ut was removed, the form was built up to the next
strut, at front and back, and braced to the sheeting, so that, by the
time the entire length of the section had been carried up to the level
of the first line of struts, forms were ready at one end for the
succeeding layers. The layers of concrete never exceeded 8 in. in
height, and at times there were slight delays in the concreting while
the carpenters made ready the next lift of forms, but such delays were
rarely long enough to permit the concrete to take its initial set.

[Illustration:
Fig. 7.
SKETCH SHOWING FORMS FOR, AND METHOD OF, CONCRETING RETAINING WALLS
IN TRENCH.]

After a section of concrete had firmly set, both back and front forms
were removed, and the thrust from the sides of the trench was
transferred directly to the finished wall. The face of the wall was
rubbed with a cement brick to remove the marks of the plank, and was
then coated with a wash of thin cement grout. The water-proofing and
brick armor were then continued up the back of the wall, the spaces
between the lines of braces being first water-proofed and bricked, and
the braces transferred to the finished surface, after which the omitted
panels were completed. The water-proofing consisted of three layers of
Hydrex felt, of a brand known as Pennsylvania Special, and four layers
of coal-tar pitch. The pitch contained not less than 25% of carbon,
softened at 60 deg. Fahr., and melted at a point between 96 deg. and 106 deg. Fahr.
The melting point was determined by placing 1 gramme of pitch on a lead
disk over a hole, 5/16-in. in diameter, and immersed in water which was
heated at the rate of 1 deg. per min.; the temperature of the water at the
time the pitch ran through the hole was considered as the melting point.

In order to prevent the water-proofing from being torn at the joint
between sections when they contract from changes in temperature, a
vertical strip of felt, 6 in. wide, was pitched over each joint, lapping
3 in. on each concrete section. The back of this strip was not pitched,
but was covered with pulverized soapstone, so that the water-proofing
sheet was free from the wall for a distance of 3 in. on either side of
each joint.

Concreting was continued during the severest weather, one section being
placed when the thermometer was 5 deg. above zero. When the thermometer was
below the freezing point both sand and stone were heated by wood fires
in large pipes under the s