Review of Gregory (1844), “On railway cuttings and embankments”

By Michael Bennett, P.E., M.ASCE (Gannett Fleming TranSystems: Audubon, PA)

 

The late 1830s and early 1840s were a key, if often overlooked, period of intensive technological change.  Political events such as Queen Victoria ascending the British throne and William Henry Harrison becoming – just a month into his term – the first US President to die in office garnered more headlines, but the world hummed all the while with new inventions heralding the dawn of the Industrial Revolution.  Americans Samuel Morse and Charles Goodyear created the telegraph and the vulcanization of rubber, respectively, while Louis Daguerre of France developed the first reliable photographic process.  Perhaps most notably, UK engineers such as Robert Stephenson and Isambard Brunel made their nation a leader in the swiftly maturing technology of railroads.  Civil engineers around the globe looked to their British counterparts for the latest railroading advances and even imported British locomotives for their own lines (Daniel 2004, Library 2020, RR Museum PA 2023, Somma 2014).

 

 

The advances in British railroad engineering at the turn of the 1840s formed but one part of the broader developments playing out in UK civil engineering at the time.  Members of Britain’s Institution of Civil Engineers were advancing their field at a truly unprecedented rate, and the Institution began publishing its Transactions in 1836 to spread word of these breakthroughs to all its members.  The next year, ICE debuted its Minutes of Proceedings, which would circulate for nearly a century.  Geotechnics remained decades away from being a stand-alone civil engineering subdiscipline, but articles on geotechnical topics regularly appeared in Minutes of Proceedings from its outset.  One sterling example was Charles Gregory’s 1844 paper “On railway cuttings and embankments.”  During his remarkable 60-year career, Gregory created semaphore signaling for railroads, consulted on projects across the British Empire, and served a then-standard two-year term as ICE’s president.  Queen Victoria eventually knighted Gregory in recognition of his many achievements (Gregory 1844, ICE 1898, ICE 2024).

 

 

Gregory focused his piece on slope stability, a topic that remains of keen interest to geotechnical practitioners.  He concentrated his writing on a 75- to 80-foot railroad cut near New Cross, a town that now forms part of southeastern London.  The cut transected a layer of heterogeneous, fissured, highly permeable clay 55 to 60 feet thick, now commonly known as London brown clay, underlain by a strata of relatively strong, impermeable clay 15 to 20 feet thick, now known as London blue clay.  The railroad tracks lay flush with the bottom of the blue clay layer.  Notably, Gregory used Imperial units to describe the cut’s stratigraphy; the UK wouldn’t widely metrify its measurement system until the mid-1970s.  In general, the geotechnical properties of London brown clay are: w = 31%, LL = 82, PI = 52, clay fraction = 55%, γSAT = 120 pcf, ɸ’ = 20°, and su ranging from 1,500 psf at 5 feet deep to 3,300 psf at 30 feet deep (Gregory 1844, Lewisham Council 2024, Skempton 1977, UKMA 2024).

 

The London and Croydon Railway opened its double-track line through the cut in 1839.  Two years later, on November 2, 1841, the cut’s London brown clay strata underwent a 50,000 cubic yard slope failure (in British geotechnical parlance, a “slip”), leaving about 350 yards of track covered in 10 to 12 feet of clay.  Railroad workmen cleared the slide in just over two weeks, but two further slope failures in the London brown clay blocked the tracks later in November and took almost until Christmas to clear.  Yet another slope failure in the London brown clay blocked the cut in early January 1842 and took about another month to clean up.  Gregory mentioned no fatalities or derailments due to the failures, but they must have been immensely expensive, as the London and Croydon had to pay its laborers to work around the clock to clear each slide.  The railroad also lost considerable revenue due to passenger and freight traffic it forfeited over the period of closures (Gregory 1844, Prockter 2021).

 

 

The New Cross slides’ financial burdens made it imperative for the London and Croydon Railway to avoid repeat incidents.  Its civil engineers got to work implementing a series of simple, practical solutions.  The first involved recognizing the ancient, sound principle that – in engineering as in medicine – an ounce of prevention is often worth a pound of cure.  Early in 1842, laborers were excavating one of the slides when sharp-eyed crews spotted another one just getting underway.  So, while some laborers continued digging out material from the completed slide, the railroad’s civil engineers shifted others to excavate the material atop the incipient slide.  For added measure, the engineers also had the crews excavate gentler slopes for the cut and add benches to these slopes.  These tactics worked and collectively indicate that the engineers fundamentally, if not explicitly, grasped the key principle that reducing driving forces improves slope stability (Gregory 1844).

