Development of a mechanical model for soil | My Assignment Tutor

Development of a mechanical model for soil218The soil description should be based on particle size distribution and plasticity, generally using therapid procedure in which these characteristics are assessed by means of visual inspection and feel; disturbed samples are generally used for this purpose. The description should include details of soil colour,particle shape and composition; if possible, the geological formation and type of deposit should be given.The structural characteristics of the soil mass should also be described, but this requires an examinationof undisturbed samples or of the soil in­situ (e.g. in a trial pit). Details should be given of the presenceand spacing of bedding features, fissures and other relevant characteristics. The density index of sandsand the consistency of clays (Table 1.3) should be indicated.6.6 Cone penetration testing (CPT)The Cone Penetrometer is one of the most versatile tools available for soil exploration (Lunne et al.,1997). In this section, the use of the CPT to identify stratigraphy and the materials which are present inthe ground will be presented. However, the technique can also be used to determine a wide range ofstandard geotechnical parameters for these materials instead of or in addition to the laboratory tests summarised in Section 6.4. This will be further discussed in Chapter 7. Furthermore, because of the closeanalogy between a CPT and a pile under vertical loading, CPT data may also be used directly in thedesign of deep foundations (Chapter 9).The penetrometer consists of a short cylindrical element, at the end of which is a cone­shaped tip. Thecone has an apex angle of 60° and a cross­sectional area of 1000mm2. This is pushed vertically into theground using a thrust machine at a constant rate of penetration of 20mm/s (ISO, 2006). For onshore applications, the thrust machine is commonly a CPT truck which provides a reaction mass due to its self­weight(typically 15–20 tonnes). A 20­tonne (200­kN) thrust will normally permit penetration to around 30m indense sands or stiff clays (Lunne et al., 1997). For deeper investigations where higher resistances to penetration will be encountered, this may be further ballasted or temporarily anchored to the ground (Figure 6.9).As the instrument penetrates, additional push rods of the same diameter as the instrument are attached toextend the string. Cables passing up through the centre of the push rods carry data from instruments withinthe penetrometer to the surface. In a standard electrical cone (CPT), a load cell between the cone and thebody of the instrument continuously records the resistance to penetration of the cone (cone tip resistance qc),and a friction sleeve is used to measure the interface shearing resistance (fs) along the cylindrical body of theinstrument. Different types of soil will exhibit different proportions of sleeve friction to end resistance: forexample, gravels generally have low fs and high qc, while clays have high fs and low qc. By examining anextensive database of CPT test data, Robertson (1990) proposed a chart which may be used for identifyingsoil types based on normalised versions of these parameters (Qt and Fr), which is shown in Figure 6.10.For a standard CPT cone, qt is approximated by qc. During penetration, however, excess pore waterpressures around the cone will increase, particularly in fine­grained soils, which artificially reduce qc.More sophisticated piezocones (CPTU) include localised measurement of the excess pore water pressures around the cone which are induced by penetration. These are most commonly measured immediately behind the cone (u2), though measurements may also be made on the cone itself (u1) and/or at theother end of the friction sleeve (u3), as shown in Figure 6.11. When such measurements are made, thecorrected cone resistance qt is determined using(6.2)The parameter a is an area correction factor depending on the penetrometer, and typically varies between0.5–0.9. As before, different soils will experience different changes in pore water pressure during penetration: coarse­grained soils (sands and gravels) will exhibit little excess pore water pressure generation due totheir high permeability, while fine­grained soils of lower permeability typically exhibit larger values of u2.Ground investigation219 Thrustmachine(CPT truck) 20mm/sPush rodsPenetrometer Friction sleeveConeFigure 6.9 Schematic of Cone Penetrometer Test (CPT) showing standardterminology.Zone Soil behaviour type1. Sensitive, ne grained2. Organic soils–peats3. Clays–clay to silty clay4. Silt mixtures; clayey silt to silty clay5. Sand mixtures; silty sand to sand silty6. Sands; clean sands to silty sands7. Gravelly sand to sand8. Very stiff sand to clayey sand9. Very stiff ne grainedQt =qt – σvoσ′voFr =qt – σvofs� 100%10001001010.1 11324567QtFr (%)Normally consolidated8910Figure 6.10 Soil behaviour type classification chart based on normalised CPT data(reproduced after Robertson, 1990).Development of a mechanical model for soil220If the soil is heavily overconsolidated, u2 may be negative. The pore water pressure measurement u2 therefore provides a third continuous parameter which may be used to identify soil types, using the chart shownin Figure 6.12 (Robertson, 1990). If CPTU data are available, both Figures 6.10 and 6.12 should be used todetermine soil type. In some instances, the two charts may give different