土木工程畢業(yè)設(shè)計外文翻譯--土方工程的地基勘察與施工

1、<p>　　DESIGN AND EXECUTION OF GROUND INVESTIGATION FOR EARTHWORKS </p><p>　　PAUL QUIGLEY, FGS</p><p>　　Irish Geotechnical Services Ltd</p><p><b>　　ABSTRACT</b></

2、p><p>　　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 engi

3、neering fill increase. An outline of ground investigation methods which can augment ‘traditional investigation methods’ particularly for glacial till / boulder clay soils is presented. The issue of ‘geotechnical certifi

4、cation’ is raised and recommendations outlined on its merits for incorporation</p><p>　　1. INTRODUCTION</p><p>　　The investigation and re-use evaluation of many Irish boulder clay soils presents

5、 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 boul

6、der 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 susce</p><p>

7、　　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 boulde

8、r clay soils to take-in free water is well established and poor planning of earthworks often amplifies this.</p><p>　　The fine soil constituents are generally sensitive to small increases in moisture content

9、 which often lead to loss 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

10、 at the selection stage, but good planning shows that they can in fact fulfil specification requirements in terms of compaction and strength.</p><p>　　The selection process should aim to maximise the use of

11、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

12、neither too wet to be stable and trafficable or too dry to be properly compacted.</p><p>　　High moisture content / low strength boulder clay soils can be suitable for use as fill in low height embankments (i

13、.e. 2 to 2.5m) 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 pr

14、operties of the soils, as for many projects this is effectively governed by the trafficability of earthmoving equipment.</p><p>　　2. TRADITIONAL GROUND INVESTIGATION METHODS </p><p>　　For road p

15、rojects, 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 cur

16、rent ground investigations for road works includes a combination of the following to give the required geotechnical data:</p><p>　　Trial pits</p><p>　　Cable percussion boreholes</p><p

17、>　　Dynamic probing</p><p>　　Rotary core drilling</p><p>　　In-situ testing (SPT, variable head permeability tests, geophysical etc.)</p><p>　　Laboratory testing</p><p&g

18、t;　　The importance of ‘phasing’ the fieldwork operations cannot be overstressed, particularly when assessing soil suitability from deep cut areas. Cable percussion boreholes are normally sunk to a desired depth or ‘refus

19、al’ with disturbed and undisturbed samples recovered at 1.00m intervals or change of strata.</p><p>　　In many instances, cable percussion boring is unable to penetrate through very stiff, hard boulder clay s

20、oils due to cobble, boulder obstructions. Sample disturbance in boreholes should be prevented and loss of fines is common, invariably this leads to inaccurate classification.</p><p>　　Trial pits are consider

21、ed 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

22、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.</p><p>　　A su

23、itably 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

24、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 a

25、ssessment of its in-situ condition.</p><p>　　3. SOIL CLASSIFICATION</p><p>　　Soil description and classification should be undertaken in accordance with BS 5930 (1999) and tested in accordance w

26、ith 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’) difficulti

27、es arise with descriptions and assessing engineering performance tests.</p><p>　　As outlined previously, Irish boulder clays usually comprise highly variable proportions of sands, gravels and cobbles in a si

28、lt 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 constituen

29、ts which govern the engineering behaviour now takes priority over particle size.</p><p>　　A key parameter (which is often underestimated) in classifying and understanding these soils is permeability (K). Ins

30、pection 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

31、 drainage characteristics of the soils accurately.</p><p>　　Low plasticity boulder clay soils of intermediate permeability (i.e. K of the order of 10-5 to 10-7 m/s) can often be ‘conditioned’ by drainage mea

32、sures. 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 c

33、an become suitable as engineering fill.</p><p>　　4. ENGINEERING PERFORMANCE TESTING OF SOILS</p><p>　　Laboratory testing is very much dictated by the proposed end-use for the soils. The engineer

