8

THE MEMPHIS BRIDGE.

ing the entire progress of the work at the bridge. The readings of thetwo gauges have been platted together and are given on Plate 7. Thisplate is specially instructive as it shows the great difference in fallbetween those two gauges at different stages in the river, the fall varyingfrom 2.46 feet at extreme high water to 0.2 foot at low water. The totalrange at the bridge line is, therefore, 2.26- feet less than at the Govern-ment gauge. These local variations are not uncommon on sinuous riverslike the Mississippi.

The highest water ever observed at Memphis was on March 25th,1890, when it reached 218.36 at the Government gauge and 216.2 atthe bridge site. The height of the top of the stringer on the bridge wasfixed at 296.25, the track being made level from the East Abutment toPier IV. The elevation of the lowest point of the superstructure is291.0, this being the bottom of a lateral pin. The clearance between ex-treme high water of 1890 and the lowest point of iron-work is 74.8 feet.At the time the plans of the bridge were approved the highest water atthe Government gauge was 218.01, occurring in 1887, this being 0.35lower than the water of 1890 ; if the fall of the river was the same thenas in 1890, the clearance would have been 75.15 feet. Low water, whichis a very rare occurrence, is 181.76 feet at the Government gauge. Nosuch water was observed during the construction of the bridge, but theestimated corresponding height at the bridge is 181.6 feet.

The rainfall in this portion of the Mississippi Valley is abundantand the climate of Memphis rather damp. While some snow falls nearlyevery winter, the amount of frost is not enough to have any essentialinfluence on construction. During three of the four winters while thebridge was building no ice was seen in the river.

The river has remained without change for a long series of years atthe site of the bridge, and, as always happens in such cases with silt-bear-ing rivers, it has become narrow and deep. The low-water width is about2000 feet, though the west-shore line has been more or less variable.

Although the Memphis bridge is the first bridge across the Missis-sippi River proper and is built within the limits of the alluvial delta, itsfoundations do not rest in the alluvial sand. As already stated, the bor-ings which were made by Mr. Duryea showed that a substantial clay ex-tended completely across the river. About the time that these boringswere made a number of artesian wells were bored in Memphis to supplythe city with water. These showed that the whole country was under-laid with a clay about 150 feet thick, which is perfectly water-tight, andunder which is an indefinite thickness of saturated sand and gravel. The

clay is now known as the La Grange formation, and the underlying sandand gravel come to the surface about 50 miles east of Memphis, and prob-ably at a greater distance west. The clay and the underlying sand andgravel are of pre-alluvial character. The clay is very substantial, and itscontinuity is shown by the fact that the water in artesian wells at firstrose about seven feet above the water in the Mississippi River.

The general arrangement of the bridge proper is given on Plate 4.It extends from the east abutment to Pier V, a total length of 2682 feet?or of 2698 feet from the east face of the abutment to the centre of PierV. The track is level from the abutment to Pier IV, and the deck-spanwest of Pier IV is built on a vertical curve which connects with thegrade on the viaduct.

The general plan of the viaduct is also given on Plate 4. It is 2290feet 7i inches long, and is built throughout on a 1.25 per cent grade (66feet per mile).

The total length of the permanent structure from the east face ofthe abutment to the west end of the iron-work of the viaduct is 4988feet 9 inches.

In this report I have thought it best to divide the bridge proper intotwo parts, the Substructure and the Superstructure, the Substructureembracing everything below the top of the masonry, the Superstructureembracing all of the metal work as well as the floor.

III.

SUBSTRUCTURE.

The Substructure includes nine piers, eight of which are of masonryconstruction and one is principally of concrete. These nine piers are asfollows:

East Abutment,

Pier A,

Pier B,

Anchorage Pier,

Pier I,

Pier II,

Pier III,

Pier IV,

Pier V.

The East Abutment is shown on Plate 9. It has a concrete founda-tion and is designed at once to form the retaining-wall at the end of theembankment and to afford office accommodations for telegraph-operator,toll-collectors, and others. The weight of superstructure carried is slight.The foundation is of concrete, and the Abutment below the level of thetrack is built hollow, there being a passageway completely through fromnorth to south and staircases leading from this passageway to bothoffices above. This particular arrangement was adopted because it wasthought that at some future time it might be desirable to open the bridgeto foot travel and lead pedestrians to either side without crossing thetrack; this, however, is not likely to be done so long as the bridge isopen for highway traffic. The masonry of the Abutment below the levelof the track is built of Bedford limestone. The two small buildingsabove the floor level are of brick, the facing being of St. Louis pressedbrick; they are entirely of incombustible construction, excepting theroof. Some furniture from the engineer’s office has been moved to thesecond story of the north building, and a set of blue prints of the plansof the bridge have been placed there.

Piers A and B are shown on Plate 10. They are small stone piershaving concrete foundations. Each of them simply carries the weight ofone panel point. The foundations are of concrete, the copings of granite,and the remainder of the work of Bedford limestone.

The Anchorage Pier is also shown on Plate 10. This pier likewisehas a concrete foundation; the upper courses are of granite, and thegreater part of the pier of limestone. The principal feature of this pieris the method in which it is made to serve as an anchorage. Imbeddedin the concrete foundation is a platform of steel I beams which carriesthe entire weight of masonry above it; under this platform are placedsteel washer-plates with conical sockets through which the anchor-rodspass, the weight being transferred to the anchor-rods on conical counter-sunk heads; the anchor-rods are built solid into the masonry, and runthrough steel plates on top; the top of each rod is turned down, andscrews of two different diameters are cut on it; the nuts on the lowerscrews, 3J inches in diameter, screw up against the plates on top of themasonry, thus taking up any possible slack in advance of strain; theupper and smaller screws, 3£ inches in diameter, carry two nuts, besidesa check-nut on top, and these two nuts hold between them the cross-blocks which carry the pin on which the outside x’ods attach; under eachtruss there are 16 rods in the masonry, these rods connecting with eightblocks which carry the single pin on which four anchor-bare connect.