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(3) METHODS EMPLOYED AND RESULTS SECURED IN THE TREATMENT OF SLIDING CUTS AND FILLS AND SOFT SPOTS IN EXCAVATIONS
AND EMBANKMENTS C. A. DALEY, Chairman;
R. D. GARNER, C. W. BROWN,
E. G. HEWSON, R. A. RUTLEDGE,
Sub-Committee. Your Committee wishes to draw the attention of the menibers to the report of the Roadway Committee under the Chairmanship of Mr. George H. Bremner, contained in Volume 10, Part 2, 1909, where a large amount of very valuable information on this subject was submitted and from which the conclusions now in the Manual were deduced:
The treatment of slides and soft spots which are the result of water pockets formed in the track has not been considered in this report, as this subject has already been well covered in recent investigations and reports by the Roadway Committee. This report covers siices and soft spots, the causes of which are of different origin and are more deeply seated.
In order to obtain information as to the different methods employed and the results obtained under different geological and climatic conditions, a questionnaire was sent out to fifteen of the larger ra:lreads in different parts of the United States and Canada, the riplies received being embodied in the foltowing report: Slides Slides will be treated in two classes, namely:
Class "A"-Surface slides of small volume.
Class “B”-Slides or slips of great volume which do not as a rule come as an avalanche but are oi slow movement.
("A") Class “A”-Surface slides occur in nearly every class of material and combination or mixture of materials, except rock, in both excavations and embankments und are caused largely by the ground becoming saturated. The extra weight of the water in the ground upsets the equilibrium and reduces the cohesion and a slide is the result. In North climates the action of frost greatly facilitates the action of the water by opening up crevices and making the ground porous.
TREATMENT IN EXCAVATIONS.-Surface water should be kept from the face of cuts as much as possible by the use of intercepting ditches constructed well back from the top of cut. These ditches should be constructed with care and should be well maintained so as not to let pockets form to hold water that will eventually soak into the ground and may be the cause of a slide.
Good results have been obtained in deep cuts where excavations have been taken out in such manner as to permit it by the construction of ditches on a table or shelf part way up the slope and the water carried off.
Where cuts have a large number of springs on their surface, small concealed tile drains have been laid down the slopes to the ditches in order to keep the water from the numerous springs and saturating the soil.
In many cases excellent results have been obtained by facing cuts with layer of engine cinders or fine slag about six inches deep. This tends to distribute any water reaching the face of the cut and prevents cutting, and also lessens the action of the frost. Where this method is used the cut should be faced to a reasonably true plane before the application of the coating.
In cities and towns sodding of slopes has been carried out with good success, but on account of the cost, this method of treatment has been confined mostly to densely inhabited districts where the esthetic value of this method is given consideration.
The planting of vegetation (not trees) on the slopes of cuts has proven very successful in the cure of slides. A mixture of seed that has been found very desirable for this purpose, in climates where it will grow, is two parts by weight of alsyke, two parts alfalfa and one part blue grass seed. Where the soil is poor it is desirable to enrich it with street sweepings or manure. Small wooden stakes should be driven into the ground at about twelve inch centers to hold the dirt and vegetation until the roots are long enough to get a good hold in the original soil. It is also well to sow oats or rye with the seed as this shades and protects the smaller grass. The real success, however, in obtaining growth of vegetation on cuts to assist in preventing slides is to properly care for the young grass for the first couple of years. Mowing should be done frequently and the grass should not be allowed to go to seed Japanese honeysuckle and Bermuda grass have also been planted on cuts with good results.
EMBANKMENTS.-Small slides of embankments are treated in practically the same manner as above mentioned in cuts except for the drainage. The slopes have been sodded and faced with cinders in a manner similar to the treatment of cuts and with equal success. The flattening of the slope and the weighting of the toe of the slope with derrick rip rap where it is practical to do so, and where the toe of the slope is subject to the action of running or standing water, has proven successful. Embankments that have given trouble for a long time when treated in this manner have become stable. In one case it was necessary to place rip rap well out in the bed of the stream to accomplish the desired result. Where the embankment is not subject to the action of water, the use of either two or three man rip rap will answer the purpose equally well. The same result has been accomplished by the construction of a false berm approximately one-third the height of the embankment.