34、ing parameters set out in Table 6.1 pf the NRA SRW include a combination of the following:</p><p>　　Moisture content</p><p>　　Particle size grading</p><p>　　Plastic Limit</p>

35、<p><b>　　CBR</b></p><p>　　Compaction (relating to optimum MC)</p><p>　　Remoulded undrained shear strength</p><p>　　A number of key factors should be borne in mind

36、when scheduling laboratory testing:</p><p>　　Compaction / CBR / MCV tests are carried out on < 20mm size material.</p><p>　　Moisture content values should relate to < 20mm size material to

37、 provide a valid comparison.</p><p>　　Pore pressures are not taken into account during compaction and may vary considerably between laboratory and field.</p><p>　　Preparation methods for soil te

38、sting must be clearly stipulated and agreed with the designated laboratory.</p><p>　　Great care must be taken when determining moisture content of boulder clay soils. Ideally, the moisture content should be

39、related to the particle size and have a corresponding grading analysis for direct comparison, although this is not always practical.</p><p>　　In the majority of cases, the MCV when used with compaction data

40、is considered to 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

41、the soil suitability directly after excavation. MCV calibration can then be carried out in the laboratory at various moisture content increments. Sample disturbance can occur during transportation to the laboratory </

42、p><p>　　IGSL has found large discrepancies when performing MCV’s in the field on low plasticity boulder clays with those carried out later in the laboratory (2 to 7 days). Many of the aforementioned low plastic

43、ity boulder clay soils exhibit time dependant behaviour with significantly different MCV’s recorded at a later date – increased values can be due to the drainage of the material following sampling, transportation and sto

44、rage while dilatancy and migration of water from granular lenses can lead to d</p><p>　　This type of information is important to both the designer and earthworks contractor as it provides an opportunity to u

45、nderstand the properties of the soils when tested as outlined above. It can also illustrate the advantages of pre-draining in some instances. With mixed soils, face excavation may be necessary to accelerate drainage work

46、s.</p><p>　　CBR testing of boulder clay soils also needs careful consideration, mainly with the preparation method employed. Design engineers need to be aware of this, as it can have an order of magnitude di

47、fference 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, c

48、uring of compacted boulder clay samples is important as this allows excess pore water</p><p>　　5. ENGINEERING CLASSIFICATION OF SOILS</p><p>　　In accordance with the NRA SRW, general cohesive

49、fill is categorised in Table 6.1 as follows:</p><p>　　2A Wet cohesive</p><p>　　2B Dry cohesive</p><p>　　2C Stony cohesive</p><p>　　2D Silty cohesive</p><p>

50、;　　The material properties required for acceptability are given and the design engineer then determines the upper and lower bound limits on the basis of the laboratory classification and engineering performance tests. Ir

51、ish boulder clay soils are predominantly Class 2C.</p><p>　　Clause 612 of the SRW sets out compaction methods. Two procedures are available:</p><p>　　Method Compaction</p><p>　　End-

52、Product Compaction</p><p>　　End product compaction is considered more practical, especially when good compaction control data becomes available during the early stages of an earthworks contract. A minimum Ta

53、rget Dry Density (TDD) is considered very useful for the contractor to work with as a means of checking compaction quality. Once the material has been approved and meets the acceptability limits, then in-situ density can

54、 be measured, preferably by nuclear gauge or sand replacement tests where the stone content is low.</p><p>　　As placing and compaction of the fill progresses, the in-situ TDD can be checked and non-conformin

55、g areas quickly recognised and corrective action taken. This process requires the design engineer to review the field densities with the laboratory compaction plots and evaluate actual with ‘theoretical densities’.</p

56、><p>　　6. SUPPLEMENTARY GROUND INVESTIGATION METHODS FOR EARTHWORKS</p><p>　　The more traditional methods and procedures have been outlined in Section 2. The following are examples of methods which

57、 are believed to enhance ground investigation works for road projects:</p><p>　　Phasing the ground investigation works, particularly the laboratory testing</p><p>　　Excavation & sampling in

