Geotechnical Engineering 101 and more…

Building firm foundations

Soil Exploration

Posted by Kshitija Nadgouda on September 25, 2010

Lecture 1 on Soil Exploration:
Soil Exploration Part I
Lecture 2 on Soil Exploration:
Soil Exploration Part II
Please note: All figures used have been downloaded from various sources on the internet.

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Earthquake prediction and structure safety

Posted by Kshitija Nadgouda on February 28, 2008

Predicting earthquakes has been very controversial subject. Seismologists can only predict that movement is “possible” along a certain fault line or it is imminent. The two aspects of the prediction – where and when – are not easy to determine. So typically, what seismologists give, are “forecasts” – not predictions. They give an estimated location, and time of an earthquake of possible magnitude.

So when structures are being built, one needs to study the “seismic hazards” that the structure will likely face. The seismic hazards that can affect a structure due to a nearby earthquake are classified as primary and secondary. Primary hazards includes ground rupture, while secondary hazards includes ground shaking, ground lurching, liquefaction, etc.

Ground rupture occurs at the surface of active faults. Since the location of a fault, deep within the earth may not be known accurately or since there may be several small faults at a given location, “fault hazard zones” are demarcated on the surface by geologists and seismologists. Any construction should be avoided within these zones.

Ground shaking occurs in all earthquake prone areas, and to mitigate the effects of shaking, all structures must be built and designed as per local building codes and using sound engineering judgment.

Since it is not practical and uneconomical to build structures that resist maximum possible earthquakes with minimum damage, the building codes typically follow a few principles. The Uniform Building Code (UBC) lays down the following requirements:

  • The structures should be able to resist minor earthquakes without damage
  • The structures should be able to resist moderate earthquakes without structural damage but with some nonstructural damage
  • The structures should be able to resist major earthquakes without collapse but with some structural as well as nonstructural damage.

Although this does not guarantee that severe structural damage will not occur, at least we can expect that the structure will not collapse.

Here are some interesting sites that talk about earthquake prediction:

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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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Construction Netcast

Posted by Kshitija Nadgouda on February 17, 2007

I recently received an email from Matt Handal of Trauner Consulting Services regarding the use of net casting in the Construction and Civil industry.

Check out their online training webpage.

It is a one-of-a-kind site that currently offers two training videos: one on “Common Scheduling Terms” (terms used commonly in Critical Path Method Scheduling) and another on the importance of using the correct mood (Imperative) and voice (Active) during “Specification Writing”.

It a great tool – technology being put to optimal use! We would love to see more such broadcasts – especially about on-site issues or training for field professionals.

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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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Consolidation of soil

Posted by Kshitija Nadgouda on November 20, 2006

I talked about compaction of soil and how removal of the air voids in the soil makes it more dense. Consolidation is the removal of water from soil. In other words, it is the squeezing out of water from the soil to make it more dense.

In case of coarse grained soils like sands and gravels, the removal of this pore water is easy since water freely moves from one region to another within these soil types. However, in case of fine grained soils like silty or clayey soils, consolidation is a time consuming process.

As an analogy, consider soil mass to be like a sponge that is slightly wet. If we press the sponge, it will deform by compressing the air out of it. If we squeeze it further, water will be removed and the sponge will be compressed further. If the sponge (soil mass) is completely wet or soaked, it is termed as saturated. This is the condition when all voids are filled with water and no air voids exist.

Soil-Water Phase
Courtsey: Dr. Kamal Tawfiq

In case of fine grained soil on which a structure is to be built, high water content is not desired as the weight of the structure may cause sinking (consolidation settlement) of the structure in due time. Typically the permeability (ability of water to move through the soil voids) of fine grained soils is low, hence it takes a long time for consolidation process. So two aspects of consolidation settlement are important: the rate at which the consolidation is taking place and the total amount of consolidation.

It is very important to note that unlike settlement in sands and other coarse grained soil, consolidation settlement of fine grained soil does not occur immediately. Hence, it is common practice to ensure that the consolidation process is expedited and that most of the consolidation takes place during the various phases of construction.

If the soil is such that it has never experienced pressure of the current magnitude in its entire history, it is called a normally loaded soil. The soil is called pre-consolidated (or over-consolidated) if at any time in history, it has been subjected to a pressure equal to or greater than the current pressure applied to it. In case of normally consolidated soils, the consolidation will be greater than that for a pre-consolidated soil. That is because the pre-consolidated soil has previously experienced greater or equal pressure and has undergone at least some consolidation under that pressure. So a pre-consolidated soil is preferred over a normally consolidated soil.

The rate at which consolidation will take place, will depend on the nature of soil, the degree of saturation (how many percent voids are filled will water), the amount and nature of the load on the soil, the soil history (normally or over -consolidated), etc.

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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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