The British Geotechnical Association (BGA) with support from the Engineering Group of the Geological Society (EGGS) will be hosting a major two-day international conference on Engineering in Chalk on 17th and 18th September 2018 at Imperial College in London.

The conference aims to bring together the knowledge and experience of industry and academia by presenting research and case histories to provide a definitive up to date perspective on engineering in Chalk.

Visit the event website for more information. To register for tickets, please click here.

Balazs Rigler of Fugro discusses the group’s efforts to improve the efficiency and accuracy of hydrological analyses for nearshore and onshore foundation engineering in soft geology.

The hydrological properties of sandy, chalk or clay geology are of interest to designers of foundations, tunnels and other underground structures. Without a robust understanding of porosity and permeability it is not possible to fully predict soil behaviour under different loading and groundwater regimes and it is harder to determine depth of weathering and risk of dissolution in chalk. But by the very nature of their diffusive relationship with water, these ground types present a range of physical and interpretative challenges in deriving accurate data on porosity, permeability and associated characteristics.

To date, engineers requiring porosity and permeability data have mostly relied on outputs from core analyses and packer testing techniques, which are limited by factors including core loss, low sample density, feasibility and cost. No single method has been effective in isolation, progress rates are slow and investigation of soft ground such as clays, is particularly problematic.

To address these challenges, Fugro is making significant progress with a downhole technique more usually associated with mining, and offshore oil and gas prospecting – Nuclear Magnetic Resonance (NMR).

A first important point to make is that NMR does not employ any radioactive source material, as its name might seem to suggest. Using the same principle applied in Magnetic Resonance Imaging (MRI) for medical scanning, NMR logging analyses the spin of hydrogen nuclei under the influence of magnetic forces.

Innovative

Though well-established in the oil and gas and mining sectors, NMR’s application for engineering and land geotechnical projects is quite innovative. Fugro has implemented the technology as part of its borehole geophysics offering because it can provide valuable hydrogeological insight for near-surface site characterisation projects.

The NMR technique effectively logs both porosity and permeability and can distinguish between mobile, capillary-bound and clay-bound water. It is effective in air, water or drilling mud and is less sensitive to variations in borehole conditions than other techniques.

The technique can be used in uncased or plastic/fibreglass cased holes (minimum = ~75 mm) even when grouted, eliminating the risk of borehole collapse, which often prevents the deployment of methods reliant on a radioactive source, such as neutron porosity.

Crucially, the NMR technique samples the ground formation away from the borehole wall and, unlike conventional downhole methods, is unaffected by borehole fluid, borehole wall geometry, mud cake or drilling induced disturbance of the near borehole formation.

NMR is thus well-suited to a wide variety of ground conditions including soft superficial sediments, clays, carbonates (limestone and chalk), sandstone and other sedimentary rocks.  The method can provide unique information in certain geologies where parameters derived from other downhole techniques could be fundamentally flawed due to borehole conditions, complexity of data processing and (sometimes empirical) interpretational procedures.

Though they are of most interest in terms of hydrological properties, sandy soils can be the most difficult to core and are often imperfectly recovered, hence NMR provides an immediate fix for this type of problem.

Lithology

In addition, parameters derived from NMR measurements are independent of variations in lithology and do not rely upon empirical or lithology specific ‘corrections’.  This is an important advantage for designers needing accurate, robust and reliable data on the saturated water volume and hydraulic properties of the ground for the foundations of river bridges, tunnels and other structures.

Using NMR data, formation porosity variations can be accurately measured and there is great potential to estimate or derive hydraulic permeability to provide information. Porosity and permeability as engineering parameters are typically provided as a continuous wireline geophysical log, thus avoiding the need for guesswork or interpolation in the event of limited core recovery or low packer test sample density.

Clay volume of formation can also be determined with a high level of confidence with some support from natural gamma log data.

NMR measurements and deliverables can be summarised as:

■             Calculation of total water content

■             Porosity (if water saturated) independent of rock matrix

■             Pore space distribution

■             Distribution of clay bound/capillary bound/mobile water

■             Permeability/hydraulic conductivity

■             Moisture content in the unsaturated zone – dry density can be derived with support from gamma-gamma density logging

Transition

NMR’s transition to onshore geotechnical applications and smaller diameter boreholes has only been possible with the recent introduction of appropriate slim hole tools.

