=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. [Illustration: FIG. 74.--Sketch Showing Failure of Roof Arch by Opening at Crown.] =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.