PAT 253 GEOTECHNIC INTRODUCTION TO GEOTECHNICAL ENGINEERING 1

PAT 253 GEOTECHNIC INTRODUCTION TO GEOTECHNICAL ENGINEERING 1

TOPICS n n n Issues in Geotechnical Engineering Purpose and planning of site investigation Methods of sampling In situ testing Preparation of site investigation report 2

Introduction to Geotechnical Engineering n n n Branch of civil engineering concerned with the engineering behavior of earth materials. Uses the principles of soil mechanics and rock mechanics to investigate subsurface conditions and material, evaluate stability of natural slopes and man made soil deposit, assess risk posed by site conditions, design earthwork and structure foundation, etc. Typical geotechnical engineering projects begins with a review of project needs to define the required material properties. 3

Introduction to Geotechnical Engineering n n In true engineering terms, the understanding of geotechnical engineering as it is known today began early in the 18 th century (Skempton, 1985). Based on the emphasis and the nature of study in the area of geotechnical engineering, the time span extending from 1700 to 1927 can be divided into four major periods (Skempton, 1985): Preclassical Period of Soil Mechanics (1700– 1776) n n Preclassical Period of Soil Mechanics (1700– 1776) Classical Soil Mechanics—Phase I (1776– 1856) Classical Soil Mechanics—Phase II (1856– 1910) Geotechnical Engineering after 1927

Introduction to Geotechnical Engineering n Preclassical Period of Soil Mechanics (1700– 1776) n n n Classical Soil Mechanics—Phase I (1776– 1856) n n n French scientist Charles Augustin Coulomb (1736– 1806) used the principles of calculus for maxima and minima to determine the true position of the sliding surface in soil behind a retaining wall. William John Macquorn Rankine (1820– 1872), a professor of civil engineering at the University of Glasgow provided a notable theory on earth pressure and equilibrium of earth masses. Rankine’s theory is a simplification of Coulomb’s theory. Classical Soil Mechanics—Phase II (1856– 1910) n n n Henri Gautier (1660– 1737), studied the natural slopes of soils when tipped in a heap formulating the design procedures of retaining walls. The first laboratory model test results on a 76 mm high retaining wall built with sand backfill were reported in 1746 by a French engineer, Francois Gadroy (1705– 1759), who observed the existence of slip planes in the soil at failure. Albert Mauritz Atterberg (1846– 1916), a Swedish chemist and soil scientist, defined clay size fractions as the percentage by weight of particles smaller than 2 microns in size. He explained the consistency of cohesive soils by defining liquid, plastic, and shrinkage limits. He also defined the plasticity index as the difference between liquid limit and plastic limit Karl Terzaghi (1883– 1963) of Austria (Figure 1. 3) developed theory of consolidation for clays as we know today. Geotechnical Engineering after 1927 n n Karl Terzaghi is known as the father of modern soil mechanics. Research covered wide range of topics; Effective stress, Shear strength, Testing with Dutch cone penetrometer, Consolidation, Centrifuge testing, Elastic theory and stress distribution, Preloading for settlement control, Swelling clays, Frost action, Earthquake and soil liquefaction , Machine vibration

Issues in Geotechnical Engineering Leaning Tower of Pisa n The structure weighs about 15, 700 metric tons and is supported by a circular base having a diameter of 20 m n Recent investigations showed that a weak clay layer exists at a depth of about 11 m n stabilized by excavating soil from under the north side of the tower.

Issues in Geotechnical Engineering

Issues in Geotechnical Engineering

Issues in Geotechnical Engineering

Issues in Geotechnical Engineering

Issues in Geotechnical Engineering

Issues in Geotechnical Engineering

Issues in Geotechnical Engineering

SITE INVESTIGATION 14

Definition The process of determining the layers of natural soil deposits that will underlie a proposed structure and their physical properties is generally referred to as site investigation. 15

Importance of Site Investigation 1. 2. 3. 4. 5. To assess the suitability of proposed site and the environment with the proposed project. To provide adequate information for design purpose with optimum cost. (including temporary work) To plan for the best construction method. To predict and avoid delay during construction. To estimate possible changes to the soil profile/condition caused by construction of project. To suggest the suitability of alternative construction site if needed. 16

The purpose of a soil investigation program 1. Selection of the type and the depth of foundation suitable for a given structure. 2. Evaluation of the load bearing capacity of the foundation. 3. Estimation of the probable settlement of a structure. 4. Determination of potential foundation problems (for example, expansive soil, collapsible soil, sanitary landfill, and so on). 5. Establishment of ground water table. 6. Prediction of lateral earth pressure for structures like retaining walls, sheet pile bulkheads, and braced cuts. 7. Establishment of construction methods for changing subsoil conditions. 17

EXPLORATION PROGRAM The purpose of the exploration program is to determine, within practical limits, the stratification and engineering properties of the soils underlying the site. The principal properties of interest will be the strength, deformation, and hydraulic characteristics. The program should be planned so that the maximum amount of information can be obtained 18 at minimum cost.

METHOD/PLANNING OF SITE INVESTIGATION 1. 2. 3. 4. 5. 6. 7. Desk Study n Compilation of the existing information regarding the structure. n Collection of existing information for the subsoil condition. Reconnaissance of the area Preliminary Assessment Detail site investigation Laboratory test – index properties, consolidation test, direct shear test, triaxial test, unconfined compression test, etc. Analysis of test results 19 Preparation of site investigation report.

