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Basics of Geotechnical Engineering

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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Copyright Kshitija Nadgouda.

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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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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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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.

reinforced.gif
(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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Caissons

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.

Schematic
(Courtsey Carol Denney)

Some interesting information:

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