Diggers Diary – 12th August 2010
Today I learned how to profile a trench and produce a section drawing.
Wide at the top and narrow at the bottom, it was the north extension of trench 3. When Richard stood in it you couldn’t see his head. He said he once helped to dig a 40 ft well. This must be where he developed his hydraulic arms.
All over the site as we dig we uncover different layers of soil. Each layer is numbered and described and a small label is hammered into the trench to locate and separate it from the others. For example layer No 53 is described as Green, brown, silty clay. So far we are up to layer 56.
Some layers are thin in one place and thick in another.
They do not appear consistently in the same order around the site. That is, No 40 might be near the top of trench 2 but close to the bottom of trench 4.
Some layers are shaped like pudding basins with others attached. It’s all very confusing. When it rains, we the volunteers, can see them clearly. When it’s dry only the experts seem to know where they are. We are learning a lot as we go along.
This is how you achieve – I think! – a section drawing – according to Richards method and my kitchen language – You can go to sleep now if you want to!
You need two people. Four six inch nails or if preferred metal spikes. A length of string. Tape measure. Plumb line. Graph Paper and Pencil.
Take the 6” nail or metal spike and attach the length of string. Bang the nail/spike into the ground at the top of the trench at one end. Do the same at the other end of the trench with the same piece of string attached. Hang the spirit level in the middle and make sure the string is level all along its length.
Run the tape measure along the side of the trench about 6” below the string, or any convenient location and secure at both end with nails and that is your DATUM.
I have been to the dictionary again for all those in my ignorance club. A datum is a fixed starting point of scale or point of reference. The ratio of the Datum in this case was 1- 10 that is 1 metre on the ground is equivalent to 10 cm on paper.
Drop the plumb line from the string at appropriate intervals relating to the soil deposits all along the trench using the tape measure to read off, above, or, mostly below the tape, the different levels of soil deposits. Read these out to the other person, who should plot them on to the graph paper with dots. When the dots are joined up you get the shape of the soil deposit appearing, as if by magic, on the paper.
Each level is plotted separately so if you have lots of levels it can be very confusing.
This took about one hour. No names mentioned here but it took four people nearly all day to plot one of the other trenches.
Time to wake up.