In low-lying country where there is a large amount of surface and ground water, considerable trouble has been experienced in holding fills. To overcome this ditches have been dug near the right-of-way line some distance from the toe of the slope and the ground water level lowered with exceedingly good results. The depth of the ditches to be dug depends on the fall that can be obtained.
In some cases where the slides are of a minor nature a timber crib or loose stone retaining wall built in the toe of the slope on an angle approximately equal to the angle of slope has proven effective.
("B") Class “B”—Slides or slips of great volume are hard to remedy. Each case must be studied and the cause determined, if possible, and this cause then removed. In nearly every case it is underground water or seepage that causes the trouble.
When a study has been made of the movement and its cause determined it is generally found that the slipping occurs on a strata of inclined rock or clay. Sometimes the strata of clay is only an inch or so thick. Water reaching the strata of rock or clay is retained and forms a lubricant which upsets the equilibrium of the mass and the slide starts. The remedy is, of course, to intercept the water and to prevent it following the strata.
The following are some interesting experiments which have recently been carried out:
1. In which the water was intercepted on a strata of clay about a hundred feet below the surface of the ground and which has proven successful in curing serious slide is reported by the Kanawha and West Virginia Railway:
"Holes were drilled about 75 ft. back of the top of slope of the cut and 75 ft. apart. A total of 11 holes were drilled ranging from 95 ft. to about 130 ft. deep. These holes were from 110 to 190 ft. from the center line of the main track. They were drilled from 8 to 10 ft. into a soapstone formation on top of solid rock 20 to 25 ft. below grade. It was necessary to case the top part of the hole in the sand clay and boulders, the balance of the hole standing in good shape without casing. As soon as the hole was drilled it was sprung and immediately loaded and shot. About 100 lb. of dynamite was placed in the bottom of rock shot and about 75 lb. in the upper or blue clay shot which was placed about at the grade of the track or a little below. The holes were filled between the lower and upper shots with sand. The holes had to be loaded and shot quickly after cleaning them, as the water ran in very rapidly. The casing was pulled after the hole was loaded and ready to shoot.
When the holes were shot there was no distortion of the face of the cut or the ground around the hole. The face of the cut was immediately dried up with the exception of a small amount of water that came over from the top in the time of heavy rain. The test is now over two years old and has so far been very satisfactory, and as the slope is now getting well covered with vegetation, no further trouble is anticipated.
The total length of holes drilled was about 1,232 lin. ft. The cost of drilling and shooting was $1,816.92, which represents a cost of approximately $1.47 per lin. ft. of hole. The work was started in May, 1919, and completed in July, 1919."
Another case of intercepting the ground water and thus curing a bad slide of an embankment built on a hillside is reported by the Southern Pacific Company as follows:
"The fill in question was on a side hill in the Santa Lucia Mountain District. It had always given trouble, but during the winter of_1909-10 it became very serious, due to the heavy rains of that winter. The settlement was so great that a very large force of men was constantly engaged, day and night, in picking up the track after the passing of each train, a steam shovel being required to furnish the material. It is estimated that the fill settled during the few months considerably over 100 ft. The sliding material was a quarter of a mile long, the original culvert broken up and carried along with the sliding material, the ground moving along in big folds and waves.
In attempting to find a solution for the trouble, we noted the absolute absence of water above the tracks, except in rainy weather, and then only in small quantities. The upper portion of the sliding fill for about 100 ft. in depth was absolutely dry, but below this 100 ft. water was traced coming out of the rim of the slide; that is, where the sliding material joined the solid hillside; below this point the hill was saturated with water. We reasoned that somewhere about 100 ft. below the track there must be a live spring of water, coming out of the hillside, possibly on the contact between two strata formations. A shaft was sunk above the track and a well defined contact was discovered. The next thing to do was to go down on the rim in the solid material, a short distance below where the water came out, and start tunnels on such a grade as to connect with the shaft above. This was done and a large amount of water picked up, one spring cut into flowing at the rate of 900 gal. per minute, and during ordinary summer conditions the water, measured at the portal of the main drain tunnel, amounted to over 1,200 gal. per minute.
