Experimental Study On Strength Characteristics Of Hydraulically Dredged Mud Slurry Treated By Physicochemical Composite Method

Enormous amount of unwanted dredged mud is produced every year in the projects of navigation channel construction and ecological dredging of inland water areas.The dredged mud has poor engineering characteristics and is difficult to be directly utilized.In general,the dredged mud is first disposed into dumping site and then recycled at an appropriate time point.In the process from dredging to recycling,there are two scenarios commonly involving chemical stabilization of dredged mud at extra high water content.On the one hand,the dredged mud is slurry-like,and the surface of newly dredged mud slurry filled in the dumping site is not able to meet the requirements for the access of workers and equipments involved in subsequent soil treatment,so the chemical solidification method can be used to form a working platform on the surface of dumping site.On the other hand,after natural deposition the dredged mud in the dumping site can be recycled as filling materials,but its mechanical properties need to be improved in advance by chemical treatment.It is worth noting that in both scenarios,the water content of the dredged mud to be solidified is very high.If cement or some other traditional stabilizing agent is used directly,the solidification efficiency is very low or even marginal,leading to significant increase in engineering cost.Based on the traditional cement solidification method(PCSM),flocculation conditioning technique,fiber reinforcement technique and vacuum preloading technique,new physicochemical treatment procedures respectively applicable to above two scenarios are proposed in this study to achieve efficient solidification of dredged mud at extra high water content and reduce engineering cost.The proposed treatment procedures involves multiple physicochemical processes such as flocculation,solidification,fiber reinforcement(optional)and vacuum preloading(optional).Then,a large number of laboratory experiments are performed in this thesis to demonstrate the feasibility of the proposed physicochemical treatment procedures.The main experimental work and results can be summarized as follows:(1)Simple sedimentation tests are conducted to determine the reasonable flocculant dosage range suitable for flocculation-solidification combined method(FSCM)and some laboratory model tests are carried out to identify the strength characteristics of FSCM treated dredged mud.The results show that the PAM solution shows good flocculation performance within the range of(0.137%,0.197%)and the reasonable PAM dosage is around 0.157%.In comparison with PCSM,the undrained shear strength of the mud treated by FSCM is larger than 5 times of that treated by the corresponding PCSM.(2)A series of laboratory preloading tests are performed,on mud samples treated by the fiber reinforcement-flocculation-solidification combined method(FR-FSCM)which is an extention of FSCM by admixing polypropylene fiber into the mixture,to find out the optimal fiber addition scheme and to explore the strength characteristics of FR-FSCM treated dredged mud.The results indicate that the optimal fiber addition scheme is 6mm in length and 0.6% in dosage.In comparison with FSCM,the FR-FSCM can generally increase the unconfined compressive strength of the samples with the maximum increase rate of 145.8%.(3)The vacuum preloading(VP)technique is further integrated with FSCM and FR-FSCM to form two extended methods named VP-FSCM and VP-FR-FSCM,in an effort to further enhance the treatment efficiency.A number of vacuum preloading tests are also conducted in this study to identify the strength characteristics of VP-FSCM and VP-FR-FSCM treated mud samples.It is demonstrated that the lower the equivalent initial water content or the higher the vacuum degree,the greater the undrained shear strength of VP-FSCM treated mud.The undrained shear strength of the samples treated by VP-FR-FSCM can be up to 10.3 times of that produced by the corresponding FSCM.(4)X-Ray Diffraction(XRD)tests and Scanning Electron Microscope(SEM)tests are also implemented in this study to analyze the microscopic strength enhancement mechanism of dredged mud treated by the proposed physicochemical methods.The results show that,PAM molecules mainly change the microstructure by means of adsorption and bridging,while fibers mainly play the role of reinforcement by cross-linking with the cementitious products.PAM molecules,fibers and cementitious products work together to form the heavily integrated and bonded soil skeleton,so as to effectively improve the strength of treated mud.