When and why the mat or raft foundations are used?

Q. When and why the mat or raft foundations are used?

A.
Mat or raft foundations are type of the shallow foundations, which covers the entire area beneath a structure and provides support to all the columns and walls. It is also a type of the combined footing where all the footings are combined in the following cases:

1. When the columns are so close such that their footings nearly touch each other.
2. When the soil is weak and having the low bearing capacity.
3. Where chances of differential settlement exist due to either the existence of the different soils or variation of the moisture content.
4. or where there is a large variation in the loading in the adjacent columns.

reference: 'Analysis and design of substructures' by Swami Saran.

thanks!

Failure Criteria for Rockmasses

Hi there!

As we know, rock-masses are the complex structures. Failure of the rock-mass occur by the development of the fractures or slip surfaces, when the stresses get increased from the strength of the rock mass.
The failure process in rock-mass is complex, and mathematically difficult to quantify. Four failure criteria are:

1. Coulomb's Criteria
2. Mohr's Criteria
3. Hoek and Bray Criteria
4. Griffith's Criteria.

First three uses empirical approach, and have considerable value for the design of excavation, while the last one has value for understanding the fracture initiation in the rock.

Tunneling in weak Rocks by Singh and Goel

Hi,
 This book by Bhawani Singh and Rajnish K. Goel is a very easy to read and useful book for the students of the UG and PG courses of Geo-technical Engineering. I got this book from my course teacher, and it is very helpful.

In this post, I want to share with you the quotes those are written in the book, at the bottom of every title line at the start of the new chapter.
I loved these quotes, and they somehow motivated me to read this book.


1. (Chapter No.1 INTRODUCTION)
"College is where you learn how to learn" - Socrates (470 - 399 B.C.)

2. (Chapter No.2  Applications of Geophysics in tunneling and site survey activities)
"And so Geology once considered mostly a descriptive and historical science has in recent years taken on the aspect of an applied science. Instead of being largely speculative as perhaps it used to be, geology has become factual, quantitative, and immensely practical. It became so first in mining as an aid in the search for metals; then in the recovery of fuels and the search for the oil; and now in engineering in the search for the more perfect adjustment of man's structure to nature's limitations and for greater safety in public works." -  Charles P. Berkey, Pioneer Engineering Geologist. 

3. (Chapter No.3 Terzaghi's Rock Load Theory)
"The Geological engineer should apply theory experimentation but temper them by putting them into the context of the uncertainty of nature. Judgement enters through engineering." - Karl Terzaghi

4. (Chapter No.4 Rock Mass Rating (RMR))
"Effectiveness of knowledge through research(E) is E = mc^2; where m is the mass of knowledge and c is communication of knowledge by publications."   - Z.T. Bieniawski

5. (Chapter No.5  Rock Mass Quality Q)
"Genius is 99 percent perspiration and 1 percent inspiration." - Bernard Shaw

6. (Chapter No.6  Rock Mass Number)
 " My attention is now entirely concentrated on rock mechanics, where my experience in applied soil mechanics can render useful services. I am more and more amazed about the blind optimism with which the younger generation invades this field, without paying any attention to the inevitable uncertainties in the data on which their theoretical reasoning is based and without making serious attempts to evaluate the resulting errors."     - Annual summary in Terzaghi's Diary

7. (Chapter No.7 Strength of Discontinuities)
"Failure is success if we learn from it."  - Malcom S. Forbes

8.(Chapter No.8 Strength enhancement of rock mass in tunnels)
 "The behaviour of microscopic systems is generally described by non linear laws. (The non-linear laws may explain irreversible phenomena like instabilities, dualism, unevolving societies, cycles of growth and decay of societies. The linear laws are only linear approximation of the non-linear laws at a point in time and space.)"    -  Ilya Prigogine, Nobel Laureate



In this book,  there are 29 chapters in total, I think it would be a length post to write all of the quotes in one post, let me know if you want the others. If you want to buy the book, go to the Amazon link I have given above. 

Thanks.

Rock Quality Designation (RQD)

Rock Quality Designation  was introduced by D.U. Deere (1964) (Practical Approach to Civil Engineering by Singh and Goel) as an index of assessing rock quality quantitatively. RQD is a modified per cent core recovery which incorporated only sound rock core pieces that are 100 mm or greater in length along the core axis.

RQD  = (Sum of core pieces >= 10 cm)/ total drill length *100

Following are the methods of obtaining RQD :

a. Direct Method

International Society for Rock Mechanics(ISRM) recommends a core size of at least NX (size 54.7 mm) drilled with double-tube core barrel using a diamond bit. All the artificial fractures should be ignored while calculating the core length for RQD.
The relationship between RQD and engineering quality of the rock mass as proposed by Deere in 1968 is given as below:

S.No.          RQD (%)           Rock Quality
 1.                 <25                      Very Poor
 2.                 25-50                   Poor
 3.                 50-75                   Fair
 4.                 75- 90                  Good
 5.                 90- 100              Excellent

RQD determination - Direct Method ( Photo credit- A Practical Approach to Civil Engineering by Singh and Goel)

b. Indirect Method:
1. Seismic Method : The seismic survey method makes use of the elastic properties of strata that affect the velocity of the seismic waves travelling through them. This method is cheap, rapid and relatively easy to apply. We can find out the following information from this method:
a) Location and configuration of bed rock and geological structures in the subsurface.
b) The effect of discontinuities in rock mass may be estimated by comparing the in-situ compressional wave velocity with laboratory sonic wave velocity of intact drill core obtained from the same rock mass.
                RQD(5) = (Vi/Vl)^2 * 100
Where, Vi is the in-situ compressional wave velocity and Vl is compressional wave velocity in the intact rock core.

