Laboratory Testing of Soils, Rocks and Aggregates

by Nagaratnam Sivakugan, A. Arulrajah, M.W. Bo

Overview

Testing rocks and aggregates are rarely covered in soil testing books and there are no separate books on rock or aggregate testing. Laboratory Testing of Soils, Rocks and Aggregates includes laboratory testing methods for most tests for soils as well as rocks and aggregates, which are becoming increasingly common in professional practice and university teaching. Part A gives a general overview of laboratory measurements, equipment, units, safety and standards. Part B covers soil tests from grain size distribution to consolidation, triaxial and direct shear tests. Part C covers rock tests, which includes the indirect tensile strength test and point load test. Part D covers the common tests carried out routinely on aggregates, which includes the aggregate impact value test and Los Angeles abrasion test. Each test consists of the following descriptive parts: Objective, Standards, Introduction, Procedure, and Cost. References are made to ASTM International (ASTM), Australian (AS), British (BS) and International Society of Rock Mechanics (ISRM) standards and any differences are noted.

Product Details

ISBN-13:	9781604277142
Publisher:	Ross, J. Publishing, Incorporated
Publication date:	06/01/2011
Sold by:	INDEPENDENT PUB GROUP - EPUB - EBKS
Format:	eBook
Pages:	240
File size:	10 MB

About the Author

Dr. Nagaratnam Sivakugan is an associate professor and the head of Civil & Environmental Engineering in the School of Engineering and Physical Sciences at James Cook University, Townsville, Australia. He is a co-author of Geotechnical Engineering: A Practical Problem Solving Approach, a popular textbook adopted by many universities worldwide. He graduated from the University of Peradeniya, Sri Lanka, with first class honors and received his MSCE and PhD from Purdue University, Lafayette, Indiana. As a chartered professional engineer and registered professional engineer of Queensland, he does substantial consulting work, including extensive laboratory and in situ tests for geotechnical and mining companies throughout Australia as well as internationally. He is a Fellow of Engineers Australia. Dr. Sivakugan has supervised eight PhDs to completion and has published 65 scientific and technical papers in refereed international journals, 65 more in refereed international conference proceedings, and six book chapters. He serves on the editorial board of the International Journal of Geotechnical Engineering and is an active reviewer for more than 10 international journals. He developed a suite of geotechnical PowerPoint slideshows that is being used worldwide as an effective teaching and learning tool. Dr. A. Arulrajah is an associate professor in geotechnical engineering at Swinburne University of Technology, Melbourne, Australia. He completed his BSc in civil engineering at Purdue University, Lafayette, Indiana, in 1992; MEngSc at the University of Malaya, Malaysia, in 2003 and PhD from Curtin University, Australia, in 2005. Dr. Arulrajah is a Fellow of Engineers Australia, and a Chartered Professional Engineer in Australia. Prior to his first academic appointment in 2006, he worked for 14 years for various engineering consultants in Australia, Singapore, and Malaysia. Dr. Arulrajah was the first author of a journal paper on ground improvement that won the Telford Premium Prize (2009) from the Institution of Civil Engineers, United Kingdom. He is also the recipient of the Shamsher Prakash prize for excellence in the practice of geotechnical engineering (2010) and the recipient of three Swinburne University Vice-Chancellor Awards. He is the author of nearly 40 scientific and technical papers in refereed international journals and 40 more in refereed international conference proceedings. He has supervised three PhD candidates to completion in these areas of research. He is an editorial board member of the International Journal of Geotechnical Engineering and is an active reviewer for several international journals. Dr. Myint Win Bo (BO Myint Win) is a senior principal/director (Geo-Services) at DST Consulting Engineers, Canada. He graduated with a BSc (geology) from the University of Rangoon and received a postgraduate diploma in hydrogeology from University College, London and an M.Sc degree from the University of London. He obtained his PhD in civil engineering (specializing in geotechnics) from the Nanyang Technological University, Singapore. He is a Fellow of the Geological Society, London, and a Fellow of the Institution of Civil Engineers, UK. He is also a professional engineer, professional geoscientist, international professional engineer, United Kingdom, and a chartered geologist, scientist, engineer, environmentalist, European geologist, and European engineer. Dr. Bo is also serving in several national and international professional societies as a committee member. Dr. Bo is an experienced practicing engineer as well as educator, and he has been giving several special lectures and workshops at international conferences, tertiary institutions, and professional associations. Additionally, Dr. Bo is an adjunct professor at the University of Ottawa and Lakehead University in Canada as well as adjunct professor at the Swinburne University of Technology in Australia. He is the author of more than 50 scientific and technical