Shortly after the completion of these drain tunnels the track stopped settling, and only ordinary maintenance has been required since then. Several other slides in the immediate vicinity were corrected in a similar manner.'
A report from the North Pacific Coast is as follows:
"Throughout the North Pacific Coast territory slides of large volume are of frequent occurrence. The attention of the Committee is called to an article which will appear in a future number of the proceedings of the American Society of Civil Engineers by Mr. S. Murray, Assistant Chief Engineer, 0.-W. R. R. & N. Company, concerning a slip of large volume which has been under treatment for a good many years. This slip is in the gorge of the Columbia River, about forty-five miles East of Portland. At this point the river cuts through the Cascade Range. On the north side of the river the present slip extends back three or four miles from the edge of the river, but the ground along the river bank has become practically stable so that no difficulty is encountered by the railroad line on that side of the river.
On the south side of the Columbia River the break from the top of the range is only a few thousand feet from the river bank. Great portions of the ground in this vicinity for a stretch of three miles along the river bank are slowly slipping toward the river, carrying with it the railroad which lies close to the river's edge.
Measurements made of this slip during the past fifteen years show a movement of 10 or 15 ft., up to 30 or 40 ft., and at some points the original stakes to mark the ground have gone beyond this point into the river and are, of course, obliterated.
This slip has caused considerable difficulty in maintaining the track. The method of treatment as outlined in the paper mentioned in the foregoing has been to run tunnels back into the slipping ground above the track to tap the veins of water and drain the small ponds and reservoirs lying in depressions on the surface. The details of construction and the results obtained are thoroughly covered by Mr. Murray in his paper and the Committee will no doubt find interesting material for study in it.
A slip of a different character has occurred in Eastern Oregon on a spur of the Blue Mountains. At this point an area of some three or four hundred acres lying along the track' for a distance of about a mile has been gradually slipping down the side of the mountain, carrying the track with it. The movement has been very slow ordinarily, although at times it has been of sufficient amount to endanger the track. A study of the character of the ground at this point shows a surface strata of rock and clay to a depth of 2 or 3 ft. Beneath this strata is another of yellow clay 12 to 2 in. in thickness; below the yellow clay, coarse boulders and blue clay.
The yellow clay has acted as a lubricant on the surface strata, permitting it when wet to move bodily. The treatment in this case has been to run a trench through the natural drainage channels up the side of the mountain, shore it with heavy timbers and extend branches at frequent intervals from it over the slipping area. By this means it is hoped to keep the strata of yellow clay sufficiently free from water to prevent saturation and thus stop the slipping."
In a good many cases where the volume of the slip amounts to several thousand yards, the movement has been stopped by the use of concrete retaining walls, or pile bulkheads. A concrete retaining wall can be used only where a good foundation is procurable and its use is, therefore, considerably limited. In many cases, pile bulkheads have been the correct solution.
In the case of such slips the movement usually occurs only during the rainy season and, of course, the saturation of the ground decreases the angle of repose. Ordinarily the material is very nearly stabilized and only a small force is required to counteract any tendency to slip. For this reason pile bulkheads effectively offer the necessary resistance to hold a seemingly large mass in equilibrium.
Concrete retaining walls and piles have been used in an effort to cure slides of this class but in most cases the best that can be said for this method of treatment is that it affords only temporary relief, for the retaining wall will eventually slide at the base or tip over and the piles crowd out
SOFT SPOTS.-Soft spots in excavations and embankments, the origin of which is the result of geological formation, are hard to contend with and no uniform treatment can be applied to all cases.
A method that has been successfully used in both excavations and embankments is the use of long ties.
One of the railroads in the Northwest reports an effective method of treatment by the construction of a reinforced concrete slab 12 in. thick, on which the track is carried. The foundation for the concrete slab is prepared by leveling off the sub-grade and applying a 12-in. layer of good engine cinders. The cinders furnish good drainage and the concrete slab uniformly distributes the load.