Ground investigation reports have been called many things over the years, most of them unprintable! Ask any civil engineer, project manager, site manager or contractor to describe their ground investigation report and you will likely get a confused grimace in return. Somehow ground investigation reports have become something to fear and avoid. Many construction professionals allow the unfamiliar geotechnical jargon to put them off using their ground investigation report as a practical tool for construction.
UK civil engineers dread ground investigation reports. Without a good understanding of their contents, ground investigations can make or break your construction project. Whether you’re facing expensive delays, redesigns or even total project failure; your ground investigation report should be your first port of call. In this article we reveal how to understand your ground investigation report and how you can use it to confidently progress your construction projects.
So, what is a ground investigation report?
In the UK ground investigations are carried out to British Standard BS 5930: 2015+A1:2020 (Code of Practice for Ground Investigations) and Eurocode 7 (BS EN 1997). This may sound quite intimidating but essentially these investigations are carried out to define the ground conditions; identify any hazards associated with the ground; provide parameters for the design of foundations, retaining structures and earthworks; and ultimately minimise risk and uncertainty.
There are typically two phases of investigation that make up a ground investigation in the UK. The desk study; review of old maps, geological surveys, environmental records and past site investigations conducted near your site. This non-destructive study gives you an idea of what may be lurking beneath your building or civil engineering project. Then there is the intrusive investigation; drilling of boreholes, digging of trial pits, on-site tests and offsite laboratory tests of samples. All these tests work together to create your ground investigation report.
Digging deeper with your ground investigation report
We’ve covered how to read your ground investigation report and common pitfalls and warning signs in previous articles. However, knowing how to interpret your ground investigation report is only half the battle. This final article in our ground investigation series explains how you can use your report to inform design decisions around your construction project, covering topics like foundation design, groundworks, ground improvement, and construction methodology.
Foundation design
Ground investigation reports tell you what’s below ground level so that you can select the most appropriate foundation type for your building or structure. Most reports include design guidance and recommendations which are based on soil strengths and groundwater levels identified in the report. We’ve covered stripping and pad foundations previously, however your ground investigation report can help you decide when they’re suitable.
Firm to stiff clay to shallow depth with no significant groundwater issues
Suppose your ground investigation report identifies firm to stiff clay (SPT N-values of 8-30, undrained shear strength of 40-150 kPa) from near ground level to a moderate depth (typically 1.5 – 3 metres). Groundwater is well below foundation level with no signs of perched water tables or contaminated aquifers in your local area. In that case, it’s likely that simple strip or pad foundations will be suitable for most low-rise buildings.
Allowable bearing pressures provided in the report can be applied directly (typically 75-150 kN/m² for firm to stiff clay at shallow depth) when designing foundations, provided the actual foundation size matches that assumed by the geotechnical engineer preparing the report.
If your subsoil conditions match this description along with access to a good UK groundworks company you will typically require foundation depths of between 1.0-1.5 metres dug to form level, laid in reinforced concrete strip foundations or pad foundations.
This is probably the most common ground condition encountered on housing and light commercial developments in the UK and will provide the most economic foundation solution for clay sites.
Made Ground/soft clay overlaying competent natural strata at moderate depth
The next scenario deals with reports identifying challenging shallow materials (Made Ground/soft clay/loose sand) overlying competent natural ground at moderate depth (typically 2-4 metres from ground level).
You basically have three options:
- Remove and replace Made Ground/soft clay all the way down to competent natural strata. Provided the Made Ground or soft clay layer is not too deep (say 1-2 metres deep) this can be a relatively economic solution. Excavated material will need to be disposed of so replacement with engineered granular fill (material brought in from outside the site) may be preferred.
- Strip or trench fill foundations can be taken down to the competent natural strata to avoid/deal with the Made Ground/soft clay/shale altogether – reasonable solutions for depths up to around 2.5-3.0 metres depending on conditions
- It may be necessary to consider piled foundations (either bored or driven), these can transfer the loads from the structure through the poor material down to competent strata below. Although more expensive than a shallow foundation solution piled foundations may be the most economic solution if the poor ground conditions are deep.
