BS1377 Methods of Test for Soils for Civil Engineering Purposes

posted Jun 16, 2018, 8:07 PM by jeffery jim

BS1277: 1990 consist of 9 Parts: Part 1: General requirements and sample preparation; Part 2: Classification tests; Part 3: Chemical and electro-chemical tests; Part 4: Compaction-related tests; Part 5: Compressibility, permeability and durability tests; Part 6: Consolidation and permeability tests in hydraulic cells and with pore pressure measurement; Part 7: Shear strength tests (total stress); Part 8: Shear strength tests (effective stress); Part 9: In-situ tests.

 

3.1.1 Part 1. General requirements and sample preparation

Part 1 of this standard contains general information relating to the tests, common calibration and specification requirements and general requirements for testing laboratories and field work. It also includes details of procedures for the preparation of disturbed and undisturbed samples, where these are common to more than one type of test.

 

3.1.2 Part 2. Classification Tests

Part 2 describes tests for the classification of soils according to moisture content, Atterberg limits, density, particle density and particle size distribution. No changes in principle have been made in the test procedures, but some additional tests are included. In the preparation of cohesive soils for testing, the requirement of using the soil in its natural state, without drying, has been introduced. The main additions and amendments are as follows.

a) Moisture content. Determination of the saturation moisture content of chalk has been added. The alcohol method and the sand bath method have been deleted.

b) Liquid limits. A one-point cone penetration test has been added.

c) Shrinkage limit. This is an addition, and two procedures are given.

d) Bulk density. Determination by direct measurement has been added.

e) Particle density (previously specific gravity). The pyknometer jar method has been reintroduced as a site

f) Particle size distribution (sedimentation). Procedures have been rationalized and amended where necessary. Pretreatment is not now a requirement.

Summary of Part 2

Determination of moisture content: soil is dry when water is removed not more than 110⁰C using oven-drying method (105-110⁰C).

The liquid limit is the empirically established moisture content at which a soil passes from the liquid state to the plastic state. It provides a means of classifying a soil, especially when the plastic limit (see clause 5) is also known. Two main types of test are specified. The first is the cone penetrometer method, which is fundamentally more satisfactory than the alternative because it is essentially a static test depending on soil shear strength. It is also easier to perform and gives more reproducible results. The second is the much earlier Casagrande type of test which has been used for many years as a basis for soil classification and correlation of engineering properties. This test introduces dynamic effects and is more susceptible to discrepancies between operators.  Wherever possible the test shall be carried out on soil in its natural state. With many clay soils it is practicable and shall be permissible to remove by hand any coarse particles present.

The plastic limit is the empirically established moisture content at which a soil becomes too dry to be plastic. It is used together with the liquid limit to determine the plasticity index which when plotted against the liquid limit on the plasticity chart (see BS 5930) provides a means of classifying cohesive soils. It is recognized that the results are subject to the judgment of the operator, and that some variability in results will occur.

Shrinkage due to drying is significant in clays but less so in silts and sands. These tests enable the shrinkage limit, ws, of clays to be determined, i.e. the moisture content below which a clay ceases to shrink. They also provide ways of quantifying the amount of shrinkage likely to be experienced by clays, in terms of the shrinkage ratio, volumetric shrinkage and linear shrinkage. These factors are also relevant to the converse condition of expansion due to wetting. Three types of test are specified. The first is the definitive method in which volumetric measurements are made on a cylindrical specimen, usually of undisturbed soil, as it is allowed to dry. The second is the subsidiary method, in which disturbed soil is mixed to a paste with water to form a small pat for the same purpose. Both procedures enable the shrinkage limit of the soil, the shrinkage ratio, and the volumetric shrinkage for a given change of moisture content, to be determined. In the third test only the total linear shrinkage of a soil paste is measured. The second and third procedures are carried out on the fraction of the soil sample passing a 425 μm test sieve.

This method covers the determination of the linear shrinkage of the fraction of a soil sample passing a 425 μm test sieve from linear measurements on a bar of soil.

In this standard, density is expressed in terms of mass density. The bulk density of a soil, r, is the mass per unit volume of the soil deposit including any water it contains. The dry density, rd, is the mass of dry soil contained in a unit volume. Both are expressed in Mg/m3, which is numerically the same as g/cm3. Three methods are specified. The first applies to soils that can be formed into a regular geometric shape (linear measurement method), the volume of which can be calculated from linear measurements. In the second the volume of the specimen is determined by weighing it submerged in water (immersion in water method). In the third the volume is measured by displacement of water (water displacement method).

In this standard the term particle density is used instead of the term specific gravity, which was used in previous editions of this standard, to comply with current usage in other standards. It is denoted by the symbol rs. In this standard particle density is quoted in Mg/m3, which is numerically equal to the specific gravity. Three methods are described. The first is a gas jar method suitable for most soils including those containing gravel-sized particles. The second is the small pyknometer method which

is the definitive method for soils consisting of clay, silt and sand-sized particles. The third is a large pyknometer method, suitable for soils containing particles up to medium gravel size. The last is less accurate than the other two and is more suitable as a site test or when a result of lower accuracy is acceptable.

