Laboratory investigation of interface shearing in chalk

Abstract

Chalk, a soft fine-grained Cretaceous limestone, is encountered across northern Europe where recent offshore windfarm, oil, gas and onshore developments have called for better foundation design methods, particularly for driven piles whose shaft capacities are controlled by an effective stress Coulomb interface failure criterion. Interface type and roughness is known to affect both interface friction angles, δ′ and the magnitude of dilation required for shaft failure to develop. Site-specific interface ring-shear tests are recommended for offshore pile design in sands and clays to account for driven pile shaft materials, roughnesses and shear displacements. However, few such tests have been reported for chalks and it is also unclear whether δ′ angle changes contribute to the striking axial capacity increases, or set-up, noted over time with piles driven in chalk. This paper describes an interface shear study on low-to-medium density chalk from the St. Nicholas-at-Wade research test site in Kent, UK, where extensive field driven pile studies have been conducted [1, 2]. Direct shear and Bishop ring shear apparatus were employed to investigate the influences of interface material and surface roughness, as well as ageing under constant normal effective stresses (σn'). It is shown that the high relative roughness of the interface compared to the chalk grain size results in the ultimate interface shearing angles falling close to the chalk-chalk shearing resistance angles. The δ′ angles also increased by up to 5° over 38 days of ageing.

Effective stress regime around a jacked steel pile during installation ageing and load testing in chalk

“…Ring shear interface tests by the Authors in the Bishop apparatus, using mild steel interfaces to represent field pile roughness (average roughness, R a ≈ 10-15µm) values, indicate residual δ ୰ ʹ angles between 30 and 31°, similar to those reported by Le et al (2014) and Ziogos et al (2016). Bishop ring shear tests carried out by Chen (2017), on samples from the test site, indicated δ ୰ ʹ angles of between 26 and 31.5° (depending on normal effective stress level) using stainless steel interfaces prepared with roughness, R a of 1.22 µm, similar to those of industrial CPT friction sleeves.…”

“…The water table is reported 11.6 mbgl below the current quarry base, but the degree of saturation remains between 90 and 100% up to ground level. Chalk specimens crushed from quarry samples indicate predominantly silt sized grains, see Figure 2 after Bialowas et al (2016) and Chan (2017). The median grain size, D 50 for crushed chalk samples tends to vary with the method of sample preparation and grinding (Bundy 2013).…”

“…Moving to slightly higher δ′ ୡ୮୲ = 30.5º, giving 100 < σ′ ୰୫ < 350 kPa at h/R = 5.5. The value of δ′ ୡ୮୲ = 30.5º was chosen based on the results of interface ring shear tests on stainless steel interfaces with similar roughness to the CPT cone sleeves at a confining stress of 200kPa, compatible with the mean f s value(Chan 2017). The near pile tip values of σ′ ୰୫ discussed above are compared below to the local radial effective and…”

Characterisation of sand-steel interface shearing behaviour for the interpretation of driven pile behaviour in sands

“…3, offers a robust tool for characterising soil-soil and soil-structure residual shear strengths. Site-specific ring shear interface tests are recommended by [1] as an effective approach for deriving representative interface friction angles for the detailed design of driven piles in clays [13], sands [14][15][16][17] and other geo-materials including chalk [18]. [19] summarise the important aspects of upgrade and modifications implemented to the Bishop ring shear apparatuses at Imperial College London (ICL) and Norwegian Geotechnical Institute (NGI), and present a unified database for sand/silt-steel interface shear tests that followed the 'ICP' procedures [1].…”

Behaviour of piles driven in chalk

“…A check test on one previously failed pile showed that it could not recover, on re-testing, the same age trend as the 'virgin' piles; see the open symbol on Figure 8. Parallel laboratory testing at Imperial College by Chan et al (2019) investigated the potential for changes over time in the effective-stress residual interface shear strengths of chalk. Samples from SNW were remoulded and subjected to interface ring shear testing after ageing under appropriate normal stress loading.…”