WIXLIB

An OverviewofSoil MechanicsDr. P. K. BasudharDept of Civil EngineeringIIT Kanpur

Soil Problems&SolutionsA Preview ofSoil BehaviorPioneers inSoil Mechanics

WHERE ?CIVIL ENGINEERENCOUNTERS SOIL AS A– FOUNDATION– CONSTRUCTION MATERIAL SOIL RETAINING SPECIAL PROBLEMSSOIL

FOUNDATIONS WHAT ARE FOUNDATIONS? TYPES OF FOUNDATIONS– SHALLOW FOUNDATIONS– DEEP FOUNDATIONS MAIN PROBLEM IN THE DESIGN– TO PREVENT SETTLEMENT TOTAL SETTLEMENT DIFFERENTIAL SETTLEMENT

SHALLOW FOUNDATIONS Structural loads arecarried by the soildirectly under thestructure

DEEP FOUNDATIONS Used to carryca y thet eloads to firm soil atpsome depth

Classic case of bad foundation Fig.Figg. shows the Palacio de lasBellas Artes, Mexico City The2mdifferentialsettlement between the streetand the building on the rightnecessitated the steps whichwere added as the settlementoccurred The general subsidence ofthis part of the city is 7 m (Photograph compliments ofRaul Marsal)

Example of Shallow foundations Fig shows the MITstudents centre Mat foundation Floatation technique

Main Factors1. Just how deep into the soil should the building beplaced?2. Would the excavation have to be enclosed by a wallduringg construction to pprevent cavecave--ins of soil?3. Would it be necessary to lower the water table inorder to excavate and construct the foundation and,if so,so whath t meansme ns shouldsho ld be usedsed to accomplishomplish thislowering of the ground water (dewatering)?4. Was ttheree e a dadangerge oof dadamageage to adjaceadjacentt bbuildings?d gs?5. How much would the completed building settle andwould it settle uniformly?6. For what stresses and what stress distribution shouldthe mat of the building be designed?

Example of Deep foundations MIT material centre hasdeeppppile foundation Reasons– Basement space notdesirable– No sand and gravel attheh sitei– Not to disturbunderground utilities Point bearing pile Friction pilep Augering

Main FactorsI What type of pile should be used?I.2. What was the maximum allowable load for a pile?3 At what3.h t spacingp in shouldh ld ththe pilpiles bbe dridriven?n?4. How should the piles be driven?5. How much variation from the vertical should bepermitted in a pile?6. What was the optimum sequence for driving piles?7. Would the driving of piles have an influence onadjacent structures?

Example of Embankmenton Soft Soil 10.10.7 m embankmenton a 9.8m layer ofsoft soil Preloading technique Shear rupture shouldnot occur

Main Factors1. How high a fill could be placed?2. How fast could the fill be placed?3. WhatWh t werer theth maximumm im m slopesl p forf r theth fill?4. Could the fill be placed without employing specialt h itechniquest containtot i or draind i theth softft foundationfd tisoil?5. HowH muchh wouldld theh fill settle?l ?6. How long should the fill be left in place in orderthath theh foundationfd ib compressedd enoughbeh topermit construction and use of the tank?

Example of Foundation Heave Occurs when foundation soil expands when theconfining pressure is reduced and / or the water contentof the soil is increasedg Arid regions Presence of montmorillonite

Main Factors Proper size ,capacity,capacity length and spacing of thepiles The pile should be long enough to extend belowthe depth of the soil that would expand TheTh depthd h selectedld ini suchh a way thath thehconfining pressure from the soil overburdenpluslminimumi iloadl d isi sufficientffi ito preventexpansion

CONSTRUCTION MATERIAL Select proper type of soilMethod of placementControl of actual placementFillingg

Example of an Earth Dam Two main zones– Clayy core– Rock toe Gravel filter Rock facing Zoned earth dam & homogeneous earth dam

