INTRODUCTION TO ROCK MECHANICS

INTRODUCTION  TO  ROCK MECHANICS

  Types of  Rocks

 A. Igneous rocks 

B. Metamorphic rocks 

C. Sedimentary rocks 

A. Igneous Rock: They  are  formed  from the  cooling  of magma deep  inside  the earth; they  are  visible to the eye  and have  large  crystals. Magma is a  mixture  molten or semi-molten rocks as well  as some  gases and volatile  elements.  It  gets  hot Deeper  and mantle surrounds the  earth core.   If  the magma  cools underground  at high temperature, its process would be  slow and crystals take  time to develop e.g.    Granites, but  if magma erupts and cools rapidly, its produces volcanic  rocks e.g. basalt. Volcanic rocks are  called  extrusive rocks. Types of a  igneous Rock Volcanic bomb Basalt Dacite Gabbro Diorite Periodotile Metamorphic  Rock These  are  rocks that undergo metamorphosis. There  either sedimentary  or igneous rock. They  are  made  by  either  heating  up or squashing  the earth  crust. They  are  found in mountainous regions.  It occurs as pre-existing  rock. The  original Rock is subjected to very  high  heat and pressure, which caused obvious  physical and  chemical changes. They  can  be  formed by  pressure deep inside  the earth, by  tectonic  processes sources continental collisions or when they  are heated up by  and intrusion of hot molten Rock called magma. Examples of  these  Rock times include I. Marble II.  Slate III.  Schist Sedimentary  Rocks They  are  formed by  deposition and subsequent cementation of  materials at  the earth surface  and within water  bodies. This  rock is formed by  accumulating  particles called sediment.   Before  being  deposited the  sediments were  formed by  weathering  and erosion from the  surface area  and then transported  to the  place  of the  deposition by  water,  wind, ice,  Mass movements or glaciers, which  are  called  agent of denudation. The  sedimentary  rocks cover the  Continent of the earth extensively  (about  73%). 

Sedimentary  rocks mainly  only  thin veneer over a  crust consisting  mainly  of igneous  rock  and metamorphic rocks. The  rocks are  deposited in layers and Strata,  forming  a  structure  called bedding.   Sedimentary  rocks include  some  limestone  deposits, halite, gypsum  and  anhydrate. Engineering Properties  of  Rock When Rock sis used as foundation for engineering  structure, the relative property  are  mass density  compressibility. Rock  Strength In  general,  a  rock can be  subjected to three  types of stress compressive, tensile, compressive stress tend to decrease  the  volume  of the  rocks by  the forces acting  in opposite  direction to each other. Shear stress is caused by  two  equal forces  acting  in opposite  directions as couple and tensile  forces tends to pull  a  substance  apart outwardly. Compressive, tensile  and shear forces are illustrated below. The  compressive  strength is the  compressive  stress needed to break the spacing  measured in N/mm2. The  unconfined pressure  strength (uniaxial) compressive  strength  is given by  where  p=failure  load in it  A =  cross sectional area  of sample  (m2). The  unconfined  compressive strength of rod ranges from 100kpa-280,000kpa. Example  1 A rock core  of limestone  is 76.2mm  in diameter and 152.4mm  long.  It is loaded to failure  in an unconfined testing  machine.  If the  failure  load is 276KN. What is the unconfined strength of the limestone  sample. Solution        p=276kN, d=76.2/100= 0.00762m   A=  =   = 276/456x10-3  =  6.05 x  10-4 3.142x0.762/4 

TRIAL AXIAL COMPRESSION TEST    They  are  performed on rocks to determine  how it  will  behave  under different confining  pressure. The  axial load required  causing  failure  of  rock increases  with increasing  confining  pressure  for the test, by  performing  2  or 3 tests  of different confining  pressures, the  relationship between axial compressive  strength  and confining  pressure  can be  determined by  that rock material. Data are  plotted on shear and  compression strength known as ‘Mohr  diagram’. Elasticity of  Rocks   Compressibility  is the  amount  of a  soil that will reduce  in length under load. Some of  the deformation will be recovered when the load is removed but a  portion will not  be  regained.  The recoverable is called elastic  while the  non-recoverable is called the plastic  deformation.   Commonly, the  elastic deformation of rock is directly  proportional to  the applied stress. When this is  the case, the proportionality  constant K  can  be  determined  as follows