DESIGN AND EXECUTION OF GROUND INVESTIGATION FOR EARTHWORKS
茄子的英文单词PAUL QUIGLEY, FGS
Irish Geotechnical Services Ltd
ABSTRACT
The design and execution of ground investigation works for earthwork projects has become increasingly important as the availability of suitable disposal areas becomes limited and costs of importing engineering fill increa. An outline of ground investigation methods which can augment ‘traditional investigation methods’ particularly for glacial till / boulder clay soils is prented. The issue of ‘geotechnical certification’ is raid and recommendations outlined on its merits for incorporation with ground investigations and earthworks.
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1. INTRODUCTION
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The investigation and re-u evaluation of many Irish boulder clay soils prents difficulties for both the geotechnical engineer and the road design engineer. The glacial till or boulder clay soils are mainly of low plasticity and have particle sizes ranging from clay to boulders. Most of our boulder clay soils contain varying proportions of sand, gravel, cobbles and boulders in a clay or silt matrix. The amount of fines governs their behaviour and the silt content makes it very weather susceptible.
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Moisture contents can be highly variable ranging from as low as 7% for the hard grey black Dublin boulder clay up to 20-25% for Midland, South-West and North-West light grey boulder clay deposits. The ability of boulder clay soils to take-in free water is well established and poor planning of earthworks often amplifies this.
The fine soil constituents are generally nsitive to small increas in moisture content which often lead to loss in strength and render the soils unsuitable for re-u as engineering fill. Many of our boulder clay soils (especially tho with intermediate type silts and fine sand matrix) have been rejected at the lection stage, but good planning s
hows that they can in fact fulfil specification requirements in terms of compaction and strength.
The lection process should aim to maximi the u of locally available soils and with careful evaluation it is possible to u 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 compacted.
High moisture content / low strength boulder clay soils can be suitable for u as fill in low height embankments (i.e. 2 to 2.5m) but not suitable for trafficking by earthwork plant without using a geotextile parator 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. TRADITIONAL GROUND INVESTIGATION METHODS
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intimeFor road projects, a principal aim of the ground investigation is to classify the suitability of
the soils in accordance with Table 6.1 from Series 600 of the NRA Specification for Road Works (SRW), March 2000. The majority of current ground investigations for road works includes a combination of the following to give the required geotechnical data:
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▪ Cable percussion boreholes
▪ Dynamic probing
eiia▪ Rotary core drilling
▪ In-situ testing (SPT, variable head permeability tests, geophysical etc.)
▪ Laboratory testing
The importance of ‘唯一英文phasing’ the fieldwork operations cannot be overstresd, particularly when asssing soil suitability from deep cut areas. Cable percussion boreholes are normally sunk to a desired depth or ‘refusal’ with disturbed and undisturbed samples reco
vered at 1.00m intervals or change of strata.
In many instances, cable percussion boring is unable to penetrate through very stiff, hard boulder clay soils due to cobble, boulder obstructions. Sample disturbance in boreholes should be prevented and loss of fines is common, invariably this leads to inaccurate classification.
Trial pits are considered more appropriate for recovering appropriate size samples and for obrving 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 obrve water ingress and its effects.
A suitably experienced geotechnical engineer or engineering geologist should supervi the trial pitting works and recovery of samples. The characteristics of the soils during trial pit excavation should be cloly obrved as this provides information on soil nsitivity, especially if water from granular zones migrates into the fine matrix material. Very often, t
he condition of soil on the sides of an excavation provides a more accurate asssment of its in-situ 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 bad on its particle size grading, supplemented by plasticity for fine soils. For many of our glacial till, boulder clay soils (i.e. ‘mixed soils’) difficulties ari with descriptions and asssing engineering performance tests.