Groundwater levels in these cases can also have a significant impact so your report should allow you to identify depths to good bearing strata..
Deep soft clay/peat deposits
Reports often identify deep deposits of soft clay or peat (which may extend to depths of 4-5 metres or more below ground level). In this scenario shallow foundations will likely be unsuitable and piled foundations will be necessary.
Driven or bored piles through the compressible soils/sands into denser sands/gravel/foundation rock beneath are the most common solution. Special consideration will be required for pile design to consider negative skin friction (downwards drag force) as the soft compressible soils consolidate/settle.
Ground improvement techniques like vibro replacement (stone columns) /dynamic replacement may be an option as may pre-loading with surcharge (loading the ground with a temporary weight to force consolidation before construction). These methods work well but tend to be most economic on large sites where costs can be spread over multiple buildings or structures.
In certain circumstances raft foundations with limited ground improvement may be acceptable (some settlement is expected but loads from the structure are spread out to minimise differential movement).
Deep compressible deposits (soft clay or peat) are challenging from a groundworks perspective and will require input from a geotechnical engineer to develop the most economic solution.
Dense sand and gravel
This next scenario couldn’t be easier! Dense sand and gravel (SPT N-values greater than 30) from just below ground level presents excellent bearing capacity and will allow high allowable bearing pressures of 200-400 kN/m² or more.
These soils present excellent ground conditions that will allow for shallow foundations to be economically constructed beneath most buildings or structures.
As mentioned earlier granular soils present their own issues if excavating – groundwater conditions will need to be assessed to determine if excavations may go “quick” at any point (requiring either dewatering or sheet piling), also dense/cemented sands and gravels can be difficult/expensive to excavate if mechanical breaking/blasting is required.
Ground investigation reports for granular soils should indicate groundwater levels/depth to water table and excavatability of the sands/gravels.
Rock!
You guessed it – rock provides excellent bearing capacity BUT like dense sands and gravels comes with its own problems when excavating!
Reports should identify rock strength and provide information on any weathering or fracturing (and likely excavatability).
Very weak rock that can be excavated by mechanical means may present few issues if foundations are permitted to sit on rock (or driven piles are used). Strong intact rock may require blasting/mechanical breaking which can be expensive and difficult when undertaking foundation works.
Again your report should comment on rock strength/weathering/fracture spacing etc which should allow you to gauge how easy/hard it will be to excavate through.
Earthworks – what can you keep/use from site?
Most ground investigation reports will tell you about the various layers of soils encountered on your site along with their particle size distribution, plasticity characteristics and moisture content.
This information will help you decide what you can reuse from site as engineered fill – if you’re importing fill onto site it makes sense to reuse what you can from excavation to minimise costs.
Tip: Clay soils with very high plasticity are typically unsuitable for reuse along with Made Ground and organic deposits.
Ground investigation reports should classify your soils based on plasticity/moisture content/particle size distribution allowing you to classify what can be reused as fill based on MCHW specification or whichever specification your project uses.
Granular soils with very little fines (<15% passing the 0.063mm sieve) are typically excellent for reuse as fill as they possess good compaction characteristics and allow water to drain through. Clay soils of low to medium plasticity can also be used as fill materials although moisture control can be difficult and they don’t compact as easily as granular fills.
Ground improvement
Assuming your ground investigation report has excluded poor ground conditions that require piled foundations you may still be faced with poor ground conditions – loadsheets that aren’t great for shallow foundations but not so bad that they require piles.
Ground improvement methods can provide cost-effective solutions to allow shallow foundations to be constructed in marginal ground conditions.
Ground improvement methods for granular soils include vibro-compaction / dynamic compaction. These methods of ground improvement work best in clean sands and gravels with little fines content. Particle sizes distribution along with densities should be included in your ground investigation report allowing you to gauge the potential for ground improvement via compaction.
Soft cohesive soils can be improved via vibro replacement (stone columns), preload surcharging (loading the ground with a temporary weight to force consolidation before construction of your building) or vertical drains to accelerate consolidation.
Sites with particularly variable ground conditions could benefit from soil mixing/jet grouting to create columns/pads of improved material beneath your foundations. These techniques are typically more expensive than others on this list and may require specialist input from a geotechnical engineer.
