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Archive for May, 2006

Landslides – Causes

Posted by Kshitija Nadgouda on May 30, 2006

A landslide is defined as the rapid movement of landmass over a slope. When a natural slope or man-made slope becomes unstable, a landslide results. The main causes of landslides are:

  1. gravity
  2. groundwater table changes
  3. earthquakes or other vibrations
  4. construction on top of slope, etc.

Vancouver slide
(Courtesy Huge Landslide)

The resistance to a landslide is offered by the type of soil and the geometry of the slope. Preventive and remedial measures include modifying the geometry of the slope, controlling the groundwater, constructing tie backs, spreading rock nets, etc.

Although landslides may not be preventable, their devastating effect on humans and their property is avoidable and can be mitigated.

For this post, let us discuss landslide causes in detail.
The basic cause of a slope failure is when the driving forces (forces causing the downward movement) exceed the resisting forces. When heavy rains occur, the rain water infiltrates into the soil and groundwater table is raised. The pressure exerted by the water thus increases, causing the driving force to exceed the resisting force, hence resulting in a landslide. In an extreme case, the soil may become so saturated with water, that it behaves as a fluid and flows downwards. This is called a mudslide.

Mudslide
(Courtesy Daily Bruin)

Earthquakes cause vibrations and ground shaking, which induce landslides. Other man-made vibrations such as movement of traffic, opertions of heavy machinery, pile driving and other construction-related vibrations may also cause landslides. Particularly, if these man-made vibrations occur on top of the slope, a landslide could be triggered.

Kobe slide
(Courtesy Landslide at Kobe)

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Significance of Geotechnical Engineering – Part II – Total Settlement

Posted by Kshitija Nadgouda on May 5, 2006

This is a continuation, rather the next part, of the topic I posted earlier on Differential Settlement.

Total settlement is the uniform “sinking” of the structure due to various factors such as the self-weight of the structure, the loads imposed on it, the nature of the soil on which the structure’s foundation rests, etc.

The settlement of any structure can occur immediately (during or post-construction) or it may take years to show up (or sink in!), depending on nature of the soil.

Immediate Settlement

The immediate settlement occurs due to re-organization of the soil particles in response to the weight imposed on it. This is typically observed in sandy soils. Sandy soil or coarse-grained soil is permeable to water, that is, it allows water to move through it easily. When the foundation of any structure rests on coarse-grained soil, the air gaps (voids) are either compressed to a small extent or removed by the re-arrangement of the soil particles, causing the soil to become more dense. The water within the voids, since it cannot be compressed, will move away. This high permeability attributed to these soils results in immediate (quick) settlement.

Long term Settlement

The long term settlement (also called consolidation settlement) is a phenomenon exhibited by fine-grained, saturated, clayey soils, in simpler words, sticky, muddy soil saturated with groundwater. Fine-grained clay soil shows low permeability, that is, it takes very long for the water to move through it from one point to another. This causes a time lag in the settlement to occur! Silty soils are the gray area between the sandy and clayey soils. Silty soil may appear to be similar to very fine sand, but exhibits many properties like clay.

However, in nature, soil is not often “purely sandy” or “purely clayey”. In most cases, soil will be a mixture of sandy, silty and clayey particles. So estimation of the total settlement is important, and limit it within safety standards.

For safety and aesthetic reasons, the total settlement is typically limited to 25 mm (1 inch).

It is the responsibility of the Geotechnical Engineer to determine the soil properties, assess the predicted load imposed on the soil due to the foundation and super-structure and estimate the total settlement.

Palace Of Fine Arts, Mexico City

A classic and drastic example of total settlement is the Palacio De Bellas Artes (Palace Of Fine Arts), cultural centre in Mexico City, which has sunk more than 15 feet (4.6m) since its construction in the early 20th century.

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