Mechanical Behaviour of Completely Decomposed Granite Soil with Tire Rubber Granules and Fibres

Mixing soil with waste tire rubber granules or fibres is a practical and promising solution to the problem of global scrap tire pollution. Before successful applications, the mechanical behaviour of the soil–rubber mixture must be thoroughly investigated. Comprehensive laboratory studies (compaction, permeability, oedometer and triaxial tests) were conducted on the completely decomposed granite (CDG)–rubber mixtures, considering the effects of rubber type (rubber granules GR1 and rubber fibre FR2) and rubber content (0–30%). Results show that, for the CDG–rubber mixture, as the rubber content increases, the compaction curves become more rubber-like with less obvious optimum moisture content. The effect on permeability becomes clearer only when the rubber content is greater than 30%. The shape effect of rubber particles in compression is minimal. In triaxial shearing, the inclusion of rubber particles tends to reduce the stiffness of the mixtures. After adding GR1, the peak stress decreases with the increasing rubber content due to the participation of soft rubber particles in the force transmission, while the FR2 results in higher peak stress especially at higher rubber contents because of the reinforcement effect. For the CDG–GR1 mixture, the friction angle at the critical state (φ’cs) decreases with the increasing rubber content, mainly due to the lower inter-particle friction of the CDG–rubber interface compared to the pure CDG interface, while for the CDG–FR2 mixture, the φ’cs increases with the increasing rubber content, again mainly due to the reinforcement effect.

Biochar as a Soil Amendment: Reduction in Mercury Transport from Hydraulic Mine Debris

Mercury mining and its use in gold mine operations left a legacy of contamination in northern California. Contaminated sediments and water continue to affect local and downstream ecosystems. To assess the efficacy of biochar-amended soils on decreasing Hg transport, biochar was used to amend rock and sediment columns and mesocosms to decrease suspended sediment and associated mercury (Hg) in storm water runoff from Sierra Nevada hydraulic mines. Mercury-contaminated storm water runoff and hydraulic mine debris were collected from two hydraulic mine sites in the Yuba River, California watershed. Mercury concentrations and turbidity were analyzed from storm water samples and hydraulic mine debris in three simulated storm runoff experiments using decomposed granite columns, sediment columns, and sediment mesocosms amended at 0%, 2%, or 5% biochar by weight. Columns containing hydraulic mine debris and mixed with 5% biochar had a significant (p < 0.05) reduction in filter-passed mercury (FHg) in the outflow as compared to control columns. To simulate saturated hydraulic mine debris runoff, mesocosms were filled with mine sediment and saturated with deionized water to generate runoff. Five percent biochar in mesocosm trays decreased FHg significantly (p < 0.001), but, because of the angle of the tray, sediment also moved out of the trays. Biochar was effective at reducing FHg from hydraulic mine discharge. Biochar in laboratory columns with decomposed granite or mine sediments was more effective at removing Hg than mesocosms.

Soil Fabric and Transitional Behavior in Completely Decomposed Granite: An Example of Well-Graded Soil

Unlike sedimentary soils, limited studies have dealt with completely decomposed granite (CDG) soils, even though they are plentiful and used extensively in several engineering applications. In this paper, a set of triaxial compression tests have been conducted on well-graded intact and disturbed CDG soils to study the impact of the fabric on soil behavior. The soil behavior was robustly affected by the soil fabric and its mineral composition. The intact soil showed multiple parallel compression lines, while a unique isotropic compression line was present in the case of disturbed soil. Both the intact and disturbed soils showed unique critical state lines (CSL) in both the e-log p′ and q-p′ spaces. The intact soil showed behavior unlike other transitional soils that have both distinct isotropic compression lines ICLs and CSLs. The gradient of the unique ICL of the disturbed soil was much more than that of the parallel compression lines of the intact soil. In the intact soil, the slope of the unique CSL (M) in the q-p′ space was higher than that of the disturbed soil. The isotropic response was present for both the intact and disturbed soils after erasing the inherited anisotropy as the stress increased with irrecoverable volumetric change. Soil fabric is considered the dominant factor in the transitional behavior and such a mode of soil behavior is no longer restricted to gap-graded soil as previously thought.

Use of saturation diving technique for tunnel boring machine cutterhead intervention in the Tuen Mun–Chek Lap Kok Link Sub-sea Tunnel Project, Hong Kong

The entire Tuen Mun–Chek Lap Kok Link (TM-CLKL) was commissioned on 27 December 2020 and it comprises a 9km-long dual 2-lane carriageway between Tuen Mun and North Lantau, Hong Kong. Construction of the 5km-long sub-sea tunnels was carried out by two 14m diameter Tunnel Boring Machines (TBMs). The tunnel alignment for the TM-CLKL sub-sea tunnel section is in mixed ground condition with the first 500 m in mixed geology of slightly to moderately decomposed granite and completely decomposed granite (CDG), followed by soft ground condition with CDG, alluvial sand, alluvial clay and marine deposit. This mixed ground geology requires regular TBM cutterhead interventions to change the worn-out cutting tools during the tunnelling operation. As the tunnel alignment is up to 55 m below the sea level with the deepest seabed level at -21 mPD, in order to maintain the cutting face stability during the intervention, the intervention pressure could be up to 6 bars. This paper describes different techniques used for TBM interventions under the sea such as trimix bounce mode and saturation mode that appears first time in Hong Kong under a high hyperbaric pressure to change the worn-out cutting tools at the TBM cutterhead.