Geotechnical Engineering 101 and more…

Building firm foundations

Modular Retaining Wall Components

Posted by Kshitija Nadgouda on July 26, 2006

Since my article on Retaining Walls, I received queries regarding the components of a modular block retaining wall. So I deceided to write this post to supplemnt my earlier article.

(Courtsey Versa-Lok)
A modular block (or segmental) retaining wall consists of the following components:

  1. Modular blocks (Facing Unit)
  2. Levelling pad
  3. Drainage Material
  4. Engineered Fill
  5. Geosynthetic Reinforcement
  6. Impervious Fill
  7. Retained Backfill

Modular blocks are the facing units – the aesthetic component of the wall that meets the eye. There are several companies like VERSA-LOK, KEYSTONE, ALLAN BLOCK, MESA BLOCK , etc that sell these inter-locking units.
Levelling Pad refers to the base of the “founding” element of the retaining wall. It provides a level base for the retaining blocks.
Drainage material is placed immediately behind the facing units to ensure that any moisture or groundwater is drained away and doesn’t exert excess pressure on the facing units.
Engineered Fill refers to the soil that replaces the material excavated from the natural slope. Typically a silty sand is used as engineered fill.
Geosynthetic Reinforcement are the sheets or grids of geosynthetic reinforcement that are used to strengthen the system. Tensar and Mirafi are two of the popular companies that manufacture geosynthetic reinforcement sheets and grids in the US, while Techfab India provides these materials in India,
Impervious fill is used to cap the backfill behind the facing units so that rain water or other surficial water doesn’t permeate into it.
Retained backfill is the part of the natural slope or natural terrain that is still undisturbed.

Geogrid Retaining Wall Systems, Incorported has an excellent series of photographs showing the step-by-step construction of a segmental retaining wall.

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Posted by Kshitija Nadgouda on July 15, 2006

This post is about foundations of structures built over the sea, in response to two questions posted by Sachin Choughule on the post about the Bandra Worli Sea Link.

Most bridge foundations over the sea are caissons that are embedded into the river-bed (or ocean floor) until a suitable firm stratum is encountered. A caisson foundation, in simple words, is like a can (although it need not be circular!). It is a watertight chamber that facilitates the operation of construction equipment by workers within it.

Small Caisson
(Courtsey Manitoba Labour and Immigration)

The workers operate construction equipment and excavate the soil within the caisson walls thus sinking the caisson into the river-bed (or ocean floor). Compressed air is pumped into the caisson and regulated to ensure that the caisson remains stable and soil/mud/water do not rush in through the bottom.

(Courtsey Carol Denney)

Some interesting information:

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Landslide along Mumbai-Pune Expressway

Posted by Kshitija Nadgouda on July 6, 2006

Heavy rains on Monday caused a landslide along the Mumbai-Pune Expresway near Khandala. It would be more accurate to call it a rock slide rather than a landslide since the hillslope is primarily a deccan trap rock formation. Big boulders and stones came toppiling down on to the expressway, obstructing traffic along two of the six lanes.

Simple preventive or mitigation measures such as nets should be proposed along the expressway in potential shallow landslide prone areas. Geologic mapping of the area should be carried out to determine such potential landslide prone areas or aras that have seen landslides historically.

Landslides along the Mumbai-Pune expressway have unfortunately become an annual feature with at least one landslide each year in the monsoons. The concerned agencies must take preventive and proactive mitigation measures to ensure this doesn’t continue. Will the authorites wake up only after loss of life occurs due to these landslides?

Get more information about the Mumbai-Pune expressway at:
or at the Road Traffic Technology site.
And photos at:
Amit Kulkarni’s site
or Shantanu’s gallery
The Pune Lifestyle webpage also has some great photos.

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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.


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.

(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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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.

(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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Significance of Geotechnical Engineering – Part I – Differential Settlement

Posted by Kshitija Nadgouda on April 5, 2006

Geotechnical Engineering is the study of soil in relation to man-made structures that stand on it. Every structure that is built, has a foundation that rests on the soil underneath and transfers the load (weight) from the structure to the ground. It is the job of the geotechnical engineer to assess the properties of the soil and determine wheather the structure is feasible at the proposed location.

Soil in its natural state is very heterogenous and non-uniform. It can vary greatly from place to place and also with depth. Soil investigations need to be performed to determine the type of soil present at a location. Based on the collected data, the geotechnical engineer determines the soil bearing capacity and estimated settlement of the structure.

The bearing capacity of the soil is simply the capacity of the soil to bear the weight (load) of the structure built on it, without undergoing failure. The settlement of the structure is the amount the structure will “sink” during and afer construction. It is the role of the geotechnical engineer to ensure that this settlement is within tolerable limits.

Settlement is broadly classified as total settlement and differential (uneven) settlement. Total settlement refers to the uniform settlement of the entire structure and occurs due to weight of the structure and imposed loads. Differential or uneven settlement can occur if the loads on the structure are unevenly distrbuted, variations in the soil properties or due to construction related variations.

Probably the most talked about and classsic “failure” in terms of differential settlement is La Torre Di Pisa (The Tower of Pisa) in Italy. Imagine a building meant for habitat (residential or commercial) structure showing so much inclination!!!

Leaning Tower Of Pisa

Leaning Tower of Pisa
(Courtsey Wikipedia )

The Leaning Tower of Pisa is the bell tower of the Cathedral. Its construction was commenced in 1173 and contiued haltingly over a period of 200 years! The tower began “leaning” soon after construction began in 1173. The inclination of the tower is attributed to the non-uniform, sponge-like saturated clay soil on which the foundation of the tower rests. The softer area within this strata has settled more causing the tilt.

Several engineers have proposed plans to “straighten” the tower. However, with its 800+ years of “leaning” history, locals do not want the tower to be straightened. Every few years some form of restoration is performed to ensure that the tower does not become unstable or collapse.

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Sea Link Protests

Posted by Kshitija Nadgouda on March 28, 2006

The Bandra-Worli sea link and the western freeway sea link project on the whole has come into the limelight again today, when the Indian Nightingale, Lata Mangeshkar announced that she would leave Mumbai if the flyover at Peddar Road was constructed.

It is sad to note that Lata Mangeshkar, a former Rajya Sabha member, is protesting this development only for personal issues -the proposed flyover alignment will obstruct the view from her first floor window.

The Peddar Road Residents Association (PRRA) is planning to take legal action against the PWD (Public Works Department) for witholding traffic survey data for the same flyover. The PRRA are of the opinion that the sea link project will ease traffic and congestion issues even without the Peddar Road flyover which was evident from traffic suvey that was withheld from them. They plan to meet Member of Parliament Milind Deora and also take a delegation to the central environmental ministry.

These two forms of protest caught my eye – both protesting the same issue, but in very different ways and for different reasons! Development versus Environment has always been a big issue worldwide. While everyone agrees that development is essential for the development of human race, nature and environment should not be sacrificed as a result.

India needs to have stricter Environmental laws and more stringent implementation of these laws. The Ministry of Environment and Forestry (MoEF) issued an environmental clearance for the project in February 2003. Several issues of concern pointed out by the Indian People’s Tribunal of Environment and Human Rights that I talked about on Friday, have been addressed.

The current quality of air, water and sound pollution have been studied and the impact on them during and after construction have been predicted. It will now be necessary to monitor these during and after construction and if they are exceeding the permissible limits, corrective action needs to be taken dynamically.

The question that arises is: Are the concerned Indian Government agencies and Departments pro-active and dynamic enough to handle this? It will remain to be seen…

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