Mr. W. R. Hill, then chief engineer of the Croton Aqueduct Commission, favored extending the core walls to solid rock. He took exception to the manner of obtaining samples of sand by means of pipe and force-jet of water, claiming that only the coarsest sand was obtained for examination. He did not consider fine sand through which three men could run a ¾-in. rod 19 and 20 ft. to rock without use of a hammer, very stable material upon which to build a wall.
North Dike of the Wachusett Reservoir,
Boston.
The North Dike of the Wachusett Reservoir is another large public work in progress at the present time. It is of somewhat unusual design and the preliminary investigations and experiments which led to its adoption are interesting in the extreme.[3]
The area to be explored in determining the best location for the dike was great, and the preliminary investigations conducted by means of wash drill borings, very extensive. A total of 1,131 borings were made to an average depth of 83 ft., the maximum depth being 286 ft. The materials were classified largely by the appearance of the samples, though chemical and filtration tests were also made. The plane of the ground water was from 35 to 50 ft. below the surface, and the action of the water-jet indicated in a measure the degree of permeability of the strata.
In addition to these tests experimental dikes of different materials, and deposited in different ways, were made in a wooden tank 6 ft. wide, 8 ft. high and 60 ft. long. The stability of soils when in contact with water was experimented with, as shown in [Fig. 21], in the following manner:
An embankment ([Fig. 21]) was constructed in the tank of the material to be experimented with, 2 ft. wide on top, 6 ft. high, with slopes 2 on 1, and water admitted on both sides to a depth of 5 ft. The top was covered with 4-in. planks 2 ft. long and pressure applied by means of two jack screws resting upon a cross beam on top of the planks.
With a pressure of three tons per square foot, the 4-in. planks were forced down into the embankment a little more than 6 ins., resulting in a very slight bulging of the slopes a little below the water level. Immediately under the planks the soil became hard and compact. A man’s weight pushed a sharp steel rod, ¾-in. in diameter, only 6 to 8 ins. into the embankment where the pressure was applied, while outside of this area the rod was easily pushed to the bottom of the tank.
These results corroborate in a general way the practical experience of the author, both in compressed embankments, where he found it necessary to use a pick vigorously to loosen the material of which they were composed, and in embankments made by merely dumping the material from a track, in which case the earth is so slightly compressed that an excavation is easily made with a shovel.
Fig. 21.