Search

The long-term loading of the trains on the subgrade in service is the cyclic loading when the train passes and the intermittent stage when there is no train passing. However, in most studies, continuous loading was used to investigate the dynamic characteristics of a subgrade soil, which ignores the effect of loading intermittence. The aims of this research were first to study the cumulative permanent deformation behaviors under intermittent cyclic loading of trains, then to establish the classification criterion for cumulative permanent strain behaviors, and finally to propose a practical method for estimating the critical dynamic stress in the intermittent loading mode. For this purpose, according to the actual conditions of the subgrade of Shuo-Huang Heavy-Haul Railway, the monotonic triaxial test and the cyclic triaxial test with intermittent loading were performed. Based on the shakedown theory, the cumulative permanent strain behaviors under intermittent loading can be classified into three categories of plastic shakedown, plastic creep, and incremental collapse. Then, the classification criterion was established based on the permanent strain rate. Finally, an empirical formula of critical dynamic stress for intermittent loading, which could also consider the varying moisture content, was proposed.

Since there is an interval time between two trains traveling in the same direction on an automatically blocked line, the dynamic loading on the railway subgrade is composed of periodic vibration when the train passes and an intermittent stage after a train has passed. Take into account this characteristic, a series of continuous-stopping repeated load triaxial tests were conducted to study deformation characteristics such as plastic strain and elastic strain of fine-grained soil under intermittent loading. Compared to continuous loading, it is found that the intermittent stage has a significant effect on the deformation characteristics of subgrade. The excess pore water pressure accumulated during the loading stage dissipated during the intermittent stage. The development of plastic strain in subsequent loading stages slowed down significantly, and the axial strain of the sample rebounded in the intermittent stage, so cumulative plastic strain was reduced. The intermittent effect on reducing elastic strain was, however, limited. An increase in intermittent time can also improve the resistance of the sample to subsequent cyclic loading. An alternating continuous-stopping loading mode can better simulate intermittent loading of trains in actual train operation and produce more practical results, providing a theoretical basis and guidance for further understanding of deformation characteristics of railway subgrade.

A novel gene encoding an endoglucanase designated Cel5D was cloned from a marine bacterium Martelella mediterranea by genomic library. The gene had a 1,113 bp opening reading frame encoding a 371-amino-acid protein with a molecular mass of 40,508 Da and containing a putative signal peptide (41 amino acids). Cel5D had low similarity (48-51% identity) with other known endoglucanases and consisted of one single catalytic domain, which belonged to the glycosyl hydrolase family 5. The maximum activity of Cel5D was observed at 60 degrees C and pH 5.0. Cel5D displayed broad pH stability within the range of pH 3.0-11.0 and retained hydrolytic activity in the presence of a wide variety of metal ions and some chemical reagents. These characteristics suggest that the enzyme has considerable potential in industrial applications.

Bacillus subtilis endo-beta-1,4-glucanase (Cel5A) hydrolyzes cellulose by cleavage of the internal bonds in the glucose chains, producing new ends randomly. Using directed evolution techniques of error-prone polymerase chain reaction (PCR) and DNA shuffling, several Cel5A variants with improved catalytic activity had been screened from the mutant library, which contained 71,000 colonies. Compared with the wild-type enzyme, the variants (M44-11, S75 and S78) showed 2.03 to 2.68-fold increased activities toward sodium carboxymethyl cellulose (CMC), while the M44-11 also exhibited a wider pH tolerance and higher thermostability. Structural models of M44-11, S75, S78, and WT proteins revealed that most of the substitutions were not located in the strictly conserved regions, except the mutation V255A of S75, which was closed to the nucleophile Glu257 in the catalytic center of the enzyme. Moreover, V74A and D272G of M44-11, which were not located in the substrate binding sites and the catalytic center, might result in improved stability and catalytic activity. These results provided useful references for directed evolution of the enzymes that belonged to the glycoside hydrolase family 5 (GH5).