Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

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Chapter 4 LABORATORY TESTING FOR SOILS 4.4 PRINCIPLES OF EFFECTIVE STRESS The consolidation process demonstrates the very important principle of effective stress, which will be used in all the remaining modules of this course. Under an applied load, the total stress in a saturated soil sample is composed of the inter-granular stress and porewater pressure (neutral stress). As the porewater has zero shear strength and is considered incompressible, only the inter-granular stress is effective in resisting shear or limiting compression of the soil sample. Therefore, the inter-granular contact stress is called the effective stress. Simply stated, this fundamental principle states that the effective stress (σ’) on any plane within a soil mass is the net difference between the total stress (σ t ) and porewater pressure (u). When pore water drains from soil during consolidation, the area of contact between soil grains increases, which increases the level of effective stress and therefore the soil’s shear strength. In practice, staged construction of embankments is used to permit increase of effective stress in the foundation soil before subsequent fill load is added. In such operations the effective stress increase is frequently monitored with piezometers to ensure the next stage of embankment can be safely placed. Soil deposits below the water table will be considered saturated and the ambient pore pressure at any depth may be computed by multiplying the unit weight of water (γ w ) by the height of water above that depth. For partially saturated soil, the effective stress will be influenced by the soil structure and degree of saturation (Bishop, et. al., 1960). In many cases involving silts and clays, the continuous void spaces that exist in the soil behave as capillary tubes of variable cross-section. Due to capillarity, water may rise above the static groundwater table (phreatic surface) as a negative porewater pressure and the soils may be nearly or fully saturated. 4.5 OVERBURDEN STRESS The purpose of laboratory testing is to simulate in-situ soil loading under controlled boundary conditions. Soils existing at a depth below the ground surface are affected by the weight of the soil above that depth. The influence of this weight, known generally as the overburden stress, causes a state of stress to exist which is unique at that depth for that soil. When a soil sample is removed from the ground, that state of stress is relieved as all confinement of the sample has been removed. In testing, it is important to re-establish the in-situ stress conditions and to study changes in soil properties when additional stresses representing the expected design loading are applied. In this regard, the effective stress (grain-to-grain contact) is the controlling factor in shear, state of stress, consolidation, stiffness, and flow. Therefore, the designer should try to re-establish the effective stress condition during most testing. The test confining stresses are estimated from the total, hydrostatic, and effective overburden stresses. The engineer’s first task is determining these stress and pressure variations with depth. This involves determining the total unit weights (density) for each soil layer in the subsurface profile, and determining the depth of the water table. Unit weight may be accurately determined from density tests on undisturbed samples or estimated from in-situ test measurements. The water table is routinely recorded on the boring logs, or can be measured in open standpipes, piezometers, and dissipation tests during CPTs and DMTs. The total vertical (overburden) stress (σ vo ) at any depth (z) may be found as the accumulation of total unit weights ( t ) of the soil strata above that depth: σ vo = t dz= ∑ t ∆z (4.1) March 2009 4-3
Chapter 4 LABORATORY TESTING FOR SOILS For soils above the phreatic surface, the applicable value of total unit weight may be dry, moist, or saturated depending upon the soil type and degree of capillarity (see Table 4.2). For soil elements situated below the groundwater table, the saturated unit weight is normally adopted. The hydrostatic pressure depends upon the degree of saturation and level of the phreatic surface and is determined as follow: Soil elements above water table: u o = 0 (Completely dry) (4.2a) = w (z-z w ) (Full capillarity) (4.2b) Soils elements below water table: u o = w (z-z w ) (4.2c) where z = depth of soil element, z w = depth to groundwater table. Another case involves partial saturation with intermediate values between (4.2a and 4.2b) which literally vary daily with the weather and can be obtained via tensiometer measurements in the field. Usual practical calculations adopt (4.2a) for many soils, yet the negative capillary values from (4.2b) often apply to saturated clay and silt deposits. The effective vertical stress is obtained as the difference between (4.1) and (4.2): σ vo ’ = σ vo - u o (4.3) A plot of effective overburden profile with depth is called a ’ v diagram and is extensively used in all aspects of foundation testing and analysis (see Holtz & Kovacs, 1981; Lambe & Whitman, 1979). 4.6 TESTS FOR GEOTECHNICAL PARAMETERS A wide range of tests has been used to determine the geotechnical parameters required in calculations for example, of bearing capacity, slope stability, earth pressure and settlement. Geotechnical calculations remain almost entirely semi-empirical in nature; it has been said that when calculating the stability of a slope one uses the ‘wrong’ slip circle with the ‘wrong’ shear strength to arrive at a satisfactory answer. For this reason testing requirements differ considerably from region to region. The new British Standard (BS 1377:1990.) is divided into nine separate parts: Part 1 Part 2 Part 3 Part 4 Part 5 Part 6 Part 7 Part 8 Part 9 General requirements and sample preparation Classification tests Chemical and electro-chemical tests Compaction-related tests Compressibility, permeability and durability tests Consolidation and permeability tests in hydraulic cells and with pore pressure measurement Shear strength tests (total stress) Shear strength tests (effective stress) In situ tests. 4-4 March 2009
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GOVERNMENT OF MALAYSIA DEPARTMENT O
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DID MANUAL Volume 6 Foreword The fi
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DID MANUAL Volume 6 List of Volumes
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DID MANUAL Volume 6 Registration of
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DID MANUAL Volume 6 List of Symbols
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CHAPTER 1 GENERAL
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Chapter 2 GEOTECHNICAL DESIGN PROCE
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CHAPTER 3 FUNDAMENTAL PRINCIPLES
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CHAPTER 7 RETAINING WALL
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PART 2: SOIL INVESTIGATION
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PART 3: ENGINEERING SURVEY
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