AN EXPERIMENTAL INVESTIGATION OF PROTECTIVE FILTERS

THE MOST IMPORTANT SINGLE ITEM IN THE DESIGN OF DAMS ON PERVIOUS FOUNDATIONS IS THE PROVISION OF SAFEGUARDS AGAINST FAILURE DUE TO GRADUAL OR SUDDEN EROSION OF THE FOUNDATION. THE PROTECTION USUALLY PROVIDED AGAINST EROSION CONSISTS IN THE PROVISION OF LAYERS OR BLANKETS OF PERVIOUS MATERIAL, EACH LAYER BEARING A FIXED RELATION TO THE GRADING OF THE PREVIOUS AND SUCCESSIVE LAYERS. LABORATORY STUDY WAS CONDUCTED BASED ON THE PERMEABILITY TEST WHICH IS EMPLOYED IN SOIL TESTING TO DETERMINE THE COEFFICIENT OF PERMEABILITY OF SOILS. A PROTECTIVE FILTER MUST FULFILL THE FOLLOWING REQUIREMENTS: (1) IT MUST BE MANY TIMES MORE PERVIOUS THAN THE SOIL WHICH IS TO BE PROTECTED IN ORDER THAT THE GRAIN STRUCTURE OF THE FILTER WILL REMAIN ABSOLUTELY STABLE, AND (2) ITS PORES MUST BE SUFFICIENTLY FINE SO THAT SOIL GRAINS WILL NOT BE WASHED THROUGH THE FILTER. SOILS WERE PLACED IN TEST CYLINDERS IN TWO LAYERS, ONE LAYER BEING COMPOSED OF FINER GRAIN SIZES THAN THE OTHER. A PERMEABILITY TEST WAS THEN CONDUCTED WITH THE WATER FLOWING THROUGH THE SOIL, PASSING FROM THE FINER LAYER INTO THE COARSER LAYER. A COMPARISON OF THE GRAIN SIZE DISTRIBUTION CURVES BEFORE AND AFTER EACH TEST WAS MADE TO DETERMINE THE CHANGE WHICH HAD TAKEN PLACE IN THE GRADING CHARACTERISTICS OF EACH LAYER OF SOIL. THE TOTAL WEIGHT OF DRY SOIL IN THE INDIVIDUAL LAYERS WAS DETERMINED BEFORE AND AFTER A TEST. THE DIFFERENCE BETWEEN THE INITIAL AND FINAL WEIGHTS GAVE A QUANITATIVE INDEX OF THE MOVEMENT OF THE SOIL GRAINS. AIR SEGREGATION FROM THE WATER USED FOR PERMEABILITY TESTS IS AN IMPORTANT FACTOR WHICH MUST BE CONSIDERED IN ALL PERMEABILITY STUDIES OF SOILS. THE USE OF DISTILLED WATER IS RECOMMENDED FOR ALL TESTS TO AVOID CLOGGING BY SUSPENDED MATERIAL AND TO ESCAPE ANY DETRIMENTAL EFFECTS WHICH MAY BE CAUSED BY SOLUBLE SOLIDS. FILTER TESTS OF UNIFORM SANDS SHOULD LEAD TO THE FOLLOWING CONCLUSIONS: (1) THE MINIMUM CRITICAL RATIO OF THE 15 PER CENT SIZE OF THE FILTER AT THE LIMIT OF STABILITY IS IS APPROXIMATELY 9, (2) THE CRITICAL RATIOS ARE PRACTICALLY INDEPENDENT OF THE SHAPE OF THE SOIL GRAINS. (3) THE CRITICAL RATIOS ARE FAIRLY CONSTANT FOR THE RANGE OF HYDRAULIC GRADIENTS FROM 6 TO 20 WHICH WAS INVESTIGATED, (4) THE CRITICAL RATIOS UP TO WHICH MOVEMENT OF SOIL GRAINS IS VERY SLIGHT ARE REPRESENTED BY SHARPLY DEFINED RATIOS OF THE GRAIN SIZE OF THE BASE MATERIAL AND THE FILTER MATERIAL, (5) THESE CONCLUSIONS ARE APPLICABLE ONLY TO FILTERS AT LEAST 50 PERCENT COMPACTED.

  • Supplemental Notes:
    • Publ 267, SOIL MECHANICS SERIES, NO 7, 21 PP, 26 FIG, 4 TAB
  • Corporate Authors:

    Harvard University

    Graduate School of Engineering
    Cambridge, MA  United States  02138
  • Authors:
    • Bertram, G E
  • Publication Date: 1940

Subject/Index Terms

Filing Info

  • Accession Number: 00234444
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Dec 9 1994 12:00AM