Rock weathering is a common geological hazard. It often destroys stone cultural relics and geological remains, and affects the stability of rock slopes. Rock fracture is regarded as the first way for rainwater to infiltrate into the rock, and is considered as the dominant effect in accelerating weathering process through freeze-thaw cycles, chemical and biological erosions, etc. (Mckay et al. 2009; Sel and Binal, 2021).
Several researchers had pointed out that permeability reduction in term of fracture healing was an useful strategy for weathering mitigation, and developed various types of chemical healing materials (Cardiano et al. 2005; Guo et al. 2009). Classified by chemical composition, chemical healing materials can be divided into inorganic materials and organic materials. However, both the two types of healing materials are not always preferable in site applications because there are some disadvantages. For example, inorganic materials with large particles such as Portland cement have low permeability and thus are difficult to be penetrated into microfractures, other inorganic materials with low viscosity normally cannot form effective bonding strength among fractures, and organic materials usually have poor weather resistance and durability (Naeimi and Haddad 2020). It thus important to propose a new method with low viscosity, high bonding strength, good weather resistance and durability, and carbon emission and eco-friendly.
Since the discovery that microbially induced calcium carbonate precipitation (MICP) can be applied for the improvement of soil foundation as a novel, green, effective, and sustainable microbial geotechnical engineering technology ( Ivanov and Chu 2008; DeJong et al. 2010; Van Paassen et al. 2010), a small number of researchers have attempted to verify the feasibility of healing rock fractures using MICP technology, and to explore the hydraulic and mechanical performance. According to the literature review, these researches can be divided in to three scales including small-scale, borehole-scale, and field scale.
For the small-scale conditions, a series of planar flow experiments were carried out on varying single smooth artificial fractures by etching the fractures using transparent rock-like materials or rock materials (Phillips et al. 2013; EI Mountassir et al. 2014; Minto et al. 2016). Borehole-scale experiments were mainly radial flow experiments conducted on single artificial rock fractures those were constructed by hydraulic fracturing or saw cutting (Phillips et al. 2013; Minto et al. 2016). Moreover, after Cuthbert et al. (2013) presented a first field experiment applying MICP to reduce a single dacite fracture permeability approximately 25 m below ground level, a few researchers, i.e., Cunningham et al. (2014), Phillips et al. (2016 and 2018), and Kirkland et al. (2020) furthly performed MICP field healing experiments on subsurface single fractures near wellbores for wellbore integrity purpose.
In these studies, all researchers adopted a similar injection strategy namely repeated bacterial injection strategy to ensure an even calcium carbonate precipitation. The repeated bacterial injection strategy means that injecting bacteria solution (BS) first, followed by the cementation solution (CS, Ca2+and urea), and the injection process was repeated several cycles until the experiment was completed. The three scales of experiments demonstrated that MICP technology accompanied with a reasonable injection strategy had shown great potential to heal single smooth rock fractures in small-laboratory-scale and to reduce hydraulic properties of single rock fractures in large-field-scale. Micro-structure analysis on small-laboratory-scale experiments also showed that for the horizontal single smooth fractures, gradual reduction in fracture apertures due to calcium carbonate precipitation was the main healing mechanism. It was influenced by hydrodynamics (i.e., velocity, flow rate, and aperture) and the properties of the bacteria solution and the cementation solution. Although researchers would like to design a good injection strategy to precipitate calcium carbonate evenly, it's actually difficult. Ultimately, the precipitated calcium carbonates reduced each fracture to a number of smaller tortuous pathways along the upper and lower fracture surfaces. Part of the CaCO3 crystals, especially at the locations near inlet port bridged across the fracture aperture and formed a hydraulic barrier, resulting in a significant reduction in the hydraulic conductivity.
However, shallow ground weathering fractures in nature have more complex geometric characteristics than these studied smooth single fissures, such as varying surface roughness, abrupt changes in aperture, existing branches, shallow developmental depth. To date, there is limited information related to the MICP healing performance on shallow ground weathering fractures in nature, the feasibility and the underlying healing mechanism remain poorly understood, which are essential criteria to be investigated for weathering mitigation.
Sandstone is a wide-spread construction, sculpture and monuments material all over the world. These sandstone construction, sculpture and monuments are facing serious weathering hazard, and weathering fracturing is one of the important diseases (Turkington et al. 2005). It is thus necessary and urgent to study the healing problem of sandstone weathering fractures.
Therefore, this study took sandstone as an example, and was a first attempt to investigate the hydraulic performance of adopting MICP to heal nature-weathering-like rough fractures (NWLRF) and to reveal the corresponding micro-healing mechanism. A repeated mixture injection strategy (detailed information can be found in Section of injection strategy) was proposed and a series of laboratory MICP injection experiments were carried out on four types of NWLRF including single fractures with broad aperture, single fractures with fine aperture and multiple fractures with penetrated branch fracture and non-penetrated branch fracture. Accompanied with the conducted relevant observational analysis and hydraulic tests, the spatial distribution of the calcium carbonate precipitation, apparent fracture healing ratio and fracture transmissivity reduction were evaluated, which are key aspects controlling the effect of rock weathering mitigation. In addition, fracture healing mechanism, morphology features of calcium carbonate and effects of fracture aperture, CS concentration and branch fracture were discussed. The research results have important theoretical significance and technical guidance value for the disaster prevention and mitigation of rock weathering.