CHAPTER XII. EXCAVATING TUNNELS THROUGH SOFT GROUND; GENERAL DISCUSSION; THE BELGIAN METHOD. GENERAL DISCUSSION.

It may be set down as a general truth that the excavation of tunnels through soft ground is the most difficult task which confronts the tunnel engineer. Under the general term of soft ground, however, a great variety of materials is included, beginning with stratified soft rock and the most stable sands and clays, and ending with laminated clay of the worst character. From this it is evident that certain kinds of soft-ground tunneling may be less difficult than the tunneling of rock, and that other kinds may present almost insurmountable difficulties. Classing both the easy and the difficult materials together, however, the accuracy of the statement first made holds good in a general way. Whatever the opinion may be in regard to this point, however, there is no chance for dispute in the statement that the difficulty of tunneling the softer and more treacherous clays, peats, and sands is greater than that of tunneling firm soils and rock; and if we describe the methods which are used successfully in tunneling very unstable materials, no difficulty need be experienced in modifying them to handle stable materials.

Characteristics of Soft-Ground Tunneling.

—The principal characteristics which distinguish soft-ground tunneling are, first, that the material is excavated without the use of explosives, and second, that the excavation has to be strutted practically as fast as it is completed. In treacherous soils the excavation also presents other characteristic phenomena: The material forming the walls of the excavation tends to cave and slide. This tendency may develop immediately upon excavation, or it may be of slower growth, due to weathering and other natural causes. In either case the roof of the excavations tends to fall, the sides tend to cave inward and squeeze together, and the bottom tends to bulge or swell upward. In materials of very unstable character these movements exert enormous pressures upon the timbering or strutting, and in especially bad cases may destroy and crush the strutting completely. Outside the tunnel the surface of the ground above sinks for a considerable distance on each side of the line of the tunnel.

Methods of Soft-Ground Tunneling.

—There are a variety of methods of tunneling through soft ground. Some of these, like the quicksand method and the shield method, differ in character entirely, while in others, like the Belgian, German, English, Austrian, and Italian methods, the difference consists simply in the different order in which the drifts and headings are driven, in the difference in the number and size of these advance galleries, and in the different forms of strutting framework employed. In this book the shield method is considered individually; but the description of the Belgian, German, English, Austrian, Italian, and quicksand methods are grouped together in this and the three succeeding chapters to permit of easy comparison.

THE BELGIAN METHOD OF TUNNELING THROUGH SOFT GROUND.

The Belgian method of tunneling through soft ground was first employed in 1828 in excavating the Charleroy tunnel of the Brussels-Charleroy Canal in Belgium, and it takes its name from the country in which it originated. The distinctive characteristic of the method is the construction of the roof arch before the side walls and invert are built. The excavation, therefore, begins with the driving of a top center heading which is enlarged until the whole of the section above the springing lines of the arch is opened. Various modifications of the method have been developed, and some of the more important of these will be described farther on, but we shall begin its consideration here by describing first the original and usual mode of procedure.

Excavation.

—Fig. 68 is the excavation diagram of the Belgian method of tunneling. The excavation is begun by opening the center top heading No. 1, which is carried ahead a greater or less distance, depending upon the nature of the soil, and is immediately strutted. This heading is then deepened by excavating part No. 2, to a depth corresponding to the springing lines of the roof arch. The next step is to remove the two side sections No. 3, by attacking them at the two fronts and at the sides with four gangs of excavators. The regularity and efficiency of the mode of procedure described consist in adopting such dimensions for these several parts of the section that each will be excavated at the same rate of speed. When the upper part of the section has been excavated as described, the roof arch is built, with its feet supported by the unexcavated earth below. This portion of the section is excavated by taking out first the central trench No. 4 to the depth of the bottom of the tunnel, and then by removing the two side parts No. 5. As these side parts No. 5 have to support the arch, they have to be excavated in such a way as not to endanger it. At intervals along the central trench No. 4, transverse or side trenches about 2 ft. wide are excavated on both sides, and struts are inserted to support the masonry previously supported by the earth which has been removed. The next step is to widen these side trenches, and insert struts until all of the material in parts No. 5 is taken out.

When the material penetrated is firm enough to permit, the plan of excavation illustrated by the diagram, Fig. 68A, is substituted for the more typical one just described. The only difference in the two methods consists in the plan of excavating the upper part of the profile, which in the second method consists in driving first the center top heading No. 1, and then in taking out the remainder of the section above the springing lines of the arch in one operation, while in the first method it is done in two operations. The distance ahead of the masonry to which the various parts can be driven varies from 10 ft. to, in some cases, 100 ft., being very short in treacherous ground, and longer the more stable the material is.

Strutting.

