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Numerical Modeling of Dynamic Compaction Operation in Sandy Soils Deposits in Order to Provide a Design Approach

Jahangiri, Ghodrat | 2009

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 40122 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Pak, Ali
  7. Abstract:
  8. Dynamic compaction is a widely used soil deep improvement method in dry and/or saturated soils. Despite its vast application, numerical and /or experimental modeling studies of dynamic compaction due to impact are little. In order to clarify the ambiguities in process and to present a comprehensive design approach, numerical modeling of dynamic compaction in dry soils with PISA (A fully coupled dynamic finite element code: Program for Incremental Stress Analysis) has been done. Because the geometry and loading configuration are symmetric around the falling tamper (load centerline), a two-dimensional axisymmetric simulation has been used. Here for sake of simplicity, initial velocity of tamper nodes that can be determined from free fall equation has been used as input of program (Rigid Body Method). Granular soil densification behaviour due to body wave propagation is modelled using cap model of models introduced by Dimaggio and Sandler (1971). In order to verify the constitutive law formulation and numerical modelling procedure for dry soils, comparisons have been made with Desai et al. (1981) experimental model test and Takada & Oshima (1994) centrifuge model test. There were good agreements between numerical and experimental results. To assessment the capabilities of mesh update formulation available in software a comparison has been made with Gu & Lee (2004) numerical studies with updated-lagrangiam large deformation formulation. Average difference between two solutions was about %11. A comprehensive parametric study has been done that result in a rational design approach. Parameters were: energy, momentum, tampers radius, soil type and initial relative density. According to parametric studies two design methods has been suggested: 1) Determination of depth of improvement when energy and momentum are known. 2) Determination of Necessary energy and number impact with introduced depth. Comparison with Takada & Oshima (1994) centrifuge test and Chow et al. (1992) field operation showed that suggested methods have acceptable accuracy. With assumption of non-plastic model for a certain depth and applying super position law, a method for determination of grid spacing has been introduced. Choice of none-plastic model for deeper zone seems reasonable due to the large distance between that and energy source (impact point). Farther more elastic analysis showed that soils plastic behaviour after a certain depth was similar to elastic behaviour. Two design curves, one for near the surface (zone with high density) and other for far from the surface (zone with low density) are established. These curves summarised the influence of print spacing on the effectiveness of dynamic compaction in densification the soil to reach a certain improvement to certain depth. One reported case history of dynamic compaction is analyzed, and the solution is shown to be in good agreement with those obtained from the field.
  9. Keywords:
  10. Dynamic Compaction ; Design Technique ; Numerical Analysis ; Impact Load ; Grid Spacing

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