Read an Excerpt

CHAPTER 1

Part A

Introduction

Laboratory testing of soils, rocks, and aggregates is an integral part of the geotechnical design. The design parameters are derived from laboratory and in situ testing of the geomaterials. During the site investigation program, when the in situ tests (e.g., standard or cone penetration) are being carried out, it is a common practice to take samples from the ground at various locations for further laboratory tests.

The laboratory tests have certain advantages over in situ tests that include:

• Well defined boundary conditions that can also be controlled

• More rational interpretation

• Higher degree of accuracy in the measurements

On the other hand, in situ tests are quicker, test a larger volume of soil, and are relatively inexpensive. However, their boundary conditions are not well defined, and their interpretation is often empirical or semi-empirical. Does it make one better than the other? Not really. A good site investigation program will include both in situ and laboratory tests that complement each other. The use of one should not be at the expense of the other.

DISTURBED AND INTACT SAMPLES

The grains of a gravelly, sandy, or silty soil are equidimensional, where the dimensions in all three mutually perpendicular directions are of the same order of magnitude. In addition, they are nonplastic and noncohesive. Their packing density is measured by relative density (Dr) defined as:

Dr = emax - e/emax - emax x 100 (A.1)

where emax = void ratio at the loosest state and emin = void ratio at the densest state (e = current void ratio at which the relative density is computed). Dr varies between 0 and 100%. At the loosest state, it is 0 and at the densest state it is 100%. Classification of granular soils based on packing density of the grains is shown in Figure A.1. The behavior of a granular soil subjected to external loading is highly dependent on the grain size distribution, relative density, and the angularity of the soil grains.

Clay particles are one- or two-dimensional with shapes of needles or flakes and have a large specific surface (i.e., surface area per unit mass, measured in m2/g). For example, montmorillonite clays can have a specific surface of 800 m2/g. They are electrically charged with a net negative charge. In the presence of water, they are sticky and can exhibit plasticity and cohesion. Clayey soils can be classified on the basis of their plasticity and unconfined compressive strength as shown in Figures A.2 and A.3, respectively. Undrained shear strength (cu) is half of the unconfined compressive strength (qu). The term relative density is not applicable to clays.

Undisturbed samples literally means that the samples have the same characteristics (e.g., temperature and stresses) as in the in situ state. It is impossible to remove material from the ground without causing a disturbance to at least one of these characteristics. Intact samples, on the other hand, are the ones obtained using the best possible methods to minimize disturbance and still acknowledge that there is a degree of disturbance. The terms samples and specimens are misused commonly in practice. Samples are what we collect from the site, and from these samples we select specimens for specific laboratory tests. Specimen is a subset of sample.

It is difficult to collect good quality intact or undisturbed samples in granular soils. If required, there are some special techniques such as ground freezing, using resins, or attaching a core catcher to the sampling tube, among others. When laboratory tests are required on granular soils, it is common to perform them on reconstituted samples. Here, the granular soil grains are repacked to densities that are representative of the field samples and hence replicate the field situation. Reconstituted cohesive soil specimens can be prepared in the Harvard miniature compaction device (Wilson 1970). This device provides a kneading action on the soil placed in layers and simulates compaction by a sheepsfoot roller. It can be used in the laboratory to produce specimens that can be tested directly in a triaxial or uniaxial compression setup without further preparation. Reconstituted clay samples can also be prepared by sedimenting the clay slurry, typically mixed at a water content of 1.5 to 2.5 times the liquid limit.

