694 Mr. A. Henry Savage Landor [June 18, 



endanger the Gatun Dam when the artificial lake is filled. The 

 composition of the npper stratum does allow water to pass through, 

 and it will be deemed advisable, even imperative, to drive a cut-off 

 wall of sheet piling right across the valley through this permeable 

 material. According to American engineers this will prevent per- 

 colation and afford additional resistance. 



On the sound construction of, and the eventual resistance offered 

 by this wall will, I personally think, rest the safety of the entire dam. 



Tests of permeability and frictional resistance have been made 

 with the various rocks predominating in the vicinity of Gatun. We 

 have there blue sandstone, volcanic ash, volcanic tufa, dark and 

 brown conglomerate, light l)lue-grey conglomerate, argillaceous, grey 

 and light brown sandstone. 



These tests were made in order to determine the permeability of 

 the rocks under various pressures and the loss by erosion when 

 exposed to a pressure of 40 lb, (equal to a head of 1)2 "o), the 

 maximum head to which they will be exposed in contact with the 

 waters of the lake. The weight and specific gravity per cubic foot 

 when dried at a temperature of 212° F. and when saturated was 

 accurately registered, as well as the absorption in 24 hours, and the 

 abrasion, showing the loss in 15 minutes under a 40 lb. pressure. 



Permeability and aln-asion tests have also been made to determine 

 the frictional resistance of the soil to water in the Gatun region. 



The coarse-grained blue sandstone in which volcanic ash could be 

 detected was highly porous, brittle, and crumbled easily. Water 

 passed through freely. 



The soft grey volcanic ash was somewhat firmer. It contained 

 a considerable amount of sand and gravel. It eroded easily. 



The hard compact volcanic tufa, resembling ryolite, became soft 

 and disintegrated when exposed to the air. It proved impervious 

 under a pressure of 90 lb., and eroded but little in contact with 

 water. 



The dark conglomerate, resembling argillaceous sandstone with a 

 small percentage of large gravel, was practically impervious, but 

 eroded easily. So did the heavy brown conglomerate of gravel 

 cemented by volcanic ash. 



Then we find the soft light grey volcanic ash from the Gatun 

 lock site ; very fine-grained, easily crushed between one's fingers, and 

 easily washed away when openly exposed to pressure. 



The dark blue argillaceous sandstone from the Spillway test-pit 

 was of medium hardness, but disintegrated rapidly when exposed to 

 the air. Its large percentage of clay made it impervious, but easily 

 eroded. 



Both the light blue sandstone and the light brown sandstone 

 from the Spillway test-pit were only moderately pervious but easily 

 eroded. 



Various interesting practical tests have been made with experi- 



