A COMPARISON OF SHEAR TESTS USING CARR-WALKER AND SCHULZE ANNULAR SHEAR CELLS ON COMMERCIAL AGRICULTURAL CRUSHED LIMESTONE OF NEW ZEALAND

Crushed agricultural limestone is used extensively throughout New Zealand to raise the pH of the country's largely acidic soils. The materials used are quarried from areas of varying mineralogy and levels of purity and are measured in calcium carbonate neutralizing equivalence, which must be greater than 65%. About half of the agricultural limestone used in New Zealand is applied by aircraft due to the terrain of the pasture land in production. The flow properties of limestone are of interest, as a number of accidents have occurred as a result of the limestone failing to discharge from the hoppers of the aircraft applying it. A copy of the Carr-Walker annular shear cell was used to measure the strength characteristics of crushed agricultural limestones from New Zealand. Measurements were also made on samples of the same materials using a Schulze RST-XS 200 mL tester, and the results were compared. Five limestones were tested and multipoint loci were produced, from which four point powder flow functions were obtained. In addition, wall yield loci were compared, and hopper opening and half angle were calculated to compare the design differences resulting from the yield loci of each shear cell. The results from the Carr-Walker cell were less conservative for cohesive materials than those obtained from the Schulze cell, which performed more closely to the Jenike shear cell. These results are consistent with earlier findings that the Jenike shear cell did not offer design criteria that provided agreement with engineering practice for some materials.