losses in individual layers.

h = h₁ + h₂ + h₃ + … + h_n (i)

If k_z denotes average permeability perpendicular to bedding plane, applying Darcy’s law, we have –

How to Test Compaction of Soil?

Compaction test is conducted in the laboratory to determine the relation between the dry density and the water content of the given soil compacted with standard compaction energy and to determine the OMC corresponding to the MDD.

The OMC obtained from the laboratory compaction test will help in deciding the amount of water to be used for compaction in the field. The MDD obtained from the laboratory compaction test helps in knowing the dry density achievable in the field compaction and also as a check for quality control.

Based on the compacted energy used in compacting the soil in the laboratory test, the laboratory compaction tests are of two types:

1. Standard Proctor test.

2. Modified Proctor test.

1. Standard Proctor Test:

In this test, the soil is compacted in three layers, each layer subjected to blows of a rammer with falling weight of 5.5 pounds (2.6 kgf) falling through a height of 12 in. in a cylindrical mold of internal diameter of 4 in. and effective height of 4.6 in.

The compaction parameters in a standard Proctor test are – W is the weight of rammer blow = 5.5 pounds, h is the height of fall = 12 in. = 1 ft, n is the number of blows per layer = 25, l is the number of layers = 3, and V_m is the volume of the mold = 1/30 cubic ft. The total compaction energy imparted on the soil per unit volume in this test is –

IS – 2720 (Part 7) – 1980 recommends the Indian Standard light compaction method based on standard Proctor test in metric system. The standard Proctor test or IS light compaction can be used as a criteria for the compaction of subgrades on highways and earth dams, where light rollers are used.

2. Modified Proctor Test:

Advances in construction technology resulted in the development of heavier rollers, which impart higher compaction energy during field compaction. To provide a laboratory control criterion for the higher compaction energy, the modified Proctor test was developed and standardized by the American Association of State Highway (and Transportation) Officials (AASHO or AASHTO) and by US Army Corps of Engineers for airfield construction.

In modified Proctor test, the soil is compacted in the same mold as in standard Proctor test, which has internal diameter of 4 in. and affective height of 4.6 in. giving a total internal volume of 57.805 cubic in. or 1/30 cubic ft. The rammer is bigger with a hammer of weight 10 pounds (4.5 kgf) falling through a height of 18 in. The soil is compacted in five layers, each layer being given 25 blows.

The compaction parameters in modified Proctor test are as follows – W is the weight of hammer blow = 10 pounds, h is the height of fall = 18 in. = 1.5 ft, n is the number of blows per layer = 25,l is the number of layers = 5, and V_m is the volume of the mold = 1/30 cubic ft.

The compaction energy imparted to the soil, per unit volume, in the modified Proctor test is:

Thus, the compaction energy in the modified Proctor test is (56250/12375) 4.55 times that in standard Proctor test.

If the soil fraction retained on 20 mm sieve is more than 5%, a bigger mold of 5.9 in. (15 cm) internal diameter and 5 in. (12.73 cm) internal height giving a total volume of 137.04 cubic in. or 1/12.611 cubic ft (2250 cm³) is used. When the bigger mold is used, the soil is compacted with 56 blows for each layer.

IS – 2720 (Part VII) – 1980 and Part VIII-1983 have recommended procedures corresponding to these two tests as follows:

1. IS light compaction test.

2. IS heavy compaction test.

1. IS Light Compaction Test:

The objective of the IS light compaction test is to determine the relation between the water content and the dry density of compacted soil and to determine the MDD and OMC from this test. The compaction energy used to compact the soil corresponds to that of standard Proctor test.

The compaction parameters in IS light compaction test are as follows – W is the weight of hammer blow = 2.6 kgf, h is the height of fall = 31 cm, n is the number of blows per layer = 25, and I is the number of layers = 3, and V_mis the volume of the mold = 1000 cubic centimeter (cc). The total compaction energy imparted on the soil in this test is –

E = Whnl = 2.6 × 31 × 25 × 3 = 6045 kgf cm

The total compaction energy imparted on the soil per unit volume in this test is –

E = 6045/1000 = 6.045 kgf cm/cm³

2. IS Heavy Compaction Test:

The IS heavy compaction test is similar to IS light compaction text except for the following differences:

1. A heavy rammer of 4.9 kgf falling weight that falls through a height of 45 cm is used for compacting the soil in IS heavy compaction.