Determination of particle size distribution. Two methods of sieving are specified. Wet sieving is the definitive method applicable to essentially cohesionless soils. Dry sieving is suitable only for soils containing insignificant quantities of silt and clay. Two methods of determining the size distribution of fine particles down to the clay size by sedimentation are specified, namely the pipette method and the hydrometer method, in both of which the density of the soil suspension at various intervals is measured. Combined sieving and sedimentation procedures enable a continuous particle size distribution curve of a soil to be plotted from the size of the coarsest particles down to the clay size.

 

3.1.3 Part 3. Chemical and electro-chemical Test

Part 3 describes chemical tests on soils and on water. Existing test procedures have been retained, with some modification, and additional tests have been included for the determination of the following.

a) Loss on ignition; b) sulphate content of soil and ground water; c) Carbonate content; d) Chloride content; e) Total dissolved solids; f) pH value; g) resistivity; h) redox potential

Tests have also been included for the assessment of the corrosivity of soils; these are the determination of the electrical resistivity and of the redox potential. In-situ methods of these two tests are given in Part 9.

 

3.1.4 Part 4. Compaction-related test

Part 4 describes those tests that refer in some way to the compaction of soils. These include existing procedures for determining compaction parameters; additional tests for measurement of the limiting densities of non-cohesive soils; and tests which are related to the control and behaviour of soil placed in-situ as fill, comprising the CBR test and two procedures which have been added. These are the moisture condition test, and the chalk crushing value test, both of which require use of the same apparatus. Attention has been given to several methods of sample preparation appropriate to different soil types prior to compaction tests and the compaction of samples for the CBR test.

Summary of Part 4

Determining dry density and moisture content relationship: For a given degree of compaction of a given cohesive soil there is an optimum moisture content at which the dry density obtained reaches a maximum value. For cohesionless soils optimum moisture content might be difficult to define. The objective of the tests described in this clause is to obtain relationships between compacted dry density and soil moisture content, using two magnitudes of manual compactive effort, or compaction by vibration. Three types of compaction test are described, each with procedural variations related to the nature of the soil. The first is the light manual compaction test in which a 2.5 kg rammer is used. The second is the heavy manual compaction test which is similar but gives a much greater degree of compaction by using a 4.5 kg rammer with a greater drop on thinner layers of soil. If there is a limited amount of particles up to 37.5 mm size, equivalent tests are carried out in the larger California Bearing Ratio (CBR) mould. NOTE 2: If more than 30 % of material is retained on a 20 mm test sieve the material is too coarse to be tested. The third type of test makes use of a vibrating hammer, and is intended mainly for granular soils passing a 37.5 mm test sieve, with no more than 30 % retained on a 20 mm test sieve. The soil is compacted into a CBR mould. NOTE 1: The amount of water to be mixed with soil at the commencement of the test will vary with the type of soil under test. In general, with sandy and gravelly soils a moisture content of 4 % to 6 % would be suitable, while with cohesive soils a moisture content about 8 % to 10 % below the plastic limit of the soil would usually be suitable.

Determination of maximum and minimum dry densities for granular soil: An indication of the state of compaction of a cohesionless (free-draining) soil is obtained by relating its dry density to its maximum and minimum possible densities (the limiting densities). The tests described in this section enable these parameters to be determined for cohesionless soils. Two tests are described for the determination of maximum density, one for sands and one for gravelly soils. In both tests the soil is compacted under water with a vibrating hammer.

Determination of the moisture condition value: The procedures cover the determination of the moisture condition value (MCV) of a sample of soil and the determination of the variation of MCV with changing moisture content. A rapid procedure for assessing whether or not a sample of soil is stronger than a precalibrated standard is also included.

Determination of the chalk crushing value: The chalk crushing value (CCV) is determined by using the chalk impact crushing test, which measures the rate at which a sample of chalk lumps crushes under impacts from a free-falling rammer. The chalk crushing value can be used, together with the saturation moisture content of the intact chalk lumps (see 3.3 of BS 1377-2:1990), to classify chalk in relation to its behaviour as a freshly placed fill material.

Determination of the California Bearing Ratio (CBR): Principle. This method covers the laboratory determination of the California Bearing Ration (CBR) of a compacted or undisturbed sample of soil.

The principle is to determine the relationship between force and penetration when a cylindrical plunger of a standard cross-sectional area is made to penetrate the soil at a given rate. At certain values of penetration the ratio of the applied force to a standard force, expressed as a percentage, is defined as the California Bearing Ratio (CBR).

 

3.1.5 Part 5. Compressibility, permeability and durability tests

Part 5 describes test procedures in which the presence or drainage or flow of water within the pore spaces of the soil is the significant factor, but without requirinq the measurement of pore water pressure. These include the one-dimensional oedometer consolidation test, which incorporates some additional requirements. Tests for determining the swelling and collapsing characteristics have been added. Further additional test procedures are as follows.

a) Determination of soil permeability (constant-head method).

b) Determination of erodibility and dispersive characteristics of fine-grained soils.

c) Determination of potential frost heave for which reference is made to BS 812-124.

