RE: SOWACS: pavement moisture monitoring
Wed, 04 Dec 2002
From: Lea Dodds <lea.dodds@delta-t.co.uk>
 
Hello Sowacs Listers,
Here is a belated response to the posting below. I have been waiting for
permission to arrive from the consultants and the end users involved before
citing the work. We have supplied many ThetaProbes for this type of
application, but this is the best documented example. 
The Consultant was Robert Armitage of Scott Wilson Pavement Engineering Ltd,
and his client was the UK Highways Agency - thanks to both for consenting to
the attached report being made available for posting.
The ThetaProbes were located under an existing pavement (actually installed
from the sides) with cables running to the roadside data logger which also
had temperature measurement. Ideally they would have also installed a rain
gauge, and they hope to add one in the future. Please note that where the
report refers to TDR Probes, it actually means ThetaProbes (which are FDR
rather than TDR devices).

Lea Dodds
Delta-T Devices Ltd

6.0 TDR MOISTURE PROBES

The six TDR moisture probes described in previous reports were read in August 2001 and found to be functioning correctly. The moisture results for the period March to August 2001, in terms of Volumetric Moisture Contents, have been combined with earlier data and are shown in Figure 3, along with Air Temperature data from a sensor in the roadside cabinet (from March 2001 only). Each probe is consistent with the data for May 2000 - February 2001 shown in the Second Progress Report.

Figure 3: TDR Volumetric Moisture Probe Data and Air Temperature

The Volumetric readings have been converted to the more commonly reported Gravimetric readings in Figure 4, which show Gravimetric Moisture Contents in the range 6 to 22%. TDR 5 is consistent in showing relatively low readings, although it follows the same trends as the other probes, suggesting a calibration error. In some cases there are 'spikes' of higher moisture levels over short periods, for example TDR 6 shows 7 and 9 day spikes in early April and mid May 2001, where moisture levels rose by 2%, suggesting a subgrade wetting/drying cycle after rain. It should be noted that TDR 6 was in the shoulder, whilst all other sensors were below Lane 1 where moisture changes would be expected to be more seasonal.


Figure 4 : TDR Gravimetric Moisture Probe Data and Air Temperature

When combined with previous data, TDR 6 appears to show higher moisture content during the Winter rather than the Summer; TDR's 2 and 4 appear to be stable (but at the highest, possibly saturated, moisture content); and TDR's 1 and 3 appear to show higher moisture content in Summer rather than the Winter. This implies an out of phase movement of water from verge to pavement subgrade, occurring over several months.

Air temperature data taken daily by the logger at 1:30pm is also shown in Figures 3 and 4. The temperatures range from 2oC (March 2001) to 34oC (June 2001); the unseasonably high upper values were probably distorted by heat building up inside the steel cabinet that houses the temperature sensor. The temperature data otherwise shows the expected variation during the Spring and Summer, with higher temperatures often preceding periods of higher moisture in TDR 6. (i.e. Temperatures are often higher during wet and mild weather, and this could result in the subsequent increase in subgrade moisture).