58、deep trial pits</p><p>　　Large diameter high quality rotary core drilling using air-mist or polymer gel techniques</p><p>　　Small-scale compaction trials on potentially suitable cut material<

59、/p><p><b>　　PHASING</b></p><p>　　Phasing ground investigation works for many large projects has been advocated for many years – this is particularly true for road projects where signifi

60、cant amounts of geotechnical data becomes available over a short period. On the majority of large ground investigation projects no period is left to ‘digest’ or review the preliminary findings and re-appraise the suitabi

61、lity of the methods.</p><p>　　With regard to soil laboratory testing, large testing schedules are often prepared with no real consideration given to their end use. In many cases, the schedule is prepared by

62、a junior engineer while the senior design engineer who will probably design the earthworks will have no real involvement.</p><p>　　It is highlighted that the engineering performance tests are expensive and o

63、f long duration (e.g. 5 point compaction with CBR & MCV at each point takes in excess of two weeks). When classification tests (moisture contents, particle size analysis and Atterberg Limits) are completed then a mor

64、e incisive evaluation can be carried out on the data and the engineering performance tests scheduled. If MCV’s are performed during trial pitting then a good assessment of the soil suitability can be immediat</p>

65、<p>　　DEEP TRIAL PITS</p><p>　　The excavation of deep trial pits is often perceived as cumbersome and difficult and therefore not considered appropriate by design engineers. Excavation of deep trial pit

66、s in boulder clay soils to depths of up to 12m is feasible using benching techniques and sump pumping of groundwater.</p><p>　　In recent years, IGSL has undertaken such deep trial pits on several large road

67、ground investigation projects. The data obtained from these has certainly enhanced the geotechnical data and provided a better understanding of the bulk properties of the soils.</p><p>　　It is recommended th

68、at this work be carried out following completion of the cable percussion boreholes and rotary core drill holes. The groundwater regime within the cut area will play an important role in governing the feasibility of excav

69、ating deep trial pits. The installation of standpipes and piezometers will greatly assist the understanding of the groundwater conditions, hence the purpose of undertaking this work late on in the ground investigation pr

70、ogramme.</p><p>　　Large representative samples can be obtained (using trench box) and in-situ shear strength measured on block samples. The stability of the pit sidewalls and groundwater conditions can also

71、be established and compared with levels in nearby borehole standpipes or piezometers. Over a prominent cut area of say 500m, three deep trial pits can prove invaluable and the spoil material also used to carry out small-

72、scale compaction trials.</p><p>　　From a value engineering perspective, the cost of excavating and reinstating these excavations can be easily recovered. A provisional sum can be allocated in the ground inve

73、stigation and used for this work.</p><p>　　HIGH QUALITY LARGE DIAMETER ROTARY CORE DRILLING</p><p>　　This system entails the use of large diameter rotary core drilling techniques using air mist

74、or polymer gel flush. Triple tube core drilling is carried out through the overburden soils with the recovered material held in a plastic core liner.</p><p>　　Core recovery in low plasticity boulder clay has

75、 been shown to be extremely good (typically in excess of 90%). The high core recovery permits detailed engineering geological logging and provision of samples for laboratory testing.</p><p>　　In drumlin area

76、s, such as those around Cavan and Monaghan, IGSL has found the use of large diameter polymer gel rotary core drilling to be very successful in recovering very stiff / hard boulder clay soils for deep road cut areas (wher

77、e cable percussion boreholes and trial pits have failed to penetrate). In-situ testing (vanes, SPT’s etc) can also be carried out within the drillhole to establish strength and bearing capacity of discrete horizons.</

78、p><p>　　Large diameter rotary drilling costs using the aforementioned systems are typically 50 to 60% greater than conventional HQ core size, but again from a value engineering aspect can prove much more worthw

