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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 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 slopes, karst features, disused / abandoned mine workings, ground improvement systems employed, etc).