Temporary works & Construction methodology
Ground investigation reports rarely seem to get a mention when it comes to thinking about temporary works requirements or construction methodology – perhaps because these aren’t considered at the outset of a project.
However they can have huge cost and programme implications if not considered early on!
Excavation stability
If you’re excavating on your site temporary works requirements will vary massively depending on ground conditions.
Reports should provide information on soil strength which can be used to design temporary works (undrained shear strength for clays and angle of friction for sands and gravels).
Deep excavations in soft soils or any excavation below the water table will need supporting via trench sheets /sheet piles etc. but shallow excavations in stiff clay/dense sands well above groundwater may not require any temporary support at all!
Groundwater control
Once again your ground investigation report should help you plan for groundwater control – any excavations below the water table will either need excluding (using sheet piles /diaphragm walls) or dewatered (lowering the water table via pumping).
Soil types and particle size distribution will give you an idea of ground permeability which affects how easy/hard (and costly) it will be to dewater your site. Highly permeable gravels may require deep wells/high capacity pumps whereas low permeability clays may not be dewaterable meaning you’ll have to exclude water from the excavation.
Knowledge of groundwater levels on your site can also be used to determine if you’ll need to use groundwater control methods.
Excavatability/plant requirements
Likewise the ground investigation report should help you assess excavation plant requirements – weak soils/clays are excavated easily using standard construction plant (excavators/dozers).
Very stiff/clays/dense sands will require ripping (usually with dozers or hydraulic excavator breakers). Removing rock typically requires blasting/mechanical breaking – significantly increasing costs and potentially having an impact on programme and site logistics.
Ground investigation reports should identify material strength. Look out for the terms “very stiff” / “very dense” / “weak rock” which should ring alarm bells that you’ll require additional/plant other than excavators/dozers to undertake your groundworks. Reports commenting on rock should include information on UCS (unconfined compressive strength) which is a good indicator of excavatability.Where do you go from here?
Practical Workflow: Report to Design
In order to make sure you get the most out of your ground investigation report and don’t run into the problems highlighted above, below is a suggested workflow for reviewing investigation reports and putting them to use:
NOTE: This step-by-step guide has been written specifically with UK civil engineers, project managers and contractors in mind.
Step 1 – Review for Red Flags
As soon as you’ve received your ground investigation report, perform a quick review looking for red flags which may impact the feasibility/design/approach/budget of your project.
Read through the executive summary and conclusions first, then review the borehole logs and trial pit records to get a good feel for the ground conditions throughout the site.
Look out for things like:
- Made Ground/ fill materials (depth & composition?)
- Soft/ loose materials at shallow depth
- Peat/high organic content soils
- High groundwater tables relative to depth of construction
- Highly plastic clays that could shrink
- Aggressive ground conditions (high sulfates, pH extremes)
- Obstructions/voids/unusual features
- Large variations in ground conditions across the site
If any major red flags are present you may need additional investigations/specialist input/design alterations before continuing.
Step 2 – Review parameters/test results
Now read through the laboratory test results and review parameters that will be used for design purposes:
- SPT N values and variation with depth/site
- Undrained shear strengths for cohesive soils
- Atterberg Limits/plasticity
- Particle sizes/distributions
- CBR values if designing pavement
- Chemical results/exposure classifications
- Groundwater levels / permeability
Compare these values to what you would typically expect to see for the types of soils encountered and watch for any anomalies/unexpected results. Large variations in SPT N values or discrepancies between SPT N values and say laboratory CU strength results could be red flags that warrant discussion with the ground investigator.
Step 3 – Review recommendations
Read through the recommendations, noting any explicit assumptions and limitations stated. Key items to check include:
- Allowable bearing pressures offered and assumptions they’re based on (depth, width, founding stratum, etc.)
- Recommended foundation depths/types
- Specification for any buried concrete (sulfate resistant? Type of cement?)
- Groundwater lowering requirements
- Ground improvement recommendations
- Limitations of investigation (borehole spacing, depth reached, time of year etc.)
Match your intended design to the recommendations and limitations. If you intend to design foundations differently than what they designed and reviewed (different depth/founding stratum/types of loading) their recommendations may not apply and could be considered invalid.
