The ends of a culvert are usually expanded into end walls for the retention of the embankment. For the larger culverts, this may develop into two wing walls which act as retaining walls to prevent the embankment from falling over into the bed of the stream. An end wall is especially necessary on the upstream end of the culvert, so as to avoid the danger that the stream will scour the bank and work its way behind the culvert walls. The end wall is also carried up above the height of the top of the culvert, so as to guard still further against the washing of earth from the embankment over the end of the culvert into the stream below. All of these details are illustrated in the figures shown.

Fig. 75. Double Box Culvert, 4 by 3 Foot.

Fig. 75. Double Box Culvert, 4 by 3-Foot.

Box culverts are sometimes constructed as dry masonry - that is, without the use of mortar. This should never be done, except for very small culverts and when the stones are so large and regular that they form close, solid walls with comparatively small joints. A dry wall made up of irregular stones cannot withstand the thrusts which are usually exerted by the subsequent expansion of the earth embankment above it.