—The longitudinal method of strutting, with the poling-boards running transversely of the tunnel, is always employed in the Belgian method of tunneling. In driving the first center top heading, pairs of vertical posts carrying a transverse cap-piece are erected at intervals. On these cap-pieces are carried two longitudinal bars, which in turn support the saddle planks. As fast as part No. 2, Fig. 68, is excavated, the vertical posts are replaced by the batter posts A and B, Fig. 69. The excavation of parts No. 3 is begun at the top, the poling-boards a and b being inserted as the work progresses. To support the outer ends of these poling-boards, the longitudinals X and Y are inserted and supported by the batter posts C and D. In exactly the same way the poling-boards c and d, the longitudinals V and W, and the struts E and F, are placed in position; and this procedure is repeated until the whole top part of the section is strutted, as shown by Fig. 69, the cross struts x, y, z, etc., being inserted to hold the radial struts firmly in position. The feet of the various radial props rest on the sill M N. These fan-like timber structures are set up at intervals of from 3 ft. to 6 ft., depending upon the quality of the soil penetrated.

Centers.

—Either plank or trussed centers may be employed in laying the roof arch in the Belgian method, but the form of center commonly employed is a trussed center constructed as shown by Fig. 70. It may be said to consist of a king-post truss carried on top of a modified form of queen-post truss. The collar-beam and the tie-beam of the queen-post truss are spaced about 7 ft. apart, and the posts themselves are left far enough apart to allow the passage of workmen and cars between them. The tie beam of the king-post truss is clamped to the collar-beam of the queen-post truss by iron bands. On the rafters of the two trusses are fastened timbers, with their outer edges cut to the curve of the roof arch. These centers are set up midway between the fan-like strutting frames previously described. They are usually built of square timbers. The tie beams are usually 6× 6 in., and the struts and posts 4× 4 in. timbers. The reason for giving the larger sectional dimensions to the tie beams, contrary to the usual practice in constructing centers, is that it has to serve as a sill for distributing the pressure to the foundation of unexcavated soil which supports the center. Sometimes a sub-sill is used to support the center upon the soil; and in any case wedges are employed to carry it, which can be removed for the purpose of striking the center. After the arch is completed, the centers may be removed immediately, or may be left in position until the masonry has thoroughly set. In either case the leading center over which the arch masonry terminates temporarily is left in position until the next section of the arch is built.

Masonry.

—The masonry of the roof arch, which is the first part built, is of necessity begun at the springing lines, and the first course rests on short lengths of heavy planks. These planks, besides giving an even surface upon which to begin the masonry, are essential in furnishing a bearing to the struts inserted to support the arch while the earth below them, part No. 5, Fig. 68, is being excavated. As the arch masonry progresses from the springing lines upward, the radial posts of the strutting are removed, and replaced by short struts resting on the lagging of the centers, which support the crown bars or longitudinals until the masonry is in place, when they and the poling-boards are removed, and the space between the arch masonry and walls of the excavation is filled with stone or well-rammed earth.

Considering now the side wall masonry, it will be remembered that in excavating the part No. 5, Fig. 68, of the section, frequent side trenches were excavated, and struts inserted to take the weight of the masonry. These struts are inserted on a batter, with their feet near the center of the tunnel floor, so that the side wall masonry may be carried up behind them to a height as near as possible to the springing lines of the arch. When this is done the struts are removed, and the space remaining between the top of the partly finished side wall and the arch is filled in. This leaves the arch supported by alternate lengths or pillars of unexcavated earth and completed side wall. The next step is to remove the remaining sections of earth between the sections of side wall, and fill in the space with masonry. Fig. 71 is a cross-section, showing the masonry completed for one-half and the inclined props in position for the other half; and Fig. 72 is a longitudinal section showing the pillars of unexcavated earth between the consecutive sets of inclined struts and several other details of the lining, strutting, and excavating work.

The invert masonry is built after the side walls are completed. This is regarded as a defect of this method of tunneling, since the lateral pressures may squeeze the side walls together and distort the arch before the invert is in place to brace them apart. To prevent as much as possible the distortion of the arch after the centers are removed, it is considered good practice to shore the masonry with horizontal beams having their ends abutting against plank, as shown by Fig. 71. These horizontal beams should be placed at close intervals, and be supported at intermediate points by vertical posts, as shown by the illustration. Since the roof arch rests are for some time supported directly by the unexcavated earth below, settlement is liable, particularly in working through soft ground. This fact may not be very important so long as the settlement is uniform, and is not enough to encroach on the space necessary for the safe passage of travel. To prevent the latter possibility the centers are placed from 9 ins. to 15 ins. higher than their true positions, depending upon the nature of the soil, so that considerable settlement is possible without any danger of the necessary cross-section being infringed upon. In conclusion it may be noted that the lining may be constructed in a series of consecutive rings, or as a single cylindrical mass.

Hauling.

—Since in this method of tunneling the upper part of the section is excavated and lined before the excavation of the lower part is begun, the upper portion is always more advanced than the lower. To carry away the earth excavated at the front, therefore, an elevation has to be surmounted; and this is usually done by constructing an inclined plane rising from the floor of the tunnel to the floor of the heading, as shown by Fig. 72. This inclined plane has, of course, to be moved ahead as the work advances, and to permit of this movement with as little interruption of the other work as possible, two planes are employed. One is erected at the right-hand side of the section, and serves to carry the traffic while the left-hand side of the lower section is being removed some distance ahead and the other plane is being erected. The inclination given to these planes depends upon the size of the loads to be hauled, but they should always have as slight a grade as practicable. Narrow-gauge tracks are laid on these planes and along the floor of the upper part of the section passing through the center opening mentioned before as being left in the centers and strutting.