Step 4 – Design!
Design your foundations taking into account what you learned from the ground investigation report. If you have a simple project and good ground conditions, you may be able to directly apply allowable bearing pressures and recommendations from the report.
Complex projects, poor ground conditions, or projects where allowances are tight may require that you develop detailed geotechnical design calculations. These could include:
- Bearing capacity calculations utilizing soil strength parameters from investigation report
- Settlement calculations to ensure calculated deformations will be acceptable
- Slope stability calculations if relevant
- Retaining wall designs which incorporate soil parameters from the investigation report
- Pile design including capacity calculations and settlement
Make sure that whatever calculations you’re doing are using appropriate soil parameters gathered from the investigation report. Also ensure that you’re using the correct safety factors for your design calculations (see Eurocode 7).
For complicated geotechnical designs, having another chartered geotechnical engineer review your calculations and approve your design is always a good idea.
Step 5 – Plan Temporary Works & Construction Approach
Ground investigation reports are also very useful when planning construction methodology and designing temporary works. Things to consider include:
- Excavation stability and required temporary support
- Groundwater lowering requirements
- Type of plant required based on excavatability
- Special requirements such as dewatering, ground improvement etc.
- Foundation inspections to verify ground conditions
A good UK groundworks contractor will also have years of experience digging around your local area. Speak to them as part of your initial planning stages to gain insight into practical construction methods and any additional issues they anticipate encountering.
Step 6 – Foundation Inspection & Verification
Another stage where ground investigation reports can be extremely useful is during foundation inspection and verification. Before pouring concrete you should excavate to foundation level and verify that the ground conditions meet the assumptions made during design.
Things to verify during foundation inspection include:
- Confirming that you’re founding on expected material (natural clay/dense sand/rock etc.)
- Confirming that you’re at the right founding level
- Checking for any unexpected features (soft/powdery spots, obstructions, water etc.)
- Checking that the formation has been properly prepared and there is no loose/degraded material sitting at the bottom of the excavation
If the conditions don’t match what was expected, DO NOT continue with construction. Go back to the drawing board and figure out why things aren’t matching up – your design may need to be changed or you may need additional investigation.
Taking a few pictures of each foundation excavation can be useful if questions come up later. For larger projects, consider having your engineer fill out a formal foundation inspection report.
Step 7 – Post Construction Monitoring
In some cases, it may be wise to monitor your structure after construction. This is usually only required when your structure is founded on soft soils, Made Ground, shrinkable clays or other soils that may move.
Monitoring can take many forms including:
- Settlement monitoring via precise levelling surveys
- Crack monitoring grids if minor cracking is expected
- Groundwater monitoring if required
- Structural monitoring at regular intervals
Whatever the case may be, monitoring helps catch issues early so that they can be remediated before they get out of hand. Post construction monitoring is typically not required for most standard construction projects on good ground conditions but having regular visual inspections is always recommended.
Conclusion: Decision Making with Confidence
There’s little doubt that the ground investigation report is simultaneously one of the most valuable and most misunderstood documents you’ll come across during your career.
If you’re a UK civil engineer, project manager, site manager or contractor then you should now have a toolbox of techniques and tips that you can use to confidently interpret your ground investigation reports. Knowing what to look for in a GI report and what SPT N-values, Atterberg Limits and allowable bearing pressures really mean for you and your project empowers you to make decisions with confidence right from the initial design stages through to the completion of construction.
So next time you receive a ground investigation report, don’t sigh and pass it along to someone else to worry about. Take a few minutes to review it with the checklists above. Highlight any issues you’re unsure about and work through the data to translate the findings into real, pragmatic decisions. Talk to a good UK groundworks company about how the conditions may impact construction. Consult the relevant specialists if you need to. Check conditions as you build. If you take the time to properly understand your ground investigations report you’ll easily recoup your time many times over through reduced risk, avoided problems and happy clients.
Far from being a source of stress, your ground investigation report is your opportunity to gain insight into the world beneath your site. Learn how to read it effectively and you’ll be designing and building better (safer, stronger and cheaper) long into the future.