In cohesive soils, good quality intact samples are required for laboratory tests such as triaxial, direct shear, consolidation, and permeability tests. They can be obtained from boreholes or trial pits. The samples from boreholes are recovered from thin walled Shelby tubes™ or special samplers such as a piston sampler. Piston samplers are effective in very soft clays and organic soils such as peat. Hvorslev (1949) introduced area ratio AR as an indirect measure of the degree of mechanical disturbance that can be expected. It is defined as:

AR = D2o - D2i/D2i x 100 (A.2)

where Di and Do are the inner and outer diameter of the sampler. For a good quality intact sample, AR should be less than 10%. In very stiff or hard strata, it may be necessary to use tubes with greater wall thicknesses and, thus, significantly higher values of AR. It is common to see double or triple tube core barrel samplers used in very stiff or hard soils for obtaining good quality intact samples. Hvorslev suggested that the length of the intact sample be limited to 10 to 20 times the diameter of the tube in cohesive soils. ASTM International (ASTM) D1587 suggests the maximum length to be 910 mm (36 in) for 50 to 75 mm (2 to 3 in) diameter tubes, and 1450 mm (57 in) for 127 mm (5.0 in) diameter tubes. The sampling tubes are made of mild, galvanized, or stainless steel, or brass. When sampling in environmental projects, to avoid chemical reactions with the metal, epoxy-coated steel or plastic liners can be used.

Sample disturbance occurs due to two separate factors, namely, mechanical disturbance and stress-relief. Mechanical disturbance is caused by the drilling equipment and the sampler that is inserted into the borehole. As seen in Equation A.2, the mechanical disturbance increases with the wall thickness. As a result, the outer annular region in the sample recovered from the borehole can be disturbed. Under such circumstances, it is a good practice to discard the annular region and use the core from the center. To overcome the effects of stress relief, the sample can be reconsolidated to the estimated in situ overburden stress. Block samples can be recovered from a wall or base of an excavation or trial pit.

Disturbed or remolded samples are adequate for the soil classification, Atterberg limits, water content, and specific gravity determination. They can also be used for compaction and California Bearing Ratio tests if available in sufficient quantity. The split-spoon sampler used in a standard penetration test has AR in excess of 100%, and hence the samples are highly disturbed, rendering them suitable only for visual identification purposes. The split-spoon sampler can be equipped with a liner that will hold the samples intact. The liner can then be sealed and waxed at the ends before it is transported to the laboratory to preserve the natural water content and avoid oxidation. Sealable plastic bags and glass jars with air-tight lids are useful for preserving the samples at their natural water content. When filled with samples, they have to be properly labeled indicating the project, geographic location, sample depth, date, and other relevant information.

For safety reasons, it is a common practice to fill the borehole or trial pit with soil once the sampling and the associated in situ tests are completed.

ACCURACY, PRECISION, AND RESOLUTION

Let's define some simple terms associated with laboratory measurements. Resolution is the smallest change the measuring device can display (e.g., 0.01 g in a digital balance). In a digital display, the resolution is simply one digit of change in the last digit. The term accuracy can be split into two components: precision and bias. Precision is a measure of scatter about an average value of the measurements that need not be close to the true value. The difference between this average of the measured values and the true value is known as the bias of the instrument. Sensitivity refers to the response of a device to a unit input (e.g., 100 milli volts per mm movement of a linear variable differential transformer). Sensitivity applies to measuring devices such as transducers, load cells, or dial gages; resolution applies to the readout or display devices. Repeatability is slightly different from reproducibility. Repeatability is a qualitative measure of the variability between test results when the tests are repeated on identical specimens by the same person in the same laboratory under the same conditions. Reproducibility is a measure of variability between the test results when the tests are repeated in different laboratories, by different operators using different equipment.

The test measurements and computed results should be reported to appropriate significant digits. When reporting the mass of a soil sample as 142.3 g, there are four significant digits. Reporting too many digits can give a false sense of accuracy and demonstrates a lack of appreciation for the quality of measurements. The significant digits should be based on the sensitivity of the instrument, resolution of the measuring device, specimen size, and measured quantity. In geotechnical laboratory testing, it is sometimes specified by the relevant standards. For example, liquid or plastic limit and plasticity index are generally rounded to the nearest integers. Densities (in t/m or Mg/m) and specific gravities are reported to 0.01.