2. The soil is compacted in five layers of equal thickness in the compaction mold.

3. The initial water content to be used in the first trial is 3%-5% for sandy and gravelly soils and 12%-16% below plastic limit for cohesive soils.

To increase the accuracy of the test results, it is desirable to reduce the increment of water in the region of OMC.

Shifting and Tilting of Well Foundations

Shifting and tilting problems occurs generally during sinking process of well foundations. If proper care is not taken, they will cause serious problems and weaken the stability of foundations. Precautions to avoid shifting and tilting, limitations and rectifying methods are discussed below.

Shifting and Tilting of Well Foundations

When the well is moved away horizontally from the desired position, then it is called shifting of well foundation.
When the well is sloped against vertical alignment,it is called tilting of well foundation.

Precautions to Prevent Shifting and Tilting

It is safer and economical to avoid tilting and shifting of wells by adopting the following preventive measures:

The outer surface of the well curb and steining should be level, straight, and smooth.
The radius of the well curb should be kept 2-4 cm more than the outer radius of the well steining.
The cutting edge should be sharp and of uniform thickness.
The steining should be built in lifts and the entire steining height should be built in one straight line from bottom to top at right angles to the plane of the curb.
Dredging should be uniform on all sides of the well. For a twin well, dredging should be uniform in both the wells.
The well should be constructed in stages of small height increments.
The magnitude and direction of sinking of wells should be properly and carefully monitored on a continuous basis to identify any tilt or shift and adopt appropriate corrective measures immediately to rectify the same.
If the well shows a tendency for tilting, dredging should be done on the higher side. If this does not bring required improvement, sinking should be stopped and should be resumed only after the tilting is corrected.
Dredged material should not be deposited unevenly around the well.
When a kentledge is used to provide additional sinking effort, it should be placed evenly on the loading platform.

Limitations

The maximum tilt allowed in case of well foundation is 1 in 60.
The shift in well foundation should not be more than 1 % of depth of sunk.
Beyond the above limits, well foundation is considered as dangerous and in such a case, remedial measures to rectify shifting and tilting should be followed.

Rectifying Methods

Rectifying methods for Rectification of shifting and tilting problems in well foundations are as follows:

Eccentric loading
Excavation on higher side
Water jetting
Pulling the well
Using hydraulic jacks
Using struts
Excavation under cutting edge
Wood sleeper under cutting edge

1. Eccentric loading

The well tilt can be rectified by placing eccentric loading on the higher side. Higher side is nothing but the opposite side of tilt or lower side.
A loading platform is constructed on the higher side and load is placed on it.
This eccentric load will increase downward pressure on higher side and correct the tilt.
The amount of load and eccentricity is decided based on the depth of sinking.
Greater is the depth of sinking of well, larger will be the eccentricity and load.

2. Excavation on Higher Side

When well is tilted to one side, excavation should be increased on the other side which is opposite to tilted side.
This technique is useful only in the initial stages of well sinking.

3. Water Jetting

Water jetting on external surface of well on the higher side is another remedial measure for rectifying tilt.
When water jet is forced towards surface of well, the friction between soil and well surface gets reduced and the higher side of well becomes lowered to make well vertical.

4. Pulling the Well

The well can be pulled towards higher side using steel ropes.
One or more steel ropes are wound around the well with wooden sleepers packed in between well and ropes to prevent damage to the well steining by distributing load over to larger area of steining.
Pull should be carefully done otherwise,shifting of well foundation may occur.

5. Pushing using Jacks

Another method to rectify tilting and shifting of well foundation is using hydraulic jacks or mechanical jacks, the tilted well can be pushed from lower side to higher side.
Neighbor vertical well foundations or suitable arrangements made will give support to the jack system.
Care should be taken while pushing the well otherwise the well may shifts.

