Special Engineering Properties Research Articles

Research on the expansion, shrinkage properties and fracture evolution of red clay stabilised with phosphogypsum under dry-wet cycles.

In view of the special engineering properties of red clay and the waste of phosphogypsum resources, the expansion and contraction deformation and fissure evolution of phosphogypsum stabilized red clay under different conditions were investigated by laboratory tests and image processing system. The research results show that: (1) the absolute expansion and absolute shrinkage of phosphogypsum stabilized red clay are positively correlated with the compaction degree, the number of dry and wet cycles and the cement dosage, and negatively correlated with the initial water content and the phosphogypsum dosage; (2) the fissure rate increases with the increase of the number of dry and wet cycles, and decreases with the increase of the initial water content, the compaction degree, the cement, and the phosphogypsum dosage; (3) The relationship among absolute expansion rate (absolute shrinkage), degree of compaction and fracture rate can be fitted by the equation f(x,y) = ax+by+cx2+dy2+e; (4) Phosphogypsum has an obvious inhibiting effect on the expansion, shrinkage and cracking of the mix. It is recommended that the cement mixing amount of 6% and phosphogypsum: red clay = 1:1~1:2 as roadbed filler.

Engineering Characteristics and Application Analysis of Red Sandstone in the Lanzhou Metro

The water-rich red sandstone strata at the Lanzhou Metro site area have special engineering properties and vary greatly in their speed of disintegration when exposed to water. There is an urgent need for a comprehensive and systematic study of the engineering properties of red sandstone and their classification. From the disintegration speed of red sandstone encountered during the excavation of Lanzhou metro lines 1 and 2, the relationship between physical parameters such as particle size, composition, dry density, and permeability coefficient as well as mechanical parameters such as shear wave speed, dynamic penetration test (DPT), natural uniaxial compressive strength, and disintegration speed of red sandstone was analyzed through indoor and outdoor tests and geological exploration data statistics, and classification guidelines for red sandstone are given. The results show a significant correlation between dry density, permeability coefficient, natural uniaxial compressive strength, and disintegration speed. The red sandstone can be classified as I, II, and III according to the disintegration speed, dry density, permeability coefficient, and natural uniaxial compressive strength. The design of the foundations is differentiated according to the classification, and different support systems are used for the deep foundation pits of the metro stations. The category I red sandstone pit is supported by diaphragm walls with internal bracing, the category II pit by bite piles with internal bracing, and the category III pit by row piles with internal bracing. The study results can provide technical support and experience reference for the investigation, design, and construction of metro projects in similar red sandstone distribution areas.

Experimental Study on Nanomechanical Properties of Yunnan Red-Bed Mudstone

As a natural building material and building environment, red-bed mudstone has special engineering properties due to its special formation conditions. The engineering disaster caused by these special properties is becoming increasingly prominent. Therefore, it is of great theoretical significance and practical engineering value to study the macro- and micromechanical properties of red-bed mudstone. Nanoindentation mechanical tests are carried out on the red-bed mudstone from Chuxiong, Jingdong, and Nanhua in Yunnan. The microstructure, residual indentation morphology, and nanomechanical parameters of red-bed mudstone in different areas are compared and analyzed. The results show that the load-displacement curves can be divided into four stages, which are the compaction stage, linear elastic stage, holding stage, and unloading stage. Under the same loading and holding time conditions, sample 3 (Nanhua) has the deepest residual indentation, and SEM tests show that the damage to indentation is obvious. Sample 2 (Jingdong) has the shallowest residual indentation. The indentation morphology is complete and the size is small by SEM tests. The residual indentation depth of sample 1 (Chuxiong) is between sample 2 and sample 3. The average elastic modulus of sample 1 (Chuxiong) is 38.0 ± 2.29 GPa. The average hardness is 1.36 ± 0.42 GPa, and the fracture toughness is 0.966 ± 0.048 MPa·m0.5. The average elastic modulus of sample 2 (Jingdong) is 40.1 ± 2.03 GPa. The average hardness is 1.71 ± 0.35 GPa, and the fracture toughness is 1.142 ± 0.053 MPa·m0.5. However, the average elastic modulus of sample 3 (Nanhua) is 25.1 ± 2.53 GPa. The average hardness is 0.61 ± 0.21 GPa, and the fracture toughness is 0.507 ± 0.043 MPa·m0.5. Overall, the elastic modulus and the hardness of sample 2 (Jingdong) are the greatest because of its high quartz crystal content. It shows that the nanomechanical properties of red-bed mudstone in Jingdong are better than those in Chuxiong and Nanhua. The results can provide a basis for revealing the regional differences in macroscopic mechanical properties of Yunnan red-bed mudstone.

Experimental research on geotechnical engineering characteristics of coral reef in Xisha Islands

Coral reefs are widely distributed in Xisha Islands, which have special engineering properties. Coral reef geology from top to bottom is coral sand, coral gravel, coral reefs calcareous rock. Coral sand and coral gravel have the characteristics of mixed size particles, alternating accumulation of sand and gravel, great change of density and gradually coarsening from top to bottom. Coral reefs calcareous rocks are composed of loose bioclasts cemented, some of them are dense, and some of them are semi-cemented and weakly cemented. The geological age of the Xisha Islands within 50m was determined by carbon-14 radioisotope isotopic test. The bearing capacity and deformation characteristics of coral sand and coral gravel were analyzed by plate loading test, heavy dynamic penetration test, extra-heavy dynamic penetration test and standard penetration test. The physical and mechanical characteristics of coral reefs calcareous rock were studied by longitudinal wave velocity test, density test, porosity test and uniaxial compressive strength test.

