I would imagine that the average front line soldier in the trenches of WWI became intimately acquainted with earthworks. I further imagine that they must have become fairly proficient at building and maintaining earthworks in all kinds of conditions. When these soldiers went home that skill didn't go away. I would imagine that creative people with need could come up with uses for earthworks outside of warfare.
Is there any evidence to suggest that there was a wave of earthwork construction by veterans of WWI?
At a push maybe Anderson shelter construction in the UK in the late 30s/early 40s might be an example. These were corrugated iron air raid shelters for up to 6 people, supplied in kit form (free of charge to the less well off, at a fee to the wealthier) to be built and partially buried in the back gardens of family homes. Over 3.5 million were produced. Many of the men constructing them for their families would have been WW1 veterans, and although there were basic instructions to be followed, if you do a Google image search for period examples of the shelters you can see that some people were going above and beyond with the construction, with impressive obliquely entrenched entrances to act as blast walls and thorough sandbagging: this suggests to me a familiarity with earthworks carried over from personal experience in the trenches, but I couldn't offer any definite evidence for this.
It's worth considering that while the average (British WW1) soldier would be intimately and miserably familiar with the entrenching tool and pick axe, they were typically working under instruction rather than free initiative. Trench digging was part of basic infantry training and all infantrymen could expect to be called on for work details at the front, which would usually take place under the cover of darkness and involve digging or repairing forward trenches, saps and wire emplacements; but it's not necessarily accurate to extrapolate from this that they were widely skilled in the construction of earthworks, as they were really working as labourers under direction of their officers, who had instructions from further up the chain of command. Although infantrymen might dig rudimentary cover trenches after taking ground and would have understood the principles of improving and maintaining them, the actual details of improvement, revetting, drainage, sanitation, direction of traverse, link-up with other fortifications etc. was all planned higher up by senior staff and the Royal Engineers. Serious earthworks, such as strong points and redoubts, were again planned and mainly constructed by the RE based on the tactical requirements of local brigade or divisional commanders.
A number of manuals were published, with the The Manual of Field Engineering (1911, with amendments) being the main authority. Another of the better known was Field Entrenchments: Spadework for Riflemen by John Solano (first published in 1914, again with updates throughout the war) (available for download here if you'd like to read it). The name is a little misleading, though: I've not found any evidence that the average rifleman read manuals like these, nor would they have much personal freedom to put the information into practice on their own initiative. They were reading for officers, and particularly officers of the Royal Engineers.
One slightly unusual one, ex-soldiers were involved in a number of major archaeological excavations at British historic monuments in the immediate post war period. At the excavation of Rievaulx Abbey in Yorkshire in the early 1920s a large force of ex soldiers took part, using their earth moving expertise to clear many meters of fallen debris to reach the mediaeval archeological layers. The soldiers even had a war surplus miniature railway that they used for moving the overburden of spoil. We have pictures of this in the photos of the excavations.