Main FactorsI. What should be the dimensions of the dam to give the mosteconomical, safe structure?2. What is the minimum safe thickness for the gravel layers?3. How thick a layer of gravel and rock facing is necessary tokeep any swelling of the clay core to a tolerable amount?4. What moisture content and compaction technique should beemployed to place the gravel and clay materials?5. What are the strength and permeability characteristics of theconstructed dam?6. How would the strength and permeability of the darn varywith time and depth of water in the reservoir?7. How much leakage would, occur under and through the dam?8. What,Wh if any, speciali l restrictionsi ion theh operationioff thehreservoir are necessary?

Example of a Reclamation Structure NonNon-availabilityy ofgood building sites Harbor and terminalfacilities Hydraulic filling

Main FactorsI. How deep should the sheet pile wall penetrate the foundationsoil?2. How should these piles be braced laterally?3. What is the most desirable pattern of fill placement i.e., howshould the exit of the dredge pipe be located in order to getthe firmer part of the fill at the locations where the maximumfoundation loads would be placed?p4. What design strength and compressibility of the hydraulic fillshould be used for selecting foundations for the tanks,buildings,g , and pumpingp p g facilities to be pplaced on the island?5. Where did the soil fines in the dirty effluent which went out ofthe island over the spillway ultimately settle?

Example of Highway Pavement Most common use of soil asconstr ction materialconstruction Pavements– Rigid– Flexible

Main Factors1. How tthickc sshouldo d tthee variousva o s cocomponentspo e ts oof ttheepavement be to carry the expected loads?pmixture of additives for2. What is the optimumstabilizing the desert sand?3. Is the desert sand acceptable for the construction ofthe wearing surface?4. What grade and weight of available asphalt maketheh most economical,i l satisfactoryifwearingi surface?f ?5. What type and how much compaction should beused?sed?

SLOPES AND EXCAVATIONS(a) Natural Slope(c) Excavation for Pipe(b) Excavation for Building(d) Canal

UNDERGROUND AND EARTHRETAININGTSTRUCTURESTT SoilSoil--structure interaction Examples– PipePi shellsh ll– Basement walls of the building– Sheet pile wall– Tunnels– Drainage structures

Example of Earth retaining structure Anchored bulkhead Take care of lateralstressestr Stability against shearrupture

Main Factors1. WhatWh t typet p off wallll (material(m t ri l andnd crossr section)ti n) shouldh ld beb used?d?2. How deep must the wall penetrate the foundation soil in order toprevent the wall from kicking out to the left at its base?3. AtA whath heighth i h on theh wallll shouldh ld theh anchorh tiei beb located?ld?4. How far from the wall should the anchor tie extend?5. What type of anchoringg system should be employed at theonshore end of the anchor tie? (One way to anchor the wall is touse a large mass of concrete, i.e., dead manman. Another way is to usea system of piles ;including some driven at a slope with thevertical;; such a sloping pile is termed a batter pile)vertical6. What was the distribution of stresses acting on the wall?7. What type of (drainage system should be installed to prevent allargediffdifferentiali l water pressure fromfd l i on thedevelopingh insidei id offthe wall?8. How close to the wall should the loaded crane (578 kN whenf ll loaded)fullyl d d) beb permitted?pitt d?9. What restrictions, if any, are necessary on the storage of cargo onthe area back of the wall?