Layered Grouting Technology Based on a Comprehensive Water‐to‐Cement Ratio for the Overlying Loess Stratum of Urban Shallow Tunnels

Different from sand and clay, loess has special engineering properties; hence, existing soil grouting theories are not suitable for the disaster treatment of shallow loess tunnels. In this study, a fine grouting reinforcement test system was developed, and the Yuhan Road tunnel overlying loess was used as the injection medium. An orthogonal test based on slurry dry density, moisture content, water‐to‐cement ratio, and grouting pressure was conducted. Results revealed that the loess samples have high integrity after grouting, and the cohesion and compressive strength improved significantly. The stress‐strain curve showed that the strengthened samples have greater ultimate and residual strengths than samples before grouting. Through a range analysis, it was determined that water‐to‐cement ratio and moisture content are the main factors affecting loess cohesion and compressive strength. Therefore, a comprehensive test of the water‐to‐cement ratio and moisture content as a single variable was conducted. It was found that their influence on loess cohesion and compressive strength is not a single linear relationship but a combined balance. To characterize the joint effect of water in loess and in slurry on reinforcement, the concept of a comprehensive water‐to‐cement ratio is proposed, and the cohesion and compressive strength curves with respect to this ratio were drawn. An optimal comprehensive water‐to‐cement ratio, which corresponds to the maximum cohesion or compressive strength, was found. Based on this ratio, we further propose a method to calculate the water‐to‐cement ratio of slurry and suitable grouting amount for the Yuhan Road tunnel reinforcement project, in which all solution parameters can be measured via field tests. In the project, a surface layered grouting scheme, based on the optimal comprehensive water‐to‐cement ratio, was designed. After grouting, loess strength was improved significantly, permeability was reduced greatly, and the overall reinforcement effect was suitable; these results provide a reference for similar projects.

Laboratory Tests for the Consolidation Property of Dredger Fill in the Shenzhen Area

In order to analyze the special engineering properties of the super soft dredger fill, experimental studies had been done on the physical-mechanical properties of that in Shenzhen, especially for its consolidation properties. The test results indicated that the super soft dredger fill had the properties of high water content, high void ratio and high clay content; When the confining pressure applied to the sample was less than 200kPa, the change of deformation and void ratio of it was more obvious; When the confining pressure applied to the sample was less than 80kPa, the chang of permeability coefficient of that was more significant.

A New Method based on Centrifuge Model Test for Evaluating Ground Settlement Induced by Tunneling

Current methods for evaluating the ground settlement due to tunneling in loess do not fully consider the special engineering properties of loess. This paper first studied the ground settlement due to tunnel construction in loess by conducting a uniquely designed centrifuge model test. Then the ground settlements were calculated using a currently widely used method and compared with the measured values from the centrifuge model test. The results indicated a substantial difference between the calculated and measured settlement profiles. For the stratum at shallow depth, the calculated settlement profiles were flatter with smaller maximum settlement Smax at the tunnel centerline and larger settlement far from the centerline than those from the centrifuge model test; while for the stratum near the tunnel crown, the opposite was obtained. The main reason for the difference is because of the large void structure and high compressibility of loess which causes additional settlement at shallow depth. Based on the centrifuge model test results, a new method which properly considers the special engineering characteristics of loess was proposed to evaluate the settlement due to tunnel construction in loess. The calculated settlement profiles using the new method were in good agreement with those from the centrifuge model test. Finally, the new method was used to evaluate the ground settlement caused by the construction of a Metro tunnel in loess and satisfactory results were obtained.

Experimental Study on Road Properties of High Liquid Limit Soil in Yunfu Highway

There are a large number of high liquid limit soil distributed in southern China. As one of the special soils commonly seen in highway construction, high liquid limit soil cannot be used in subgrade filling directly due to its special engineering properties. With the idea of saving engineering investment and protecting environment, high liquid limit soil must be improved to be qualified subgrade filling material. First, detailed laboratory tests are carried out to reveal the special properties of high liquid limit soil. Then, based on the characters of high liquid limit soil, the improving tests of liquid plastic limit, unconfined compressive strength, CBR and compaction are completed by adding different percentages of quicklime and white lime to the high liquid limit soil to study the change in the physical and mechanical properties and strength. The results show that, after being improved by lime, the high liquid limit soil can be used as subgrade filling of expressway as its liquid limit, plastic index being reduced, the strength increased and the value of CBR increased remarkably, and modified effectiveness of quicklime is better than that of white lime. Through comparison and analysis of the test results, it is found that high liquid limit soil with 5% quicklime can be used as subgrade filling material for highway since it is not only economical but also qualified to meet the requirement of improvement and strength.

Combinatorial synthesis approach employing graph networks

The paper proposes a methodology to assist the designer at the initial stages of the design synthesis process by enabling him/her to employ knowledge and algorithms existing in graph network theory. The proposed method comprises three main stages: transforming the synthesis problem into a graph theoretic problem; devising the topology possessing special engineering properties corresponding to the system requirements; finding the geometric configuration of that topology that will possess the desired properties. To clarify the idea and to demonstrate its generality, the approach is presented through three synthesis case studies from different engineering domains: electrical networks, statics and kinematics. As is highlighted in the paper, the approach of employing graph theory in the synthesis process offers several unique advantages. Among these advantages are: gaining a general perspective on different synthesis problems from different engineering domains by transforming them into the same graph problem; employing the same graph algorithms for different synthesis problems; establishing the existence of configurations with special properties solely from the topology of the system; transferring knowledge and methods between different engineering disciplines for both the topology and the geometry generation steps.