Unconventional hydrocarbon resources found across the world are driving a renewed interest in mudrock hydraulic fracturing methods. However, given the difficulty in safely measuring the various controlling factors in a natural environment, considerable challenges remain in understanding the fracture process. To investigate, we report a new laboratory study that simulates hydraulic fracturing using a conventional triaxial apparatus. We show that fracture orientation is primarily controlled by external stress conditions and the inherent rock anisotropy and fabric are critical in governing fracture initiation, propagation, and geometry. We use anisotropic Nash Point Shale (NPS) from the early Jurassic with high elastic P wave anisotropy (56%) and mechanical tensile anisotropy (60%), and highly anisotropic (cemented) Crab Orchard Sandstone with P wave/tensile anisotropies of 12% and 14%, respectively. Initiation of tensile fracture requires 36 MPa for NPS at 1‐km simulated depth and 32 MPa for Crab Orchard Sandstone, in both cases with cross‐bedding favorable orientated. When unfavorably orientated, this increases to 58 MPa for NPS at 800‐m simulated depth, far higher as fractures must now traverse cross‐bedding. We record a swarm of acoustic emission activity, which exhibits spectral power peaks at 600 and 100 kHz suggesting primary fracture and fluid‐rock resonance, respectively. The onset of the acoustic emission data precedes the dynamic instability of the fracture by 0.02 s, which scales to ~20 s for ~100‐m size fractures. We conclude that a monitoring system could become not only a forecasting tool but also a means to control the fracking process to prevent avoidable seismic events.

A number of key processes, both natural and anthropogenic, involve the fracture of rocks subjected to tensile stress, including vein growth and mineralization, and the extraction of hydrocarbons through hydraulic fracturing. In each case, the fundamental material property of mode‐I fracture toughness must be overcome in order for a tensile fracture to propagate. While measuring this parameter is relatively straightforward at ambient pressure, estimating fracture toughness of rocks at depth, where they experience confining pressure, is technically challenging. Here we report a new analysis that combines results from thick‐walled cylinder burst tests with quantitative acoustic emission to estimate the mode‐I fracture toughness (KIc) of Nash Point Shale at confining pressure simulating in situ conditions to approximately 1‐km depth. In the most favorable orientation, the pressure required to fracture the rock shell (injection pressure, Pinj) increases from 6.1 MPa at 2.2‐MPa confining pressure (Pc), to 34 MPa at 20‐MPa confining pressure. When fractures are forced to cross the shale bedding, the required injection pressures are 30.3 MPa (at Pc = 4.5 MPa) and 58 MPa (Pc = 20 MPa), respectively. Applying the model of Abou‐Sayed et al. (1978, https://doi.org/10.1029/JB083iB06p02851) to estimate the initial flaw size, we calculate that this pressure increase equates to an increase in KIc from 0.36 to 4.05 MPa·m1/2 as differential fluid pressure (Pinj − Pc) increases from 3.2 to 22.0 MPa. We conclude that the increasing pressure due to depth in the Earth will have a significant influence on fracture toughness, which is also a function of the inherent anisotropy.

The collapse of an old masonry wall on Hong Kong Island in 1994 prompted research by the Hong Kong SAR Government into the use of modern, non-invasive, geophysical investigative techniques for site characterization. Hong Kong has thousands of retaining walls and during periods of high rainfall, some old masonry walls have failed, damaging property, restricting access and occasionally leading to loss of life. Occasional catastrophic failure of old masonry walls has been linked to combinations of lack of design, high pore pressures, leaking utilities, substandard construction, void development and changes in the land use around the structure. In Hong Kong, the identification of anomalous features in or behind a wall, using conventional investigations such as drilling and trial pits, is expensive and time consuming. Because of the discrete nature of these intrusive methods, they are also less likely than geophysical methods to intersect anomalous features that could adversely affect the stability of a masonry wall. One of the main objectives of the research was to identify efficient, non-invasive, geophysical tools that could assess the geometry and structure of old masonry walls, provide information on the hydrogeological conditions and continuous images of the subsurface and, if required, guide conventional investigation methods. The main conclusion is that a combination of ground penetrating radar and electrical imaging has the potential to identify ‘thin’ old masonry retaining walls in Hong Kong provided that attention to detail during data acquisition and the correct application of analytical techniques is made.

This paper compiles new and existing information relating to features frequently referred to as drift-filled hollows located within London. The key aim of this paper is to update the article written by Berry (1979), producing a resource for both engineering projects and academic research. Fifty-four additional drift-filled hollows have been identified and their physical characteristics are tabulated where available information allows. A case study of the Nine Elms area is presented. The drift-filled hollows have been identified through examination and critical quality assessment of historical borehole records, site investigation records, construction records and published articles. This enlarged dataset illustrates the high level of variability between features and as a result it is proposed that these features did not form due to a single process, but to differing processes.

The measurement of strain is a fundamental and widely studied parameter in engineering, rock mechanics, construction and materials testing. Contact sensors often used in these fields require contact with the target surface throughout the duration of a strain event. Non-contact methods typically require that that the measurement surface is prepared and often coated prior to testing. This paper considers the potential application of near infrared spectroscopy as a non-contact technique for the measurement of strain on natural surfaces. Excellent correlation was found between surface measurements of visible-NIR spectra and longitudinal strain taken during indirect Brazilian Disc Test for samples of sandstone, marble and basalt.