Summary of Part 5

Determination of the one-dimensional consolidation properties: This method covers the determination of the magnitude and rate of the consolidation of a saturated or near-saturated specimen of soil (see note 1) in the form of a disc confined laterally, subjected to vertical axial pressure, and allowed to drain freely from the top and bottom surfaces. The method is concerned mainly with the primary consolidation phase, but it can also be used to determine secondary compression characteristics. The compressibility characteristics may be illustrated by plotting the compression of the specimen as ordinate on a linear scale against the corresponding applied pressure p (in kP/Pa), as abscissa on a logarithmic scale. Other information made available are; Coefficient of consolidation and Coefficient of secondary compression.

Determination of swelling and collapse characteristics: The tests comprise the following; a) Measurement of swelling pressure. For a soil which has a swelling capability when allowed access to water, the swelling pressure, ps, is the vertical pressure on the specimen in an oedometer ring required to prevent it swelling. The swelling pressure is usually the starting point and finishing point for the series of pressures applied to a soil of this type in a consolidation test. b) Measurement of swelling. This test enables the swelling characteristics of a laterally confined soil specimen to be measured when it is unloaded from the swelling pressure in the presence of water. c) Measurement of settlement on saturation. In this test the amount by which an unsaturated laterally confined specimen settles due to structural collapse on the addition of water is determined.

Determination of permeability by the constant-head method: The permeability of a soil is a measure of its capacity to allow the flow of water through the pore spaces between solid particles. The degree of permeability is determined by applying a hydraulic pressure gradient in a sample of saturated soil and measuring the consequent rate of flow. The coefficient of permeability is expressed as a velocity. Permeability tests on undisturbed samples using triaxial cell and hydraulic consolidation cell apparatus are described in BS 1377-6:1990. The test procedure described in this clause covers the determination of the coefficient of permeability using a constant-head permeameter in which the flow of water through the sample is laminar. The volume of water passing through the soil in a known time is measured, and the hydraulic gradient is measured using manometer tubes.

Determination of dispersibility: Certain fine-grained soils that are highly erodible are referred to as dispersive soils. Dispersive soils cannot be identified by means of conventional soil classification tests, but the qualitative tests described below enable them to be recognized. However, it does not follow that soils classified by these tests as non-dispersive are not susceptible to erosion in some circumstances. These methods are not applicable to soils with a clay content of less than 10 % and with a plasticity index less than or equal to 4. Three tests are described as follows; a) The pinhole test, in which the flow of water under a high hydraulic gradient through a cavity in the soil is reproduced. b) The crumb test, in which the behaviour of crumbs of soil in a static dilute sodium hydroxide solution is observed. c) The dispersion method (double hydrometer test), in which the extent of natural dispersion of clay particles is compared with that obtained with the use of standard chemical and mechanical dispersion.


3.1.6 Part 6. Consolidation and permeability test on hydraulic cells and with pore pressure measurement

Part 6 is a major addition to this standard. It describes tests for the determination of consolidation and permeability parameters using equipment in which the measurement of pore water pressure is an essential feature. These comprise the following.

a) Determination of consolidation properties in a hydraulic consolidation cell. For samples of large diameter, either vertical or horizontal (radial) drainage can be used.

b) Determination of consolidation properties in a triaxial cell under isotropic conditions.

Summary of Part 6

Two types of equipment are used: (a) hydraulically loaded one-dimensional consolidation cell; (b) a triaxial consolidation cell. Consolidation or triaxial cells of large diameter enable large specimens to be tested so that some account can be taken of the effects of the soil fabric.

Definitions:  Diaphragm pressure of a hydraulic - consolidation cell the pressure applied to the fluid above the flexible loading diaphragm. Applied total stress - the mean pressure actually transmitted to thesurface of the specimen. Free strain loading - application of a uniformly distributed pressure to the surface of the specimen from the flexible diaphragm. Equal strain loading - application of pressure to the surface of the specimen through a rigid disc so that the surface always remains plane. Pore pressure ratio - the ratio of the incremental change in pore pressure to the applied increment of vertical stress when drainage is not allowed. Cell pressure (σ3) - the pressure of the cell fluid which applies isotropic stress to the specimen in a triaxial cell. Back pressure (ub) - pressure applied directly to the pore fluid in the specimen voids. Effective cell pressure - the difference between the cell pressure and pore water pressure. Effective consolidation pressure (σ’3 ) - the difference between the cell pressure and the back pressure against which the pore fluid drains during the consolidation stage. Pore pressure coefficients A and B - changes in total stresses applied to a specimen when no drainage is permitted produces changes in the pore pressure in accordance with the equation.

