The new debt facility from funds managed by Ares Capital Europe will help RSK achieve its strategy of doubling in size over the next few years.
The latest deals see RSK acquire the TBF Contracting businesses of TBF Traffic and TBF Scaffolding and tree surgeons the BTS Group.
RSK now employs 2,500 staff globally and has an annual turnover of £200m.
Founder and CEO Dr Alan Ryder said: “The financing gives us the financial clout to continue our track record of acquiring businesses built by entrepreneurs that have a strong culture and loyal staff.
“RSK is delighted to join forces with BTS and TBF and their extremely skilled workforce. They are excellent businesses which have a good cultural fit with RSK.
“Their teams have an impressive track record of delivering innovative solutions in a wide range of sectors.
“By bringing them under our umbrella we can release synergies, taking new services to our existing clients and taking our pre-existing services to the clients of our newly acquired businesses.”
BTS and the TBF companies are owned by the same shareholders.
Headquartered in Suffolk, they have a combined turnover of £22m, and 200 staff and 160 subcontractors in offices in Needham Market, Telford, Newark-on-Trent and Elgin, Scotland.
TBF Traffic provides road and traffic management services to a wide range of utility and civil engineering firms, while TBF Scaffolding provides services to a range of utility and construction firms on projects of all sizes.
BTS and the TBF companies will join the agriculture, land and property management division of RSK.
BTS and TBF managing director Halley MacCallum, who will join RSK and continue to lead the businesses together with their three current directors, said: “Joining forces with RSK offers us market access to work on HS2 and other contracts which we’re keen to get on tender lists for.
“This is an exciting opportunity for our staff and our businesses to expand into new fields.”
RSK also plans to use the financing to expand its fleet of equipment and add capabilities in its laboratories.
It invests millions each year into new tools and specialist kit, including its fleet of drilling equipment which already comprises the largest collection of site investigation equipment in the UK.