Ground compaction due to vibrodriving of piles
Civil engineering construction frequently requires the use of piles to carry structural loads to stronger ground strata or to control lateral ground movements. A variety of techniques are available to install piles into the ground. Of central interest to this research is the vibratory hammer, or vibrodriver, which is the preferred method used to drive piles into granular soils. .The installation of sheet and bearing piles by vibrodriver causes periodic vibration in the adjacent ground which is severe very close to the piles, but attenuates with distance. A potential consequential effect of the vibrations that are caused by vibrodriving is ground compaction, which may be observed as differential surface settlement. It is desirable that vibration induced ground compaction settlement should be estimated for contracts where loose to medium-dense granular soils occur, especially when buildings on shallow foundations or poorly bedded service pipes are adjacent. It is unlikely that a simple in-situ soils test will allow accurate, specific estimates, but rather that a range of vibratory tests should be performed which can then be used as a knowledge base. Settlement trends and associated parameters can then be identified which will allow the prediction of settlement with reference to the in-situ soil and the ground vibration data. This argument forms the basis of the laboratory test programme. A range of granular soils were studied using an adapted 150mm Rowe cell (a hydraulic oedometer). Use of the Rowe cell enabled samples to experience compaction under effective stress conditions that are appropriate for equivalent soils in the field. The complete cell was mounted on an electromagnetic shaker and after static consolidation, the samples were vibrated under maintained hydraulic load, at frequencies and accelerations that are appropriate for soils adjacent to vibrodrivers. Change in sample height was recorded for controlled vertical (and horizontal) vibrations, typically in the range of 0.lg to 5.0g at 25Hz and 40Hz. Soils were tested under a range of effective stresses and moisture content. The results of the laboratory programme and subsequent data analysis are presented in tables and diagrams. Expressions that describe a good relationship between acceleration, soil type, relative density and static load allow upperbound estimates of vibratory settlements to be made for accelerations of up to 6.0g. An additional expression is presented that accounts for the influence of moisture content, ground vibration frequency and vibration duration. Summary tables are presented that define categories of vibration induced ground compaction settlement based on settlement potential, risk and severity. The use of the settlement equations and the influence of various parameters are demonstrated for a range of example applications, hi addition, data is abstracted from case studies found in the literature and sites that were visited during the research. The abstracted data are then used to perform settlement estimates which are compared to the reported examples. Good correlation between observed and calculated settlement is demonstrated in many cases. However, in some instances, it appears that ground settlements were exacerbated by at least one additional mechanism, such as cumulative pore water pressure increase, or lateral movement of sheet piles, in addition, extraction of piles by vibrodriver appears to contribute significantly to the reported cases of ground settlement.