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Archive for June, 2006

Retaining Walls

Posted by Kshitija Nadgouda on June 23, 2006

A Retaining Wall is simply a wall that retains (holds back) soil or sometimes water behind it. Here, we will discuss only walls that retain soil behind them. A retaining wall is typically constructed when change in elevation is sudden and requires a vertical (or almost vertical) grade change. The most common example would be along roads in hilly regions. It is not always feasible (economically or due to lack of space) to gradually change the level (elevation) from the road to the hill top and create a slope. In such cases, it is necessary to build a wall that will maintain the hillside behind it (retain the soil) while building a road in front of the wall.

Retaining Wall
(Courtsey Brockman Engineering Contractors, Inc.)

You can see such walls commonly along the new Mumbai-Pune Expressway and NH-4. The height of the wall may vary from 2 feet to as high as 25 feet. There have been special cases where the height of the wall was as high as 15 m ( roughly 49 feet) for the Ladera Ranch project in California.

Retaining walls can be classified as:

  1. Cast-in-place concrete walls
  2. Pre-cast walls
  3. Modular Block Walls or Segmental Retaining Walls (SRW)
  4. Mechanically Stabilized Earth Walls
  5. Other walls such as timber walls, sheet pile walls, brick walls, stone walls, etc

Gravity Walls are typically made from a large mass of concrete and rely only on its self weight to retain the soil behind it. Although more costly than most other options, these are particularly used when the area of soil behind the wall is not enough for reinforcement. These walls are cast-in-place and usually unreinforced.

Gravity Walls
(Courtsey Concrete Network)

Pre-cast Walls are also typically gravity walls that are assembled on site. Since they are pre-cast, they are used in projects where time is essential.

Modular Block Walls or Segmental Retaining Walls (SRW) are the most extensively used retaining wall type. A SRW consists of interlocking blocks of concrete that are placed over a levelling pad. Since these blocks are available in a variety of sizes, colours, textures, shapes, etc., construction of these walls can be done in the most aesthetic manner even in a cramped space. SRWs being cheap, durable, flexible and easy to install, this are the most preferred retaining wall system used in and around homes for landscaping where the wall heights are typically low.

Segmental Retaining Wall
(Courtsey Tensar Corp.)

Landscaping Walls
(Courtsey Keystone Walls)

Mechanically Stabilized Easth Walls (MSE) are SRWs with typically geosynthetic reinforcements placed within the retained soil. They are primarily a “gravity” type retaining wall, but since they are reinforced with geosynthetics, the effective width and weight of the soil mass that resists overturning or sliding increases greatly. MSE walls utilize the advantages of a SRW for taller walls with structural uses. The 15 m ( roughly 49 feet) wall being built for the Ladera Ranch project in California is a MSE wall.

Most other type of retaining walls such as timber walls, sheet pile walls, brick walls, stone walls are used for non-structural use, such as for landscaping or where these materials are available easily and more economically than other types.

Sheet Pile Wall
(Courtsey H.B. Fleming)

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Landslides – Mitigation

Posted by Kshitija Nadgouda on June 17, 2006

Mitigation of a landslide means reducing the effects or the intensity of the landslide. Most methods of mitigation overlap with preventive measures so the subject matter in this post will overlap with the post on Landslides-Prevention. However, in my opinion, the importance of landslide mitigation calls for a separate post.
After a landslide occurs, the first task is to remove the mass of soil that has been displaced from its original position so that human life, if disrupted, can get back to normal. The next task is to determine the exact cause of the landslide in order to decide on the mitigation plan.

First rule of thumb for landslide mitigation is to stabilize the slope.

The slope can be stabilized by one or a combination of any or all of the following methods:

  1. Remove the landslide soil material and replace it will engineered fill
  2. Shear keys with drainage
  3. Buttress
  4. Removal of top
  5. Retaining Walls
  6. Steel nets
  7. Soil Nails

I discussed all these points in the previous post (Landslides-Prevention), but let us discuss them in detail here.

Remove and replace

This is a common technique specially used if the landslide area is small and if construction is ongoing in the area of the landslide.