Determination of consolidation properties using a hydraulic cell - These procedures cover the determination of the magnitudes and rates of consolidation of soil specimens of relatively low permeability using hydraulically loaded apparatus. They provide a convenient means of testing large specimens, and enable drainage in either the horizontal or vertical directions to be investigated. The specimen is in the form of a cylinder confined laterally, subjected to vertical axial pressure applied hydraulically. Types of test. In this type of cell, pressure may be applied to the surface of the specimen either directly from the flexible diaphragm (giving a uniform stress distribution, the “free strain” condition), or through a rigid loading plate which ensures that the top surface remains plane (the “equal strain” condition). With either type of loading the following drainage conditions are possible. The various configurations are indicated diagrammatically in Figure 1, as follows: (a) vertical drainage to the top surface only, with measurement of pore pressure at the centre of the base; (b) vertical drainage to both top and bottom surfaces; (c) radial drainage outwards to the periphery only, with measurement of pore pressure at the centre of the base; (d) radial drainage inwards to a central drain with measurement of pore pressure at one or more points off centre. Each method requires its own curve-fitting procedure and multiplying factors for deriving the relevant coefficient of consolidation. The factors also depend on whether data are derived from pore pressure measurements at a single point, or from “average” measurements (volume change or settlement) for the specimen as a whole.

Determination of permeability in a hydraulic consolidation cell - This method covers the measurement of the coefficient of permeability of a laterally confined specimen of soil under a known vertical effective stress, and under the application of a back pressure. The volume of water passing through the soil in a known time, and under a constant hydraulic gradient, is measured. The direction of flow may be either vertical (parallel to the specimen axis) or horizontal (radially outwards or inwards). The method is suitable for soils of low and intermediate permeability. Types of test. Two types of permeability test are described. The first (4.8.3) is for the determination of permeability in the vertical direction, in which water is made to flow vertically downwards through the specimen. The second (4.8.4) is for the determination of horizontal permeability in which water is made to flow radially, either outwards from the centre to the periphery or inwards to the centre.

Determination of isotropic consolidation properties using a triaxial cell - These procedures cover the determination of the magnitude and rate of consolidation of soil specimens when subjected to isotropic stress conditions in a triaxial cell. In this test the soil specimen is subjected to increments of equal all-round confining pressure, i.e. s1 = s2 = s3. Each pressure increment is held constant until virtually all the excess pore pressure due to that increment has dissipated. During this process water drains out from one end of the specimen, and its volume is measured. At the same time the pore water pressure at the other (undrained) end is monitored. These measurements are used for the determination of the relationship between voids ratio and effective isotropic stress for three-dimensional consolidation, and for the calculation of volumetric coefficients of consolidation and compressibility. The usual arrangement is for drainage to take place vertically upwards to the top face, while pore pressure is measured at the base.

Determination of permeability in a triaxial cell - This method covers the measurement of the coefficient of permeability of a cylindrical specimen of soil in the triaxial apparatus under known conditions of effective stress, and under the application of a back pressure. The volume of water passing through the soil in a known time, and under a constant hydraulic gradient, is measured. The method is suitable for soils of low and intermediate permeability.

 

3.1.7 Part 7. Shear strength test (total stress)

Part 7 describes methods for the determination of the shear strength of soils in terms of total stress, or (in the case of drained direct shear tests) in terms of effective stress when equal to total stress. A test for determining unconfined compressive strength using standard laboratory apparatus has been added. For very soft soils the laboratory vane test has been added. Direct shear tests using the shear box and the ring shear apparatus have been added, and include the determination of drained and drained-residual shear strength parameters. The triaxial compression test procedure has been augmented by the addition of a multi-stage method which is appropriate under certain conditions.

Definitions: Unconfined compression strength (qu) the compressive strength at failure of a specimen subjected to uniaxial (unconfined) compression. Sensitivity - the ratio of the undrained shear strength of an undisturbed clay specimen to that of the same specimen after remoulding at the same moisture content. Vane shear strength (τv) the shear strength of a soil as determined by applying a torque in the vane shear test. Undrained shear strength (Cu) the shear strength of a soil under undrained conditions, before drainage of water due to application of stress can take place. Residual strength - the shear strength which a soil can maintain when subjected to large shear displacement after the peak strength has been mobilized.

Summary of Part 7

Direct shear tests (clauses 3 to 6) comprise: (a) laboratory vane test procedure, for soft to firm cohesive soils; (b) small shearbox procedures for determining the angle of shear resistance of cohesionless soils, and the drained peak and residual shear strength parameters of cohesive soils; (c) large shearbox procedures for determining similar properties of gravelly soils, or on large block samples; (d) small ring shear procedure for drained residual shear strength parameters of remoulded clays. Compression tests (clauses 7 to 9) comprise: (e) unconfined compression test procedure, in the laboratory and in a portable apparatus for use on site; (f) triaxial compression test procedure from which the undrained shear strength is derived; (g) triaxial compression test procedure in several stages on one specimen, for deriving undrained shear strength. The unconfined compression test procedure using portable apparatus, and the single-stage triaxial compression test, are similar in principle to those given in the 1975 Standard. All the other procedures are new additions.