6. Using Struts

By providing struts as supports on the lower side or tilted side of well, further tilting can be prevented.
Wooden sleepers are provided between struts and well steining to prevent damage to well steining and to distribute pressure to larger area.
Struts are rested on firm base having driven piles.

Fig 6: Strutting the Well From Lower Side

7. Excavation under Cutting Edge

This technique is used for hard strata soils. In this method, the well is de-watered first and open excavation is carried out exactly under the cutting edge on the higher side.
If de-watering is not possible, soil strata is loosened using suitable equipment with the help of professional divers.

8. Wood Sleeper under Cutting Edge

If tilting towards lower side is increasing,then wooden sleepers are placed under cutting edge on lower side to control the tilting temporarily.
When well is corrected to vertical level, this sleepers can be removed.

Lime & Cement Stabilisation Process

Introduction

Soil stabilisation, in terms of pavement construction, is the process of (usually insitu) pulverising and moisture conditioning by mixing various binders with soil, compaction and trimming as necessary. This improves soil characteristics preferred for construction in terms of moisture content, density, strength (CBR%), permeability, plasticity index and shrink swell characteristics. Most material types, clay through to crushed rock, are suitable for stabilisation. Seeking advice early during the design/feasibility stage enables planning for efficient use of stabilisation.

Stabilisation

Lime and/or cement stabilisation is often used to improve the properties of site won materials, to enable their use in a pavement and other like areas, such as dam foundations and building pad sites. Lime stabilisation of clay material reduces entrance cracking, whilst increasing the hardness of the material by up to ten times. The use of cement as a binder, after lime, can further increase the strength and durability towards that of concrete. Various binder blends, away from lime and cement, such as slag or fly ash, are commonly utilised for further benefit dependant on site conditions and requirements.

General Benefits of Soil Stabilisation

Saturated, wet sites can be treated to provide a working platform within a day for project continuation during wet periods/seasons.
Stabilisation recycles existing pavement by pulverising the existing pavement to 25mm down. Lime and or cement or other binders are then mixed with water as necessary. No imported materials and increased production rates means cost savings.
Strength gains often over CBR 15% or 5 times the previous strength are the result of the realignment of particles and adjustment of moisture content allowing compaction at optimum moisture content.
Reduce Plasticity Index (PI) in cohesive materials. For example a material with PI 20 will typically stabilise to PI < 10, say 8.

General Stabilisation for Lining Systems and Cohesive Expansion Material

Reduce or eliminate the need for imported clay liner by stabilising insitu materials.
Reduce permeability.
Reduce Linear Shrinkage rate up to 10%.
Environmental benefits of reduced geotextile, borrow pit clay and quarry import.
Additional environmental benefits from reducing extra excavation and disposal by modification to suitable material.
Improved structural stability through realignment of soil particles by ionic exchange between clay and lime.
Increased Strength and durability.
Reduced dispersion means reduced dispersion piping failure and increased erosion protection.
Pulverisation to 40mm down of clay, extremely weathered limestone, mudstone and siltstone provides smaller diameter conglomerates and homogenous material throughout the stabilised layer eliminating lenses, streaks, rock fissures and faults providing reduced seepage.

Lime

Note that there are many variations of lime available but only quicklime is considered suitable for lime stabilisation in the pavement construction industry and general field construction activities. Quicklime is calcium oxide (CaO) supplied commercially in a dry powder form. Agriculture Lime is a calcium carbonate (CaC03) and not suitable for pavement construction. Hydrated Lime is calcium hydroxide (Ca(OH)2) often used in the laboratory for lime saturation testing, not generally used on site for pavement construction.

Hydrated lime (calcium hydroxide), is produced by reacting water with quicklime (calcium oxide). CaO + H2O => Ca(OH)2. When calculated using the atomic weights, this converts practically to 5t Quicklime + 3t Water => 7t Hydrated lime + 1t Water Evapouration.

