dc.description.abstract | In this study, Danang Sand was used as the main material for backfilling in water. The purpose of this study is focused on the mechanical properties, liquefaction mechanisms and improved effects of cement treated sand after the procedure of water sedimentation. A optimum ratio model was established as a suggestion for reclamation in water or a prevention method of liquefaction hazards. The first step of research was to control initial dry weight or initial relative density, and then wet tamping method and water sedimentation method was used to prepare soil specimens under the condition of different cement content, curing time, fine content, cement type, salinity and coagulant, respectively. A series of unconfined compression tests and dynamic triaxial tests were carried out.
The results of this study showed that the shear strength of cement treated sand prepared by wet tamping method increased significantly as the increase of initial dry unit weight, cement content and curing time, respectively. On the other hand, due to the difference between the control of main variable and types of fine materials, there exists different tendencies with the increase of fine content. Under the control of initial relative density and adding Danang fines in cement treated sand, the unconfined compression strength raised continuously with the increase of fine content. When the control of main variable was initial relative density, the unconfined compression strength increased first and then decreased with the increase of fine content, whether adding Danang fines or Kaolinite. Furthermore, the unconfined compression strength presented a clear trend to reduce with the increase of salinity of environment when curing time exceeded or was equal to 28 days.
The cement treated sand can provide great shear strength for land reclamation. However, for the projects of reclamation in water, the cement treated sand will occur a separation between cement and sand particles, and that caused a significant loss of about 90 % of shear strength. Therefore, in this study, some coagulants were used and added into the cement treated sands. Because the coagulant can coat the sand particles and cement effectively, to prevent cement treated sand from separating by charge adsorption and bridging, and hydration of cement can act effectively and continuously. Then the shear strength of cement treated sand can be enhanced. Among them, the best type of coagulant is polyacrylamide which can effectively reduce the loss of unconfined compression strength, and the lowest loss rate can be reduced to 41 %.
In addition, under the condition of using the Portland cement type I, Danang fine and Danang sand, this study recommends a model of unconfined compression strength for cement treated sand. This model includes five parameters, such as initial dry unit weight, cement content, fine content, salinity and curing time. The unconfined compression strength of cement treated sand can be assessed accurately.
In the scope of this study, when the specimens were prepared by wet tamping method, under the conditions of initial relative density of 40 %, fine content of Kaolinite of 10 %, salinity of 3.5 %, curing time of 28 days and cement content of 1.5 % or when using the water sedimentation method, under the conditions of cement content of 6 %, adding ratio of polyacrylamide of 344 mg/kg and curing time of 28 days, the liquefaction resistance of the cement treated sand mentioned previously can be significantly improved.
In view of most of the presented excess pore water pressure ratio curve models are established by pure sand, these models cannot present the characteristics of excess pore water pressure of cement treated sand properly. Therefore, this study recommends a modified model of excess pore pressure ratio curve which has the advantages of model of both DeAlba et al. (1975) and Sakai et al. (1994), and can improve the shortcomings of this aforementioned two models. So, the modified model mentioned in this study can offer a better exploration of the excess pore water pressure ratio of cement treated sand.
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