Example of Buried Pipeline FlexibleFl ibl andd RigidRi id PipesPip Failures– Faulty construction– Excess construction load– Sagging of pipe Select– Properp thickness of thepipe wall– Workout and supervisetheh installationill i

SPECIAL PROBLEMS VibrationsExplosions and earthquakesSStorageoff industriali d i l fluidsfl id ini earthh reservoirsiFrostRegional subsidence

Oil storage

Frost Heave

SOLUTIONSSOIL MECHANICSStress-strainproperties Theoreticalanalyses forsoil massesGEOLOGY,EXPLORATIONComposition of actualsoil massesEXPERIENCEECONOMICSENGINEERINGJUDGEMENT

Why Soil problems are UNIQUE?1. Soil does not possess a linear or unique stressstress-strainrelationship2. Soil behavior depends on pressure, time, andenvironment3. TheTh soilil att essentiallyti ll every locationl ti isi differentdifft4. In nearly all cases the mass of soil involved is underunder-ground and cannot be seen in its entirety but must beevaluated on the basis of small samples obtainedfrom isolated locations5. MostMsoilsil are very sensitivei i to disturbancedi bffromsampling, and thus the behavior measured by alaboratoryy test mayy be unlike that of the in situ soil

An Overview Particulate Nature of Soil Nature of Soil Deformation Role of Pore Phase¾ Chemical Interaction¾ Physical Interaction¾ Sharing the Load A brief look at Consolidation

Particulate Nature off Soil Soil is composed of microscopic or macroscopic discreteparticles, which are not strongly bonded together as crystals Soil particles are relatively free to move with respect to another,pless fluent than the movement of fluid particles Particulate system pertains to a system of particles, and thescience dealing with the stress-strain behavior of soils is referredas Particulate Mechanics

Nature of Soil Deformation Contact forces develop due toapplied forces Contact forces are resolved intonormal N and tangential T forces The usual types of deformation inthe vicinity of contact forces¾ Elastic strain¾ Plastic strain¾ ParticleP i l crushinghi underd highhi h stress

Contact area enlarges due tothe deformations,deformations and thus thecenter of the particles comecloser (Fig. a) Plate like particles bend toallowllrelativel imovementbetween adjacent particles((Fig.g b)) Interparticle sliding occurswhen the shear force at thecontact surface exceeds theshear resistance of soil particle(Fig. c)

Overall strain of a soil mass is the combined effect of particledeformation and interparticle sliding.sliding Relative sliding of soil particles result in rearrangement of soilparticles , which is a nonlinear and irreversible phenomena, thusresulting in a non-linear and irreversible stress-strain behavior ofsoils. FFrictionali til andd adhesiondh i forcesfare alsol effectiveff ti ini producingd iparticle deformation There are 5 million contacts within 1 cm3 of sand mass. Hence,defining stress-strain relation of soil at each of the contacts isimpossible, and thus one has to rely on experimental results

If the box has rigid walls, andthe vertical load is increased,increasedthe soil particles will nestlecloser and closer. This is calledVolumetric Compression Slidingg failure will occur atindividual contacts, but the soilmass will not undergo anoverall shear failure Removal of the load will resulti Expansion or Swell off soilinilmass through a reverse processdue to rearrangement ofparticles

If the box has flexible walls,,the entire soil mass willundergo an overall shearfailure The load at which failureoccurs is called the ShearStrength of Soil Shear strength is determinedby the resistance to slidingbetween particles movinglaterally to each other

Role of Pore Phase : Chemical Interaction The spaces among the soil particles are calledPore Spaces The spaces are usually filled with air and/or water(with or without dissolved matter) Soil is a Multiphase system¾ Mineral Phase (Mineral Skeleton)¾ FluidFl id PhasePh(P(PoreFl id)Fluid) Pore fluid influences the magnitude of the shearresistance existing between two particles byintroducing chemical matter to the surface ofcontact Pore fluid intrudes particle spaces and acts intransmission of normal and tangential forces

Role of PorePo e Phase : Physical InteInteractionaction Hydrostaticycondition of waterpressure The pressure in the pore waterat any point is equal to the unitweight of water times thedepth of the point below thewater surface In this case, there is no flow ofwater

Water pressure at the base ofbox is increased, whileoverflows hold the watersurface constant Upward flow of water takesplace, the amount of which iscontrolled by excess pressureat base and Permeability of thesoil mass The more the permeable a soil,the more water will flow for agiven excess pore pressure