The downhole sensor characteristics can be optimised based upon borehole diameter and geology to investigate concentric shells around the borehole; this allows discrete evaluation of the ground outside the zone disturbed by drilling. The specific measured zone lies within a 200 mm to 400 mm diameter of the sensitive shell (see Fig 2).

NMR logging works in continuous or stationary mode and, unless very large diameter wash-outs are present, borehole conditions have no effect on the measurements.  The presence of such wash-outs can easily be determined by standard caliper downhole measurements.

The set-up is relatively low cost and portable, with a very wide range of probe sizes. The typical tool diameter for slim hole operations is 60 mm and maximum logging depth is ~1500 metres.

With the plastic/fibreglass sleeve ensuring data can be successfully gathered from a borehole without risk of collapse, the technique is saving time and money for over-water work, particularly where extra boreholes and jack-up platform/vessel time may be needed to use conventional techniques.

NMR is the only method which provides direct and lithology-independent information about water, moisture content and permeability as a continuous log.

The lateral investigation depth of the method is based on the well-defined cylinder about the borehole, with a minimum and maximum diameter from the borehole axis as a function of the magnet and coil array geometry and frequency of B1. High frequency tools have better signal-to-noise ratios but are sensitive closer to the tool, while low frequency tools yield data further away from the borehole. Thus, tool selection can be designed in a way that the readings are not influenced by the fluid in the borehole, the mudcake or the strongly altered, disturbed formation close to the borehole.

This greatly facilitates the collection of guaranteed porosity data readings along the full length of the borehole, even in unstable geology.

Permeability and hydraulic data

NMR also provides time and cost advantages when it comes to capturing hydraulic permeability data.

There is a limit to the retrieval of permeability data using other types of discrete tests, often leading to the additional need for comparatively expensive pumping tests.

With conventional methods, unless transit is estimated, permeability will require additional in situ logging based on packer testing and/or injection testing which involves complicated hardware and heavy equipment. This type of test only takes point measurements, while NMR provides a continuous log the full length of the borehole. In order to capture permeability and hydraulic transit data, several distinct surveys will be necessary involving separate hardware assemblies. Through the use of different size probes and frequencies, NMR logging captures all required data using one set-up, to save time and cost.

Improvement in the accuracy of permeability data is a particular focus of Fugro’s advancements in the use of NMR for civil engineering.

Calculation of porosity is straightforward, derived by separating the water content of the target strata into bound (clay or capillary) and free/mobile water content.

Maximised value

With insightful early planning with the client and designers, value from NMR can be maximised by clever selection of complementary downhole investigation techniques to facilitate measurement of permeability and other required hydrogeological data.

In a recent Fugro project, an NMR approach afforded greater surety of data capture across the target strata (chalk) than would have been possible using traditional (neutron porosity) radioactive source based methods. As well as the clear HSE benefits, the method provided valuable and comprehensive hydrogeological data critical to facilitate a robust characterisation of risk to foundation design within the chalk.

Borehole CCTV or an optical televiewer, for example, are often chosen to assess water inflow into a borehole, and outflow via fractures and joints. As a minimum, both methods need clear water to be effective since image quality suffers in murky water – to achieve this is often a challenging and time consuming operation in soft sedimentary geology.

For a current bridge foundation project, Fugro has used NMR combined with an acoustic televiewer to overcome the problem of ‘milky’ water within the borehole due to chalk geology. Use of a conventional borehole CCTV system would have involved flushing out the cloudy water and leaving the borehole to stand for anything between 24 and 72 hours to achieve the right conditions for optical inspection.  This approach would mean taking two or three extra days of jack-up time per borehole and would also increase the risk of borehole collapse and associated requirement for redrilling operations. Both factors would result in a significant increase in cost and time required for the site characterisation programme.

Conclusion

Current advances in downhole NMR are especially exciting for near surface investigations for infrastructure development.

As discussed, the technique is effective whatever the water volume of the ground formation in the saturated zone, or whether its representation is clay bound, capillary bound or mobile water. NMR offers the potential to derive accurate, robust and reliable data on porosity and its relationship with permeability in order to provide information directly related to engineering properties and other important values.

With further work and evaluation, Fugro anticipates it could contribute much to characterise chalk and engineering properties wholly based on porosity, water content, clay content, acoustic and gamma density data.

As well as being of value for foundation design and tunnelling, the NMR method also offers potential for other applications such as water abstraction, flood alleviation schemes, grain size distribution, pit slope stability, open and closed fracture identification, tailings dams, mining and environmental investigations.

