Discussion of “Rock Fracture Spacings, Openings, and Porosities”

Mr. Houlsby Congratulates the Author on His Application of Several Disciplines to the Problem of Foundation Permeability and on His Compilation of Data From Case Histories. The Method Proposed Presumes a Relationship Between Grout-Take and Permeability, So That After Determination of Fracture Porosity, Spacing and Opening, the Relationship Can Be Used to Predict Grout-Takes From This Data. However, When Designing the Grouting for a Large Foundation, the Engineer Has No Need to Resort to Indirect Methods Such as This. Assessment of Foundations in Terms of Lugeons Has Been Used by Many Grouting Engineers, and Is Now a Well Established Method. It Is Essentially a Crude Test, but Its Simplicity and Relatively Low Cost Are Compatible With the Unavoidably Inaccurate Nature of Most Grouting Operations. Mr. Scott Observes That Since the Method Depends on a Significant Proportion of Null Values of Water-Make, the Test Lengths Must Necessarily Intersect, on Average, Only a Small Number of Fissures. However, This Is No Great Limitation Because of the Extent of Existing Site Data Available Which Are Presented. There Are Occasions When It Is Practicable and Desirable to Have a Finer Texture to the Water-Make Pattern. The Writer Describes a Procedure to Rate the Widths of Individual Water-Bearing Fissures and Thereby Provide a Criterion Governing Whether or Not to Attempt Some Element of Cement Grouting Prior to a General Phase of Chemical Grouting. Mr. Wittke Observes That the Author Attempts to Develop a Method by Which Fracture Porosities and Openings Can Be Calculated From the Results of Pressure Tests. Despite the Relatively Good Applicability of an Isotropic Model in This Case, One Should Not Forget That Neglect of Anisotropy Can Lead to Grave Mistakes in Other Problems of Rock Mechanics. For This Reason, Exact Investigations Must Be Made With Anisotropic Models as Well. The Estimation of Fracture Spacings From the Percentage of Zero Discharges in Pressure Tests Is a Good Idea and Will Be Useful on Large Dam Sites. The Writer Has Found Fracture Spacings for Calculating Porosities and Openings at German Dam Dites Using the Results of Television Camera Probings. This Is Also the Best Way to Get Knowledge of the Position of Fractures, Which Is Most Important for Calculations of Stability. For Estimation of Fracture Openings It Is Assumed That Flow Only Occurs in Horizontal Fractures. For This Case a Formula by Baker Can Be Employed. References: Rock Fracture Spacings, Openings and Porosities, David T. Snow, Asce (Proc. Paper 5736), January 1968.