Determination of shear strength by the laboratory vane method - This method covers the measurement of the shear strength of a sample of soft to firm cohesive soil without having to remove it from its container or sampling tube. The sample therefore does not suffer disturbance due to preparation of a test specimen. The method may be used for soils that are too soft or too sensitive to enable a satisfactory compression test specimen to be prepared. The shear strength of the remoulded soil, and hence the sensitivity, can also be determined.

Determination of shear strength by direct shear (small shearbox apparatus) - In the direct shear test a square prism of soil is laterally restrained and sheared along a mechanically induced horizontal plane while subjected to a pressure applied normal to that plane. The shearing resistance offered by the soil as one portion is made to slide on the other is measured at regular intervals of displacement. Failure occurs when the shearing resistance reaches the maximum value which the soil can sustain. By carrying out tests on a set of (usually three) similar specimens of the same soil under different normal pressures, the relationship between measured shear stress at failure and normal applied stress is obtained. The shearbox apparatus can be used only for carrying out drained tests for the determination of effective shear strength parameters. There is no control of drainage and the procedure cannot be used for undrained tests. The test specimen is consolidated under a vertical normal load until the primary consolidation is completed. It is then sheared at a rate of displacement that is slow enough to prevent development of excess pore pressures. Test data enable the effective shear strength parameters c’ and ø’ to be derived. The residual shear strength parameters c’R, ø’R can be obtained by extending the tests to give large cumulative displacements by reversals and re-shearing.

Determination of shear strength by direct shear (large shearbox apparatus) - The principle of this method is the same as that described in clause 4 for the small shearbox apparatus. The large shearbox referred to in this standard is designed for carrying out tests on soil specimens up to 305 mm square and 150 mm high.

Determination of residual strength using the small ring shear apparatus - The ring shear apparatus enables an annular specimen of remoulded cohesive soil of 5 mm thickness with internal and external diameters of 70 mm and 100 mm to be subjected to rotational shear while subjected to a vertical stress. In this test it is assumed that c’r is zero.

Determination of the unconfined compressive strength  - In the unconfined compression test a cylindrical specimen of cohesive soil is subjected to a steadily increasing axial compression until failure occurs. The axial force is the only force applied to the specimen. The test is normally carried out on 38 mm diameter specimens, but can also be performed on specimens up to 100 mm diameter. The test provides an immediate approximate value of the compressive strength of the soil, either in the undisturbed or the remoulded condition, it is carried out within a short enough time to ensure that no drainage of water is permitted into or out of the specimen. It is suitable only for saturated, non-fissured cohesive soils. Failure criteria. The maximum value of the compressive force per unit area which the specimen can sustain is referred to as the unconfined compressive strength of the soil. In very plastic soils in which the axial stress does not readily reach a maximum value, an axial strain of 20 % is used as the criterion of failure. Types of test. Two methods are given for determining the unconfined compressive strength. The first is the definitive method using a load frame, in which specimens of any suitable diameter can be tested. The second makes use of an autographic apparatus.

Determination of the undrained shear strength in triaxial compression without measurement of pore pressure (definitive method) - This method covers the determination of the undrained strength of a specimen of cohesive soil when it is subjected to a constant confining pressure and to strain-controlled axial loading, when no change in total moisture content is allowed. Tests are usually carried out on a set of similar specimens, subjected to different confining pressures.

The test is carried out in the triaxial apparatus on specimens in the form of right cylinders of height approximately equal to twice the diameter. Specimen diameters range from 38 mm to about 110 mm.

Determination of the undrained shear strength in triaxial compression with multistage loading and without measurement of pore pressure - This method covers the determination of the undrained compressive strength of a specimen of cohesive soil when it is subjected to a constant all-round confining pressure and to strain-controlled axial loading, when no change in total moisture content is allowed. The method provides a means of determining the relationship between undrained shear strength and confining pressure from a single specimen. The method shall not be used for brittle or sensitive soils.


3.1.8 Part 8. Shear strength test (effective stress)

Part 8 is a major addition to this standard, namely the determination of effective stress shear strength parameters in the consolidated-drained and consolidated-undrained triaxial compression tests.

