The mineralogy and fabric of ‘Brickearths’ in Kent, UK and their relationship to engineering behaviour

Abstract

Mineralogical and petrographical investigation of two loessic brickearth profiles from Ospringe and Pegwell Bay in north Kent, UK have differentiated two types of brickearth fabric that can be correlated with different engineering behaviour. Both sequences comprise metastable (collapsing) calcareous brickearth, overlain by noncollapsing 'non-calcareous' brickearth. This study has demonstrated that the two types of brickearth are discretely different sedimentary units, with different primary sedimentary characteristics and an erosional junction between the two units. A palaeosol is developed on the calcareous brickearth, and is associated with the formation of rhizolithic calcrete indicating an arid or semi-arid environment. No evidence has been found for decalcification being responsible for the fabric of the upper 'non-calcareous' brickearth. Optically-stimulated dates lend further

Effect of depositional water content on the collapsibility of a reconstituted loess

“…they are stable at their insitu water contents. However, they can be susceptible to hydro-collapse or wetting-induced collapse, a process where they undergo a rapid reduction in volume when they are loaded and wetted (Feda, 1988;Northmore et al, 1996;Milodowski et al, 2015).…”

“…Although loessic soils are transported by the wind, they can be deposited under a range of moisture conditions, including (i) dry deposition due to a reduction in wind speed or increase in surface roughness; (ii) wet deposition due to precipitation; (iii) formation and gravitational settling of grain aggregations formed when atmospheric moisture causes agglomeration of individual particles (Pye, 1995;Iriondo and Kröhling, 2007); (iv) reworked and redeposited by fluvial processes (Northmore et al, 1996). The fabric of the initially deposited loess will have a major influence on the post-depositional processes of loessification, such as the accumulation of illuviated clay and/or calcite crystal formation at clay bridges between particles formed during deposition (Smalley and Marković, 2014;Milodowski et al, 2015).…”

Collapsibility in Calcareous Clayey Loess: A Factor of Stress-Hydraulic History

“…Thawing results in dispersion and illuvial migration of fines in depths and formation of loose clay bridges [11]. Depending on the particle size and stress levels, wetting may either result in flocculation, softening and destruction of metastable fabrics, or leaching and decalcification.…”

“…Drying may develop a negative pore water pressure, which draws clay fragments into the grains' contacts. Drying may also contribute in the precipitation of carbonates ( [11,12], which increases the collapsibility. Solifluction is responsible for the formation of reworked loess.…”

“…Solifluction may disrupt clay buttresses and cryogenic fabrics [13] and form more stable structures [14]. Ecological changes may decrease the collapsibility by formation of nodular calcretes after erosion of roots [11,15], bio-mineralization [16], algae photosynthesis [17], and bacterial Ureolysis [18]. Collapsibility however may slightly increase through sulphate uptake by vegetation in wetlands.…”