Abstract
Serviceability and ultimate limit state design of micropiles requires knowledge of the mechanical properties of the center rod, bond strength between the grout and surrounding geomaterial, the physical and mechanical properties of the geomaterial and the load transfer mechanism to the surrounding geomaterial. Subsurface exploration is not alway possible at proposed locations for micropile installation, therefore the development of a computer model that can interpret drill data and provide installers and the engineer with sufficient information to make appropriate design adjustments in real time could enable the construction of micropiles in these situations. As part of this model, characterization of the shear resistance between the grout and surrounding geomaterial is required. This study focuses on the development of an experimental technique to correlate the interface shear strength between grout and rock geomaterial through direct shear testing of fabricated textures using the empirical relationship eqn. 1 as described in Barton et. al. 1977. Testing includes a complimentary uniaxial compressive strength (UCS) test program.
τ = σn tan[JRC log (JCS/σn) + Φb] (eqn. 1)
Cylindrical test specimens are fabricated measuring 50 mm diameter by 55 mm long using grout and synthesized rock mated at a textured circular plane oriented approximately perpendicular to the longitudinal axis of the cylinder. The grout consists of cement Type I/II paste and is cast directly onto synthesized rock made from manufactured high strength construction grout. The grout recipes consist of 0.45 and 0.60 fresh water to cement ratio and 0.45 and 0.60 seawater to cement ratio. The roughness between the two materials is created by casting the synthesized rock onto silicone molds crafted to approximate relatively smooth, low, medium and high textures like those described by Barton et. al. 1977. The method for fabrication of the textured silicone molds is discussed. A Barton rock comb is used to measure and record the texture of each test specimen for comparison before and after shear testing. Normal stresses of 5, 10, 30, 40 and 60 kPa are applied to each recipe and texture series. Results include graphical representations of friction angle vs. normal stress, the joint roughness coefficient vs friction angle and comparisons among the textures, grout recipes and shear displacement.
The complimentary Unconfined Compression (UCS) testing is performed on 50 mm diameter by 100 mm diameter cylindrical specimens in which each grout recipe is paired with each texture and grout ages of 3, 7, 14, and 28 days. The synthesized rock is aged a minimum of 30 days.