The pozzolanic reaction between lime with water and the silica and alumina in clay results in an ionic exchange, which permanently realigns the clay particles forming friable conglomerates. The new alignment of the particles provides less ability for the clay to absorb water around the particles. This makes the clay more waterproof, less expansive and therefore reduces the plasticity and linear shrinkage. The PI is often more than ½ and the shrinkage is often 10% of what it was. Practically this results in improved permeability less shrinkage cracking providing less chance of piping failure and seepage.

In a lime saturated environment (typically 3% to 4% quicklime), the clay-alumina and clay-silica become available to react with the free calcium to form calcium aluminates or silicates. The pozzolanic reaction is illustrated by the following equations:

Ca2+ + OH- + Available Clay Aluminium Calcium Aluminate Hydrate (CAH)
Ca2+ + OH- + Available Clay Silica Calcium Silicate Hydrate (CSH)

Stabilisation Process & Machinery

Insitu stabilisation procedures vary depending upon the type of project and the binder used. All machinery suitable for the process is purpose built for stabilisation. A range of purpose built equipment has been developed according to specific requirements of various site conditions and design specifications for the process to be effective. Adaptation of agricultural equipment and other equipment does not meet specification requirements and results in a process failure.

Preparation

Spreader

the Stabil-Lime Group operates a range of purpose built lime and cement spreader trucks including an articulated 4×4 all terrain spreader for particularly boggy sites. This enables the supply and distribution of a full range of binder products suitable for the insitu stabilisation process. Leading technologies are incorporated into all trucks to ensure accurate binder spread rates and containment of dust.

On board computers linked to load cells and farm scan distance measuring devices assist in assuring accurate spread rates.
All spreader trucks have sealed bulk bins to ensure the product does not start to react until it is on the ground ready to be mixed into the pavement. Spread rates (kg/m2) must vary in accordance with varying ground conditions.
Additional mat test can be carried out in order to confirm and adjust the spreading rate.

Water Truck

Especially during the drier months, water must be added to ensure optimum moisture content is maintained for compaction. Depending on soil conditions and moisture content water can be added before and or after spreading any binder or directly into the mixing chamber by linking the water truck to the mixer where appropriate. Not only is water vital to ensure optimum moisture content at compaction, water initiates the necessary chemical reactions with most binders.

Stabilising Machine

Prior to stabilisation commencing it is important to ensure the surface is prepared for stabilisation ahead of stabilisation. Preparation of a surface for stabilisation includes pegging out for stringing as necessary, trimming to approximate levels and shaping to shed water and sufficient drainage to prevent water ponding where possible. Note that due to the addition of binders and density changes, some bulking may occur, however this may also be balanced by other factors such reducing the moisture content or increasing the density of the underlying material during compaction. Immediately prior to stabilisation, the surface should be ripped to the required depth to identify and remove unsuitable material such as obstacles, organics and material too hard to stabilise. Rotary hoe type attachments to bob cats, tractors and the like are not accepted by the Industry for pavement construction as they are not mixing chambers, they do not ensure homogenous mixing or accurate depth control amongst other faults. Research shows pavements mixed with such machines often fail within 1-3 years because the binder and moisture have not been mixed thoroughly.

Compaction

Compaction commences after mixing. Typically stabilised materials are compacted to 95% standard, however higher compaction standards are achievable. Insitu mixing up to 400mm in a single layer requires compaction equipment large enough to achieve density throughout a layer this thick. Typically large self propelled vibratory padfoot rollers are used initially for deep compaction followed by a similar smooth drum to complete compaction of the full layer.

Final Trimming

It is normal to commence trimming the pavement before the completion of the compaction operation, ensuring good bonding of any corrected shape before is finished.

Considerations for Stabilisation

By seeking our advice early during the development stages of a project we can ensure savings are maximised by optimising the use of stabilisation in designs to reduce double handling and import and export of materials. We employ a number of qualified engineers and project managers offering sound advice based on years of experience.

