Almost everything a geotechnical engineer does day-to-day traces back to a fairly small group of people who developed the theory in the first place. Some of it is centuries old. Most of the modern framework was built in a few decades of the 20th century. Here are the names behind the methods you're probably using this week.
Charles-Augustin de Coulomb (1736-1806)
Coulomb is better known for his work on electricity and magnetism, but in 1776, he published the first proper analytical theory of earth pressure on retaining walls. His wedge theory considered a soil mass sliding against a wall, including friction between the soil and the wall itself. Nearly 250 years later, Coulomb's theory remains one of the two standard methods (alongside Rankine's) for calculating lateral earth pressure today.
William Rankine (1820-1872)
Rankine developed his own earth pressure theory in 1857, using a simpler approach based on stress within a soil element rather than a sliding wedge. It's less accurate in some situations than Coulomb's method but far easier to apply, which is why both are still taught and used side by side depending on the problem.
Karl von Terzaghi (1883-1963)
Terzaghi is generally regarded as the founder of soil mechanics as a formal discipline. His 1925 textbook, Erdbaumechanik, was the first to put soil behaviour on a proper scientific footing. His biggest single contribution was the effective stress principle: the idea that a soil's strength and compressibility are governed by the stress carried by the soil skeleton, rather than the total stress, including pore water pressure. That principle underpins consolidation and settlement analysis as practised today, and his bearing capacity theory is still a reference point for foundation design.
Arthur Casagrande (1902-1981)
Casagrande worked with Terzaghi at MIT before developing much of the soil classification framework still in use. He designed the mechanical liquid limit device that bears his name and used it to develop the plasticity chart, which classifies fine-grained soils by liquid limit and plasticity index. Anyone running an Atterberg limits test on a project this week is using his apparatus.
Ralph B. Peck (1912-2008)
Peck co-authored Soil Mechanics in Engineering Practice with Terzaghi, still considered a foundational text. His own lasting contribution was the observational method, developed on the Chicago subway project in the 1940s and formalised in his 1969 Rankine Lecture: design conservatively, monitor as you build, and have a fallback plan ready if conditions differ from what you assumed. It's the theoretical basis for much of the staged construction and monitoring work on modern infrastructure projects.
Alec Skempton (1914-2001)
Skempton's pore pressure coefficients A and B, published in 1954, describe how pore water pressure changes in response to changes in total stress. They're still standard tools for predicting effective stress in undrained conditions, such as during rapid drawdown of an embankment or staged loading on soft clay, and they sit directly behind a lot of shear strength testing interpretation.
G.G. Meyerhof (1916-2003)
Meyerhof extended Terzaghi's bearing capacity theory, publishing his own version in 1951 that better accounted for foundation depth, shape, and load inclination. Meyerhof's and Terzaghi's methods are the two most commonly referenced approaches for calculating bearing capacity of shallow foundations, and most geotechnical software gives you the option of either.
Laurits Bjerrum (1918-1973)
As director of the Norwegian Geotechnical Institute, Bjerrum led much of the research on sensitive quick clays that cause sudden, large landslides in Scandinavia. His work established that undrained shear strength, rather than effective stress parameters, should govern short-term stability assessments in these clays, and his corrections to vane shear testing are still applied when interpreting in situ strength data in soft, sensitive soils.
A.W. Bishop (1920-1988)
While working at Imperial College London, Bishop published his simplified method of slices in 1955, a way of dividing a potential slope failure into vertical slices and solving for the factor of safety, while more accurately accounting for interslice forces than earlier methods. It remains one of the standard approaches used in slope stability analysis, both by hand and in modern software.
Nilmar Janbu (1921-2013)
A Norwegian professor and contemporary of Bjerrum, Janbu developed his own slope stability method in 1954 and refined it in 1973. Unlike Bishop's method, Janbu's approach handles non-circular failure surfaces, which makes it useful for irregular slopes, reinforced cuts, and other geometries where a simple circular failure assumption doesn't hold.
H. Bolton Seed (1922-1989)
Seed, at UC Berkeley, is considered the founder of geotechnical earthquake engineering. Following his investigations into the 1964 Alaska earthquake and other major events, he and his students developed methods for evaluating soil liquefaction risk that are still the basis of liquefaction assessment procedures used worldwide, including in Australian seismic design practice.