Tsutomu Namikawa

Effects of Spatial Correlation on the Compression Behavior of a Cement-Treated Column

The strength of soil treated by cement deep mixing varies greatly, even at a single column, because of the variability of in situ soil properties, the variability of mixing effectiveness, and other factors. The variation in strength would affect the entire behavior of the cement-treated column. This paper presents a numerical experiment that investigates the effect of spatial correlation on the unconfined compressive strength of cement-treated columns. Finite element analyses incorporating random-field theory were performed to simulate the unconfined compression behavior of full-scale column samples in which the material properties vary with respect to spatial autocorrelation. The numerical results indicate that spatial autocorrelation affects the compression behavior of full-scale cement-treated columns. The overall strength of the sample in the absence of spatial autocorrelation is slightly lower than that of the uniform sample of mean strength. On the other hand, the overall strength of the sample in the presence of spatial autocorrelation is significantly lower than that of the uniform sample of mean strength. These results suggest that spatial autocorrelation should be considered when evaluating the overall strength of full-scale cement-treated columns within a relevant design procedure.

Conditional Probabilistic Analysis of Cement-Treated Soil Column Strength

AbstractThe quality of cement-treated soil columns is usually assessed by examining the strength of cored samples. This paper presents a finite-element analysis approach with random field theory for assessing the quality of a cement-treated soil column from which cored samples are retrieved. Monte Carlo simulations with conditional distribution (conditional simulation) were conducted to generate spatial dependence random fields with the data known at some locations. Simulations without the known data (unconditional simulation) were also performed for comparison. Using the random field samples generated from the Monte Carlo simulations, finite-element analysis was performed to simulate the compression behavior of a full-scale column in which material properties vary with a presence of spatial autocorrelation. Finite-element analysis with conditional simulation predicts the overall strength of a targeted cement-treated soil column appropriately, and the overall strength variability obtained with the conditi...

Influence of pulling out existing piles on the surrounding ground

Teardowns of social infrastructure, including civil structures, have been increasing in number in recent years because these structures have aged and their utilization has decreased along with the decrease in population. The number pile foundations being pulled out is now far greater than that being newly installed. However, after a pile foundation is pulled out, the mechanical characteristics of the surrounding ground may be affected by the existence of the resulting hole formed by pulling out. There are no regulations yet on injecting fillers into pull-out holes, and the influence of filler strength on the surrounding ground is yet to be elucidated. This study considers the influence of a pull-out hole on the static and dynamic characteristics of the surrounding ground using two-dimensional dynamic finite-element analysis. The special qualities required by fillers injected into such holes are also clarified.

Rapid Compression Test of Cement-treated Sands

During earthquakes, dynamic loads caused by inertia forces are applied to ground improvements by deep mixing method. Therefore, the internal stability of cement-treated soil ground subjected to the seismic loading should be examined in the seismic design of the ground improvement. The speed of the loading caused by the inertia forces during earthquakes is much faster than that in the standard unconfined compression test. It is well known that the loading speed affects the strength of materials. Therefore, the strength under the rapid loading conditions is required to design the ground improvements rationally. The unconfined compression tests of cement-treated soils were conducted under the rapid loading conditions in this study. The rapid load in the shape of a solitary wave is applied to the specimens. The unconfined compression tests with the strain rate of 1%/min were also conducted for comparison. The experimental result indicates that the rapid loading increases the strength by about 20% as compared with the strain controlled loading. The test results were analyzed with the statistical manner to evaluate the variation of the strength of the cement-treated soil specimens prepared in a laboratory. The statistical analysis provides the good understanding of the influence of the data variation on the relationship between the strengths obtained from the rapid loading and strain controlled tests.

Literature survey and experimental study on the direct tension test on rocks

Direct tension tests are indispensable for obtaining an accurate understanding of the deformation and failure characteristics of rocks in tension. For this technical note, the authors collected original papers published in international journals and surveyed the previously reported methods and results of direct tension tests on rocks. Some important factors affecting the strength and stress–strain curve are discussed, such as the shape, size, anisotropy, and water content of the specimen; the loading rate; and the confining pressure. The relationship between the results of the direct tension tests and those of other tests also is discussed. Although various types of testing apparatuses have been adopted in direct tension tests on rocks, the dependence of test results on the apparatuses has not been investigated. In this study, direct tension tests were conducted using various testing apparatuses in seven laboratories. The results of the direct tension tests showed that direct tensile strengths were not dependent on the type of test apparatus used or whether a flexible linkage system was used. The results of the tuff in this study showed that the coefficient of variation of the direct tensile strengths is smaller than that of the Brazilian tensile strengths. Moreover, the results revealed that the coefficient of variation of the direct tensile strengths is comparable to that of the uniaxial compressive strengths of the tuff. The authors noted that to obtain the stress–strain curve from the pre-failure region to the post-failure region, it is necessary to conduct a test with a high-stiffness machine and without a flexible linkage system. This technical note summarized the results of the standardization activity by the authors in the Japanese Geotechnical Society.