Determination of Rock Mass Modulus for Foundation Design

This paper reports on the ground investigation carried out for each test pile for a series of nine 30MN full-scale load tests of piles founded on rock. The work was carried out to provide data for the design of over 20km of the Kowloon-Canton Railway Corporation West Rail Project Phase I by foundation contractors. The ground investigation techniques used included Goodman Jack, High Pressure Dilatometer, Self-boring Pressuremeter and Geophysics as well detailed logging of the rock cores and laboratory testing for presentation in Rock Mass Rating (RMR) format. The advantages and limitations of each of the testing techniques are discussed. The paper concludes that the use of published correlations between stiffness and rock mass rating parameters such as RMR and GSI would result in a fivefold overestimate of the stiffness. It is recommended that the stiffness of the rock may be determined using conventional RMR data with the water and joint orientation ratings set to zero. A new coefficient Modified Rock Mass Rating (RM²) is proposed for deep foundations on weathered rock.