土方工程地基勘察与施工中英文对照外文翻译文献Word格式文档下载.docx

1、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 increase. An outline of ground investigation methods which can augment traditional in

2、vestigation methods particularly for glacial till / boulder clay soils is presented. The issue of geotechnical certification is raised and recommendations outlined on its merits for incorporation with ground investigations and earthworks.1. INTRODUCTIONThe investigation and re-use evaluation of many

3、 Irish boulder clay soils presents difficulties for both the geotechnical engineer and the road design engineer. These 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

4、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.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, Sou

5、th-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 sensitive to small increases in moisture content which often lead to loss

6、 in strength and render the soils unsuitable for re-use as engineering fill. Many of our boulder clay soils （especially those with intermediate 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

7、in terms of compaction 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 t

8、hat 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 use as fill in low height embankments （i.e. 2 to 2.5m） but not suitable for trafficking by earthwork plant without using a geotextile

9、 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. TRADITIONAL GROUND INVESTIGATION METHODS For road

10、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 followi

11、ng to give the required geotechnical data: Trial pits Cable percussion boreholes Dynamic probing Rotary core drilling In-situ testing （SPT, variable head permeability tests, geophysical etc.） Laboratory testingThe importance of phasing the fieldwork operations cannot be overstressed, particularly wh

12、en assessing soil suitability from deep cut areas. Cable percussion boreholes are normally sunk to a desired depth or refusal with disturbed and undisturbed samples recovered at 1.00m intervals or change of strata.In many instances, cable percussion boring is unable to penetrate through very stiff,

13、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 observing the pr

14、oportion 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 w

15、ater 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 sensitivit

16、y, 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 in-situ condition.3. SOIL CLASSIFICATIONSoil description and classification should be undertaken in accordance

17、 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 （i.e. mixed soils） difficulties arise with descriptions and asse

18、ssing engineering performance tests.As outlined previously, Irish boulder clays usually comprise 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 re

19、cognises these difficulties in describing mixed soils the fine soil constituents which govern the engineering behaviour now takes priority over particle size.A key parameter （which is often underestimated） in classifying and understanding these soils is permeability （K）. Inspection of the particle s

20、ize gradings will indicate magnitude of permeability. Where possible, triaxial cell tests should be carried out on either undisturbed samples （U100s） or good quality core samples to evaluate the drainage characteristics of the soils accurately.Low plasticity boulder clay soils of intermediate permea

21、bility （i.e. K of the order of 10-5 to 10-7 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 til

22、l soils can become suitable as engineering fill.4. ENGINEERING PERFORMANCE TESTING OF SOILSLaboratory testing is very much dictated by the proposed end-use for the soils. The engineering parameters set out in Table 6.1 pf the NRA SRW include a combination of the following: Moisture content Particle

23、size grading Plastic Limit CBR Compaction （relating to optimum MC） Remoulded undrained shear strengthA 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 s

24、ize material to provide a valid comparison. Pore pressures are not taken into account during compaction and may vary considerably between laboratory and field. Preparation methods for soil testing must be clearly stipulated and agreed with the designated laboratory.Great care must be taken when dete

25、rmining moisture content of boulder clay soils. Ideally, the moisture content should be related to the particle size and have a corresponding grading analysis for direct comparison, although this is not always practical.In the majority of cases, the MCV when used with compaction data is considered t

26、o offer the best method of establishing （and checking） the suitability characteristics of a boulder clay soil. MCV testing during trial pitting is strongly recommended as it provides a rapid assessment of the soil suitability directly after excavation. MCV calibration can then be carried out in the

27、laboratory at various moisture content increments. Sample disturbance can occur during transportation to the laboratory and this can have a significant impact on the resultant MCVs.IGSL has found large discrepancies when performing MCVs in the field on low plasticity boulder clays with those carried

28、 out later in the laboratory （2 to 7 days）. Many of the aforementioned low plasticity boulder clay soils exhibit time dependant behaviour with significantly different MCVs recorded at a later date increased values can be due to the drainage of the material following sampling, transportation and stor

29、age while dilatancy and migration of water from granular lenses can lead to deterioration and lower values.This type of information is important to both the designer and earthworks contractor as it provides an opportunity to understand the properties of the soils when tested as outlined above. It ca

30、n also illustrate the advantages of pre-draining in some instances. With mixed soils, face excavation may be necessary to accelerate drainage works.CBR testing of boulder clay soils also needs careful consideration, mainly with the preparation method employed. Design engineers need to be aware of th

31、is, as it can have an order of magnitude difference in results. Static compaction of boulder clay soils is advised as compaction with the 2.5 or 4.5kg rammer often leads to high excess pore pressures being generated hence very low CBR values can result. Also, curing of compacted boulder clay samples

32、 is important as this allows excess pore water pressures to dissipate.5. ENGINEERING CLASSIFICATION OF SOILSIn accordance with the NRA SRW, general cohesive fill is categorised in Table 6.1 as follows: 2A Wet cohesive 2B Dry cohesive 2C Stony cohesive 2D Silty cohesiveThe material properties required for acceptability are given and the design engineer then determines the upper and lower bound limits on the basis of the l