Steps of subsurface exploration program (Desk Study) 1. Assembly of all available information on dimensions, column spacing, type and use of the structure, basement requirements, and any special architectural considerations of the proposed building. Foundation regulations in the local building code should be consulted for any special requirements. For bridges the soil engineer should have access to type and span lengths as well as pier loadings. This information will indicate any settlement limitations, and can be used to estimate foundation loads. 20

Steps of subsurface exploration program (Site Visit) 2. Reconnaissance of the area: This may be in the form of a field trip to the site which can reveal information on the type and behavior of adjacent structures such as cracks, noticeable sags, and possibly sticking doors and windows. The type of local existing structure may influence, to a considerable extent, the exploration program and the best foundation type for the proposed adjacent 21 structure.

Steps of subsurface exploration program 3. A preliminary site investigation: In this phase a few borings are made or a test pit is opened to establish in a general manner the stratification, types of soil to be expected, and possibly the location of the groundwater table. One or more borings should be taken to rock, or competent strata, if the initial borings indicate the upper soil is loose or highly compressible. This amount of exploration is usually the extent of the site investigation for small structures. 22

Steps of subsurface exploration program 4. A detailed site investigation: Where the preliminary site investigation has established the feasibility of the project, a more detailed exploration program is undertaken. The preliminary borings and data are used as a basis for locating additional borings, which should be confirmatory in nature, and determining the additional samples required. 23

Depth of Boring The approximate required minimum depth of the borings should be predetermined. The estimated depths can be changed during the drilling operation, depending on the subsoil encoun tered. 24

Depth of Boring When deep excavations are anticipated, the depth of boring should be at, least 1. 5 times the depth of excavation. Sometimes subsoil conditions are such that the foundation load may have to be transmitted to the bedrock. The minimum depth of core boring into the bedrock is about 3 m. If the bedrock is irregular or weathered, the core borings may have to be extended to greater depths. 25

Spacing Boring There are no hard and fast rules for the spacing of the boreholes. The following table gives some general guidelines for borehole spacing. These spacing can be increased or decreased, depending on the subsoil condition. If various soil strata are more or less uniform and predictable, the number of boreholes can be reduced. 26

Spacing Boring Approximate Spacing of Boreholes 27

SOIL BORING The earliest method of obtaining a test hole was to excavate a test pit using a pick and shovel. Because of economics, the current procedure is to use power excavation equipment such as a backhoe to excavate the pit and then to use hand tools to remove a block sample or shape the site for in situ testing. This is the best method at present for obtaining quality undisturbed samples or samples for testing at other than vertical orientation. 28

SOIL BORING 29

Boring tools Auger boring Power drills 30

Boring tools 31

Boring tools 32

Boring tools 33

Boring tools 34

35

SOIL SAMPLING Two types of soil samples can be obtained during sampling disturbed and undisturbed. The most important engineering properties required for foundation design are strength, compressibility, and permeability. Reasonably good estimates of these properties for cohesive soils can be made by laboratory tests on undisturbed samples which can be obtained with moderate difficulty. It is nearly impossible to obtain a truly undisturbed sample of soil; so in general usage the term "undisturbed" means a sample where some precautions have been taken to minimize disturbance or remolding effects. In this context, the quality of an "undisturbed" sample varies widely between soil laboratories. 36

Disturbed vs Undisturbed Good quality samples necessary. AR<10% soi l area ratio sampling tube Thicker the wall, greater the disturbance. 37

Common Sampling Methods 38

Common Sampling Methods 39

Common Sampling Methods 40

Common Sampling Methods 41

ROCK SAMPLING n n n Rock cores are necessary if the soundness of the rock is to be established. small cores tend to break up inside the drill barrel. Larger cores also have a tendency to break up (rotate inside the barrel and degrade), especially if the rock is soft or fissured. 42

Rock coring 43

ROCK SAMPLING Definition 44

Rock Core Drilling n n n Done with either tungsten carbide or diamond core bits Use a double or triple tube core barrel when sampling weathered or fractured rock Used to determine Rock Quality Designation core barrel 45

Rock Quality Designation RQD 46

Rock Quality Designation RQD Rock Quality Designation (RQD) is defined as the percentage of rock cores that have length equal or greater than 10 cm over the total drill length. 47

Example on Core Recovery & RQD n n Core run of 150 cm Total core recovery = 125 cm Core recovery ratio = 125/150 = 83% On modified basis, 95 cm are counted RQD = 95/150=63 % 48

GROUND WATER TABLE LEVEL n n Groundwater conditions and the potential for groundwater seepage are fundamental factors in virtually all geotechnical analyses and design studies. Accordingly, the evaluation of groundwater conditions is a basic element of almost all geotechnical investigation programs. Groundwater investigations are of two types as follows: Determination of groundwater levels and pressures. Measurement of the permeability of the subsurface materials. 49

FIELD STRENGTH TESTS The following are the major field tests for determining the soil strength: 1. Vane shear test (VST). 2. 3. Standard Penetration Test (SPT). Cone Penetration Test (CPT). 4. Borehole Pressuremeter Test. 50

Standard Penetration Test (SPT) 51

Standard Penetration Test (SPT) 52

Standard Penetration Test (SPT) 53

54

55

Geophysical Tests