interpretations of the ground conditions. In these cases, judgement is required to correctly identify the ground conditions. CPT soundingsmay also struggle to identify inter­bedded soil layers (Lunne et al., 1997).As data are recorded continuously with increasing depth during a CPT sounding, the technique can beused to produce a ground profile showing soil stratigraphy and classification, similar to a borehole log. Anexample of the use of the foregoing identification charts is shown in Figure 6.13. The CPT test is quick, relatively cheap, and has the advantage of not leaving a large void in the ground as in the case of a borehole ortrial pit. However, due to the difficulties in interpretation which have previously been discussed, CPT dataare most effective at ‘filling in the gaps’ between widely spaced boreholes. Under these conditions, use ofthe CPT(U) soil identification charts can be informed by the observations from the boreholes. The CPT(U)data will then provide useful information as to how the levels of different soil strata vary across a site, andmay identify localised hard inclusions or voids which may have been missed by the boreholes.Given the large amount of data that is generated from a continuous CPT sounding, and that soilbehaviour types (zones 1–9 in Figures 6.10 and 6.12) fall within ranges defined by the magnitude of themeasured parameters, interpretation of soil stratigraphy from CPT data benefits greatly from automation/computerisation. It may be seen from Figure 6.10 that the boundaries between zones 2–7 are close tobeing circular arcs with an origin around the top left corner of the plot. Robertson and Wride (1998)quantified the radius of these arcs by a parameter Ic:(6.3)where Qt and Fr are the normalised tip resistance and friction ratio as defined in Figure 6.10. Figure 6.14shows curves plotted using Equation 6.3 for values of Ic which most closely represent the soil boundariesin Figure 6.10. Using a single equation to define the soil behaviour type makes the processing of CPTu3u2FrictionsleeveConeNominalcone area=1000mm2Penetrometeru1qcfsFigure 6.11 Schematic of piezocone (CPTU).Ground investigation2211000100QtBq uoqtuσvo23457 61 101–0.4 0.40.8 1.20Zone Soil behaviour type1. Sensitive, ne grained2. Organic soils–peats3. Clays–clay to silty clay4. Silt mixtures; clayey silt to silty clay5. Sand mixtures; silty sand to sand silty6. Sands; clean sands to silty sands7. Gravelly sand to sand8. Very stiff sand to clayey sand9. Very stiff ne grainedQt =qt – σvoσʹvoBq =qt – σvou2 – uoFigure 6.12 Soil behaviour type classification chart based on normalised CPTU data.0 0Porepressureu (MPa)Sleevefrictionfs(MPa)Coneresistanceqt (MPa)Soilprofile Soft clay & siltCoarse sandloose to densewith layersof fine sandSoft, normallyconsolidatedclayey siltu2uoFine sand,some silt 1 0 0 0.1 10 2010Depth below ground level (m)2030Figure 6.13 Example showing use of CPTU data to provide ground information.Development of a mechanical model for soil222data amenable to automated analysis using a spreadsheet, by applying Equation 6.3 to each testdatapoint. A spreadsheet tool, CPTic_CSM8.xls, which implements the Ic method may be found on theCompanion Website. Care must be taken when using the method, as it will not correctly identify soiltypes 1, 8 and 9 (Figure 6.10); however, for most routine use, the Ic method provides a valuable tool forthe interpretation of stratigraphic information from CPT soundings.6.7 Geophysical methodsUnder certain conditions geophysical methods may be useful in ground investigation, especially at thereconnaissance stage. However, the methods are not suitable for all ground conditions and there are limitations to the information that can be obtained; thus they must be considered mainly as supplementarymethods. It is possible to locate strata boundaries only if the physical properties of the adjacent materialsare significantly different. It is always necessary to check the results against data obtained by directmethods such as boring or CPT soundings. Geophysical methods can produce rapid and economicresults, making them useful for the filling in of detail between widely spaced boreholes or to indicatewhere additional boreholes may be required. The methods can also be useful in estimating the depth tobedrock or to the water table, or for locating buried metallic objects (e.g. unexploded ordnance) andvoids. They can be particularly useful in investigating sensitive contaminated sites, as the methods arenon­intrusive – unlike boring or CPT. There are several geophysical techniques, based on different physical principles. Three of these techniques are described below.Seismic refractionThe seismic refraction method depends on the fact that seismic waves have different velocities in different types of soil (or rock), as shown in Table 6.5; in addition, the waves are refracted when they crossthe boundary between different types of soil. The method enables the general soil types and the approximate depths to strata boundaries, or to bedrock, to be determined.1000100Qt1010.1 1Fr (%)102345673.602.952.602.05Zone Soil behaviour typeIc=1.312. Organic soils–peats3. Clays–clay to silty clay4. Silt mixtures; clayey silt to silty clay5. Sand mixtures; silty sand to sand silty6. Sands; clean sands to silty sands7. Gravelly sand to sandQt =qt – σvoσʹvoFr =qt – σvofs� 100%Figure 6.14 Soil behaviour type classification using the Ic method.


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