As they dug down, they discovered a large container placed in a bricked-up vault beneath a water cistern. The container apparently crumbled as soon as it was touched - hardly surprising considering it had probably been sealed over 600 years before.
But what they found inside was truly astounding: an extraordinary array of over 140,000 medieval coins, the largest deliberately buried hoard of coins ever found, according to Guinness World Records.
Known as the Brussels Hoard, it comprised roughly 64,000 continental coins but also 81,000 English, Scottish and Irish silver pennies. With so many examples minted in England during the reign of Henry III (1216-72), the hoard might well have originally been some sort of Royal payment for trade or military purposes.
But whatever the reason for its existence, it seems the hoard was deposited for safe-keeping back in 1267 during the unrest in the Flemish capital when the city's craft guilds armed themselves and rebelled against the authorities.
Following its rediscovery by the workmen, and an initial examinination, the coins ended up being sold at auction in October 1909 in Brussels by the coin dealer Charles Dupriez. They were divided into two lots - one for the continental coins and the other for the British issues.
At the sale, the British coins were purchased by Albert Baldwin of London dealers and auctioneers Baldwin's (the great-grandfather of Edward Baldwin, the firm's current chairman). It appears that Albert was able to buy them relatively cheaply as a thick fog had descended over the English Channel the night before the auction, which delayed other potential bidders. He paid fr15,250 (plus a commission of 10%), then equivalent to about £9000, to purchase the lot.
While a different bidder bought the continental coins and they were later melted down, the survival of the British section of the hoard has gone down in numismatic folklore.
Some of the coins were, in fact, stolen shortly before the original Brussels sale took place. However, records show that Albert Baldwin attended another sale in Belgium some months later and purchased 2000 further coins from the hoard - perhaps the recovered stolen pieces, although that has never been confirmed.
Most of the coins from the hoard have remained in Baldwin's stock ever since, although some were sold to collectors over the years and some of the poorer pieces were melted down to help pay the wages of staff during the 1920s.
Then, some 20 or so years later, when Albert Baldwin had his house burgled, handfuls of these coins from the hoard were lost along country footpaths in Orpington during a police chase. Indeed, there may be lucky residents who could still find them in their gardens one day.
Finally, in the 1950s an embargo was placed on further disposals pending the proper academic study of what remained. With over half the British portion of the hoard remaining intact, some 55,000 English coins were left awaiting the mammoth task of being analysed and fully recorded.
Now, after surely one of the longest-running pieces of research in numismatic history, the results have finally been published in a volume entitled The Brussels Hoard of 1908 written by an ex-Whitehall civil servant and historical coins devotee, Ron Churchill, and retired banker and numismatist Bob Thomas.
Effectively a new reference work on Henry III coinage, the book - which took over 12 years to complete - includes details of the discovery of the hoard, its subsequent history and a detailed analysis of the individual coins themselves.
Furthermore, Baldwin's are now selling a selection of 101 of the rarest Henry III pennies from the hoard, with estimates ranging from a few hundred pounds to over £1000.
They will be offered at an auction on September 27 during Coinex week and will be sold alongside the second part of the Bentley collection of gold sovereigns.
This initial auction is intended as a precursor to future sales, where more rarities from the hoard will be offered, while Baldwin's are planning to offer some of the 'rank and file' coins in a fixed price list in early 2013. Further information will be released in due course.
The Brussels Hoard of 1908. The Long Cross Coinage of Henry III by R. Churchill and B. Thomas (Quarto, London 2012) can be ordered from Baldwin's website at www.baldwin.co.uk