In order to assess a site accurately in terms of stabilisation, ideally the following information is considered:

Geotechnical data including site conditions, material type and depth, sub-grade and existing pavement material.
Construction conditions and loading.
Geometric site layout proposed and existing.
Proposed minimum area to be treated.
Specification requirements typically in terms of density, CBR strength or binder content if provided.
Proposed award and commencement dates.

Landslide Questions

What is a landslide?

A landslide is defined as the movement of a mass of rock, debris, or earth down a slope due to gravity. The materials may move by falling, toppling, sliding, spreading, or flowing.

Landslide Animation:

What causes a landslide?

Almost every landslide has multiple causes. Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope. Landslides can be triggered by rainfall, snowmelt, changes in water level, stream erosion, changes in ground water, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors.

What are submarine landslides?

Earthquake shaking and other factors can also induce landslides underwater. These landslides are called submarine landslides. Submarine landslides sometimes cause tsunamis that damage coastal areas.

Where do landslides occur?

Landslides in the United States occur in all 50 States. The primary regions of landslide occurrence and potential are the coastal and mountainous areas of California, Oregon, and Washington, the States comprising the intermountain west, and the mountainous and hilly regions of the Eastern United States. Alaska and Hawaii also experience all types of landslides.

How fast do landslide travel?

Landslides can move slowly, (millimeters per year) or can move quickly and disastrously, as is the case with debris flows. Debris flows can travel down a hillside at speeds up to 200 miles per hour (more commonly, 30 – 50 miles per hour), depending on the slope angle, water content, volume of debris, and type of earth and debris in the flow. These flows are initiated by heavy periods of rainfall, but sometimes can happen as a result of short bursts of concentrated rainfall or other factors in susceptible areas. Burned areas charred by wildfires are particularly susceptible to debris flows, given certain soil characteristics and slope conditions.

Why study landslides?
Landslides are a serious geologic hazard. It is estimated that in the United States they cause in excess of $1 billion in damages and from about 25 to 50 deaths each year. Globally, landslides cause billions of dollars in damages and thousands of deaths and injuries each year.

Who is most at risk for landslides?

As people move into new areas of hilly or mountainous terrain, it is important to understand the nature of their potential exposure to landslide hazards, and how cities, towns, and counties can plan for land-use, engineering of new construction and infrastructure, and other measures which will reduce the costs of living with landslides. Although the physical causes of many landslides cannot be removed, geologic investigations, good engineering practices, and effective enforcement of land-use management regulations can reduce landslide hazards.

Do human activities cause landslides?

Yes, in some cases human activities can be a contributing factor in causing landslides. Many human-caused landslides can be avoided or mitigated. They are commonly a result of building roads and structures without adequate grading of slopes, of poorly planned alteration of drainage patterns, and of disturbing old landslides.

Where can I find landslide information for my area?

The USGS National Landslide Information Center (NLIC) is a part of the U.S. Geological Survey Landslide Hazards Program that collects and distributes all forms of information related to landslides. The NLIC is designed to serve landslide researchers, geotechnical practitioners engaged in landslide stabilization, and anyone else concerned in any way with landslide education, hazard, safety, and mitigation. Every state in the US has a geoscience agency and most have some landslide information. The Association of American State Geologists provides links to the State Geologist for every state.

What was the most expensive landslide to fix in the United States?

The Thistle, Utah, landslide cost in excess of $200 million dollars to fix. The landslide occurred during the spring of 1983, when unseasonably warm weather caused rapid snowmelt to saturate the slope. The landslide destroyed the railroad tracks of the Denver and Rio Grande Western Railway Company, and the adjacent Highway 89. It also flowed across the Spanish Fork River, forming a dam. The impounded river water inundated the small town of Thistle. The inhabitants of the town of Thistle, directly upstream from the landslide, were evacuated as the lake began to flood the town, and within a day the town was completely covered with water. Populations downstream from the dam were at risk because of the possible overtopping of the landslide by the lake. This could cause a catastrophic outburst of the dam with a massive flood downstream. Eventually, a drain system was engineered to drain the lake and avert the potential disaster.

