Strength Tests and Numerical Simulations of Loess Modified by Desulfurization Ash and Fly Ash

Desulfurization ash and fly ash are solid wastes discharged from boilers of power plants. Their utilization rate is low, especially desulfurization ash, most of which is stored. In order to realize their resource utilization, they are used to modify loess in this paper. Nine group compaction tests and 32 group direct shear tests are done in order to explore the influence law of desulfurization ash and fly ash on the strength of the loess. Meanwhile, FLAC3D software is used to numerically simulate the direct shear test, and the simulation results and the test results are compared and analyzed. The results show that, with the increase of desulfurization ash’s amount, the shear strength of the modified loess increases first and then decreases. The loess modified by the fly ash has the same law with that of the desulfurization ash. The best mass ratio of modified loess is 80:20. When the mass ratio is 80:20, the shear strength of loess modified by the desulfurization ash is 12.74% higher than that of the pure loess on average and the shear strength of loess modified by fly ash is 3.59% higher than that of the pure loess on average. The effect of the desulfurization ash on modifying the loess is better than that of the fly ash. When the mass ratio is 80:20, the shear strength of loess modified by the desulfurization ash is 9.15% higher than that of the fly ash on average. Comparing the results of the simulation calculation with the actual test results, the increase rate of the shear stress of the FLAC3D simulation is larger than that of the actual test, and the simulated shear strength is about 8.21% higher than the test shear strength.

Experimental Study on the Reliability of Scaling Down Techniques Used in Direct Shear Tests to Determine the Shear Strength of Rockfill and Waste Rocks

The determination of shear strength parameters for coarse granular materials such as rockfill and waste rocks is challenging due to their oversized particles and the minimum required ratio of 10 between the specimen width (W) and the maximum particle size (dmax) of tested samples for direct shear tests. To overcome this problem, a common practice is to prepare test samples by excluding the oversized particles. This method is called the scalping scaling down technique. Making further modifications on scalped samples to achieve a specific particle size distribution curve (PSDC) leads to other scaling down techniques. Until now, the parallel scaling down technique has been the most popular and most commonly applied, generally because it produces a PSDC parallel and similar to that of field material. Recently, a critical literature review performed by the authors revealed that the methodology used by previous researchers to validate or invalidate the scaling down techniques in estimating the shear strength of field materials is inappropriate. The validity of scaling down techniques remains unknown. In addition, the minimum required W/dmax ratio of 10, stipulated in ASTM D3080/D3080M-11 for direct shear tests, is not large enough to eliminate the specimen size effect (SSE). The authors’ recent experimental study showed that a minimum W/dmax ratio of 60 is necessary to avoid any SSE in direct shear tests. In this study, a series of direct shear tests were performed on samples with different dmax values, prepared by applying scalping and parallel scaling down techniques. All tested specimens had a W/dmax ratio equal to or larger than 60. The test results of the scaled down samples with dmax values smaller than those of field samples were used to establish a predictive equation between the effective internal friction angle (hereafter named “friction angle”) and dmax, which was then used to predict the friction angles of the field samples. Comparisons between the measured and predicted friction angles of field samples demonstrated that the equations based on scalping scaling down technique correctly predicted the friction angles of field samples, whereas the equations based on parallel scaling down technique failed to correctly predict the friction angles of field samples. The scalping down technique has been validated, whereas the parallel scaling down technique has been invalidated by the experimental results presented in this study.

Study on Shear Failure and Crack Propagation Characteristics of Soil-Rock Mixture

Soil–rock mixture is a special geological material between homogeneous soil masses and fractured rock masses. In this study, the shear characteristics, movement and failure characteristics of particles and the evolution law of cracks were studied by direct shear tests and particle flow numerical simulations. The results show that the shear stress-shear displacement curves of the soil–rock mixture can be roughly classified into three stages: elastic stage, plastic stage and strain softening stage, and there was a "jump" phenomenon. The higher the rock content was, the more obvious the phenomenon. The shear strength and its indices of the soil–rock mixture did not increase with increasing rock content, but there was an "optimal rock content". According to the experimental and simulation results, particle breakage can be divided into three types: slight failure, partial failure and complete failure. The crack propagation characteristics can be divided into three stages, and the crack propagation depth increases with increasing shear displacement. It increases with increasing vertical stress and decreases with increasing block rock content.

Mechanical Properties of Frozen Glacial Tills due to Short Periods of Thawing

Due to the warming climate, glacier retreat has left massive glacial tills in steep gullies; ice in the soil is prone to change phase resulting in the decrease of the ice strength and bonding of soil particles; collapse of thawing tills can lead to debris flows with disastrous consequences for geotechnical infrastructures. To improve our understanding of the mechanics of thawing glacial tills, we conducted unconsolidated–undrained direct shear tests on glacial tills from Tianmo gully on the southeastern Tibetan Plateau. Control specimens were not subjected to freeze–thaw action. A total of 648 specimens with three different dry densities, three initial water contents, and 18 thawing times were tested. Peak shear strength, peak stress to displacement ratio (0.857), and cohesion were the highest in frozen specimens. After a thawing time of 0.25 h, there was a marked decline in shear strength; maximum friction was 2.58, which was far below the value of cohesive strength. For thawing times of 0.25–4 h, peak strength varied little with thawing time, but cohesion decreased and internal friction angle increased with increasing thawing time. Our results indicate that thawing of the solid ice in the till during the initial phase of till thawing is the key control of peak till strength; the effect of ice on cohesion is greater during the initial phase of thawing and in loose tills. Moreover, frequent sediment recharge of gullies may be explained by the decrease of cohesion with increasing thawing time caused by short-term destruction of ice bonding.

