0808 196 1460 info@pdmace.com

Field identification of soils

22nd May 2020

As a firm specialising in temporary works design for projects of every size, from housing developments to major civil engineering infrastructure schemes, we are often asked to design the foundations for a crane or structure, when no information on the ground is available.

This usually occurs on refurbishment or maintenance jobs where it was not necessary for the permanent works designer to commission a geotechnical investigation report.

When this situation occurs, we can make conservative assumptions or we can make an assessment of the soil conditions using the results of one or more trial holes dug on site. The choice would be determined by the number of foundations required and the load on the foundations.

History points the way

As with any site investigation, a good place to start is at your desk, searching the valuable resources available to us.

The first place to look is through the historic borehole logs on the British Geological Survey (BGS) website, which is a mine of information, if you excuse the pun.

The site allows us to search over a million records of boreholes, shafts and wells from all forms of drilling and site investigation work, ranging from one metre to several thousand metres deep, spread right across the UK.

Then there are mobile phone apps available such as the iGeology app, also from the BGS which provides mobile access to the geological maps of the UK, detailing information on the superficial and bedrock geology for any location.

We also use the MySoil app, which anyone can use to discover what lies beneath their feet and help the BGS build a more comprehensive geology record of the UK by submitting their own soil information.

Using a combination of all these resources can provide valuable background information on the soils and potential difficulties in the area of the project we are designing temporary works for.

Practical steps for detail

If the project area is not covered by these resources, we can develop an even more detailed picture of the likely ground conditions by digging trial holes to over 4m deep, with a readily available machine such as a JCB 3CX.

This will be sufficiently deep to indicate the soils that will be loaded by a foundation less than 2m wide. For larger foundations then borehole information would be required.

The strata are logged by looking down the hole and measuring with a tape or staff to any interface. The soil is logged once it is on the surface; never by getting into the excavated hole.

The code of practice for ground investigations, BS5930:2015 contains a useful table for identifying soils on site, whilst BS5975:2019, the code of practice for temporary works procedures has a simplified version in a modified format.

But for experienced engineers, there’s nothing quite like getting your hands dirty. Firstly, by observation it can be determined whether the soil is made ground or natural.

Secondly, by sniffing the soil, any organic material can be noted or it might be possible to observe that the soil was contaminated. An estimate of grain size can also be made, noting sand ranges from 0.06mm to 2mm and is always visible.

The smallest particles are clay and the individual particles are never visible. Silt particles are between the two in size and the particles are rarely visible, but feel smooth, like flour, when rubbed between the fingers.

Silt particles can also be felt on the tongue – not that I am recommending this test to others, but it shows the lengths to which and an engineer will go for the right results!

Simple field tests are described for fine grained (‘cohesive’) soils where an indication of strength can be determined by handling the clay. For example, very soft clay oozes through the fingers when squeezed whereas very stiff clay is possible to indent with the thumb nail.

The density of coarse-grained soils (sands and gravels) is more difficult to determine. The now superseded, BS5975:1996 also contained the chart for field identification of soils but this chart also included some description of how to differentiate between loose and dense sand by how hard each was to dig.

Of course, this is very subjective when machine excavating rather than digging by hand, but by observing the machine during excavation and asking the driver how hard it was to dig, some indication of density can be estimated.

Once the ground has been logged, the soil design parameters can be interpreted and design can be undertaken in accordance with normal geotechnical practice.

It must always be borne in mind where the geotechnical information came from and it might be necessary to adjust (increase) factors of safety or make appropriately conservative assessments of the soil parameter values, but that comes with experience measured in decades, not years.