Good Vibes, Bad Vibes

Ground conditions are always a source of potential problems and the surveyor is often called upon to assist in the solutions. Where piles are the primary means of supporting new works the involvement for the surveyor is normally placing pegs in the ground at the required locations.

Pegging the river bed isn't quite so straightforward. When the piles are to be seated on a fairly thin layer of sandstone, under which lie some metres of peat before the next rock layer is encountered then the activity becomes quite interesting.

Back in 1979 older technology still held sway, although add-on EDM was available for survey work. The problem facing surveyors on a new jetty in the Thames estuary was that of ensuring that the pile driving stopped as soon as the required loading was reached and before the pile broke through the thin rock layer and had to have a 10 metre extension welded on.

The normal procedure would have been to drive the pile until the desired set had been reached and then drive for a further half metre to ensure that this level of loading could be sustained. In the geological circumstances of the site there was a real risk that continued driving might fracture the sandstone layer. A method was therefore needed to stop the piling as soon as a sustainable loading had been reached.

Two theodolites on shore stations were used to monitor the pile drive for each quarter metre and the resulting figures plotted in real time as a load graph. Once the loading approached the design requirement a third surveyor on the piling barge moved to a specially constructed platform on the lower piling gate. (Modern day Health and Safety officers should take their sedative tablets now!)

Record cards were stuck to the pile face and using an extended pencil graphs were recorded to show the amount the pile moved with each blow of the hammer. The example illustration shows what happened. The sudden peak represents the temporary compression of the pile under the hammer load; the pile rebounding for the rest of the record so that the difference between the start and finish of each line is the amount driven. The critical item is the height of the peak. From this the actual resistance to the pile can be calculated for each blow.

With an indicative value calculated before the pile driving started, this meant that the surveyor on the piling barge could stop the piling within a few blows of the desired loading having been achieved. Thus any risk of overdriving was eliminated.

Despite the apparent risk of standing under a piling hammer the surveyor was well protected with only the small gap between the shelter and the pile being open, hence the extended pencil. Cards were only stuck onto the pile as the hammer was being lifted. It was, however, quite noisy and at the end of a long drive the wooden dolly between the hammer and the pile head might start disintegrating and shower the surveyor with wood slivers.

Piling barge Record card for pile compression whoops deary me

Now if we think those are good vibes, just imagine what bad vibes might look like. In fact you need not use your imagination, just look at the next page.

Thixotropic is a word most people have probably met only in respect of paint. (It's the claggy stuff that goes on in lumps and then miraculously becomes smooth as you brush it out). Other materials also display the same characteristic, including some forms of fly ash.

No matter how urgent the job, never take a digger onto fly ash and sit with the engine ticking over. What might support a moving machine can turn to jelly if it is vibrated at the right frequency. These unfortunate souls were working on a large development within one of the old enterprise zones when they went wandering off one Saturday morning (when supervision was thin on the ground!). The resulting "rescue" operation bore strong overtones of the old Burl Ives song "There was an old lady who swallowed a fly".