Definitions: deviator stress - the difference between the major and minor principal stresses, i.e. the principal stress difference in a triaxial test. Strain (Ł) (cumulative strain) - the change in dimension, expressed as a ratio or a percentage, of the initial reference dimension. Cell pressure - the pressure of the cell fluid which applies isotropic stress to the specimen. In axial compression tests, it is the total minor principal stress, denoted by σ3. Pore pressure (u) - the pressure of the water in the voids between solid particles as measured in the triaxial test. Back pressure (ub) - pressure applied directly to the pore fluid in the specimen voids. Effective confining pressure - the difference between the cell pressure and the pore water pressure. Effective consolidation pressure - the difference between the cell pressure and the back pressure against which the pore fluid drains during the consolidation stage. Failure - criteria for the stress condition at failure are as follows: (a) maximum deviator stress, i.e. maximum; principal stress difference; (b) maximum effective principal stress ratio; (c) when shearing continues at constant pore pressure (undrained) or with no change in volume (drained), in both cases at constant shear stress. Shear strength - the shear stress on the failure plane at failure (τf), i.e. the maximum shear resistance. Mohr circle of effective stress at failure - the Mohr circle representing the state of effective stress at failure, the diameter defined by points representing the major and minor effective principal stress at failure. Effective shear strength parameters - the slope and intercept of the Mohr-Coulomb effective stress envelope drawn to a set of Mohr circles of effective stress at failure. Angle of shear resistance in terms of effective stress (ø’) - the slope of the Mohr-Coulomb effective stress envelope. Cohesion intercept in terms of effective stress (c’) - the intercept of the Mohr-Coulomb effective stress envelope [NOTE The symbols f9and c9 are collectively referred to as the effective shear strength parameters.] Pore pressure coefficients A and B - changes in total stresses applied to a specimen when no drainage is permitted produces changes in the pore pressure. Pore pressure coefficient at failure (Af) - the value of the coefficient A at failure. stress path parameters (s’, t’) - the stress path parameters (in terms of effective stress) c.

Summary of Part 8

Two methods of carrying out the compression test are given, which are as follows (a) The consolidated-undrained triaxial compression test with measurement of pore pressure. This test gives the undrained shear strength of a specimen subjected to a known initial effective stress, and the pore pressure changes during shear from which the pore pressure coefficient A can be derived. From a set of tests the effective shear strength parameters at failure, c’ and ø’, can be derived. (b) The consolidated-drained triaxial compression test with measurement of volume change. This test gives the drained shear strength, and volume change characteristics during shear, of a specimen from which the pore water is allowed to drain freely. From a set of tests the drained effective shear strength parameters at failure, c’ and ø’, can be derived.

For many soils other than heavily-overconsolidated clays, the parameters c’ and ø’, determined from the two types of test, can be considered to be identical for most practical purposes, and are not differentiated in this standard.

Both types of test are carried out in three stages: (1) saturation (clause 5); (2) consolidation (clause 6); (3) compression (clause 7 or 8).

The first two stages saturate the specimen and bring it to the desired state of effective stress for the compression test, and are common to both types of test. The compression stage of the consolidated-undrained test is described in clause 7, and that of the consolidated-drained test in clause 8. The procedures described relate to strain-controlled apparatus for compression in a mechanical load frame, and a detachable triaxial cell. Alternatively, hydraulic triaxial cells may be used, provided that the essential principles are maintained (in which case the procedures may differ in detail).

Consolidated-undrained triaxial compression test with measurement of pore pressure - In this test, during the compression stage, the cell pressure is maintained constant while the specimen is sheared at a constant rate of axial deformation (strain-controlled compression) until failure occurs. No drainage is permitted and therefore the moisture content remains constant during compression. The resulting changes in pore pressure are usually measured at the base of the specimen, and the rate of axial deformation is applied slowly enough to ensure adequate equalization of excess pore pressures.

Consolidated-drained triaxial compression test with measurement of volume change - In this test, during the compression stage, the cell pressure is maintained constant while the specimen is sheared at a constant rate of axial deformation (strain-controlled compression) until failure occurs.

Free drainage of pore water from the specimen is allowed. The test is run slowly enough to ensure that pore pressure changes due to shearing are negligible. The required rate of strain can be much slower than that for a consolidated-undrained test on a similar specimen under similar conditions. Since the pore pressure remains virtually constant, the effective confining pressure does not vary. The volume of pore fluid draining out of or into the specimen is measured by means of the volume change indicator in the back pressure line, and is equal to the change in volume of the specimen during shear. Pore pressure can be monitored at the base as a check on the efficacy of drainage. The test procedure described in 8.2 to 8.6 relates to a saturated specimen in the triaxial cell which has been brought to the required effective stress by consolidation in accordance with clause 6.

 

3.1.9 Part 9. In-situ Test

The methods described in this Part include major additions and have been formed into groups, according to either the purpose of the test or the mode of execution. These groups are as follows:

Summary of Part 9

The methods described in this Part of this standard have been arranged in groups either according to the purpose of the test or the mode of execution. These groups are as follows. (a) Five methods for the determination of the in-situ density. (b) Three methods for the determination of penetration resistances. (c) Four methods for the determination of the vertical deformation and strength characteristics. (d) Two methods for the determination of the in-situ corrosivity characteristics.

