CHAPTER IV.

THE ENGINEERING PROBLEM.-PART I.

THE EXCAVATIONS AND DIVIDE CUTS.

HE character of the material to be removed, both wet and dry, has been accurately determined on the whole route by numerous borings penetrating to the depth required to the bottom of the canal, and at the sites of the dams, embankments, and locks.

THE EASTERN SECTION.

Leaving the San Francisco Basin, and proceeding eastward, the route cuts across a narrow neck of the intervening ridge-a spur of the main Cordillera bounding the San Juan watershed to the north-which, as a broad mass of hills, extends, on the south, to the banks of the river San Juan, often rising to elevations of 1,500 feet, while on the north it merges into the main Cordillera. But at the point selected, the spur is nearly divided on the west and east by the neighbouring valleys, whose axes lie on a generally direct line between Ochoa and Greytown, their floors being at about the same level, and

here is found the lowest gap for many miles on either flank.

In the harbour of Greytown and its approaches clean, sharp sand is the only material met with. From the harbour to Lock No. 1, and through Benard Lagoon, the materials are sand and sandy clay, underlying a thin, loamy stratum and decomposed organic matter, and from the lagoon to the lower lock, stiff clay. The harbour and this sea-level portion of the canal will be made with the floating dredge. Slopes of three horizontal to one vertical have been allowed in the estimates; but past experience gained in dredging by the Company in the first mile of canal is said to show that the material stands perfectly for several months at a much less inclination. In the excavation for the railroad through the stiff, tenacious clay predominating in this region, the material stands nearly vertical, but slopes of 1 to 1 have been estimated for. From the lower lock to the Divide cut this hard clay, with occasional boulders, is the only material found by the boring tool on the axis of the canal throughout, and also at the site of the three locks and the embankment. The clay is impervious to water, and has a large sustaining power, so that no apprehension is felt as to the character of the foundations.

In the deep cut (the Great Divide) the geological formation is clay, overlying solid volcanic rock. Diamond drill borings have been taken along the whole length of the cut to the bottom of the canal at intervals of about 1,000 feet; and the cores brought up are claimed to "settle beyond doubt the character of the material to be

removed, and dispel all apprehension that this cut might be a repetition of the disastrous experience in the great Culebra cut at Panama." The slope allowed in clay is

1 to 1, and in rock, to 1 to the level of the water, and below that point vertical. Mr. Menocal is of opinion that there is no good reason why the whole rock excavation should not be made with vertical sides, and he instances the Corinth Canal, where, with a longer and deeper excavation, and the rock less homogeneous and softer, "a slope of to 1 has been carried down to the water level, and the sides do not crumble or slide." This example is an unfortunate one to cite, as it happens that, since the expression of this opinion, a slip has occurred, and anxiety is still felt as to the future stability of the cut. From the Divide to Ochoa homogeneous clay has been found at all points; and the standard

1

In reply to a criticism made that even with the canal cross-section and the slopes proposed, 80' x 28', altered to 100' x 30', there are in it 10,000,000 cu. yds. of excavation, "of which about 3,000,000 yds. are surface soil and decomposed lava, and 7,000,000 are solid lavas and indurated or consolidated beds of volcanic ashes of about the hardness and consistency of slate rock," and that it would be interesting to compare the material with that in the great cut on the Panama Canal Mr. Menocal has said: "The Great Divide cut is no doubt a work of considerable magnitude, but there is no serious difficulty involved beyond proportional labour and expense. There is no need to discuss the suggestion that the material may prove to be similar to the Culebra cut at Panama. Diamond drill borings at intervals of 1,000 feet have been made throughout its whole length to the bottom of the proposed excavation, and nothing "suspicious" has been revealed. There is no loose rock, sand or volcanic ashes in it, stiff clay overlaying solid rock being the only material met with."-Railroad Gazette, February 24th, 1893.

section in soft material has been adopted throughout. At the site of the Ochoa dam, gravel, clay, and rock in the order named are shown by the borings.

Several important advantages in connection with this work are claimed by Mr. Menocal, all of which may be freely conceded, except perhaps the certain stability of the nearly vertical sides, discussed later.

First:-The material to be removed is mainly solid rock; and, therefore, the volume of excavation is reduced

45.0 FT.
POLSS
COROMANDEL

1.009.0
SOFT

10 0 10 20 30 40 50 60 70 80 90 100

SCALE OF FEET

CROSS-SECTION OF "GREAT DIVIDE" (NOTE: WIDTH ALTERED TO

100 FEET).

to a minimum, and the cut, when made, will remain so for ever without further expense.

Second:-The material is needed for the construction of the dam at Ochoa, for the embankments between Ochoa and Greytown, for the construction of the locks, for the breakwater at Greytown, and for pitching the sides of the canal, and the surplus can be dumped in the immediate vicinity.

Third:-The centre of distribution is most conveniently located, and were not this material available, at the sole

expense of transportation down grade, it would have to be obtained at considerable cost from quarries in the vicinity, as there is no rock easy of access between Ochoa and Greytown, except in this ridge.

Fourth-The locality is one of the healthiest in Nicaragua, the drainage is perfect, and water abundant and excellent for domestic uses.

Fifth-There is close at hand, on both sides of the ridge, an inexhaustible water-power for the economical and convenient operation of all the machinery required to do the work.

The stupendous character of this work will be apparent from the fact that with the section proposed a probable total of about 12,000,000 yards will have to be extracted and removed from a trench less than three miles long, at an average depth of 141 feet and a maximum of 328 feet. The Western Divide is insignificant in comparison, for, though it contains nearly as much excavation, it will occur in a length of nine miles, with the greatest depth only 72 feet.

The Great Divide is one of the three problems presenting any doubt whatever as to their entire practicability on the plans proposed, the other two being the Ochoa dam and the harbour at Greytown. In his report, Major Dutton gives the following account of the cut:

"At the head of the Deseado basin, and three miles distant from the upper lock, the greatest of all the individual works of the canal begins. A cut of vast proportions is necessary to force a water-way from the coastal plain and Deseado watershed into the basin of the San