SOME LABORATORY DEVICES

In this section, we discuss some common laboratory devices that are used frequently in the laboratory testing of soils, rocks, and aggregates.

Length Measuring Devices

Length measurement devices used in a geotechnical laboratory include a meter stick, ruler, measuring tape, Vernier calipers, micrometers, dial gages, and LVDTs, among others. Some of these are shown in Figure A.4. Vernier calipers and micrometers are used for precise one-off measurements of dimensions such as inner and/or outer diameters of a cylinder. Dial gages and LVDTs are used for precise continuous measurements of displacements or deformations during a test.

Mass Measuring Devices

Water content is one of the most common measurements in the geotechnical laboratory. It is carried out as a part of most laboratory tests and when collecting field samples on samples as small as 20 g in mass. A good balance (Figure A.5) capable of measuring the mass to 0.01 g resolution is preferable. This includes top pan balances and analytical balances. When dealing with larger soil samples, it may be necessary to use a coarse balance such as a semi-self-indicating scale or a heavy platform scale with lesser resolution.

Frequently, there is a requirement to measure the load acting on a specimen rather precisely. It is necessary to monitor the loads throughout the tests, especially in strength tests where the specimens are tested to failure. Load cells and proving rings are some of the common devices that are used for this purpose (Figure A.6). Pressures are generally measured by transducers.

Drying Equipment

To determine water contents and to dry soils in preparation for laboratory tests, it is necessary to dry out the soils completely. Laboratory drying ovens are useful for this purpose. They are usually set at 105 to 110°C for inorganic soils and somewhat lower for organic soils. Drying for 16 to 24 hours is usually sufficient. By plotting the dry mass with time, this can be verified. When the difference between successive weighings is less than 0.1% of the dry mass, it can be assumed that the soil has reached a constant mass. Metal trays, water content tins, and weighing bottles are suitable for drying soils in the oven. They should be transferred to the desiccator and cooled without absorbing any moisture before placing on a weighing scale or balance.

Desiccator

A desiccator (Figure A.7) is a glass enclosure that contains a desiccant (e.g., anhydrous silica gel) that keeps the air within the desiccator dry. It is used to cool a sample without absorbing moisture when removed from the oven. With its airtight seal, it can be used to contain the samples when applying vacuum. The silica gel, in the form of crystals, can be spread in a dish and kept below the perforated floor of the desiccator. The self-indicating crystals are blue in color when dry and slowly become colorless when they absorb moisture. By heating them in the oven at 110°C, they can be dried and reused. There are also cheaper nonindicating desiccants that do not change color. Desiccators can be useful for containing the soil specimens while de-airing through application of vacuum.

Constant Temperature Baths

For laboratory tests (e.g., hydrometer tests) that require a constant temperature environment for a long period of time, a constant temperature bath is useful.

Vacuum Pump

Often the laboratory tests are carried out on saturated soil specimens. De-airing water, soils, and soil-water mixes can be accomplished by applying vacuum and/or by heating. Remember that atmospheric pressure at sea level is 1 atm (= 760 mm Hg = 101.325 kPa = 1.01325 bar). Therefore, theoretically, the maximum vacuum one can apply is 101.325 kPa. A vacuum pump with a pressure gage for measuring the vacuum is a useful device in a geotechnical laboratory. Vacuum is commonly applied to specimens placed in a desiccator that can be covered by a protective cage for safety reasons.

Water Purification System

Tap water contains dissolved ions and bacteria that can react with the soil samples that are being tested. Therefore, it is necessary to use some purer forms of water such as distilled or deionized water for the laboratory tests. Distilled water has all of the impurities removed by boiling the water and condensing the steam. Deionized water has all of the mineral ions removed by running the water through a series of filters. In most soil testing systems, the tests are carried out on saturated specimens. To ensure no air is introduced into the system, de-aired distilled or deionized water is used. De-aired water does not have any dissolved air in it. De-airing can be carried out by applying vacuum to the water in a suitable vessel that can resist the atmospheric pressure from outside.

