The Clay Research Group Issue 66 – November 2010 – Pa

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The Clay Research Group

Monthly Bulletin

TheClay Research

Group

November 2010

RESEARCH AREAS

Climate Change � Data Analysis � Electrical Resistivity Tomography

Time Domain Reflectometry � BioSciences � Ground Movement

Soil Testing Techniques � Telemetry � Numerical Modelling

Ground Remediation Techniques � Risk Analysis

Mapping � Software Analysis Tools

The Clay Research Group Issue 66 – November 2010 – Page 1

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CONTENTS

Weather Data & Ground Movement

Climate Model & ‘Memory of Desiccation’

Precise Levels & GeoServ

Updates and Snippets

Headmasters House Update

ALDENHAM SCHOOLRESEARCH SITE

Our research has relied heavily on thegenerosity of Aldenham School. The site –100 acres in North London – has allowedacademics to gather and share the data theyhave collected using a wide range oftechniques.

The value has been correlating one methodwith another. Understanding which soiltesting technique is reliable by comparingthe results with precise levels for example.Determining actual soil moisture deficiencyusing the neutron probe. Reviewingemerging methods like electrical resistivityto measure moisture change over time.Sending data from site using telemetry.

Aldenham has been a generous host sincework commenced in 2006.

5 Year ReviewWhat has the last five years delivered? Too few answersunfortunately, but it has clarified some of the questionsthat we might want to ask. Next months newsletter reviewsthe work of the CRG over this period.

The CRG borrows heavily from the work of others. Forexample, the BRE were monitoring the Chattenden siteyears before the CRG was formed and have producedextensive guidance on a range of topics under the directionof Richard Driscoll, Tim Freeman and Mike Crilly. Warddefined the problem and came up with many of theanswers half a century ago. Giles Biddle measured moisturechange beneath several species of tree on a variety of soils.Both have produced tables putting trees in rank order ofrisk, as have Cutler & Richardson.

Electrolevels were in use by the BRE for monitoring theleaning Tower of Pisa. TDR sensors are in common use inagriculture around the world. Very little of our work isnew or novel. The CRG offers a platform for newmethods to be tested and reviewed by colleagues. In nextmonths edition we review progress.

Congratulations to Richard Rollit on gaining his MBA.Several years of hard work at the same time as holdingdown a demanding position at Crawford & Co., can’t havebeen easy.

We are holding our Christmas bash at Aldenham this year,and Giles is busy looking at his diary to see if it clasheswith his demanding schedule acting as an Expert Witness,dealing with TPO appeal decisions for PlanningInspectorate, desk-topping some claims … and sailing hisyacht to Florida to avoid the winter snow.

Margaret McQueen from OCA continues to drive us allforward and is heavily involved with several complex Courtcases as well as keeping abreast of developments relatingto planning and changes in the TPO legislation.

Tim Freeman has allowed us to publish his suggestionson the use of precise levels to establish causation andresolve disputes on tree related claims.

The Clay Research Group Issue 66 – November 2010 – Page 2

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TEMPERATURE

WEATHER DATA

On the face of it there seems to be littledifference between temperature plots for 2008,2009 and 2010.

In contrast, the Relative Humidity plots for thesame years reveal that the readings for 2010were much lower than preceding years.

There is little doubt that RH is one of the mostimportant factors driving transpiration andmay explain the difference in claim numbersbetween busy and normal years when othermeasures appear similar.

GROUND MOVEMENT~ August Profiles ~

Comparison plots of the ground movementprofiles beneath the Aldenham Willow for themonth of August over several years are showntop, right.

2006, 2009 and 2010 were relatively dry years.In contrast, 2007 and 2008 were much wetter.The plot reveals the influence of the weatheron root moisture uptake.

The difference occurs mainly at the rootperiphery in the case of the Aldenham Willow.Peak ground movement in dry years is aroundtwice that recorded in wet years.

SMD VALUES~ Supplied by the Meteorological Office ~

The 2003 profile started early and was maintainedthroughout the summer, revealing thecharacteristic signature of an event year. Incontrast, 2007 was a quiet year in terms of claimnumbers.

2006 was busy throughout the summer, and 2010threatened, but didn’t deliver, even though theSMD values were high for a few months, matchingthe 2003 values.

The Clay Research Group Issue 66 – November 2010 – Page 3

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Loading

Unloading

CLAIM FREQUENCY AND COUNT

We have touched on this before. How riskysectors tend to get riskier at times of surge. Thegraph below models two years, one with 38,000claims (grey dots and green moving average),and another (red dots and moving average) for55,000 claims, ranked in frequency order.

The count of claims per sector for the aboveyears suggests busy sectors double their countwhilst others will only see a small increase.

Just over 12% of the sectors have no claimsnotified at all.

Sectors with say 4 claims in normal years, on alow shrink/swell soil may receive around 5maybe. Sectors on highly shrinkable soils withsay 9 claims could see that figure reach 15 ormore.

The greatest change – the sectors at highest riskfrom Climate Change – are those on clay soils.The base figures can be flexed to match thedeveloping year, using ‘by-month’ weather andSMD data.

This model will provide insurers with some ideaof what Climate Change will deliver in terms ofwhat claims will occur and where.

