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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 costs 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 for 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 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. 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 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) 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 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 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 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 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: · Trial pits · Cable percussion boreholes · Dynamic probing · Rotary core drilling · In-situ 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 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, 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 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 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 plasticity 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 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. 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 (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 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 the 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 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 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 determining 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 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 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 MCV’s. 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 plasticity 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 storage 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 can 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 this, 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 is important as this allows excess pore water pressures to dissipate. 5. ENGINEERING CLASSIFICATION OF SOILS In 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 cohesive 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. Irish boulder clay soils are predominantly Class 2C. Clause 612 of the SRW sets out compaction methods. Two procedures are available: · Method Compaction · End-Product Compaction 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 Target 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 be measured, preferably by nuclear gauge or sand replacement tests where the stone content is low. As placing and compaction of the fill progresses, the in-situ TDD can be checked and non-conforming 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’. 6. SUPPLEMENTARY GROUND INVESTIGATION METHODS FOR EARTHWORKS The more traditional methods and procedures have been outlined in Section 2. The following are examples of methods which are believed to enhance ground investigation works for road projects: · Phasing the ground investigation works, particularly the laboratory testing · Excavation & sampling in deep trial pits · Large diameter high quality rotary core drilling using air-mist or polymer gel techniques · Small-scale compaction trials on potentially suitable cut material 6.1 PHASING Phasing ground investigation works for many large projects has been advocated for many years – this is particularly true for road projects where significant 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 suitability of the methods. 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 a junior engineer while the senior design engineer who will probably design the earthworks will have no real involvement. It is highlighted that the engineering performance tests are expensive and of 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 more 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 immediately obtained. 6.2 DEEP TRIAL PITS 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 pits in boulder clay soils to depths of up to 12m is feasible using benching techniques and sump pumping of groundwater. In recent years, IGSL has undertaken such deep trial pits on several large road 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. It is recommended that 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 excavating 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 programme. 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 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-scale compaction trials. 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 investigation and used for this work. 6.3 HIGH QUALITY LARGE DIAMETER ROTARY CORE DRILLING This system entails the use of large diameter rotary core drilling techniques using air mist 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. Core recovery in low plasticity boulder clay has 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. In drumlin areas, 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 (where 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. 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 worthwhile due to the quality of geotechnical information obtained. 6.4 SMALL-SCALE COMPACTION TRIALS The undertaking of small-scale compaction trials during the ground investigation programme 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 can provide considerable cost savings. The compaction trial can provide the following: · Achievable field density, remoulded shear strength and CBR · Establishing optimum layer thickness and number of roller passes · Response of soil during compaction (static v dynamic) · Monitor trafficability & degree of rutting. 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. Earthwork plant would normally entail a tracked excavator (CAT 320 or equivalent), 25t dumptruck, D6 dozer and either a towed or self-propelled roller. In-situ density measurement on the compacted fill 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 the 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 made with the field data (particularly moisture contents). IGSL’s experience is that with good planning a small-scale compaction trial takes two working days to complete. 7. SUPERVISION OF GROUND INVESTIGATION PROJECTS Close interaction and mutual respect between the ground investigation contractor and the consulting engineer is 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 be changed to reflect the stratigraphy and ground conditions encountered. The nature of large ground investigation projects means that there must be good communication and flexibility in approach to obtaining data. Be prepared to compromise as methods and procedures specified may not be appropriate and site conditions can quickly change. From a supervision aspect (both contractor and consulting engineer), the emphasis should be on the quality of site-based geotechnical engineers, engineering geologists as opposed to quantity where work is duplicated. 8. GEOTECHNICAL CERTIFICATION 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 investigations and construction of earthworks. Road projects involving earthmoving activities or complex geotechnical features must be certified by the Design Organisation (DO) - consulting engineer or agent authority. The professional responsibility for the geotechnical work rests with the DO. For such a project, the DO must nominate a chartered engineer with appropriate geotechnical engineering experience. 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. Section 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 together 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 site investigation data and design of earthworks. 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 complexities discovered as the investigation and design proceed. After the earthworks are completed, a geotechnical feedback report is required and is to be prepared by the DO. This addresses the geotechnical issues a