The work, which affects a stretch of High Street between Bell Lane and Church Street, is due to begin on Thursday, February 12 and is expected to last three weeks.
It will include the digging of trial pits to gather evidence of underground conditions and identify locations for tree pits and parking spaces to be included in stage two of the scheme.
The tree pits will reduce the risk of flooding by catching water and channelling it into drains.
Pedestrian barriers will be in place around the trial pits, which will take up a few square metres of pavement and should not affect shoppers.
Meanwhile, information is being gathered from CCTV and surveys of the basements of properties in the street, which will also be used to finalise the design of the second stage of the improvement scheme.
Detailed designs will be on display in the spring, with construction work on stage two to begin in the summer.
Cllr Claire Dowling, member of the scheme project board, said: “The investigative trial pits will allow us to find suitable locations for tree pits, which will go some way towards reducing flooding and protecting local businesses.
“Understanding where trees can be located is important in enabling us to finalise the design for the second stage of the improvement scheme.”
Stage two will include footway widening, repaving and the installation of new street lights, traffic signals and street furniture. This stage, which is currently being reviewed, is also looking at the number of parking spaces in the High Street.
Meanwhile, Network Rail has begun work on its new 174-space car park opposite the town railway station, work on which is due to finish in May.