Reported in TRL Unpublished Report CPR2514 (2018) by N. Hewitt, A. Cook & W. Throssel.


A traffic-speed ground penetrating radar (GPR) survey on the M27 (to be upgraded to SMART motorway) identified possible voids beneath the Eastbound M27. The pavement construction was: (i) jointed unreinforced concrete and (ii) cracked, seated and overlaid concrete. An example GPR void risk plot is shown.

TRL’s role

TRL was commissioned to verify the presence of actual voids and to understand what might be causing the results. For the investigation, eight areas of HS and Lane 1 were tested representing areas of high, medium and low voids identified from the GPR data.

Detailed investigations, carried out over four nights in Nov 2017, included:

  • Coring and Endoscope survey to identify any significant voids at transverse joints.
  • Walked longitudinal and transverse GPR surveys.
  • FWD surveys at transverse joints (Load transfer and void intercept test).
  • FWD tests at mid-slabs, plus back-analysis.
  • Dynamic Cone Penetrometer (DCP) and Unbound Material Sampling (UMS) testing.

Main Findings

The results were reported in TRL Unpublished Report CPR2514 (2018) by N. Hewitt, A. Cook & W. Throssel. Twenty-nine cores were taken. The results included the following conclusions:

  • The walked GPR and original traffic-speed GPR results correlated well, both indicating areas consistent with the presence of voids.
  • No significant voids were found apart from two locations where fine material just below the cracked & seated concrete appeared to have been ‘washed out’ leaving the coarse aggregate (natural gravel) still in place.
  • Since no significant voids were found, voiding does not appear to be a major concern at this site and the GPR results are likely to be due to another feature, probably moisture.
  • Moisture was a significant issue on the site. The endoscope surveys showed the granular material beneath the concrete was damp/wet at almost half of the locations drilled. Also one of the Lane 1 coreholes kept filling with water despite repeated emptying, implying a saturated sub-base and poor drainage, which will need corrective action within the SMART motorway pavement design. 

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