ICE news - Many breakthroughs in slope stability research were made by engineers little known outside the geotechnical community, says Mike Chrimes.
A piece I wrote in April on Telford’s involvement with the Panama Canal prompted me to look again at the contemporary accounts of the construction of that work. In an engineering sense, one of the greatest problems overcome during that project were the landslides.
Slope stability and earth pressure are among the most fundamental issues faced by civil engineers. Although Karl Terzaghi is rightly regarded as the father of modern geotechnical engineering, major contributions to this eld were also made by many who are little known outside the geotechnical community. For example, the 17th century French military engineer Vauban, and others like him, had to deal with landslides in the designs of their defensive works.
The development of analytical design methods in the 18th century by engineering scientists such as Couplet led to the classical theory of earth pressure associated with Coulomb and, in the 19th century, with Rankine.
The shortcomings of these theories, which were more appropriate for sand than clay, were evident to practising engineers, as was expressed by Benjamin Baker in the famous paper of 1881 titled The Actual Lateral Pressure of Earthwork.
Problems of slope stability in clays moved from the sphere of the military to that of the civil engineer with the construction of canals and railways, the latter on a massive scale, and also earth embankments for water retention.
Practical experience of dealing with slips soon developed among leading British engineers such as Stephenson and Brunel. However, a satisfactory understanding of the process was lacking.
The first engineer to tackle slips in a modern way was Alexander Collin. He observed a number of clay slips while working on embankment dams in the 1830s, and gathered data from colleagues. He realised that failures typically occur on a curved slip surface, cycloidal in section, and may take place during or soon after construction, or may be delayed for several years while cohesion is eroded by water ingress. Collin’s analysis was accompanied by tests to measure cohesion in a shear box, which showed the impact of water ingress on shear strength.
Despite the insight contained in Collin’s work, it had no impact on engineering design, and it was not until the early 20th century that engineers looked at the problem in a similar way. It was Swedish engineers involved in improvements to Gothenburg harbour, and Swedish railway engineers who made a major contribution to the development of modern soil mechanics.
Ground conditions in lowland Sweden are very poor, comprising soft clays. Landslips had frequently accompanied major civil engineering works, but it was the Stigberg quay wall failure of March 1916 that was the catalyst for a thorough investigation by harbour engineers K Petterson and Sven Hultin, and the G÷teborg Hamnstyrelsen (Quay Commission) of 1916.
This led to the development of a new method of analysis based on circular sliding surfaces in clay, rather than the simplified plane surface generally in use at that time.
A German member of the Commission, Professor Max M÷ller, included this in his Earth Pressure Tables (2nd edition, 1922) which introduced the methods to a wider audience.
This work was more or less contemporaneous with that of the Swedish State Railways Geotechnical Commission.
Its 1922 report, which was prompted by a series of fatal landslips, incorporated major findings regarding the equilibrium of soft clays.