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【正文】 amplifies this. The fine soil constituents are generally sensitive to small increases in moisture content which often lead to loss in strength and render the soils unsuitable for reuse as engineering fill. Many of our boulder clay soils (especially those with intermediate 西安工業(yè)大學(xué)學(xué)士學(xué)位論文 type silts and fine sand matrix) have been rejected at the selection stage, but good planning shows that they can in fact fulfil specification requirements in terms of paction and strength. The selection process should aim to maximise the use of locally available soils and with careful evaluation it is possible to use or incorporate ?poor or marginal soils? within fill areas and embankments. Fill material needs to be placed at a moisture content such that it is neither too wet to be stable and trafficable or too dry to be properly pacted. High moisture content / low strength boulder clay soils can be suitable for use as fill in low height embankments (. 2 to ) but not suitable for trafficking by earthwork plant without using a geotextile separator and granular fill capping layer. Hence, it is vital that the earthworks contractor fully understands the handling properties of the soils, as for many projects this is effectively governed by the trafficability of earthmoving equipment. 2. Tradtional Ground Investigation Methods For road projects, a principal aim of the ground investigation is to classify the suitability of the soils in accordance with Table from Series 600 of the NRA Specification for Road Works (SRW), March 2021. The majority of current ground investigations for road works includes a bination of the following to give the required geotechnical data: ? Trial pits ? Cable percussion boreholes ? Dynamic probing ? Rotary core drilling ? Insitu testing (SPT, variable head permeability tests, geophysical etc.) ? Laboratory testing The importance of ?phasing? the fieldwork operations cannot be overstressed, particularly when assessing soil suitability from deep cut areas. Cable percussion 西安工業(yè)大學(xué)學(xué)士學(xué)位論文 boreholes are normally sunk to a desired depth or ?refusal? with disturbed and undisturbed samples recovered at intervals or change of strata. In many instances, cable percussion boring is unable to perate through very stiff, hard boulder clay soils due to cobble, boulder obstructions. Sample disturbance in boreholes should be prevented and loss of fines is mon, invariably this leads to inaccurate classification. Trial pits are considered more appropriate for recovering appropriate size samples and for observing the proportion of clasts to matrix and sizes of cobbles, boulders. Detailed and accurate field descriptions are therefore vital for cut areas and trial pits provide an opportunity to examine the soils on a larger scale than boreholes. Trial pits also provide an insight on trench stability and to observe water ingress and its effects. A suitably experienced geotechnical engineer or engineering geologist should supervise the trial pitting works and recovery of samples. The characteristics of the soils during trial pit excavation should be closely observed as this provides information on soil sensitivity, especially if water from granular zones migrates into the fine matrix material. Very often, the condition of soil on the sides of an excavation provides a more accurate assessment of its insitu condition. 3. Soil Classification Soil description and classification should be undertaken in accordance with BS 5930 (1999) and tested in accordance with BS 1377 (1990). The engineering description of a soil is based on its particle size grading, supplemented by plasticity for fine soils. For many of our glacial till, boulder clay soils (. ?mixed soils?) difficulties arise with descriptions and assessing engineering performance tests. As outlined previously, Irish boulder clays usually prise highly variable proportions of sands, gravels and cobbles in a silt or clay matrix. Low plasticity soils with fines contents of around 10 to 15% often present the most difficulties. BS 5930 (1999) now recognises these difficulties in describing ?mixed soils? – the fine soil constituents which govern the engineering behaviour now takes priority over particle size. 西安工業(yè)大學(xué)學(xué)士學(xué)位論文 A key parameter (which is often underestimated) in classifying and understanding these soils is permeability (K). Inspection of the particle size gradings will indicate magnitude of permeability. Where possible, triaxial cell tests should be carried out on either undisturbed samples (U100?s) or good quality core samples to evaluate the drainage characteristics of the soils accurately. Low plasticity boulder clay soils of intermediate permeability (. K of the order of 105 to 107 m/s) can often be ?conditioned? by drainage measures. This usually entails the installation of perimeter drains and sumps at cut areas or borrow pits so as to reduce the moisture content. Hence, with small reduction in moisture content, difficult glacial till soils can bee suitable as engineering fill. 4. Engineering Performance Testing of Soils Laboratory testing is very much dictated by the proposed enduse for the soils. The engineering parameters set out in Table pf the NRA SRW include a bination of the following: ? Moisture content ? Particle size grading ? Plastic Limit ? CBR ? Compaction (relating to optimum MC) ? Remoulded undrained shear strength A number of key factors should be borne in mind when scheduling laboratory testing: ? Compaction / CBR / MCV tests are carried out on 20mm size material. ? Moisture content values should relate to 20mm size material to provide a valid parison. ? Pore pressures are not taken into account during paction and may vary considerably between laboratory and field. ? Preparation methods for soil testing must be clearly stipulated and agreed with the designated laboratory. 西安工業(yè)大學(xué)學(xué)士學(xué)位論文 Great care must be taken when determining moisture content of boulder clay soils. Ideally, the moisture content should be related to the particle size and have a corresponding grading