In-situ density tests

This clause specifies five methods for determining the in-situ density of soil, four of which use the direct measurements of mass and volume, the choice of which depends upon the type of material, and one method uses gamma rays. The last named also includes the measurement of moisture content with nuclear gauges that combine both facilities. Sand replacement method suitable for fine- and medium-grained soils (small pouring cylinder method). This method covers the determination in-situ of the density of natural or compacted fine- and medium-grained soils for which a 115 mm diameter sand-pouring cylinder is used in conjunction with replacement sand (see note 1). The method is applicable to layers not exceeding 150 mm in thickness (see note 2). Sand replacement method suitable for fine-, medium- and coarse-grained soils (large pouring cylinder method). This method covers the determination in situ of the density of natural or compacted soil containing coarse- grained particles which make the test described in 2.1 difficult to perform. It is an alternative to that test for fine- and medium-grained soils and should be used instead of that test for layers exceeding 150 mm, but not exceeding 250 mm in thickness (see note). With granular materials having little or no cohesion, particularly when they are wet, there is a danger of errors in measurement of density by this method. These errors are caused by the slumping of the sides of the excavated density hole and always result in an over-estimation of density. Water replacement method suitable for coarse-grained soils. This method covers the determination in-situ of the density of natural or compacted coarse-grained soil using a circular density ring on the ground surface and a flexible plastics sheet to retain water to determine the volume of an excavated hole. The method is used in coarse and very coarse soils when the other methods for determining the field density are unsuitable because the volume excavated would be unrepresentative. Core cutter method for cohesive soils free from coarse-grained material. This method covers the determination of the density of natural or compacted soil in-situ. NOTE This method may be less accurate than the sand replacement method test (see 2.2) and is not recommended unless speed is essential, or unless the soil is well compacted but sufficiently soft for the cutter to be driven easily.

In-situ penetration tests

This clause describes methods for determining three different types of penetration resistance of soil. All are empirical methods of testing the strength of soil at various depths below a particular location. The cone penetration test and the dynamic probing test are usually carried out independently of the borehole and other tests; the former being the more precise while the latter uses much simpler apparatus. The standard penetration test is for use in a borehole.

Determination of the penetration resistance using the fixed 60° cone and friction sleeve (static cone penetration test CPT). This method covers the determination of the resistance of soils in situ to the continuous penetration at a slow uniform rate of a series of push rods having a cone at the base, and measuring continuously or at selected depth intervals the penetration resistance of the cone and, if required, the local friction resistance on a friction sleeve and pore pressure in the vicinity of the cone and sleeve. This method requires the use of a penetrometer tip with electrical sensors as defined in 3.1.2.4, thereby permitting continuous readings and an instant read-out. This is not intended to prohibit the use of the older type of mechanical penetrometer, where readings are taken through inner push rods thrusting against load capsules mounted on the thrust machine. It should be noted that the mechanical penetrometer does not give precisely the same readings as would be obtained by the electrical penetrometer tip, which is now specified as standard. In submitting reports, the type of penetrometer and penetrometer tip which has been used should always be given. Determination of the dynamic probing resistance using the 90° cone (dynamic probing DP). This method covers the determination of the resistance of soils in situ to the intermittent penetration of a 90° cone when driven dynamically in a standard manner. A continuous record is provided with respect to depth of the resistance of the cone in contrast to the standard penetration test (see 3.3), but there are no sampling facilities. Two different sizes of apparatus are specified. Dynamic probing can be used to detect soft layers and to locate strong layers, e.g. in cohesionless soils for end-bearing piles. The results of dynamic probing should normally be checked by boring in conjunction with sampling, particularly with respect to the competence of a bearing stratum. When interpreting the test results obtained in cohesive soils and in soils at depth, caution has to be taken when friction along the extension rods becomes significant. Determination of the penetration resistance using the split-barrel sampler (the standard penetration test SPT). This method covers the determination of the resistance to soils at the base of a borehole to the penetration of the split-barrel sampler when driven dynamically in a standard manner, and the obtaining of a disturbed sample for identification purposes. The test is used mainly in sands. NOTE The test can also be used in gravels or gravelly sand in which case the drive shoe may be replaced by a solid 60° cone, but when this accessory is used in any type of ground the result should be reported separately from the standard test using the open drive shoe, and with the preface: SPT(C).

