The mechanics of dynamic compaction are studied in this paper using two-dimensional finite element analyses with a large-strain dynamic formulation and a cap model for soil behaviour. Comparison with centrifuge model results shows that stress wave attenuation and improvement effects are realistically predicted. The analyses show that, in the initial blows, stress wave propagation induces transient elasto-plastic K0 compression due to lateral inertia. This preserves the plane wavefront and reduces the attenuation rate of the dynamic stresses with depth. With multiple blows, the effect changes to one of triaxial compression: this sets a limit on the degree of improvement that can be achieved in the near field. Deeper down, the wavefront adopts a bullet shape, and the attenuation rate rises: this sets a limit on the depth of improvement. Both these phenomena are consistent with the existence of a ‘threshold’ state that has been noted in previous literature on dynamic compaction. The results also show that th...

Ground response to dynamic compaction of dry sand

Field study on the reinforcement of collapsible loess using dynamic compaction

Recent experimental studies revealed that when the drainage condition of loose sand is switched from drained to undrained it can manifest runaway instability prior to attaining the failure stress state To study this type of unstable behavior (referred to as prefailure instability) under more general conditions, the strain-path testing technique was used to constrain the strain development of a triaxial sample along a constant strain-increment-ratio path Medium-loose to dense samples were used in this study Provided certain conditions are met, prefailure instability can occur even for a dense sample The factors affecting the occurrence of instability are established from test results Two instability conditions are established for predicting of instability under a general stress-controlled condition These two conditions require input parameters that are relatively simple to obtain yet can accurately predict the occurrence of instability For the special case of undrained condition, these two instability conditions degenerate to those proposed by previous researchers

Instability of Granular Soils under Strain Path Testing

A simplified model based on the one‐dimensional wave equation, which accounts for the interaction of the pounder and the soil, and the propagation of stress wave in the soil during the dynamic compaction of loose granular soil is presented. The soil beneath the pounder is represented by a nonlinear soil column, while the surrounding soil is represented by a series of springs and dashpots. The spring simulates the dynamic soil stiffness and the dashpot accounts for the radiation damping effect. The input soil parameters of the model can be determined in the laboratory or estimated from correlations with measured field data. In spite of the various simplifying assumptions in the model and the predictive method, computed results, such as pounder penetration and degree and depth of improvement, show an encouraging measure of agreement with available field measurements from two dynamic compaction projects. The proposed model is potentially useful for the analysis of dynamic compaction of loose granular soil.

Dynamic Compaction Analysis

The strain-softening response of a granular soil along a wide spectrum of strain paths is investigated experimentally. Direct strain-path control is achieved by controlling the ratio of volume to axial strain increment using a digital pressure/volume controller. The questions associated with the definition of strain softening are examined. Provided suitable experimental techniques are used to delay the onset of nonhomogeneous deformation, strain softening as a true material property of a soil element can be observed along certain strain paths. Two types of strain softening, prefailure and postfailure types, have been identified. The conditions for the occurrence of prefailure or postfailure strain softening were established. During postfailure strain softening, the effective stress state of the sample traces along the failure curve toward the origin. The sample, however, will suffer from an irrecoverable loss in shear strength if sheared at a confining stress close to the value prior to onset of postfailure softening. Thus, the shear strength of a sample, although not affected by unidirectional, prefailure shearing, is dependent on postfailure strain history.

Strain‐Softening Behavior of Granular Soil in Strain‐Path Testing

A world-wide review of heavy tamping projects indicated that highly organic clays which due to their secondary compression characteristics may not be amenable to treatment by surcharge and vertical drain installation alone, may nevertheless be stabilized by applying high energy impact on installed dynamic replacement (DR) sand columns using a conventional dynamic consolidation plant. Field trials were thus conducted to enforce primary, as well as to negate secondary, compression of in situ peaty clay deposits by this form of treatment application, in the course of which dynamic replacement and mixing (DRM) of such soils with sand charges was originally identified as a distinct ground-improvement mechanism. From the field results and review, it is also perceived that ground-improvement mechanisms resulting from the application of a wide variety of heavy tamping plant and procedures to soils ranging from granular to highly organic cohesive materials may be generalized on a common physical basis. Further, unique relations may apparently be construed between alternative measures of degree of ground improvement on the one hand, and a collective term incorporating initial soil consistency and standard operational parameters of ground treatment on the other, thereby providing a basis for rational performance design.

Kwang Wei Lo

Papers

Discussion of "Unified Approach to Ground Improvement by Heavy Tamping"