In excavating the top center heading there is, of course, another rise to its floor from the floor of the upper part of the section. Where, as is usually the case in soft soils, this top heading is not driven very far in advance, the earth from the front is usually conveyed to the rear in wheelbarrows, and dumped into the cars standing on the tracks below. In firm soils, where the heading is driven too far in advance to make this method of conveyance adequate, tracks are also laid on the floor of the heading, and an inclined plane is built connecting it with the tracks on the next level below. In place of these inclined planes, and also in place of those between the floor of the tunnel and the level above, some form of hoisting device is sometimes employed to lift the cars from one level to the other. There are some advantages to this method in point of economy, but the hoisting-machines are not easily worked in the darkness, and accidents are likely to occur.

In the advanced top heading and in the upper part of the section narrow-gauge tracks are necessarily employed, and these may be continued along the floor of the finished section, or the permanent broad-gauge railway tracks may be laid as fast as the full section is completed. In the former case the permanent tracks are not laid until the entire tunnel is practically completed; and in the latter case, unless a third rail is laid, the loads have to be transshipped from the broad- to the narrow-gauge tracks or vice versa. It is the more general practice to use a third rail rather than to transship every load.

Modifications.

—Considering the extent to which the Belgian method of tunneling has been employed, it is not surprising that many modifications of the standard mode of procedure have been developed. The modification which differs most from the standard form is, perhaps, that adopted in excavating the Roosebeck tunnel in Germany. This method preserves the principal characteristic of the Belgian method, which is the construction of the upper part of the section first; but instead of building the side walls from the bottom upward, they are built in small sections from the top downward. The excavation begins by driving the center top heading No. 1, Fig. 73, whose floor is at the level of the springing lines of the roof arch, and then the two side parts No. 2 are excavated, opening up the entire upper portion of the section in which the roof arch is built, as in the regular Belgian method. The next step is to excavate part No. 3, shoring up the arch at frequent intervals. Between these sets of shoring the side walls are built, resting on planks on the floor of part No. 3, and then the sets of shores are removed and replaced by masonry. Next part No. 4 is excavated, shored, and filled with masonry as was part No. 3. In exactly the same way parts 5, 6, 7, and 8 are constructed in the order numbered. To prevent the distortion of the arch during the side-wall construction it is braced by horizontal struts, as indicated above in Fig. 71.

Advantages.

—The advantages of the Belgian method of tunneling may be summarized as follows: (1) The excavation progresses simultaneously at several points without the different gangs of excavators interfering with each other, thus securing rapidity and efficiency of work; (2) the excavation is done by driving a number of drifts or parts of small section, which are immediately strutted, thus causing the minimum disturbance of the surrounding material; (3) the roof of the tunnel, which is the part of the lining exposed to the greatest pressures, is built first.

Disadvantages.

—The disadvantages of the Belgian method of tunneling may be summarized as follows: (1) The roof arch which rests at first on compressible soil is liable to sink; (2) before the invert is built there is danger of the arch and side walls being distorted or sliding under the lateral pressures; (3) the masonry of the side walls has to be underpinned to the arch masonry.

Accidents and Repairs.

—One of the most frequent accidents in the Belgian method of tunneling is the sinking of the roof arch owing to its unstable foundation on the unexcavated soil of the lower portion of the section. The amount of settlement may vary from a few inches in firm soil to over 2 ft. in loose soils. To counteract the effect of this settlement it is the general practice to build the arch some inches higher than its normal position. When the settlement is great enough to infringe seriously upon the tunnel section, repairs have to be made; and the only way of accomplishing them is to demolish the arch and rebuild it from the side walls. It is usually considered best not to demolish the arch until the invert has been placed, so that no further disturbance is likely to occur once the lining is completed anew.

The rotation of the arch about its keystone, or the opening of the arch at the crown, by the squeezing inward of the haunches by the lateral pressures, is another characteristic accident. Fig. 74 shows the nature of the distortion produced; the segments of the arch move toward each other by revolving on the intradosal edges of the keystone, which are broken away and crushed together with the operation, while the extradosal edges are opened. It is to prevent this occurrence that the horizontal struts shown in Fig. 71 are employed. The manner of repairing this accident differs, depending upon the extent of the injury. When the intradosal edges of the keystone are but slightly crushed, the repairing is done as directed by Fig. 75. When the keystone is completely crushed, however, the indications are that the material of the keystone, usually brick, is not strong enough to resist the pressures coming upon it, and it is advisable to substitute a stronger material in the repairs, and a stone keystone is constructed as shown by Fig. 75. The middle stone of this keystone extends through the depth of the arch ring, and the two side stones only half-way through, their purpose being merely to resist the crushing forces which are greatest at the intrados. Sometimes, when the pressures are unsymmetrical, the arch ring breaks at the haunches as well as the crown, as shown by Fig. 75, which also indicates the mode of repairing. This consists in demolishing the original arch, and rebuilding it with stone voussoirs inserted in place of the brick in which the rupture occurred.

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