1、 DESIGN AND EXECUTION OF GROUND INVESTIGATION FOR EARTHWORKS 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 c
2、osts of importing engineering 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 certification is raised and recommendations outlined on its merits fo
3、r incorporation with ground investigations and earthworks. 1. INTRODUCTION The investigation and re-use evaluation of many 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 plasti
4、city 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. Moisture contents
5、 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 amplif
6、ies 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 re-use as engineering fill. Many of our boulder clay soils (especially those with intermediate type silts and fine sand matrix)
7、 have been rejected at the selection stage, but good planning shows that they can in fact fulfil specification requirements 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 incorp
8、orate 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 use as
9、fill in low height embankments (i.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 properties of the soils, as for many projects thi
10、s is effectively governed by the trafficability of earthmoving equipment. 2. TRADITIONAL 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 6.1 from Series 600 of the NRA Specification for R
11、oad 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: Trial pits Cable percussion boreholes Dynamic probing Rotary core drilling In-situ testing (SPT, variable head permeability tests,
12、 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 boreholes are normally sunk to a desired depth or refusal with disturbed and undisturbed samples recover
13、ed 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 inac
14、curate 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 opportun
15、ity 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
16、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 accura
17、te assessment 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 based on its particle size grading, supplemented by plastic
18、ity for fine soils. For many of our glacial till, boulder clay soils (i.e. mixed soils) difficulties arise with descriptions and assessing engineering performance tests. As outlined previously, Irish boulder clays usually comprise highly variable proportions of sands, gravels and cobbles in a silt o
19、r 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. A key p
20、arameter (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 (U100s) or good quality
21、 core samples to evaluate the drainage characteristics of the soils accurately. 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 measures. This usually entails the installation of perimeter drains and sumps
22、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 become suitable as engineering fill. 4. ENGINEERING PERFORMANCE TESTING OF SOILS Laboratory testing is very much dictated by the proposed end-use for th
23、e soils. The engineering parameters set out in Table 6.1 pf the NRA SRW include a combination 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 schedul
24、ing 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 comparison. Pore pressures are not taken into account during compaction and may vary considerably between laboratory and field. P
25、reparation methods for soil testing must be clearly stipulated and agreed with the designated laboratory. 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 analysis f
26、or direct comparison, although this is not always practical. In the majority of cases, the MCV when used with compaction data 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 rec
27、ommended as it provides a rapid assessment of 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 and this can have a significant impact on the resultant MCVs.
