ESWL - Equivalent single wheel loading.
The old airport pavement design method which was adapted from the highway California Bearing Ratio (CBR) design method, and extrapolated to cater for higher single wheel aircraft loads using Boussinesq's single layer elastic theory.
The B-29 bomber appeared in 1943-44 and, at a mass of 64 tonnes, was double the weight of previous World War II aircraft, and was supported on the first dual-wheel gears. The mass doubled again to 148 tonnes with the post-war B-36, which had dual-tandem gears. To accommodate these multi-wheeled gears the CBR method was extended using the concept of equivalent single wheel load (ESWL), which is intended to be the single wheel load which will cause pavement damage equal to that caused by a multi-wheel gear.
In practice the ESWL is defined as the load on a single tyre which will produce the same maximum deflection at subgrade level as the multiwheel load. The ESWL is also a function of pavement depth. So a 4 wheels unit off a Boeing 747, with say 20 t per wheel, is 80 tonnes on the leg. This is equivalent to a loading by 2.25 * 20 = 45 tonnes on a single wheel on the leg (at a subgrade depth of 750mm or 30 inches).
It varies with depth. Simplistically, at a very shallow depth of 1cm, where only the effect of the tyre immediately above can be felt by the soil particles, the ESWL is equivalent to a single wheel load of 20 tonnes. Down at 2 metres (or 6 feet) where the loads from all the tyres can be felt by the soil particles, the ESWL is 4 * 20 = 80 tonnes. At depths in between the two extremes, it varies.
A major consideration at the time the concept was developed was that the ESWL methodology should produce safe designs for the new multiwheel loadings, and testing of gears in the late 1960s and early 1970s confirmed that the deflection-based ESWL design method overstated the damaging effect of multi-wheel gears. This led to the introduction of pavement thickness reduction factors, or 'load repetition factors' called alpha factors. The design thickness of the pavement is reduced by multiplying the 'standard' thickness, T, by alpha (giving typically a 10-20% reduction).
Nowadays, we use layered elastic design or finite element design methods, but the ESWL is still floating around in the background and giving some pretty reasonable results. I use it as a rough check on what the other methods have computed.