 

The London and Croydon Railway’s civil engineers also approached their slope instability problems from the perspective of increasing the resisting forces.  After the laborers finished excavating the New Cross landslides, the civil engineers also had them excavate the London blue clay adjacent to the tracks in the slide zones.  The crews replaced this material with buttresses of compacted gravel, which both retained the slopes and allowed material to drain freely from their toes.  The laborers also piled the excavated blue clay on the downhill side of the gravel drains to further stabilize the slopes.  Finally, the crews laid drainage pipes along each bench of the slopes to further reduce the danger posed by moisture.  These field remedies indicate that the railroad’s civil engineers had a solid intuitive grasp of both driving and resisting forces in slope stability and the importance of maintaining drained conditions, at least to the extent possible, to improve soil shear strength.  Their solutions worked so well that the London and Croydon adopted them as standard practice for remediating slope problems on its lines (Gregory 1844).

                                                  

 

However, the London and Croydon Railway and its competitors still valued avoiding slope instability problems altogether above even the most reliable remediation techniques.  It was on the New Cross slides’ root causes that Charles Gregory presented his thoughts to ICE in 1844.  In examining the case, Gregory found it noteworthy that the cut had performed well for over two years after its construction, with no visible evidence of creep or slope failure.  He thus deduced that the introduction of some new factor to the cut must have caused the slides.  Gregory and his peers knew that water readily saturated and became trapped within the London brown clay layer.  Accordingly, Gregory hypothesized that seasonal cycles of wetting and drying propagated cracks within the layer “so that, year by year, the evil would become greater, and the tendency to slip gradually increase” until the slopes eventually gave way (Gregory 1844).

 

Following Gregory’s presentation, he and his ICE colleagues engaged in spirited debate and discussion over the New Cross landslides.  Many of his peers agreed with his hypothesis, while others put forth alternative culprits, such as dynamic loading of soil from passing trains.  Over the next century, though, Gregory appeared to have been on the money.  Karl Terzaghi himself posited during the 1936 International Conference on Soil Mechanics and Foundation Engineering a hypothesis of cohesive soil softening caused by cyclic wetting and crack expansion.  Terzaghi’s opinions on technical subjects often marked the end of discussion in geotechnical engineering’s early days, as he and his supporters were known to take unkindly at times to opposing points of view.  However, Terzaghi was often more open to diverging opinions than has been suggested.  For instance, he became good friends with Alec (later Sir Alec) Skempton of Imperial College even as Skempton challenged and eventually disproved some of Terzaghi’s conclusions, and they remained friends until Terzaghi’s death in 1963 (Cowper et al. 1844, de Boer 2005, Gregory 1844, Peck 1985, Terzaghi 1936).

 

 

Fittingly, it was Alec Skempton who, in the 1970s, finally determined the true cause of the New Cross landslides.  He had earlier studied clay behavior in depth and concluded that the limited extent of fissure-related softening observed in clays didn’t support Terzaghi’s wetting-cracking hypothesis.  Skempton also realized that he and his peers had made the “tacit assumption” – always the riskiest type in geotechnical engineering or any field – that, after cuts were excavated through clay layers, pore pressures within the clay came to equilibrium rather rapidly as the soil expanded due to stress relief.  UK researchers disproved this assumption in the early and mid-1970s by showing via instrumentation studies that negative pore pressures in London brown clay (and, thus, the clay’s shear strength) commonly take 40 to 50 years to equilibrate.  Thus, the New Cross slides were caused by the combined effects of the clay in the excavations undergoing gradual shear strength loss and the mechanism of progressive failure in mobilizing the clay’s remaining shear strength.  Ironically, the first participant in the ICE discussion of Gregory’s paper over a century earlier had come tantalizingly close to that truth when he said he “should be inclined to attribute the slips to the expansion of the clay, from the action of water” (Castellanos et al. 2015, Cowper et al. 1844, Terzaghi 1936).

 

Since 1844, the geotechnical discipline of slope stability has advanced far beyond what Charles Gregory and his contemporaries would likely even have dreamed possible.  Field and laboratory tests can be used to select appropriate shear strength parameters for soils of all kinds.  Moreover, a variety of techniques such as the Swedish circle and the respective methods of Bishop, Spencer, and Morgenstern and Price have been developed to assess slope stability using these parameters.  Over the past 50 years, reliable computer programs such as SLIDE2 and SLOPEW have come into vogue to accelerate slope stability evaluation and to assess greater numbers of potential failure surfaces; in recent decades, even newer numerical modeling methods have begun entering the geotechnical mainstream.  Simultaneously, geotechs have been introducing reliability techniques into slope stability to allow failure probabilities to be computed alongside a traditional factor of safety.  Yet a piece like Charles Gregory’s write-up on the New Cross slides, even with its incorrect technical arguments, remains a valuable reminder that modern geotechnical engineering still depends just as much on intellectual curiosity, keen observation, and data-driven judgment as did early Victorian civil engineering.  The tools of the trade have certainly changed, but its mission remains the same (Duncan et al. 2014).