How many deaths result from landslides?

An average of between 25 and 50 people are killed by landslides each year in the United States. The worldwide death toll per year due to landslides is in the thousands. Most landslide fatalities are from rock fall, debris-flows, or volcanic debris flows.

What should I know about wildfires and debris flows?

Wild land fires are inevitable in the western United States. Expansion of human development into forested areas has created a situation where wildfires can adversely affect lives and property, as can the flooding and landslides that occur in the aftermath of the fires. There is a need to develop tools and methods to identify and quantify the potential hazards posed by landslides produced from burned watersheds. Post-fire landslide hazards include fast-moving, highly destructive debris flows that can occur in the years immediately after wildfires in response to high intensity rainfall events, and those flows that are generated over longer time periods accompanied by root decay and loss of soil strength. Post-fire debris flows are particularly hazardous because they can occur with little warning, can exert great impulsive loads on objects in their paths, and can strip vegetation, block drainage ways, damage structures, and endanger human life. Wildfires could potentially result in the destabilization of pre-existing deep-seated landslides over long time periods.

How do landslides cause tsunamis?

Tsunamis are large, potentially deadly and destructive sea waves, most of which are formed as a result of submarine earthquakes. They may also result from the eruption or collapse of island or coastal volcanoes and the formation of giant landslides on marine margins. These landslides, in turn, are often triggered by earthquakes. Tsunamis can be generated on impact as a rapidly moving landslide mass enters the water or as water displaces behind and ahead of a rapidly moving underwater landslide.

What are some examples of landslides that have caused tsunamis?

The 1964 Alaska earthquake caused 115 deaths in Alaska alone, with 106 of those due to tsunamis generated by tectonic uplift of the sea floor, and by localized subareal and submarine landslides. The earthquake shaking caused at least 5 local slide-generated tsunamis within minutes after the shaking began. An eyewitness account of the tsunami caused by the movement and landslides of the 1964 Alaska earthquake.

Research in the Canary Islands concludes that there have been at least five massive volcano landslides that occurred in the past, and that similar large events may occur in the future. Giant landslides have the potential of generating large tsunami waves at close and also very great distances and would have the potential to devastate large areas of coastal land as far away as the eastern seaboard of North America.

Rock falls and rock avalanches in coastal inlets, such as those that have occurred in the past at Tidal Inlet, Glacier Bay National Park, Alaska, have the potential to cause regional tsunamis that pose a hazard to coastal ecosystems and human settlements. On July 9, 1958, a magnitude M 7.9 earthquake on the Fairweather Fault triggered a rock avalanche at the head of Lituya Bay, Alaska. The landslide generated a wave that ran up 524 m on the opposite shore and sent a 30-m high wave through Lituya Bay, sinking two of three fishing boats and killing two persons.

How soon does the danger of landslides end after the rain stops?

It’s not possible to exactly predict the number of days or weeks that landslides remain a danger after heavy rain. Residents near mountain slopes, canyons, and landslide prone areas should stay alert even after heavy rain subsides.

Why is southern California vulnerable to landslides?

Areas that have been burned by recent wildfires are highly susceptible to debris-flow activity that can be triggered by significantly less rainfall than that which triggers debris flows from unburned hill slopes.

What was the biggest landslide in the world?

The world’s biggest historic landslide occurred during the 1980 eruption of Mount St. Helens, a volcano in the Cascade Mountain Range in the State of Washington, USA. The volume of material was 2.8 cubic kilometers (km).

What was the biggest prehistoric landslide?

The world’s biggest prehistoric landslide, discovered so far on land, is in southwestern Iran, and is named the Saidmarreh landslide. The landslide is located on the Kabir Kuh anticline in Southwest Iran at 33 degrees north latitude, 47.65 degrees east longitude. The landslide has a volume of about 20 cubic kilometers, a depth of 300 m, a travel distance of 14 km and a width of 5 km. This means that about 50 billion tons of rock moved in this single event!

Source: http://www.weatherwizkids.com