In-situ vertical deformation and strength tests

This clause describes four methods for investigating in-situ strength and load settlement characteristics of soil. The plate loading test (4.1) and the shallow pad maintained load test (4.2) are particularly suited for the design of foundations or footings for buildings where it is considered that the mass characteristics of the soil would differ significantly from the results of laboratory tests, or where more precise values of settlement are required. The in-situ CBR (4.3) is generally concerned only with pavement design and the control of subgrade construction of soils with a maximum particle size not exceeding 20 mm. The determination of the vane shear strength of weak intact cohesive soils is described in 4.4. Determination of the vertical deformation and strength characteristics of soil by the plate loading test. This method covers the determination of the vertical deformation and strength characteristics of soil in situ by assessing the force and amount of penetration with time when a rigid plate is made to penetrate the soil. Uses are to evaluate the ultimate bearing capacity, the shear strength and deformation parameters of the soil beneath the plate without entailing the effects of sample disturbance. The method may be carried out at the ground surface, in pits, trenches or adits, and at depth in the bottom of a borehole. Determination of the settlement characteristics of soil for lightly loaded foundations by the shallow pad maintained load test. This method covers the determination of the settlement characteristics of soil in-situ by a test in which a constant load is applied to the ground for a period of several weeks through a pad located at shallow depth. The test is suitable for estimating the settlement caused by structures with lightly loaded shallow foundations built on filled ground and on some types of soft natural soils where the weakest ground in the profile is immediately beneath the test pad. The test should make it possible to estimate the settlement that will occur due to an applied foundation load. However, it should be recognized that there may be other causes of settlement besides weaker formations at depth, e.g. with uncompacted fills settlement may occur due to self-weight, collapse compression due to a rising water table and decay of organic matter. The test is solely confined to providing an indication of the magnitude of settlement of the ground immediately beneath the test pad. NOTE It is important that the test results are not considered to be the sole evidence on which to base the design of the foundations of the proposed structure. Precautions should be taken by means of borings or pits to ensure that the test area is representative of the weakest part of the site, also that weaker ground does not exist within the zone of influence beneath the complete structure. Determination of the in-situ California Bearing Ratio (CBR). This method covers the determination of the California Bearing Ratio (CBR) of a soil tested in situ, with a selected overburden pressure, by causing a cylindrical plunger to penetrate the soil at a given rate and comparing the relationship between force and penetration into the soil to that for a standard material. At certain values of penetration the California Bearing Ratio (CBR) is defined in the form of a percentage, as the ratio of the force exerted on the soil to a standard force that would be exerted on a specified crushed rock compacted and confined in a given manner. The CBR test may also be carried out in the laboratory on soil in a mould (see clause 7 of BS 1377-4:1990). On account of the plunger size the test is appropriate only to material having a maximum particle size not exceeding 20 mm. Hence where material of this size or larger is possibly present beneath the test surface this should be checked after making the test and reported. Determination of in-situ vane shear strength of weak intact cohesive soils. This method covers the determination in situ of the shear strength of weak intact cohesive soils using a vane of cruciform section, which is subjected to a torque of sufficient magnitude to shear the soil. The test is suitable for very soft to firm intact saturated cohesive soils.

In-situ corrosivity tests

This clause of the standard describes two methods for determining in-situ the likelihood of underground corrosion of buried metal structures. The results of these tests should be interpreted by a specialist. Determination in-situ of the apparent resistivity of soil. This method covers the determination of the electrical resistivity of soil tested in situ for a selected depth or a range of depths. (See note 1.) The test is used to assess the corrosivity of the soil towards various metals. Resistivity is the electrical resistance of an element of unit cross-sectional area and unit length. Its value indicates the relative capability of the soil to carry electric currents. Generally the severity of corrosion decreases as the apparent resistivity rises. The method consists of passing a current (see note 2) into the ground between two electrodes (A, B) and measuring the consequent apparent resistivity between another two electrodes (C, D) situated at equi-distant spacings (AC, CD and DB) and collinear between electrodes A and B. This arrangement corresponds to the conventional “Wenner”, equally spaced, four electrode configuration. Two separate measurements of the resistivity are made for a test at each selected depth with the electrodes set at approximately right angles for the two measurements. When testing in borrow areas one measurement may be made with the electrodes in the same line as the direction of the test locations and another with the electrodes set at approximately right angles to the line of tests. Determination in-situ of the redox potential of soil. This method covers the determination of the redox potential (reduction/oxidation) of soil tested in situ at a selected depth by measuring the electro-chemical potential between a platinum electrode and a saturated calomel reference electrode. The test is used to indicate the likelihood of microbial corrosion of metals by sulphate-reducing bacteria which can proliferate in anaerobic conditions. The redox potential is principally related to the oxygen in the soil, and a high value indicates that a relatively large amount is present. Anaerobic microbial corrosion can occur if a soil has a low oxygen content and hence a low redox potential.

 

In-situ density tests.

The hand-scoop method has been deleted and substituted by a new test for coarse-grained soils based on a water-replacement method. Determination of in-situ density of fine-grained, medium-grained, and coarse-grained soils by attenuation of gamma rays has been added which includes moisture content determination.

In-situ penetration tests.

The split-barrel sampler method has been revised to conform more closely to international practice. Two other test methods have been added as follows.

a) Determination of the penetration resistance using fixed 60° cone and friction sleeve (the static cone test CPT).

b) Determination of the dynamic probing resistance using a 90° cone (dynamic probing, DP).

In-situ vertical deformation and strength tests.

Three test methods have been added as follows.

a) Determination of the vertical deformation and strength characteristics of soil by the plate loading test.

b) Determination of the settlement characteristics of soil for lightly loaded foundations by the shallow pad maintained-load test.

c) Determination of the in-situ California Bearing Ratio (CBR).

In-situ corrosivity tests.

Two test methods are given as follows.

a) Determination of the in-situ apparent resistivity of soil.

b) Determination of the in-situ redox potential of the soil.

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