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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.