But technical progress like this, enhancing the site investigation (SI) industry’s capability to clarify risk for developers with benefits for programme efficiency, needs early collaboration to fully realise the benefits.

The expansion and upgrade of modern infrastructure, from city to coast, does not enjoy geographical and geological freedom; invariably its location will interact with less than ideal ground, complicated by historical use or adjacent development. Yet insufficient SI continues to rank as a major contributing factor to civil engineering and construction project overruns.

Risk and cost management in infrastructure development relies fundamentally on a robust, integrated and well planned SI and its power to create a fully evolved ground model specific to the development site. Only with this can ground risk be confidently characterised, taking full advantage of advanced and cutting edge methods such as NMR, in order to safely and successfully realise vital infrastructure development in challenging places.

Balazs Rigler is a wireline services manager at Fugro. This article was featured in the May/June issue of the AGS Magazine, which can be viewed here. 

Images:

Fig 1:  An example of logging equipment for NMR (Vista Clara)

Fig 2:  Schematic drawing of the sensitivity zone (red shell) of a NMR sonde. [Walsh 2013]

Fig 3.      Example NMR dataset with thin sandy strings in low permeability material

Data from NMR logging in an environment dominated by silts and clayey silts, but with some thin sand stringers. Note the large detected increase in the amount of mobile (large pore) water at ~9 metres. Below 9 metres there is a small but substantial fraction of mobile water which likely reflects the presence of very thin sand lens of the order of cm.

Fig 4.      Example of NMR data from chalk (in the UK)

Fig 5   Example NMR data  from London Clay and soft sediments in London

Data were acquired in East London and composite logs were made to compare results with spectral gamma and formation electrical conductivity logs. Logs demonstrate strong correlation and significant anomalies in gamma, water content, porosity and pore size distribution logs at shallow interval in the London Clay.

Lynne Llewellyn BSc (Hons) CGeol FGS
National Technical Manager, Structural Soils Ltd

I have over 15 years experience working within the ground investigation industry, much of which has been gained with various contracting and consulting firms primarily as a geotechnical engineer. I currently work for Structural Soils Ltd (an RSK company) employed as their National Technical Manager. My experience gained within the industry has been wide and varied, beginning as a lab technician and culminating in my present role of overseeing all technical aspects of ground engineering for Structural Soils Ltd.

What or who inspired you to join the geotechnical industry?
I first became interested in Geology whilst doing my A-Levels. My teacher at this time was very enthusiastic about Geology which made an impression on me (and of course, I quite liked the prospect of field trips abroad). From A-Levels, I went on to study Geology at Cardiff University. Initially finding employment as a Geologist proved quite difficult as no one was prepared to employ me with no work experience, so I began my career as a laboratory technician on minimum wage. In retrospect, I am glad I did this as it provided me with a good grounding and understanding of the geotechnical testing side of the industry. I then became a site engineer (invaluable experience) for some years. As time progressed, I became involved in larger schemes gaining valuable experience from the many varied roles required from me by my employers. During my working career I have been inspired by many dedicated hardworking professionals within the industry who continuously strive to improve quality and actively encourage improvement and innovation.

What does a typical day entail?
As National Technical Manager at Structural Soils Ltd my role is varied and can differ from one day to the next. My main role is to assist the Technical Engineering Director to oversee, co-ordinate and implement the technical output of the company nationally. Therefore my working day can be filled with anything from training and mentoring staff to designing and implementing changes in procedures. Another day I can be giving tendering advice or reporting on a ground investigation. I also work closely with other technical specialists within the RSK group.

Are there any projects which you’re particularly proud to have been a part of?
Recently Structural Soils were asked to help the BBC DIY SOS Team who had a project in Swansea for the ‘The Roots Foundation’, a charity who provides support for young adults in care or leaving care. They successfully applied to the BBC’s Children In Need fund and the BBC DIY SOS granted their wish for a new property worth £1,000,000. The building they previously operated from was very small, dilapidated and not fit for purpose with the new build on the same site was potentially underlain by coal seams or workings.

Structural Soils Ltd gladly took up the challenge, without charge, to undertake 3 rotary ‘open hole’ boreholes to prove or disprove the presence of the coal seams and investigate the possibility of unrecorded old mine workings to satisfy Coal Authority requirements. I was part of the Structural Soils team who oversaw the drilling operations and all for such a worthwhile cause.

What are the most challenging aspects of your role?