 

 

 

Acknowledgments

Sebastian Lobo-Guerrero, Ph.D., P.E., BC.GE, M.ASCE (A.G.E.S., Inc.: Canonsburg, PA), the author’s former colleague, reviewed the entry’s geotechnical content. Thomas Kennedy (Geopier: Davidson, NC), the author’s Virginia Tech classmate, co-authored a 2021 version of the entry posted on an independent webpage.

 

References

British Museum (The British Museum). 2024. “Print: Chas. H. Gregory.” The British Museum. Accessed Jul. 22, 2024. https://www.britishmuseum.org/collection/object/P_1915-0508-142

Castellanos, B.A., T.L. Brandon, and D.R. VandenBerge. 2015. “Use of fully softened shear strength in slope stability analysis.” Landslides, 13 (4), 697-709.

Chandler, R.J. 2003. “Sir Alec Westley Skempton, 4 June 1914 – 9 August 2001.” Biogr. Mems Fell. R. Soc., 49, 509-519.

Cowper, E., C.W. Pasley, P. Bruff, G.H. Phipps, F. Braithwaite, C.H. Gregory, J. Hoof, J.B. Dockray, J. Taylor, J. Smith, J. Hawkshaw, T. Sopwith, F. Forster, J.G. Thomson, G.W. Buck, J. Simpson, J.C. Clutterbuck, H.T. Delabeche, W.S. Moorsom, J. Colthurst, R. Sibley, J. Green, J. Walker, and T.W. Hughes. 1844. “Discussion of ‘On railway cuttings and embankments.’” Minut. Proc. Inst. Civ. Eng., 3, 145-173.

Daniel, M. 2004. “Daguerre (1787-1851) and the invention of photography.” Metropolitan Museum of Art (The Met). Accessed Jul. 21, 2024. https://www.metmuseum.org/toah/hd/dagu/hd_dagu.htm

De Boer, R. 2005. The engineer and the scandal: a piece of science history. New York, NY, USA: Springer.

Duncan, J.M., S.G. Wright, and T.L. Brandon. 2014. Soil strength and slope stability, 2nd ed. Hoboken, NJ, USA: John Wiley and Sons.

Gregory, C.H. 1844. “On railway cuttings and embankments; with an account of some slips in the London clay, on the line of the London and Croydon Railway.” Minut. Proc. Inst. Civ. Eng., 3, 135-145. Reprinted in Cooling, L.F., A.W. Skempton, and A.L. Little, eds. 1969. A century of soil mechanics.  London, UK: Institution of Civil Engineers, 1-13.

ICE (Institution of Civil Engineers). 1898. “Sir Charles Hutton Gregory, K.C.M.G., Past-President.” Minut. Proc. Inst. Civ. Eng., 132, 377-382.

ICE. 2024. “ICE president.” Institution of Civil Engineers. Accessed Jul. 22, 2024. https://www.ice.org.uk/about-us/our-organisation/governance/president

Lewisham Council. 2024. “New Cross.” Lewisham, Mar. 26. Accessed Aug. 11, 2024. https://lewisham.gov.uk/articles/news/a-message-from-the-mayor-and-councillors

Library (Library of Congress). 2020. “Invention of the telegraph.” Samuel F.B. Morse Papers at the Library of Congress, 1793-1919. Accessed Jul. 21, 2024. https://www.loc.gov/collections/samuel-morse-papers/

Peck, R.B. 1985. “The Last Sixty Years.” In Proc. 11th Intl. Conf. Soil Mech. Found. Eng., Golden Jubilee Vol., 123-133. San Francisco, CA, USA: ISSMFE.

Prockter, A. 2021. “Atmospheric railway (New Cross to Croydon).” Know your London, Aug. 20. Accessed Aug. 11, 2024. https://knowyourlondon.wordpress.com/2021/08/20/atmospheric-railway-new-cross-to-croydon/

RR Museum PA (Railroad Museum of Pennsylvania). 2023. “The Rocket takes off.” Railroad Museum of Pennsylvania, Aug. 11. Accessed Jul. 21, 2024. https://www.rrmuseumpa.org/the-rocket-takes-off

Skempton, A.W. 1977. “Slope stability of cuttings in brown London clay.” In Proc. 9th Intl. Conf. Soil Mech. Found. Eng., Vol. 3, 261-270. Tokyo, Japan: ISSMFE.

Somma, A.M. 2014. “Charles Goodyear and the vulcanization of rubber.” Connecticut History, Dec. 29. Accessed Jul. 21, 2024. https://connecticuthistory.org/charles-goodyear-and-the-vulcanization-of-rubber/

Terzaghi, K. 1936. “Stability of slopes of natural clay.” In Proc. Intl. Conf. Soil Mech. Found. Eng., Vol. 1, 161-165. Cambridge, MA, USA: ISSMFE.

UKMA (UK Metric Association). 2024. “Metrication timeline.” UK Metric Association. Accessed Aug. 11, 2024. https://ukma.org.uk/press/metrication-timeline/