Wax Pot And Wax

Wax is effectively used for sealing samples in tubes or other containers, protecting them against any moisture changes until the tests are carried out. To avoid damage to the samples, wax with a low melting point is preferable. Wax with a high melting point tends to be more brittle and, hence, is prone to cracking. Paraffin is the most common wax that is used for sealing sampling tubes. It is inexpensive and is available from hardware stores. Being relatively brittle, this can crack after a few days and destroy the seal. Germaine and Germaine (2009) and Lambe (1981) recommend mixing paraffin with petroleum jelly in a 1:1 ratio. An electrically heated wax pot with thermostat control can maintain the molten wax at the right temperature while coating the samples and sealing tubes. A wire brush to apply the wax and a ladle for collecting wax from the wax pot are other items that can help in the use of a wax pot and wax. Samples can also be waxed by dipping them in the molten wax. Wax should not be heated to more than a few degrees above its melting point. Overheating can affect the sealing properties and the wax can become brittle.

STANDARDS

The laboratory and field tests are generally conducted according to some standards that are developed by specialists in the area. Some of these specialists are ASTM International (ASTM), International Standardisation Office (ISO), American Association of State Highway and Transportation Officials (AASHTO), Australian Standards (AS), and British Standards (BS).

ASTM standards (http://www.astm.org) cover a wide range of materials, including steel, concrete, soils, rocks, petroleum, paints, and water. The standards are published in more than 80 volumes. Luckily, all that concerns us are available in two volumes, each of which has more than 1000 pages. The volumes are also available separately on CDs. ASTM standards for soils, aggregates, and rocks are updated regularly by Committee D18 and are published in two volumes 04.08 and 04.09 that come under Section 4 — Construction.

(Continues…)

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Table of Contents

Preface,
About the Authors,
WAV™,
Test No. Description,
Part A: Introduction,
Disturbed and Intact Samples,
Accuracy, Precision, and Resolution,
Some Laboratory Devices,
Standards,
Units.,
Laboratory Report,
Safety,
Further Reading,
Part B: Soil Testing,
Introduction,
Standards,
B1 Visual Identification and Classification of Coarse Grained Soils,
B2 Visual Identification and Classification of Fine Grained Soils,
B3 Water Content,
B4 Specific Gravity of Soil Grains,
B5 Sieve Analysis,
B6 Hydrometer Analysis,
B7 pH,
B8 Organic Content,
B9 Liquid LimitCasagrande's Percussion Cup Method,
B10 Liquid LimitFall Cone Method,
B11 Plastic Limit,
B12 Linear Shrinkage,
B13 Compaction Test,
B14 Maximum and Minimum Densities of a Granular Soil,
B15 Field Density Test,
B16 Hydraulic Conductivity of a Coarse Grained Soil,
B17 Hydraulic Conductivity of a Fine Grained Soil,
B18 One-dimensional Consolidation by Incremental Loading,
B19 Direct Shear Test,
B20 Consolidated Undrained Triaxial Test,
B21 Unconsolidated Undrained Triaxial Test,
B22 Unconfined Compression Test,
Part C: Rock Testing,
Introduction,
C1 Water Content,
C2 Density and Porosity,
C3 Uniaxial Compressive Strength,
C4 Point Load Test,
C5 Indirect (Brazilian) Tensile Strength,
C6 Schmidt Hammer Rebound Hardness,
C7 Slake Durability Test,
C8 Triaxial Test on a Rock Specimen,
C9 Direct Tensile Strength,
Part D: Aggregate Testing,
Introduction,
D1 Water Absorption of Aggregates,
D2 Flakiness Index,
D3 Fines Content,
D4 Aggregate Impact Value (AIV),
D5 California Bearing Ratio (CBR),
D6 Large Direct Shear Box Test,
D7 Los Angeles Abrasion Loss,
Part E: References,
References,