Every day is a new challenge that is why I love being part of the geotechnical industry, no two projects or days are the same and therefore the challenges keep coming which is what makes me get out of bed in the morning.

What AGS Working Group(s) are you a Member of and what are your current focuses?
I have been a member of the Senate Committee since 2014 and am also the AGS’s representative on the BSi 526/Geotechnics Committee. As a member of the senate, I recently formed part of a task group set by the committee to update the ‘AGS Guide to Geotechnical Testing’ as this document had not been updated since 1998. I have also helped to review documents for the Loss Prevention Guidance document issued in 2017.

The BSi 526/Geotechnics Committee is responsible for planning, programming and coordinating standards in the area of geotechnics and on this committee I represent the AGS. The committee also provides UK input to CEN/TC 250/SC 7 Eurocode 7 Geotechnical design, Eurocodes related documents and mirrors the CEN/TC 288 Execution of Special Geotechnical Works. I ensure that the AGS has a voice on this committee and report developments back to the Senate.

What do you enjoy most about being an AGS Member?
Being a member of the AGS is rewarding as it puts you in touch with the industry in which you work in everyday allowing new perspectives and ideas to grow. At the AGS I enjoy the fact that new friendships are formed with likeminded individuals from different sectors within the industry.

What do you find beneficial about being an AGS Member?
Being an AGS member is beneficial for me as an individual as it assists with my own continuous professional development. In turn, I can help promote the AGS within the industry to ensure the latest industry standards, guidance and perspectives are being considered. The goal is for the individual AGS members to give this advice to our own organisations and in turn the Clients we undertake work for driving the industry forward.

Why do you feel the AGS is important to the industry?
The AGS with so many member companies within the industry is important as it brings together the professional and dedicated members of various firms with specialist knowledge of both geotechnical and geoenvironmental specialism’s giving them a platform for discussing industry standards and best practice. The AGS data format is also an invaluable part of the AGS work.

What changes would you like to see implemented in the geotechnical industry?
Within the AGS meetings and the wider industry there are always ‘hot’ topics regarding certain aspects of our work which we could change to improve technical output, quality including reducing H&S incidences. By being a member of the AGS I hope to continue to support these initiatives and spread the word.

This Q&A was featured in the May/June 2018 issue of the AGS Magazine, which can be viewed here.

Let’s get some harsh truths out of the way first. Remediation is not necessarily sustainable. Excavation and disposal (ok, ok – dig ‘n’ dump) is sometimes the most sustainable option. Remediation is usually an enabling part, not the main purpose, of a project. And finally, choosing the sustainable remediation option is neither difficult nor costly.

July 2017 saw the publication of the first international standard on sustainable remediation. In September this was adopted as a British Standard: BS ISO 18504:2017. This short and even readable document begins once the risk assessment has concluded that remediation is necessary. Risk reduction objectives and overall constraints need to be established. The conceptual site model is then used as the basis for short listing technically feasible remediation strategies that will eliminate and/or control unacceptable risks in a safe and timely manner given constraints such as space, budget or other activities. Of those shortlisted strategies, the remediation strategy that is judged to be the most sustainable is the one that reduces the risks whilst optimising the environmental, social and economic value of the work.

In practice choosing the most sustainable of, say four, short listed strategies can be a simple affair. Each of the candidates is compared with every other for their relative performance in terms of environmental, social and economic effect – be that positive or negative. Often a clear winner emerges at this point – job done!
Occasionally there is no clear winner and a deeper consideration of effects is needed. In this case a set of site specific factors, or indicators, that delve deeper in to the environmental, social and economic dimensions are chosen to compare the effect of each strategy. Again simple qualitative comparisons will often identify a clear preferred strategy. Only in very rare circumstances will recourse to a much more sophisticated, costlier, fully quantitative comparison – perhaps based on Life Cycle Analysis approaches – be necessary.
BS ISO 18504:2017 seeks to improve the process of choosing a preferred remediation strategy. Once one has been selected, it can still be value engineered to reduce costs and ‘greened’ to reduce environmental impact.

The best way to test out BS ISO 18504:2017 is perhaps to consider a recent remediation project you have been involved in and see how consideration of the social, environmental and economic effects could have been taken in to account. It’s not that hard or time consuming.

Article contributed by Paul Nathanail, Managing Director of Land Quality Management Ltd and Chair of ISO Working Group on sustainable remediation.

This article was featured in the May/June 2018 issue of the AGS Magazine which can be viewed here.