Soil engineering … maximising the value of assets under our feet2 August 2022
What have soils – one of our greatest and most wide-spread resources – done for us? The answer is a great deal.
And they now hold solutions to many of our modern environmental challenges.
Which is why I think as an environmental consultant that a close look at the ground beneath us could be helpful. Specifically, I want to focus on what soil is, its characteristics, and how we can use soils both profitably and responsibly.
As Enzygo’s Geoenvironmental Department Director – and a self-confessed soil enthusiast – I would also like to explain how desk studies, on-site sampling, laboratory and in-situ testing, plus skilled interpretation, allow us to determine and classify individual soil characteristics.
With design and control investigations, we can then help clients and planners to avoid major problems before they begin, and literally lay the foundations for successful environmental and geo-technical development projects.
However, before that, and before referring to specific examples, I would also like to take brief look at what soils represent and their crucial functions in both the natural and built-environment.
Soils in action and at risk
Soils have an increasing role to play in flood control and water storage – especially with global warming – which is why flood risk assessments can be important. We now also know that soils have the capacity to hold three-times more organic carbon than living trees and plants. But depending on how we treat them, soils can be either carbon sinks or sources.
However, we also appreciate that many soils suffer a high degree of degradation, particularly in urban communities where 68% of us now live. Rain and wind-driven erosion is a major natural factor that can be compounded by salinization and acidification, but also human activities
Where humans compact soils and create large areas of hard-standing, controlling surface and sub-surface water flows in expanding towns and cities is now more difficult. Flash floods from extreme weather events are making this problem worse. Landslides are also an increasing hazard.
In many instances, an environmental impact assessment or environmental risk assessment of soil contamination is essential, with the results shown in a ground investigation report.
Enzygo’s multidisciplinary soil engineering team is able to help here (https://www.enzygo.com/geo-environmental/). As planning consultants offering planning practice guidance, we provide the accurate science-based, technical and site information, plus analysis and practical recommendations, needed to make well-informed decisions during the early project planning stage and introduce effective environmental management systems.
This service has many applications – including desiccation and soil shrinkage assessments; foundation, pavement and road designs; drainage assessments, geotechnical, geo-environmental, geo-mechanical, mining and slope stability assessments; plus frost susceptibility.
It also covers sulphate classification and concrete specifications for foundations; earthwork design for engineering fills; roads, and embankment and cutting designs.
Enzygo’s case studies show how we can put practical flesh on threatened bones. I have included two examples of desiccation and slope stability later that explain why detailed soil analysis is important.
The scope of our work as environmental consultants and planning consultants is probably well-illustrated in three particular areas:
- The first is the remediation of pre-used brownfield land sites (e.g. colliery spoil heaps) that are often highly contaminated;
- The second is straightforward construction and building projects;
- The third and increasingly important in extreme wet weather event management is sustainable drainage systems (SUDS) which provide an alternative to channelling heavy flood waters away from urban areas via overloaded rivers and estuaries to the sea. Here, soil quality can be important in replicating natural mechanisms such as shallow swales, reed beds, filter trenches and retention basins to clean and store storm water as it seeps back into the ground or wider environment.
Soil engineering – the process
Most soils are formed of water, gases, minerals (broken rock) and organic matter; good agricultural soils are roughly 50% solids, 25% air and 25% water.
The process we use to understand soil properties (so that sound project decisions can be made) has four stages.
- Preparatory investigations – involving desk studies of topographical, geological and geo-hydrological maps, aerial photograph interpretations, archives studies and site inspections. The results here lead on to the second-
- Preliminary investigations – which classify soils into three categories – cohesive soil; cohesion-less soil; and rock. Further investigation and analysis into these three categories then lead into the third-
- Design investigations – where each is split into a preliminary choice of foundation method for either pile foundations or shallow foundations. That process is repeated again going into the fourth stage-
- Control investigations – where final practical tests and checks are made to define the most appropriate solutions.
Soil engineering – in practice
More detail may be useful here.
If I can go back for a moment to our desk-based reports known as Phase 1 reports.
Phase 1 includes risk assessment and conceptual modelling and using historical Ordnance Survey plans, the British Geological Survey database, the Environment Agency database, local authority records – and the all-important physical walkover of sites by our specialist staff.
Moving on to Phase 2, we need to look at how we assess the physical, chemical and hydrological properties of soil that will be amalgamated with Phase 1 results.
Our physical characteristic measurement work on-site aims to collect data and samples that can be used to test, analyse and interpret the strength and stability of soils in-situ.
A range of disturbed bulk and small samples can be collected from trial pits, window sampling and undisturbed 100mm diameter driven tube samples, gas, groundwater and core samples from cable percussive and rotary boreholes and their installations, and core pentrometer testing (CPT) to probe mine workings.
We use these to meet different requirements – such as strength classification and pore-water pressure reporting – with different laboratory techniques to quantify the characteristics of cohesive or granular materials. These can include particle size grading analysis (PSD), California Bearing Ratio tests (CBR), triaxial, compaction and consolidation tests.
