Geotechnical EngineeringAs well as offering factual reporting SWEL can also offer interpretive report, and as such offer advice on Foundation Design, Building on Made Ground (RQA) , Building Near Trees and other common geotechnical problems.
Geotechnical Engineering FAQs
Geologists have been studying the ground for a hundred plus years, and over time have built up a system for quantifying soil strength it is based on shear strength, and although this is most accurately measured in a geotechnical laboratory, there are numerous ways a geotechnical engineer can measure these strengths in the field, to obtain an immediate and fairly accurate (some may argue more accurate) measure of strength.
One method uses the human hand, and involves squashing poking the soil to give an idea of strength as the table below shows. There are also various basic tools that can be used such as a hand vane and pocket penetrometer.
|<20 (Very Soft)||Squishes out between fingers when
squeezed in hand.
Can poke finger 25mm in to soil easily.
Will not keep its own shape, it slumps when placed on surface.
|20 - 40 (Soft)||Can be squashed in to shapes with
Finger can be pushed in 100mm or so.
Will just keep it shape when placed on a hard surface, slight slump.
|20 - 40||0.4 - 0.8
|40 - 75 (Firm)||Can be molded
with strong finger pressure.
||40 - 75||0.8 - 1.5|
|75 - 150 (Stiff)||Cannot be molded by
Can be indented slightly with thumb.
|75 - 150||1.5 - 3.0|
|150 - 300 (Very Stiff)||Can be indented by thumb nail, but not thumb.
|300< (Hard)||Normally Brittle.
Cannot be indented by thumb nail.
It is also possible to use a testing hammer on a drilling rig to assess in-situ strengths (Stroud 1989).
|Description||N - Value||Description||N - Value|
|Very Soft||<4||Very Loose||0 - 4|
|Soft||4 - 8||Loose||4 - 10|
|Firm||8 - 15||Medium Dense||10 - 30|
|Stiff||15 - 30||Dense||30 - 50|
|Hard||30 <||Very Dense||50 +|
In situ tests for granular soils are particularly important as once they have been "dug" up it is hard to tell how strong they are without laboratory testing.
If I am building near trees are there any special precautions?
Trees need water to grow, the roots in the soil suck up
water and this effects the properties of the soil.
Desiccation of soil from trees is particularly noticeable in clays with a high shrinking/swelling potential. As the water is removed from the soil it shrinks which can lead to settlement that may damage buildings.
Conversely is a tree is removed from site shortly before construction starts soil that has been artificially desiccated by trees can swell or heave again causing damage to buildings.
Geotechnical Consultants can offer advice on building near trees in line with NHBC guidelines.
How do geotechnical engineers assess rock strength?
Rock strength is assed in a number of ways, there are empirical methods based on hits clasts of rock with hammers, and there are measured methods such as in-situ testing and laboratory testing.
Some may call empirical methods estimates, but in truth when carried out by an experienced geotechnical engineer they are surprisingly accurate.
In weaker or highly fragmented rocks the good old SPT can be used to assess strength. CBR tests are also useful.
A point load test is sometimes useful , for giving an accurate gauge of rock strength.
However, all of the above methods should be used with caution, as they give a rock clast strength, for the design of foundations the really important strength value is the Rock Mass strength. For example, a highly fractured rock mass (such as you might find in northern Bristol) with in filled fractures will behave similarly to a soil in some instances. Particularly where point loading occurs with piled foundations for example this should not be overlooked.
Pile design based on geotechnical parameters.
A great many geologists and geotechnical engineers alike have written tens of books on the behavior of piles in soils and rock. To explain very basically, there are various methods of supporting a building on piles. Two of which as follows;
A long pile sunk in to London clay will create most of its load capacity via the "grip" of the clay on the wall of the pile.
|A pile in an estuarine environment may see several meters of soup like silts or muds with no bearing capacity. However, there may be bedrock suitable for an end bearings below this soup like layer. In this instance the skin of pile has little friction on the soup like upper strata instead transferring load to bedrock below.|
For more information visit UWE's website.
Eurocode 7 is a very thorough (169 page) standard for geotechnical design. EN 1997-1:2004 (Eurocode 7) is an extremely useful document when used as a guide to required report contents. From the out set it defines design requirements:
EN 1997-1:2004 (Eurocode 7) also suggests that it should be considered that knowledge of the ground conditions depends on the extent and quality of the geotechnical investigations. Such knowledge and the control of workmanship are usually more significant to fulfilling the fundamental requirements than is precision in the calculation models and partial factors. Which is good advice, which is why we would always insist that a site engineer overseas all sampling and in-situ testing work.
EN 1997-1:2004 (Eurocode 7) is a European code, and each member state, including the UK may make it own additions to the code where it is left open to interpretation. To that end it is important to read the UK National Annex to EN 1997-1:2004 (Eurocode 7), as it contains supplementary information. One example of such information included within the UK National Annex to EN 1997-1:2004 might be the Partial Resistance Factors that are included for various foundation types etc.
This Standard sets out the procedures to be followed and certificates to be used during the process of planning and reporting of all Geotechnical Works carried out on highways under the jurisdiction of the relevant Overseeing Organisation to ensure that the Geotechnical Risk is correctly managed.
The management of geotechnical risk is broken down in to various, stages. Desk Based, Intrusive and Design, there are also "Feed Back" reports which seek to verify the scheme successful completion.
This standard includes example report layouts, and sets out minimum content for the geotechnical cetificate.
A geotechnical engineering report assembled in line with
"We certify that the Reports*, Design Data*, Drawings* or Documents* for the Geotechnical Activities listed below have been prepared by us with reasonable professional skill, care and diligence, and that in our opinion:
The certificate should list any departures from standards, and also list all of the documents i.e. drawings upon which the the reports scope was based.
Southwest Environmental can issue a geotechnical certificate, as part of a full geotechnical design report.
There are various geological units that run southwest to north east across the UK, that are naturally sulfurous, and as such can create "unpleasant" or aggressive conditions for buried building elements such as concretes, grouts and structural steel. For further information please see our Aggressive Ground page.
Geotechnical Engineers Bristol
Geotechnical Engineers Exeter
Geotechnical Engineer London
Bristol - 01173 270 092
Exeter - 01398 331 258
London - 02076 920 670