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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.
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