Shear keys with drainage

Shear keys are typically used in conjunction with the “remove and replace” mitigation technique. Shear key also known as a keyway is a trench excavated into the competent soil material so that the new fill placed over the natural slope firmly keys into the existing soil. Placing a drainage pipe within a keyway futher improves the stability of the slope by reducing the effect of groundwater fluctuations.

Buttress

In simple words, a buttress is a man-made mound or hill of soil (fill slope or berm) placed at the toe of the slope. The buttress increases the resisting forces and thus prevents material from moving towards the toe of the slope. In some cases, it may also be a metal or concrete beam providing additional support to a retaining wall constructed at the toe of a slope.

Buttress Beam
(Courtsey Kansas Geological Survey)

Removal of top

An extenstion of the same principle as the fill buttress, another method is to remove the soil from the top of slope, thereby reducing the forces driving the slide. This may reduce the total height of the slope and thus help in reducing the driving forces.

Retaining Walls

Constructing a retaining wall at the toe of the slope acts principally similar to constructing a buttress. The retaining wall may be one of various kinds: gravity wall, gabion wall, modular block walls, reinforced conctrete walls, etc.

Retaining Structures
Click to see a a larger image
(Courtsey City of Anaheim)

Steel nets or wire meshes

These are usually put up on slopes where danger exists of the continually sliding mass to obstruct everyday life. The most commong example is putting wire meshes on sliding slopes that exist along roads.

Soil Nails

Soil Nails are typically steel bars inserted into the soil at close intervals to reinforce the existing slope. THe face is then typically sprayed with shotcrete.

Soil Nail wall with shotcrete
Click to see a larger image
(Courtsey USACoE, Memphis District)

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Landslides – Prevention

Posted by Kshitija Nadgouda on June 8, 2006

In my earlier post, we discussed the causes of landslides. So logically the preventive measures can be deduced directly from the causes.

Landslide
(Courtsey University of Kwazulu-Natal)

The first cause listed is gravity. Since we cannot alter gravity, what we can do, is alter geometry of the man-made slope so that the gravity effects are not detrimental. If the landslide is surficial (not too deep), the easiest way to prevent the fall of rocks and soil over the slope – is to vegetate it! However, vegetation can help only if the movement hasn’t already begun or if the landslide is deep!

Groundwater table changes are the most common cause of landslides. Heavy rains, leaking pipes, melting of snow in warm weather, floods, etc can cause changes in the groundwater table, thus inducing a landslide. Although natural phenomena such as heavy rains, melting snow, etc cannot be modified, its effect on the groundwater table can be controlled by applying the principles of hydrology and geotechnical engineering. Rain water or snow melt can be directed far away from the slopes by building drainage channels or swales that convey the water where it shall not be detrimental to the stability of the slope. Leaking pipes or leaking swimming pools can be easily fixed, once the location of the leak is determined.

Construction on top of slope
(Courtsey BBC News)

Earthquakes cause ground shaking which may directly lead to a landslide. Or, the ground shaking may cause the soil to loosen and become weak, leading to a landslide. To prevent earthquake induced landslides, the ideal solution is to design the geometry of the slope such that it has an adequate factor of safety even for seismic cases.

House on cliff
(Courtsey Emergency Management Australia)

To prevent landslides triggered due to construction on top of the slope, a setback distance should be maintained between the top of slope and construction. The distance will depend on the type of construction and geology and geometry of the slope.

Landslide on road
(Courtsey US Geological Survey)

Another cause of landslides (that I did not mention in my previous artice), is undercutting of the toe of slope. The toe of the slope plays a major role in keeping the upper portion in a stable condition. In fact, if a slope seems unstable, soil berms (counterweight fills) are placed at the toe of the slope to provide additional resistance to the potential movement of the upper part of the slope. Another aspect with similar principles would involve removing soil from the top of the slope, thus reducing the forces driving the movement.

Benching, constructing retaining walls, shotcreting, putting up steel nets, etc are some other methods of preventing or controlling landslides.

Some good information on landslides is available at:

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