Structure and Mechanical Properties of the Dueñas Clay Formation (Tertiary Duero Basin, Spain): An Overconsolidated Clay of Lacustrine Origin

The Dueñas Clay Formation is considered an example of a deposit of lacustrine continental origin. It is formed mainly by overconsolidated clays and includes feldspathic arenites, and clayey and silty levels; however, in geotechnical projects it is considered a clay unit and treated as a whole. The structure of each level was assessed in the field, in thin sections, and by SEM in the case of the clayey level. In addition, identification, strength, deformation, and durability tests were undertaken according to the nature of the samples (grain size analysis, Atterberg Limits, point load test, direct shear tests, uniaxial compression tests, swelling pressure, and unidimensional consolidation tests). The durability test was used as a criterion for dividing the levels within the formation according to their behavior as soil or rock. It was observed that the proportion and type of carbonate cementation controls the way in which the material behaves, with sparithic cement increasing the strength. The clay levels are expansive due to the presence of smectite, which also influences their behavior under shear stress. In addition, the massive and laminar structure of the layers caused by the continental conditions, in addition to the processes of post-sedimentation, explain their low compressibility.

Effect of Different Environments’ Conditioning on the Debonding Phenomenon in Fiber-Reinforced Cementitious Matrix-Concrete Joints

This paper presents the results of an experimental study conducted to understand the bond capacity through single-lap, direct-shear tests of fiber-reinforced cementitious matrix (FRCM)-concrete joints under an alkaline and hot water environment. The experimental campaign was focused on a FRCM system equipped with two different types of fibers, (PBO) and Carbon. After the conditioning, the specimens conditioned were subjected to visual inspection, and the experimental results were compared with the unconditioned specimens. Moreover, in this present work, the number of layers and the conditioning time were varied.

Mechanical Behavior of Sand Mixed with Rubber Aggregates

The main objective of this study is to compare the mechanical behavior of two sands (Hostun or Dune sands) mixed with crushed rubber obtained from used tires. However, it is essential to ensure that his geotechnical application do not result in long-term negative impacts on the environment. The chemical properties of these two sands are given by energy dispersive analysis X-ray fluorescence spectrometry. The mineral composition of these two sands is performed by X-ray diffractometry. The morphological characteristics of the sand grains are given by the analysis of the images of the two sands given by the scanning electron microscope. This study is based on 120 direct shear tests performed on sand-rubber aggregate mixtures. The results show that the rubber content of the aggregates has a significant effect on the shear strength of sand-rubber mixtures in both cases of sand. In fact, the shear strength of the sand-rubber mixture increases with increasing crushed rubber up to 20% for different normal stresses. The analysis of the test results also shows the effect of the angular shape of the sand grains on the interparticle friction. The contribution of the structure effect in the mobilized friction is analyzed by comparing the shear test results of Hostun and dune sand mixtures.

Influence of Zeolite on the Compaction Characteristics and Shear Strength Parameters of Cemented Sand

It is well known that in geotechnical engineering, soil stabilization using cement is one of the appropriate approaches for enhancing soil characteristics. With respect to zeolite, its impact on the characteristics of cemented soil has not been fully evaluated. Thus, in the current research, a set of laboratory tests including standard Proctor compaction and direct shear tests (DSTs) considering four cement contents (2, 4, 6, and 8% of sand dry weight) and four zeolite contents (0%, 30%, 60%, and 90% of cement percentage as a replacement material) was carried out. The results indicated that the zeolite reduced Maximum Dry Density (MDD) while it increased value of Optimum Moisture Content (OMC) of cemented sand. Through the DSTs, it has been found that the replacement of cement by zeolite up to 30%, leads to the highest values of shear strength parameters due to the occurrence of pozzolanic and chemical reactions, particularly the production of higher amounts of calcium aluminate and calcium silicate hydrates in comparison with zeolite-free samples.

Quantifying In Situ Tack Coat Performance Using the TackBond Tester for Quality Control

In light of the various quality assurance (QA) issues pertaining to tack coats that occur during construction, there is a need for a means of verifying interlayer bond quality in situ. Despite the immense use of tack coat as a constituent in paving, there are no construction specifications with provisions for the quantification of tack coat bond quality in laboratory or field settings. In this study, a construction QA process for tack coat bond performance was proposed. A novel field tack coat bond strength test device, TackBond, was developed and used for this purpose. The performance of engineered (new tack coat technologies that are tracking less) and conventional tack coats was also evaluated in the laboratory and the field using the developed TackBond test system. The TackBond device was improved in this study by adding features that render it more practical, portable, accurate, and better suited for a variety of pavement surface conditions. Engineered tack coat performance was compared with that of tack coats used conventionally on both milled and overlay surface types. The suitability of the TackBond Test device for capturing the true response of each tack coat was first evaluated by comparing results from TackBond laboratory tests with monotonic direct shear tests (DST) on laboratory-produced samples. Strong correlations between the two test types were achieved. Results of field and laboratory TackBond tests showed that the in situ QA control process developed in this study could be effectively used to improve the in situ tack coat bond performance.

Effect of Polypropylene Fibers on the Shear Strength–Dilation Behavior of Compacted Lateritic Soils

The stress–dilatancy relationship for fiber-reinforced soils has been the focus of recent studies. This relationship can be used as a foundation for the development of constitutive models for fiber-reinforced soils. The present study aims to investigate the effect of recycled polypropylene fibers on the shear strength–dilation behavior of two lateritic soils using the stress–dilatancy relationship for direct shear tests. Results show that fibers improved the shear strength behavior of the composites, observed by increases in the friction angle. Fibers’ orientation at the sheared interface could be observed. The volumetric change during shearing was altered by the presence of fibers in both soils. Overall, results indicate that the stress–dilatancy relationship is affected by inclusions in the soil mix. Results can be used to implement constitutive modeling for fiber-reinforced soils.