Engineering challenges in chalk

Unlike sand and clay, chalk as a geo-material is rarely covered in undergraduate or postgraduate engineering degrees, despite its prevalence in the UK and northwestern Europe.

Yet, its unique properties pose significant challenges for many engineering projects, including the iconic Channel Tunnel and modern foundation systems for offshore wind farms.

Jonathan McDonald

The Chalk 2018 conference highlighted many engineering challenges and innovations in chalk.

The recent ALPACA and ALPACA+ projects by Imperial College London and the University of Oxford have advanced the understanding of pile-chalk interaction under monotonic-and-cyclic, axial-and-lateral loading.

Additionally, the construction industry research organisation Ciria plans to update its guidance "C574 Engineering in Chalk" to reflect developments over the past two decades, with emphasis on tunnelling in chalk, slope stability, climate change and sustainability.

Engineering geology

Chalk is a soft, white, porous sedimentary rock primarily composed of calcite, formed from the accumulation of marine microorganisms over millions of years. Its distinctive white colour results from coccoliths, microscopic calcium carbonate plates secreted by marine microorganisms. These coccoliths contribute to chalk’s many other properties, such as its fine-grained texture, softness, porosity, and the ability to react with acid which indicates its high calcium carbonate content. Such properties often shape the landscape, exemplified by the iconic White Cliffs of Dover.

Chalk characterisation

Unlike sand and clay, weathering assessment based on intactness/discontinuity plays a crucial role in chalk site investigation. For example, the vertical alignment of the Channel Tunnel was designed to be at least 10m below the weathered horizon due to dissimilar mechanical properties.

Ciria's C574 guide recommends an alphanumeric classification scheme for chalk, ranging from Grade A to D, according to discontinuity aperture. Grades A to C are further subdivided based on discontinuity spacing, using a suffix from 1 to 5.

Ciria grading for chalk (reproduced from Ciria C574, 2002)

Subdivision of Grade A – C chalk (reproduced from Ciria C574, 2002)

Engineers refer to Grade A – C chalk as structured chalk, due to the presence of visible microfossils, and Grade D as structureless chalk, which is so disintegrated that it behaves more like soil than rock.

Depending on its dominant element, structureless chalk is classified as either matrix-dominated (Dm) or clast-dominated (Dc). In terms of engineering behaviour, Grade Dm chalk is treated as cohesive soil, while Grade Dc is considered as granular soil.

Subdivision of Grade D chalk (reproduced from Ciria C574, 2002)

Different design scenarios have different emphases when it comes to site investigation. For vertically loaded foundations, horizontal discontinuities are of greater concern than vertical ones; whereas for earthworks, the poorest grade determines the design (Ciria C574, 2002).

In addition to unconfined compressive strength, intact dry density is used to characterise chalk as a rock material in laboratory settings.

Challenges and solutions in engineering

Flint bands and marl seams are common bedding planes in chalk mass. In addition to being important stratigraphic markers, they significantly influence the host chalk's engineering performance (The Geological Society, 2020).

Flint, a microcrystalline rock made of silica, occurs in white chalk as either bands of dispersed black nodules or continuous sheets 5–30mm thick. Flint is much harder and more abrasive than chalk, often causing wear on drilling bits and tunnel boring machines. In some instances, specialised rock-cutting tools with tungsten carbide tips are required. It is also important to log flint size, shape and frequency during ground investigations.

Marl seams, geologically formed from volcanic ash, have great lateral extent, as well as lower shear strength than chalk, often acting as a plane of weakness. Marl seams contain significant amounts of montmorillonite, a mineral known for its tendency to swell when overburden is removed and to shrink upon application of overburden. This swelling and shrinkage can cause the surrounding chalk to fracture, potentially affecting adjacent structures. The issue can be mitigated by injecting cement stabilisers into marl seams to enhance strength and reduce volume changes.

Marl seams and flint bands in chalk mass (The Geological Society, 2020)

Both continuous flint sheets and marl seams act as aquicludes, which can lead to large volumes of perched water above them, only to be released during excavation. Additionally, surrounding chalk is often weakened by associated groundwater flow.

Chalk's soluble nature often leads to dissolution cavities, which pose subsidence or collapse hazards for geotechnical structures. These dissolution features are primarily derived from past glacial and periglacial activity, though erosion by acidic water also plays a role, with some cavities continuing to expand.

Chalk also serves as a major aquifer for potable water, with dual porosity: smaller pores within intact rock and larger pores along fractures. Therefore, it has a close-to-saturation moisture content. Typical water flow within the saturated zone of chalk ranges from 45l/s to 125l/s. Such high flow rates in caves can cause full hydrostatic head and eccentric loads, damaging excavating equipment and support structures. Thorough site investigation beforehand is thus a prerequisite.

Future perspectives

Despite the expanded experience and knowledge in chalk, certain challenges still need to be solved: difficulties in sampling and testing due to its tendency to break down into a putty-like material, issues with grouting as remoulded chalk matrix prevents adequate sealing, and so on.

With continued collaboration between industry and academia, it is expected that more unknowns in chalk will be resolved.

References

Ciria C574 Engineering in Chalk, Lord J, Clayton C, Mortimore R, 2002.

The Geological Society, 2020: Eggs Meeting: Chalk; All we need is a fracture log (youtube.com)