REVIEW OF USES OF VERTICAL SAND DRAINS

THE REVIEW PRESENTED INCLUDES A THOROUGH EXAMINATION OF AVAILABLE THEORETICAL DESIGN METHODS AND OF EXPERIENCES IN THE USE OF VERTICAL SAND DRAINS. REVIEW OF THE THEORETICAL MATERIAL SHOWS THAT THE RELIABILITY OF SAND DRAIN DESIGN ANALYSES IS LIMITED PRIMARILY BY THE CLOSENESS WITH WHICH THE ASSUMPTIONS MADE AS TO SOIL BEHAVIOR AGREE WITH THE ACTUAL PROPERTIES OF THE SOIL. THESE ASSUMPTIONS AND UNCERTAINTIES INCLUDE THOSE TERZAGHI'S CONSOLIDATION THEORY AND OTHERS SUCH AS: (1) THE EFFECT OF DISTURBANCE OF THE SOIL CAUSED BY INSTALLATION OF THE DRAINS ON THE COEFFICIENT OF CONSOLIDATION, AND (2) THE EFFECT OF MORE RAPID CONSOLIDATION, AND HENCE SETTLEMENT, OF SOIL NEAR THE SOIL DRAIN IN CAUSING ARCHING OF THE OVERLYING FOUNDATION SOIL AND FILL. THE FOLLOWING SOIL PROPERTIES AFFECTING SAND DRAIN DESIGN METHODS ARE ANALYZED: COEFFICIENT OF CONSOLIDATION, DISTURBANCE, SECONDARY COMPRESSION, AND STRATIFICATION. THE REVIEW OF INSTALLATIONS WITH UNSATISFACTORY PERFORMANCE RECORDS SHOWED THAT THE REASONS WERE: (1) FAILURE TO MAKE COMPLETE STABILITY ANALYSES, (2) IMPROPER DESIGN OF THE SAND DRAINS INCLUDING DETERMINATION OF ANTICIPATED RATES OF CONSOLIDATION, (3) LACK OF CONTROL REQUIREMENTS IN THE SPECIFICATIONS ON THE CONTRACTOR'S OPERATIONS, AND (4) VIOLATION OF THE DRAWINGS AND SPECIFICATIONS BY THE CONTRACTOR, OR INADEQUATE INSPECTION. THE REVIEW FORCIBLY DEMONSTRATES THE NEED FOR A THOROUGH INITIAL DESIGN AND LAYOUT OF SAND DRAINS ON OTHER THAN A PURELY EMPIRICAL BASIS. THE USE OF SAND DRAINS INVOLVES MANY MINOR UNCERTAINTIES WHICH NEED TO BE RESOLVED ON THE BASIS OF FULL-SCALE FIELD INVESTIGATIONS. THESE INCLUDE: (1) SMEAR, (2) DISTURBANCE, (3) PERMISSIBLE SIZES AND SPACINGS OF DRAINS INSTALLED BY DRIVEN CLOSED END MANDRELS, AND (4) SECONDARY COMPRESSION.