SIZE OF SAND GRAINS.

BORING SAMPLES FROM PORTAGE, N. Y.

Measured under microscope

Size of grains is given as diameter in decimals of an inch. with micrometer eyepiece by K. Whitman, Jr., February 19, 1908.

Sample No. 3. Mixture, coarse, .003 to .005, with finer material, .0001 to .0005.
Sample No. 4. Size, .00005 to .0001.

San ple No. 4.

Size, .002 to .006 and small amount coarser material, .03 to .1.
Mostly .00005 to .0001, with some .0005 to .002.

Size, .0001 to .0008.

Sample No. 5.

Sample No. 6.

Sample No. 7.

Size, .003 to .005.

Sample No. 1. Size, .002 to .008.

Sample No. 2. Largely .0001 to .0005 with some .001 to .002.

Sample No. 5.

Sample No. 6.

Size, .001 to .004.

Size, .00005 to .0003 with coarser material graded from ) i Sample No. 7. Graded from .0001 to .1.

REPORT ON PORTAGE RIM SAMPLE S BY W. O. CROSBY, GEOLOGIST.

MR. JOHN R. FREEMAN,

Consulting Engineer:

DEPARTMENT OF GEOLOGY,

MASSACHUSETTS INSTITUTE OF TECHNOLOGY.

BOSTON, MASS., February 4, 1908.

In accordance with your instructions, I have examined six samples of material from drill borings recently sent me from Albany and I submit herewith my conclusions as to the character and proper classification of the materials.

It will promote clearness and convenience to refer to the following:

In general usage, quartz flour (7, 8) is reckoned as clay. It is the principal constituent of even the most plastic glacial clays; but it is also often quite free from a true clay base.

SAMPLE A.

Superfine sand (6), composed almost exclusively of subangular and fairly uniform grains of quartz, with no appreciable amount of clay. The gray color indicates a source below or near the water level; and the grain is fine enough to give it a low filtration rate, or make it relatively impervious.

SAMPLE B.

A mixture of superfine sand, quartz flour and clay (6, 7, 8, 9). The sand largely predominates; and the clay does not exceed five per cent. of the whole. It is much more highly composite and more impervious than No. 1, and probably from a deeper source relatively to the water level. The composite character and especially the clay must make this material practically impervious.

SAMPLE C.

An intimate mixture of quartz flour and blue clay (8, 9). In other words, a blue or unoxidized glacial clay- a typical brick clay. Examination with the microscope and by washing shows that in spite of its plasticity it is chiefly composed of superfine quartz flour, the proportion of true clay base probably not exceeding 25 per cent.; but this is sufficient to make it highly impervious.

SAMPLE D.

Medium and fine sand (5, 6) only a trace of quartz flour and no clay. Its relative coarseness, and the uniformity of grain, make it highly pervious and a good water-bearing sandwhile the buff tint suggests a source above the water level for this sample.

SAMPLE E.

A highly composite sand, varying from fine gravel (2) to superfine sand (6), with little or no quartz flour or clay. In spite of its composite character, it is, apparently a good waterbearing sand; and the dark gray color shows that it has come from well below the water level. The statements as to the water-bearing character of this and other sands are, of course, based upon the assumption that no appreciable amounts of clay were lost in pro aring the samples.

5.

6.

7.

8.

SAMPLE F.

Superfine sand (6), devoid of clay, and essentially similar to Sample No. 1, and probably, like that, relatively impervious.

DISCUSSION OF QUICKSAND.

Quicksand is any sand, coarse or fine, in a state of complete saturation. When the water is insufficient to fill the interstices, its surface tension makes it an efficient cement, binding the grains of sand together. But when the point of saturation is reached, surface tension disappears and the water acts as a lubricant, in the same manner but more efficiently as air does in dry sand. Because the finer grades of sand (4, 5, 6) are, in general, the most uniform in size of grain, and henc the most open or porous in texture, they are, when saturated, the most typical quicksands; but they cease to be quicksands whenever the interstitial water falls below the point of saturation.

Quartz flour and clay are, like coarser sands, highly composite. For this reason and because the excessive minuteness of the grains makes friction and molecular attraction more important factors, they are "quick" only when supersaturated or reduced to mud. Short of supersaturation, they may flow under greater or less pressure, but not after the manner of quicksand.

The highly impervious character of these finer detrital materials is favorable to unequal saturation and the establishment of gliding planes or surfaces of localized movement or slipping. Broadly speaking, the condition of being supersaturated and "quick" is possible for quartz flour or clay only where they have been deposited as surface silts, and have not been compacted by subsequent compression, which would eliminate or force out the water of supersaturation.

From all this it follows that clay and quartz flour at one end of the scale, and the coarser sands and the gravels at the other, are rarely truly "quick," the materials of intermediate grade, the fine sands, are easily brought into that condition, for water penetrates them readily, the interstitial space is at a maximum, and they do not demand supersaturation. All of which is respectfully submitted.

(Signed)

W. O. CROSBY.

FILTRATION AVERAGES THROUGH SANDS OF VARIOUS SIZE.

Expressed in cubic centimeters of water per hour thru a column 6 inches long with a diameter of about 0.8 of an inch, equivalent to a cross section of 1 ten-millionth of an acre and under a head of 10 feet.

9. With this grade (clay) the filtration per hour could not be measured, the rate being practically nil.

The complete samples suffered no loss of the finer material in collecting; while in collecting the washed samples some of the finer material was lost, the consequence being a notable increase of the filtration rate, especially for the finer grades (7 and 8). The coarser grades (5 and 6) were, virtually, washed in the process of deposition, and have, therefore, even when complete, low uniformity coefficients and high filtration rates.

(Signed) W. O. CROSBY

*See report from laboratory.

CLASSIFICATION OF CERTAIN SAMPLES OF SAND OR CLAY FROM

PORTAGE, N. ` .

N. Y. BOARD OF WATER SUPPLY, LABORATORY, 147 Varick Street, February 7, 1908'

DEAR SIR.-I send the following report of examination of three (3) samples of clay marked "State Water Supply Commission No. B" and "State Water Supply Commission No. C." The samples were turned over to Mr. Gaines for the chemical and physical tests. The size of sample was insufficient to make any tests by elutriation, or ordinary mechanical analysis with sieves. I quote from Mr. Gaines' report, as follows:

44

Each of the samples responded to the tests by which clays are usually recognized, and they may be therefore classified as.clays.

"The samples were plastic in the state in which they were received, and became hard and brittle on drying. Each of the samples consisted in part of collodial substances.

After drying at 130 degrees C., Sample B was found to contain 5.54 per cent. water of hydration and Sample C 5.84 per cent. water of hydration.

Chemically, both samples consisted in part of hydrous aluminous silicate, or the predominant ingredient of ordinary clay.

"The test by water of hydration alone would be insufficient to pronounce these materials clays. Pure, high-grade clays contain a minimum of 12 per cent. water of hydration: mixed sandy clays may run as low as 3 per cent. or 4 per cent."

There are two general groups of clays; residual clays, those formed in place; sedimentary clays, those deposited from suspension in water. Naturally, the latter afford the best examples of uniform mechanical grading.

For the ordinary purposes of engineering construction I should consider these " 'clays." I should pronounce them true clays containing an intermixture of rock flour, the latter of which is probably composed of clay forming minerals not completely broken down chemically into clay, and non-clay forming minerals, probably chiefly quartz. Presumably, they are of residual origin.

Respectfully yours,

(Signed) JAMES L. DAVIS,

Assistant Engineer.