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Information regarding current issues, new projects, existing structures, etc.

Earthquake in Japan

Posted by Kshitija Nadgouda on July 18, 2007

A 6.8 Magnitude earthquake hit Japan on Monday, July 16, 2007. Considering the high magnitude, the loss of life was small – seven persons dead and hundreds injured. So what exactly is an earthquake?

Japan quake July 2007
(Image Courtesy: Environment News Service and Japan Meteorological Agency)

Wikipedia defines earthquakes as “the result from the sudden release of stored energy in the Earth’s crust that creates seismic waves”. I will try and explain that in simple words.

The earth is not a stationary, passive body. In fact, it is a very active and changes are continuously taking place inside it. The “solid” earth is actually made of four parts: the inner core which is solid, the outer core which is liquid, the mantle and the crust which are solid too.The crust is the thinnest layer and being relatively cold, it is brittle. The upper part of the mantle and the crust together make up the “lithosphere”.

Earth Core
(Image Courtesy: Nevada Seismological Laboratory)

The lithosphere is not contiguous, it is made up of several pieces like a jigsaw puzzle. However, these pieces – called tectonic plates – are continuously moving around, sliding past each other, colliding or moving away from one another. When these plates that are touching each other, get locked at the plate boundaries (while the rest of the plate is trying to move), it causes frictional stress. When this stress is exceeded beyond a certain value, these plates get unlocked and suddenly move relative to one another. This violent displacement is called an earthquake.

Here are some pictures that show devastation caused by earthquakes.

Collapse of the Hanshin Expressway Bridge in the Kobe, Japan earthquake of 1995.
Hanshin Expressway Collapse
(Image courtesy: University of Washington)

The 1906 earthquake damage in San Francisco, USA.

SFOdamage
(Image courtesy: Science Photo Library)

The earthquake in Bhuj, India in 2001

Bhuj Quake
(Image courtesy: International Federation of Red Cross and Red Crescent Societies)

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Earthquake Info for India

Posted by Kshitija Nadgouda on January 27, 2007

I came across a site created by Mr. Kishor Jaiswal that gives a lot of good information on earthquakes in general and about India in particular.

You can take a quiz on earthquakes or view an animation that shows how stress build-up leads to an earthquake.

The Seismology Division of the India Meteorological Society deals with the monitoring of earthquakes in and around India.

indiaquake.jpg
(Courtsey: Columbia News)

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Slope stability software

Posted by Kshitija Nadgouda on January 4, 2007

There is plenty of software available these days to do pretty much everything one can think of. As a geotechnical engineer, the software programs I have used most often have to be the ones available for slope stability or for geogrid design of a (soil) slope, besides using AutoCAD and gINT. Discussing AutoCAD and gINT calls for separate posts!

For a very comprehensive list on the different geotechnical and geoenvironmental software programs avaiable, check out the GGSD website. They list thousands of software programs, ranging widely in cost and applications.

The site lists a whopping 53 different programs for slope stability of soil alone!
The programs that I have most extensively used from the list are:

  1. XSTABL
  2. GSLOPE
  3. SLIDE

XSTABL is a DOS-based program and hence has not many takers. However, it does give a reasonably good graphical output. Although DOS-based, it is fairly easy to input data and has a good help feature. It allows the user to see the progress of the data while entering so you can edit any errors one may have made. It gives you an option of circular or non-circular search for failure surface. It uses Bishop and Janbu methods to calculate the factor of safety. The biggest advantage of the program is that it is relatively cheap! It costs US $450 only!! That roughly converts to approximately Indian Rs. 20,000 excluding any shipping and handling fees that may be added. A demo version is available at their website.

I primarily used GSLOPE for design of geogrid-reinforced slope stability checks. It is Windows-based and very user friendly. The program uses Bishop’s Modified method and Janbu’s Simplified method for calculating the factor of safety. It allows either method to be applied to circular, composite, and non-circular surfaces (which is not very correct – non-circular surfaces should be analyzed using Janbu method – not Bishop’s modified method). Data input can be done by either entering the co-ordinates of slope geometry or also be entered using a mouse (i.e. drawing the slope surface or subsequent soil strata). One good feature is that the analysis is real-time, i.e. if you modify a non-circular slip surface, it will update the factor of safety immediately. The program costs about Indian Rs. 45,000 (US $995) and additional cost for shipping and handling, roughly Indian Rs. 2,000. It provides a demo version for trial.
GLSOPE