We determine the chemical characteristics of Made Ground or fill soils from soil samples using various chemical laboratory analysis depending on the history, these include pH, sulphate content, asbestos and chemical analysis for various metal and fuel related chemicals. All this is used for waste classifications and to confirm what the soils are best used for and fit for their proposed use.
Here we look at the interaction of water with soils in soakaways and for their flooding potential. This covers soil property classification (cohesive/granular), permeability (low to high) and pore spacing between grains to provide soil infiltration test results that meet British standards, plus percolation test results for drainage design.
Interpretation of soil properties
This is where all our work comes together in practical recommendations. The best way to illustrate this is through case history examples.
– Desiccation – dry soils
As part of its environmental services and ground investigations, Enzygo has carried out desiccation assessments for new and historical buildings, plus building extensions. We first collect samples to a depth of 3.5m at 0.5m intervals using a window sampler rig or trial pits as close to trees or vegetation as possible because they draw up ground water. A control sample establishes natural desiccation levels without the influence of roots.
Potential desiccation is assessed using a number of parameters. For example, we look for physical evidence of soil cracking and significant surface roots. The reference criteria published by, (Driscoll, 1983), references criteria where a natural moisture content of less than 0.5 times the soil’s Liquid Limit marks the onset of desiccation and below 0.4 times the soil’s Liquid Limit the start of significant desiccation.
A moisture content profile relative to depth also shows whether soils have a moisture deficiency or dryness near the surface. Another indicator is a higher undrained shear strength at the surface. This normally increases with depth but in desiccated soils is higher nearer the surface.
We then use all this data to calculate depths where desiccation is present. Why is this important? Because future foundations must start below this depth to avoid desiccation damage.
– Dry calculations
Another way of calculating desiccation depths uses guidance in NHBC (National House Building Council) chapter 4.2 on soil ‘shrinkablity’ (low, medium, or high), measured by geotechnical laboratory testing, the water demand of trees, and the mature height of a tree versus its distance away. We can then graphically show the necessary depth of foundations.
Now we get more technical! The reference for desiccation parameter determination, and the relationship between Atterberg limits (liquid limit, plastic limit, plasticity index and moisture content) is referenced below:
Driscoll, R., 1983. The influence of vegetation on the swelling and shrinking of clay soils in Britain. Géotechnique, 33(2), pp.93-105,
Enzygo has used both methods in recent projects, including a site in Warwick for a major housing developer, where housing foundations solutions were needed in potentially desiccated zones. Here, after a detailed analysis, traditional rather than more costly piled foundations were used.
We also provided desiccation information for another site in Winscombe and won permission to use traditional foundations warranted by the NHBC and structural engineer. After trial pitting, we recommended solutions that were approved by both; excavations were inspected and approved by Enzygo on behalf of the NHBC.
– Better solutions
Low shrinkage granular soils, high water demand hedges, and relatively high groundwater meant shallow rather than piled foundations could be used. Depending on specific soil properties and groundwater interaction, desiccation cannot be present within or below these materials or groundwater.
Elsewhere, another consultant said foundations required piling. Our analysis and desiccation assessment proved 80% could be shallow traditional and not piled, with a large cost saving for the client.
Slope stability assessments
– Laboratory sample testing
In its role as an environmental consultancy, Enzygo also provides slope stability assessments for proposed or existing retaining walls. Collected soil samples undergo geotechnical laboratory analysis. This includes Atterberg limits to determine phi angles, drained strength values, and effective stress parameters.
This data is then used with topographic surveys to provide a cross section(s) of soil parameters inside and outside failure zones.
Other techniques used to plot slope failures include geomorphological mapping and 3D aerial photographic interpretation, plus onsite physical evidence such as back scars, tension and surface cracking, surface bulging, tree orientations, and fence line changes.
Installation data is also used to create piezometric groundwater profiles across slopes. This can then be entered into the slope stability software (Slope Geoslope or Oasys Slope). This is important because Interactions between the soil matrix composition and groundwater can dramatically effect slope stability and the size and extent of failures.
– Causes of failure
Most failures result from poor groundwater control and drainage, or loading upper slopes with additional weight. Once all parameters and sections have been drawn up, slope stability analysis is undertaken with specialist programmes that provide a safety factor. Above 1.3 is considered safe; anything below 1.0 is considered unstable.
A recent slope stability assessment near Cheltenham meant assessing a retaining wall failure and recommending remedial action to make the wall and site safe. This involved removing saturated material behind the wall, redesigning the water drainage system, removing groundwater, preventing water pressure behind the wall, and designing the wall for new conditions after remediation.
Other examples have involved re-profiling and re-engineering slope angles with geotextiles and gabions, and in one instance installing new drainage systems with a higher safety factor to stabilise slopes for future holiday park use.
What we have learned
Our main conclusions are that it is vital to prevent soil interaction with groundwater because it destabilises slopes. It is also important to maintain the right slope angle to minimise any failure.
Please feel free to contact me directly for more information, or to discuss any of the issues above.
Richard Hamilton – Director of Geo-environmental, Enzygo Ltd
See the LinkedIn article – https://www.linkedin.com/pulse/soil-engineering-maximising-value-assets-under-our-feet-/