Establishing the Four Corners of a Hole

It wasn’t easy.

Problem

We are putting a lower level under our existing structure so knowing where the four corners are should be easy, right?  The existing structure is being supported by concrete columns holding a double I-beam, which the existing East and West walls are bolted to.  These walls are anchored laterally to the ground, holding them solidly in place.   The support system was designed to withstand wind gusts to 100 mph.

The structure was surveyed prior to us starting any work at all, so the four corners of the existing house were identified and the coordinates established by Dunbar and Craig, licensed surveyors.  These four corners are still in place today.  The four corners of the house that were surveyed were the outside corners of the wood, which had 1/4 inch plywood and 1/2 inch moulding strips on top of.  These corners, which were probably not perfectly square to start with, have moved a bit in the almost 30 years since the existing house was built in 1969.  Plus, the house was rocked vigorously by the the 1989 Loma Prieta earthquake, which measured 6.9, as the epicenter was only 14.7 miles from 17509 Via Sereno.

In hindsight, we should have exposed the corners of the concrete slab-on-grade and surveyed those points.

A beautiful, blue-sky California day with snow on the peak of Mt. Hamilton.

A beautiful, blue-sky California day with snow on the peak of Mt. Hamilton.

Projecting the Four Corners Onto the Working Slab

Our surveyers took the existing four corners, which have not moved, and established offset marks on the working slab.  When we measured the diagonal distance between the north-east and south-west offset points, and the north-west and south-east points, we found the lengths to be different.

Different Lengths = Not Square

While the outside dimensions of the structure may not be square, the inside dimensions of the concrete walls must be perfectly square.  Having a perfectly square lower level requires the diagonal measurements to be equal.  Our architect, John Rider, calculated those diagonal measurements to be 67 ft 3 inches.

We measured the lengths of the diagonals established by the surveyor and found the lengths to be diffeerent by almost 2 inches.  Then, we measured the derived offsets established by John Rider and found those to be different by just over an inch.  Close, but not square.

Bill Brown‘s on-site foreman, Jorge Ramirez, has established the inside corners of concrete structures many times.  He knew exactly what to do and how to solve the problem.  A little tweaking and he had taught string-lines in place over the snapped chalk lines on the structural slab.  Jorge said it was square.

To compare the offset marks, Jorge and Bryan established interim offset marks 3 ft along the chalk line away from each of the four corners that Jorge had set.  The intersection of these two interim offsets became the new offset mark.

We cleared everything out of the path so we could measure the diagonal distance between the offsets.

One diagonal was 67 ft 3 inches exactly and the other was 67 ft 3 1/4 inches.

Got it – we had the four corners of the hole.

(Photos below show the scribed offset marks in the concrete from the surveyors, and Jorge’s offset marks as a blue cross with a circle.)

South-east corner, showing the offset marks for the inside corner of the concrete wall.

South-east corner, showing the offset marks for the inside corner of the concrete wall.

North-east corner, in the afternoon sunshine, with the offset marks from the concrete wall.

North-east corner, in the afternoon sunshine, with the offset marks from the concrete wall.

North-west corner, showing the offset marks.

North-west corner, showing the offset marks.

South-west corner offset mark.

South-west corner offset mark.