79、hile due to the quality of geotechnical information obtained.</p><p>　　SMALL-SCALE COMPACTION TRIALS</p><p>　　The undertaking of small-scale compaction trials during the ground investigation pro

80、gramme is strongly advised, particularly where ‘marginally suitable’ soils are present in prominent cut areas. In addition to validating the laboratory test data, they enable more realistic planning of the earthworks and

81、 can provide considerable cost savings.</p><p>　　The compaction trial can provide the following:</p><p>　　Achievable field density, remoulded shear strength and CBR</p><p>　　Establi

82、shing optimum layer thickness and number of roller passes</p><p>　　Response of soil during compaction (static v dynamic)</p><p>　　Monitor trafficability & degree of rutting.</p><p

83、>　　A typical size test pad would be approximately 20 x 10m in plan area and up to 1.5m in thickness. The selected area should be close to the cut area or borrow pit and have adequate room for stockpiling of material.

84、Earthwork plant would normally entail a tracked excavator (CAT 320 or equivalent), 25t dumptruck, D6 dozer and either a towed or self-propelled roller.</p><p>　　In-situ density measurement on the compacted f

85、ill by nuclear gauge method is recommended as this facilitates rapid measurement of moisture contents, dry and bulk densities. It also enables a large suite of data to be generated from the compacted fill and to assess t

86、he relationship between degree of compaction, layer thickness and number of roller passes. Both disturbed and undisturbed (U100) samples of the compacted fill can be taken for laboratory testing and validation checks mad

87、e with the fie</p><p>　　7. SUPERVISION OF GROUND INVESTIGATION PROJECTS </p><p>　　Close interaction and mutual respect between the ground investigation contractor and the consulting engineer is

88、considered vital to the success of large road investigation projects. A senior geotechnical engineer from each of the aforementioned parties should liase closely so that the direction and scope of the investigation can b

89、e changed to reflect the stratigraphy and ground conditions encountered.</p><p>　　The nature of large ground investigation projects means that there must be good communication and flexibility in approach to

90、obtaining data. Be prepared to compromise as methods and procedures specified may not be appropriate and site conditions can quickly change.</p><p>　　From a supervision aspect (both contractor and consulting

91、 engineer), the emphasis should be on the quality of site-based geotechnical engineers, engineering geologists as opposed to quantity where work is duplicated.</p><p>　　8. GEOTECHNICAL CERTIFICATION</p>

92、;<p>　　The Department of Transport (UK) prepared a document (HD 22/92) in 1992 for highway schemes. This sets out the procedures and documentation to be used during the planning and reporting of ground investigati

93、ons and construction of earthworks.</p><p>　　Road projects involving earthmoving activities or complex geotechnical features must be certified by the Design Organisation (DO) - consulting engineer or agent a

94、uthority. The professional responsibility for the geotechnical work rests with the DO.</p><p>　　For such a project, the DO must nominate a chartered engineer with appropriate geotechnical engineering experie

95、nce. He/she is referred to as the Geotechnical Liaison Engineer (GLE) and is responsible for all geotechnical matters including preparation of procedural statements, reports and certificates.</p><p>　　Sectio

96、n 1.18 of HD 22/92 states that “on completion of the ground investigation works, the DO shall submit a report and certificate containing all the factual records and test results produced by the specialist contractor toge

97、ther with an interpretative report produced either by the specialist contractor or DO”. The DO shall then prepare an Earthworks Design Report – this report is the Designer’s detailed report on his interpretation of the s

98、ite investigation data and design of earthworks.</p><p>　　The extent and closeness of the liaison between the Project Manager and the GLE will very much depend on the nature of the scheme and geotechnical co

99、mplexities discovered as the investigation and design proceed.</p><p>　　After the earthworks are completed, a geotechnical feedback report is required and is to be prepared by the DO. This addresses the geot

100、echnical issues and problems encountered during the construction earthworks and corrective action or measures taken. Certificates are prepared by the DO to sign off on the geotechnical measures carried out (e.g. unstable