Influence of Load Plates Diameters, Shapes of Columns and Columns Spacing on Results of Load Plate Tests of Columns Formed by Dynamic Replacement

Abstract

The dynamic replacement method is used to strengthen the subgrade of objects, usually up to 5 to 6 m thick. After the improvement process, acceptance tests in the form of load testing are carried out. Interpretation of the test results can cause some difficulties. Dynamic replacement results in a situation where columns of different shapes, loaded with plates of diameters usually smaller than the head diameter and in the vicinity of adjacent columns, are subjected to load tests. In order to demonstrate the influence of these factors, a spatial model of soil strengthened by dynamic replacement, comprising four material zones, was calibrated on the basis of load testing. The following models were used in the analysis: linear-elastic, elastic–perfectly plastic (Coulomb–Mohr) and elastic–plastic with isotropic hardening (Modified Cam-Clay). This formed the basis for 105 numerical models, which took into account the actual shapes of the columns made at various spacings, subjected to load tests with plates of various diameters. The analyses of the settlements, calculated moduli and stress distribution in the loaded system showed how the results were significantly influenced by mentioned factors. This implies that the interpretation of the results of load tests should be based on advanced spatial numerical analyses, using appropriate constitutive models and including the considered factors.

Influence of soft layer thickness on the aggregate displacement in the backfill material of dynamic replacement columns – results of laboratory model tests

“…The shapes of the columns formed by the dynamic replacement technique in the field conditions have been studied by one of the authors since 2007. The total of 65 endbearing and floating columns were investigated [5,8,11]. It has been noticed that certain relationships exist between the shapes of the end-bearing columns and the ratio of the thickness of the soft soil layer (𝐻 𝑠 ) to the height of the used pounder (𝐻 𝑝 ) [9].…”

“…The maximum final length of a typical end-bearing column is usually 5-7 m [4,6,7]. The in situ measurements of the as-built columns, conducted by one of the authors [5,8,9], proved that the ground conditions have a great influence on the final shape of the columns. One of the most important factors is the ratio of the soft soil layer thickness and the height of the pounder.…”

Dynamic Replacement: The Influence of Pounder Diameter and Ground Conditions on Shape and Diameter of the Columns

“…One involves case studies providing only the basic information on the diameters and lengths [4], [18][19][20][21], where the diameters were usually measured at the terrain level. The second group is less numerous and comprises articles where the column shapes and dimensions are determined directly from the measurements of the excavated columns [15], [22][23][24][25], or indirectly from penetration tests [5], [26] or geophysical survey [27]. Table 1 presents the summary of the literature review including the observed shapes of the columns, the D c /D p ratios and the ground profiles at the sites.…”

“…Kwiecień and Sękowski [24] identified up to five different shapes, which confirms the specific nature of this technology in relation to the other methods of column formation, where the columns are mainly cylindrical [16,17]. Kwiecień [15] indicated the influence of the soil thickness (expressed as the ratio of the weak soil thickness H s to the pounder height H p ) on the end-bearing columns' shapes: the shape of the columns changed from a cylinder (for H s /H p = 1), through a truncated cone (H s /H p = 1-1.5), a barrel (H s /H p = 1.5-2.0) and asymmetrical barrel (H s /H p = 2-2.5). For the floating columns three common shapes were identified: a cylinder, a barrel and an inverted truncated cone (regardless of the H s /H p ratio).…”

“…The acceleration of consolidation due to drainage was first noticed and described for vertical drains without regard for their stiffness [9,10,11], while the effect of the stiffness was investigated among others in [12,13,14]. Due to the specific method of execution, the DR columns are predominantly non-cylindrical [15] their cross-section area is not constant with depth. The major impact on the eventual diameter of the column (D c ) has the width or diameter of the pounder (D p ), because it determines the initial diameter of the crater that is later filled with the replacement material.…”