They are understood to have beaten off rival heavyweight bidders Balfour Beatty/Vinci and Costain/McAlpine to one of the biggest muckshifting contracts in recent years.
It will be the first significant construction contract at the Hinkley Point nuclear power station being planned by French utility giant EDF Energy with the prize contract being a £1.2bn main civils deal.
The earthworks contract will involve moving 4.5 million cu m of rock and earth to make way for the new power station in Somerset.
It is the second success for the joint venture team after signing a £4m site preparation deal with EDF last year.
Meanwhile, Hydrock has also beaten off stiff opposition to win a £1m contract to carry out remediation at the site.
It will remediate a spoil heap created during the construction of the now decommissioned Hinkley A reactor, located on the site of the proposed Hinkley C reactor. The firm will screen to extract asbestos, the primary contaminant, plus any other building materials which will then be removed from site in accordance with a comprehensive site Waste Management Plan.
Among the other big packages, Costain/Sir Robert McAlpine is one of five bidding for a £250m tunnelling contract at the site with a winner due to be announced in the next few months.
The Kier/Bam Nuttall team is also pitching and has added Bam-owned German tunnelling specialist Wayss & Freytag to beef up its bid. The others on the list are Morgan Sindall, Hochtief and Murphy.
This work will involve building a pair of 3 km long intake tunnels along with a 2 km long outfall tunnel plus a series of sea shafts.