SLIDE is a more complex program and very comprehensive. It has a steep learning curve, but can do a detailed study of the problem at hand. It can even perform probabilistic and sensitivity analysis. One can perform back-analysis of a slope that has failed, to determine the soil properties. Groundwater seepage analysis can be easily performed. Although it has a steep learning curve, the documentation and tutorials provided with the software are extremely useful.
The program has 10 different methods by which you can analyze a given slope. All analysis is performed simultaneously and the results are viewed in a separate “Interpreter” window. It has a CAD-like interface and so it is easy to draw the slope and soil strata during modeling. SLIDE doesn’t allow a demo version download and costs a whopping US $1495 plus shipping and handling, approximately Indian Rs. 67,000!
SLIDE

Some free software like STABLE is also available. My next task is to look at these freewares and review them!

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Soil Compaction

Posted by Kshitija Nadgouda on September 29, 2006

I talked about total settlement and differential settlement some time back. One of the major reasons for settlement of a structure is the presence of loose soil. So, to avoid such settlement (total or differential), it is essential to compact the soil.

Compaction of soil is the removal of air gaps (voids) from the soil. Soil, in general, is made up of three components: solid particles, air voids and water voids. Expulsion of air voids is called compaction, whereas removal of water voids is called consolidation (squeezing out of water).

Compaction
(Courtsey: Concrete Catalogue)

Compacting the soil, will increase its density and thus improve stiffness and strength of the soil. The degree of compaction will depend on several fators such as: type of soil (clay, silt, sand, organic soil, etc.), characteristics of soil (grading, plasticity, etc.), thickness of soil layer being compacted, weather conditions, amount and method of compactive effort applied, and water content of the soil at the time of compaction.

Four primary methods of applying compactive efforts are:

  • Static weight
  • Kneading action
  • Impact
  • Vibrations

Typically rollers are based on static weight and kneading action for compaction, while compactors use principles of impact and vibration to achieve compaction.

A special form of compacting is dynamic compaction where compaction is achieved by repeated dropping of a weight at a certain location and in a certain pattern over the site. More details here.

Dynamic Compaction

(Courtsey: Geoforum)

Typically Rollers perform compaction by static weight and kneading action whereas the equipment that perform compaction by impact or vibrations are called compactors.

Rollers may be further classified as tampers, smooth-wheeled or pneumatic tyred rollers. Sheepsfoot tamping compactor provides weight and kneading action.

Sheepsfoot Roller
(Courtsey: University of Missouri Extension)

Compactors may be Vibrating Roller compactors, Vibrating plate compactors and rammer compactors.

Vibrating plate compactor

(Courtsey: Haven Group)

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Update

Posted by Kshitija Nadgouda on September 12, 2006

I have accepted the post of a lecturer in my alma mater. Since it is my first attempt at teaching, I find that it is taking up more time than I expected. I am making the utmost efforts in writing articles, but the frequency has gone down dramatically. It is a promise that my next article will be up by the end of this week!

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IGS – Indian Geotechnical Society

Posted by Kshitija Nadgouda on August 3, 2006

I attended a seminar today, by Prof. Gandhi on “Failures and Remedial Measures in Geotechnical Engineering” organised by the Indian Geotechnical Society (Mumbai Chapter). It was a great learning experience! There are many more seminars/workshops on interesting topics coming up. I will encourage all students of Civil Engineering to find out about such societies as IGS (Indian Geotechnical Society), Indian wing of the ASCE, Insitute of Bridge Engineers, etc and become members. Most seminars/talks are free to members and workshops are at discounted rates.

It is not only a good way to learn about the topic under discussion, but we come into contact with lots of people, big and small, in the industry and the academics. Talking to them is in itself a learning experience. Everybody gets to contribute in the seminar, lot of discussions come up and we end up learning a lot more from them. In my opinion this is the best way for students to interact with the industry gurus and keep abreast of the latest in the best practices, research and ideas.

One drawback I have noticed in India, is that there is no access to such information for students. We have such good societies and associations that get together regularly, discuss academic and industry issues but usually students are excluded from them. This may be because they are not mature enough in terms of exposure to the industry. But the only way to change this, is to make them aware, involve them, and encourage them to participate by asking even the most basic questions. How else will they learn?

So all you students out there, whether young or old, get fired up, ask others about such societies, join them and other numerous organisations that exist out there and PARTICIPATE to get the industry knowledge first-hand!

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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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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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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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