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In some cases soft spots in cuts have been treated by driving piles along the ballast line with the idea of confining the material, and by driving pile butts from 6 to 12 ft. in length spaced from 3 to 4 ft. directly under each rail and sinking them 2 or 3 ft. below sub-grade. The latter method is reported as being more satisfactory.
Piles have been driven in embankments both along the ballast line and under the rail in a manner similar to the method reported above but the results do not appear to be as satisfactory.
An effective method reported in the treatment of soft spots under embankments is the widening of the embankment until a point is reached where the pressure, due to the load, is distributed over a very large area.
CONCLUSIONS Slides.—Your Committee recommends the substitution of the follow ing conclusions for those published in the new Manual, as follows:
(1) The primary cause of slides is the lack of proper drainage.
(2) In the construction of a new line when conditions indicative of future trouble with soft spots or slides are encountered, special attention to the diversion of the springs or streams which are likely to cause the trouble should be given. (3) Conclusion 1 in Manual becomes Conclusion 3.
Conclusion 2 in Manual becomes Conclusion 4.
ders or fine slag will prevent small slides.
Conclusion 9 in Manual becomes Conclusion 12. SOFT SPOTS.—Where soft spots cannot be effectively drained the bearing area of the track structure must be increased.
DEFINITION OF SOFT SPOT.-Soft spots are small areas in excavation or embankment, or the sub-soil under an embankment, saturated with water and having a relatively small supporting power.
(4) EFFECT OF HEAVIER POWER AND INCREASED TONNAGE UPON ROADBED PREVIOUSLY
CONSIDERED STABILIZED R. C. Gowdy, Chairman;
H. A. ROBERTS, R. K. BROWN,
E. G. TABER, H. HAWGOOD,
Sub-Committee. Your Committee has had considerable discussion with a number of officials having to do with maintenance of way work and find without exception that they are of the opinion that the use of heavier power and increased tonnage is affecting their roadbeds to a great extent. It is noticed, however, that in the West it is doubtful if all roadbeds can be considered entirely stabilized, at least to the extent that they are in the East.
Your Committee has not as yet determined upon a method of procedure but feel that the subject is an important one and while they can now only report progress, that the subject should be reassigned them for next year's work.
(5) FILLING OF BRIDGE OPENINGS J. C. WRENSHALL, Chairman; G. L. SITTON, A. S. BUTTERWORTH,
C. E. WEAVER,
Preliminary Investigation 1. Before deciding whether or not to fill a certain structure, a thorough study of costs, based on local conditions, must be made, in order that preference be given to filling structures where the greatest saving in future maintenance may be effected. This applies in greater degree to wooden trestles. It is obvious that long, low structures, where the water can be carried by comparatively small culverts, will show the greatest saving. In the case of iron structures where operating conditions such as the use of heavier power demand replacement by solid fill and where commercial dirt and ashes are available in large quantities and at the expense of the haul only, other considerations than future maintenance should govern.
2. The preliminary investigation must develop especially:
(a) The size of culvert necessary, which will generally govern the type of culvert to be used. Emphasis is laid on providing sufficient area to carry off excessive storm water.
(b) The character of foundation available for the culvert. In order to determine the foot-ton carrying capacity of the soil, which will govern the design of the culvert.
(c) The character of foundation for the new fill. The nature of the foundation may be such that it would not be economical to fill the structure in question.
(d) The condition of the moisture with regard to withstanding the stresses incident to filling and carrying traffic at the same time; for if a trestle is allowed to deteriorate too far before filling, serious difficulties will be encountered in keeping the structure safe for traffic during the progress of filling.
(e) The availability and quality of material for making the fill, as both will affect the first cost, and the latter the future maintenance of the fill. Where commercial dirt is available in large quantities, consideration should be given to its character and the cost of the haul involved.
(f) Whether or not the advantage of widening adjacent cuts from which the filling material may be excavated will by improved drainage and increased snow room justify this method of procedure as against getting all the material from one place.
(g) Consideration should be given to the necessity of providing undergrade crossings to accommodate future as well as existing highways.
(h) Elimination of fire hazard by reason of filling wooden trestles should be taken into consideration.
Construction of Culvert This phase of the work is not sufficiently different from ordinary culvert construction to require special emphasis except as follows:
(a) The culvert must be long enough. Care must be taken to determine at what slope the filling material will stand in order to insure construction of a culvert of sufficient length, which length should be determined by observation of the slope of the soil in question in other fills.
(b) The culvert must have a foundation suitable to the load.
(c) In preparing culvert foundation, care must be taken not to endanger the stability of the structure, by undermining or excavating too close to bents without proper protection,
(d) The culvert must be thoroughly protected against scour, due consideration being given to the susceptibility of scour, or the new filling material, and of the foundation material. This protection may be provided by the construction of ample head and wing walls; concrete aprons with curtain walls; and concrete floor which in extreme cases may require transverse ribs at intervals.
(e) Back-fill around and over culvert should be carefully placed so as to avoid unequal stress, and culvert should be protected from danger of damage by falling rocks during filling of structure. This protection may be provided by placing an earth cushion over culvert, but a cushion of one or two-man stone is preferable, for that provides future drainage from around the barrel of culvert as well as protection during the operation of making the fill.
Construction of Fill and Care of Structure During Filling
1. Before starting to construct fill, see if there are any springs in the area to be covered, and if so, build concrete boxes over same and pipe the water beyond the limit of the new fill, where rock is available
and fall is sufficient. French drains may be substituted for the pipe. In certain sections of the country where trestles are to be filled in marsh land, it is frequently found advantageous to lay a grillage consisting of a double layer of small tree trunks; the layers being at right angles to each other. In the absence of tree trunks, old bridge timber or sawmill slabs can be substituted. This grillage assists in preventing unequal settlement and excessive subsidence.
2. With wooden trestles, before starting new fill, the trestle must be carefully gone over and put in shape to stand the stresses incident to filling. This is very important, because if it is not done and trestle gets out of surface or line, or both, it is almost impossible to get it back to proper shape. While this item does not amount to much in a 10 or 12 ft. trestle, when dealing with a trestle over 20 ft. high, it is important. If this preliminary work is properly done, it is almost always possible to keep the structure in good line, although, of course, the entire foundation may settle and cause a settlement in the surface of the trestle. Such a settlement, however, is generally not a serious difficulty, as long as the line is good. Ordinarily, a foundation which is good enough to support a standard frame trestle will be found to give very little trouble during filling if a bearing 4 ft. wide is provided the full length of each sill. In this connection, special attention should be given to removing all soft timber from bottom of posts, from sills, and from bottom bracing, and sometimes additional posts. It is better to spend a little money preparing a trestle to stand filling than to take a chance of having to spend much more during the progress of the work. It has been found to be a fairly good rule to work by that in a trestle 20 ft. high the blocking sills, bottom of posts, and braces, should be in condition to run two years without repairs, and in a trestle 35 ft. high or over, three years, before the structure should be considered in good enough condition to stand filling.
(a) In the case of iron and steel viaducts which are usually of greater height than wooden structures, precautions must be taken to prevent distortion of the columns. This can be accomplished by encasing the columns in reinforced concrete up to a point within 20 ft. of base of rail, or where there is no danger from fire satisfactory results have been obtained by the erection of intermediate wooden bents at suitable intervals in order to reduce the load upon the towers.
(b) As the filling of the viaduct proceeds, all bracing, both longitudinal and transverse, should be cut loose from the bents as the fill reaches the connection points, for this bracing transfers a great deal of load to the bents, which increases the settlement and has a tendency to work the viaduct out of line. Vertical bracing need only be cut loose at the upper connection points.