Speaker Ken Senior - Soil Mechanics and Geology

Ken explained that he joined Holst & Co as a junior engineer. They specialised in building large structures out of reinforced concrete such as cooling towers and bridges. Ken joined what was then a small section of the group dealing with soil mechanics and geology. But this small division grew to be the largest geo-technical engineering contractor in the country.

Ken's job involved among other things checking the soil conditions below proposed major construction projects to ensure that they were to be built on sound foundations. If there were weaknesses, then proposals had to be made to strengthen the sub-strata so that it would properly support the structure.

He said that soil mechanics is the study of the sub soils (clay, sand and gravels) which overlay the rocks. He explained that roughly from the south east of England to north west the structure is clay (being drift deposits) overlaying carboniferous rocks (limestone and coal). 300 million years ago, when the UK land mass was down at the equator, coal measures were laid down in swamps and limestone was created from oceanic crustacean deposits.

Geological maps first started in modern Britain in 1815 by a Mr Smith and by 1835 the whole of the country had been surveyed. But geological mapping started much earlier. The Egyptians were mapping gold and certain other mineral deposits in 1100BC and the Romans had been very good at surveying for coal and minerals. Furthermore, it is known that the Chinese were extracting coal as long ago as 3000BC so they must have had some knowledge of geology.

The trouble in Britain is that a lot of the older mines were not recorded. Even up to 1947 when the National Coal Board was formed there was no systematic mapping of the coal mines. In earlier times timber was too expensive to be used for pit props to support the roof in a mine so instead pillars of coal were left in place to support the structure. But then as the seam became worked out the miners would rob as much of the coal from the pillars as they dare on their way back out leaving an unstable structure. Thus, today in various locations we have an unstable sub-structure over which we have then built properties or other constructions. If an old mine shaft collapses this causes pockets to bubble up towards the surface potentially creating a sink hole. Ken said that a rule of thumb is that such bubbles will not usually travel upwards more than 10 times the depth of the collapsed seam as the bubbles become progressively smaller as they head upwards. This means that a 1 meter seam needs more than 10 meters of rock above it before it should be OK to build on. If this isn't the case then the seam needs to be filled with a grout mixture. This is frequently a mixture of 10 parts Pulverised Fuel Ash (spent fly ash from a power stations) mixed with 1 part cement to bind the mixture.