2. Volumetric Joint Count Method:
The RQD can be determined by counting the number of joints(discontinuities) per unit volume Jv.  A simple relationship(given by Palmstrom, 1982), RQD = 115 - 3.3*Jv , can be used to convert Jv into RQD for clay free rock masses.

Here Jv is the number of joints per cubic meter of the rock mass.
There are few other methods, which I shall cover up in the upcoming posts.

Thank You!

Helical Auger boring and determination of water content and specific gravity

Theory:
Helical Augers are used to get the disturbed soil samples from the surface or near to the soil surface. The size of augers varies from 75mm to 200mm. These samples can be used to determine the nearest ground water level, or to determine the water content at various depths. Generally hand augers are used up to the depths of 6m but they can be used up to 30 m depths. Other properties such as particle size distribution and specific gravity can also be determined using these disturbed samples.

Hand Augers (Image source: www.bestengineeringprojects.com)

Apparatus/Equipment:

• Helical Auger bore with the handle
• Scale and sample collectors.
• Other apparatus for the water content determination and specific gravity determination, such as the oven, soil cans and pycnometer.

Procedure:

1. Fix the T-handle correctly to the Auger.
2. Press the auger into the ground and rotate the auger clockwise to get it sink into the ground to the required depth in the area under investigation.
3. When the annular space between the blades is filled with the soil, it is withdrawn and cleaned.
4. The cleaned auger is again inserted and the process is repeated.
5. Extension rods are attached when bore hole progresses downwards.
6. Take the samples from the various depths and then various tests are performed to check their properties.

Precautions:

1. The auger is rotated and pressed at uniform rate.
2. Clean the blades of the auger after the experiment is over.
3. Joining of the extension rods should be done carefully.
4. The sample should be collected at every 30cm boring and sample should be extracted whenever the strata is changed.
5. Straight driving should be done.

Limitations:

1. Helical Auger does not work in rocky strata.
2. Sample cannot be obtained under water.

Introduction to Dynamic Soil Properties

Hi, How have you been?

Soil dynamics is the study of the behavior of the soil under the dynamic loads. When the loads are larger such as the earthquake loads, Blasts or the nuclear explosions, the strain rate of the soil is from 0.01% to 0.1%, while the strain rate in the soil is comparably low of order 0.0001% to 0.001 % in case of the small loading such as the loading on the foundation of the reciprocating machines.

In order to analyse and design the structures subjected dynamic loading, we have to study the following dynamic properties of the soil:

1. Dynamic moduli, such as Young's modulus of Elasticity 'E', Shear modulus 'G', and bulk modulus 'K'.
2. Poisson's ratio
3. Dynamic Elastic constants, such as the co-efficient of uniform compression 'Cu'; Co-efficient of uniform Shear 'Ct', co-efficient of non uniform compression and Co-efficient of non-uniform shear. 
4. Damping Ratio
5. Liquefaction parameters, such as the cyclic stress ratio, cyclic deformation and pore water pressure response.

Dynamic properties of the soil are dependent upon the strain, and several laboratory and field techniques been developed to measure these properties over a wide range of strain amplitude.

Resonant column test, Ultrasonic Pulse test, Cyclic simple shear test, Cyclic torsional simple shear test, and cyclic tri-axial compression tests are the type of the laboratory tests used to determine the dynamic soil properties.

Thank you :)

Vertical Piles of a Group subjected to Eccentric Loading.

Greetings!

In order to determine the load distribution  between the vertical piles of a group subjected to eccentric loading, we have to consider the two cases of loading:

(a) When eccentricity is about one axis only

Vpi =  V/n  +-  (V.e.Xi.A)/Ig

Where,  Vpi = Load on the ith pile

              V = Total vertical load acting on the pile group.

Accentric loading on pile group

             n = Total no. of piles.

             e= amount of eccentricity w.r.t. the centre of the pile group. 

             Xi = Distance of the centre of the ith pile from the centre of the pile group, measured parallel to e. 

             Ig = Moment of Inertia of the piles about the axis normal to the direction of eccentricity. 

                 = A.X1^2 + A.X2^2 +.... + A.Xn^2

             A = Area of the pile cross-section and 

           X1, X2, ...... Xn = Distance from centre of gravity of pile group to the line of each pile, measured parallel to e. 

Since, all the piles in the group are assumed to be identical, above equation can be written as:

Vpi =  V/n  +-  (V.e.Xi.)/Sum(Xi.^2).

(b) When eccentricity is about two axes:

When there is eccentricity about both the axes, individual pile loads may be determined by the method of superposition:

Vpi =  V/n  +-  (V.ey.Yi.A)/Ix   +-  (V.ex.Xi.A)/Iy

where, V, n and A have the same meaning. 

  

Eccentric loading on pile group

ex = Amount of eccentricity w.r.t. centre of pile group measured along x-axis.

ey = Amount of eccentricity w.r.t. centre of pile group measured along y-axis.

Ix = Moment of inertia of the piles about x-axis. == A.Y1^2 + A.Y2^2 +.... + A.Yn^2

Iy = Moment of inertia of the piles about the y-axis = = A.X1^2 + A.X2^2 +.... + A.Xn^2

Xi = distance from the centre of gravity of the pile group to the line of each pile, measured parallel to the x-axis, and 

Yi = distance from the centre of gravity of the pile group to the line of each pile, measured parallel to the y-axis.

Reference: Analysis and Design of Sub-Structures by Swami Saran

Thank you :)