The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 5: Forts and Artillery.
Francis Trevelyan Miller, Ed.

James River, a pontoon bridge was laid at a point known as Chaffin's Bluff, and this was soon thereafter used to transfer reinforcements to the line between the James and Appomattox rivers, and to Petersburg.

At the same time, one company of engineer troops was detachedd with a pontoon train and sent to Petersburg, where a bridge was needed to facilitate crossing the Appomattox River at that point. Of the eleven companies of engineer troops which remained north of the James during General Grant's first operations against Petersburg, one company was in charge of the pontoon bridge at Chaffin's Bluff and ten served as infantry on what was known as the Deep Bottom line.

As soon as it became known to General Beauregard that an attempt was being made to undermine a salient point on his line, he made use of the company of engineer troops then at Petersburg, in an effort to protect the threatened point by countermining. Two pits were sunk in the trenches, from the bottom of which drifts or tunnels were extended some distance beyond the entrenchments, and a circumvallating gallery was in progress, which, if it had been completed in time, might have discovered the exact location of the underground approach of the foe: although it was subsequently ascertained that while the drift by which the Federals reached a point under the Confederate lines was about half-way between the two Confederate workings, it was at a somewhat lower level. However, the Confederate works were incomplete when, on July 30, 1864, the Federal mine was exploded.

The most lasting effect of this demonstration by General Grant was to produce a feeling of impending danger at every salient point of the Confederate line of defense; and General Lee ordered eight more companies of engineer troops from north of the James to Petersburg, and made large details from the infantry to swell their numbers, in order to expedite the work of countermining, which, from that time on, was pushed energetically, until ample protection was afforded at all points