Implications on Characterizing the Extremely Weak Sherwood Sandstone: Case of Slope Stability Analysis Using SRF at Two Oak Quarry in the UK

In the past, engineering geology mainly focused on soil and hard rocks, with little attention paid specifically to weak/soft rocks, defined by a UCS below 25 MPa (ISRM in Int J Rock Mech Min 18:85–110, 1981). Weak rock is an intermediate, which is difficult to analyze, and requires application of both soil and rock mechanics principles. The Sherwood Sandstone Group (SSG) in Nottinghamshire (UK) is often characterized as weak rock, containing extremely weak members, which display a UCS between 0.6 and 1 MPa according to the definition from BS 5930:2015 (British Standards Institution 2015). Little research has been conducted on the locally extremely weak members of the Sherwood Sandstone Group, in particular the Nottingham Castle Sandstone Formation, and engineering projects within this unit can face major design challenges. This study aims at investigating the intact material, characterizing the SSG and analysing the stability of a slope in a quarry between two water-filled silt lagoons at the Two Oak Quarry, close to Mansfield. Laboratory testing, including UCS tests, triaxial tests, tensile tests and Slake Durability tests, is conducted on the two geological units present on site, the Nottingham Castle Sandstone Formation and the Lenton Formation. From the analysis presented herein it is observed that the strength decreases as the degree of saturation increases, which can lead to a complete disintegration of the rock. In addition, the durability of the rock is determined, ranging from very low to moderately high, which has major implications on the longterm stability of the slope. The impact of the weathering on the long-term stability is difficult to establish and an estimation of the disintegration is conducted by comparing block sizes. A structural assessment confirms that failure along discontinuities is possible and requires further investigation. The Finite Element Analysis and the Limit Equilibrium Method are used for the assessment of the stability. Since similar factor of safety (FoS) are determined, both methods are considered applicable to the project, with limitations being detected when modelling discontinuities, impacting the design and the FoS. A suitable slope geometry is proposed, based on the intact material properties, the weathering characteristics, the heterogeneity of the material and structural features.