‘MEMORY’ OF DESICCATION

It is very frustrating to receive claims late in theyear, possibly associated with root induceddesiccation, only to find the tree is out of leafand knowing from experience that we may havearrived too late to record desiccation usingtraditional techniques.

The penetrometer may deliver added value inthis situation due to the recent stress history.

As we know, soils consolidate when desiccated,and don’t fully recover as we see from the graphbelow.

Clive Bennett from MatLab describes it asfollows. “Root can desiccate soils and when the tree diesor is removed (or in the winter months) the soilrecovers along the unloading line (a lot flatter than thevirgin compression line (approx 1/5)). The soil will thendisplay a bulge in its shear/unconfined compressivestrength profile due to the previous existence of the tree butthere is no suction present at the time of the test”.

We have growing confidence in thepenetrometer test, but it has to be used sensibly– in homogenous London clays with samplesfree from bands of sand, gravel and so forth.Sands and gravels will produce high readings notassociated with desiccation.

We probably all agree that in ideal conditions, thesuction test is preferable as a measure of rootactivity, but it has problems as we have seen inrecent studies.

Influence of ClimateChange on claim

frequency modelledby degree change.

The Clay Research Group Issue 66 – November 2010 – Page 4

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LEVELS –v- INVESTIGATIONS

GeoServ have provided some useful examplesof how powerful precise levels can be whendiagnosing subsidence.

Tim Freeman, the Managing Director ofGeoServ and former head of FoundationResearch at the BRE takes the view thatsometimes investigations and soil testing canbe a waste of effort. He explains “it is nosurprise when looking at a claim in North London tofind a property with foundations say 600mm deep orso, bearing onto clay, and finding desiccation in a dryyear”.

He regards it as little more than a ‘glimpse of theobvious’.

Precise levels tell us if the building is actuallymoving, and if so, by how much, and where.

It delivers the most compelling evidence andthe results are easy for the arborist andhomeowner to understand.

Tim delivered a presentation at Aston in2007 entitled “An objective framework fordealing with third party trees”, in which hesaid…

As can be seen from the pie chart, in about 85% ofvalid claims the cause was suspected to be tree-relatedshrinkage in the soil, reflecting the hot summer in2003 and the prevalence of clay soils in ourcatchment area. 103 of these claims (49%) involvedtrees belonging to Local Authorities.

By way of clarification, Tim provides expertwitness services and his use of precise levels(top pie chart) reflects the complex nature ofhis portfolio which has a bias towards NorthLondon, although his company, GeoServ,provide a nationwide service.

In an attempt to resolve the sometimes endlessexchanges between parties when dealing withRecovery claims Tim has compiled the followinggraph, outlining his recommendations for handlingroot induced clay shrinkage claims.

In summary, 40% of claims from his sample recordmovement less than 5mm over 12 months,suggesting that the tree could be retained. For thenext 40% of claims with movement between 5 –15mm, Tim suggests tree management is essential.

The balance of 20% - those with movement inexcess of 15mm - requires either tree removal orunderpinning.

This appears to be a good a starting point fordiscussion as any. If anyone would like furtherinformation, please visit the GeoServ web site.

GEO-SERV020 82084476

The Clay Research Group Issue 66 – November 2010 – Page 5

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Although the soils are saturated due to winterrecovery, the clay particles will have consolidated

and the ‘memory of desiccation’ could be revealed bythe penetrometer, but missed by the suction test.

Next Generation …We have been working on a proof-of-conceptsystem since the middle of 2009, using the AppleIphone. It’s a ‘gather data from site’ applicationwith simple selections from a few screenshots andeasy data transmission.

The Iphone already has links to the BGS mapsallowing users to determine the geology from theirlocation, a location and mapping tool, measuringdevice plus the ability to view surrounding claimsvia Google. A very basic spirit level applicationmeasures angular distortion and users are able todictate notes.

In the News …Fresh concerns have been raised about availablewater by a large team of researchers led by MartinJung from the Max Planck Institute in Germany(Nature, 21st October, 2010). The letter suggeststhat available water from evapotranspirationpeaked around 12 years ago, and has been indecline since.

The model suggests that in Australia and Africaincreases in temperature have literally drained theland. Trees are sucking moisture from the soil, butrain falls elsewhere. Not the place from which itwas taken.

The team have used weather data from around theworld, and microwave satellite observations. Theyconclude that this could be due to either ClimateChange or simply natural weather cycles.

Useful links …Displaying your data on Google Earth has justbeen made a lot easier thanks to a new website.

http://www.batchgeo.com/

Enter the postcodes you want to plot in anExcel spreadsheet, and the application deliversa file in KML - Google format for sharing anddisplay. A free, easy to use mini-GIS systemthat doesn’t require specialist knowledge forlimited ‘where is ‘x’” data display.

FILTER PAPER TESTING

It is worth checking that the laboratory is usingpolythene bags when carrying out the filterpaper test. It is easier and faster to avoid thisstep but there are important considerationsthat could influence the accuracy of the results.

The relative humidity of the laboratory caneasily distort the readings if the filter papers areweighed without the polythene bags.

The Clay Research Group Issue 66 – November 2010 – Page 6

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HEADMASTERS HOUSEUPDATE

The latest precise level readings (bottomimage) confirm that removal of the shrubshas stabilised the most significantmovement along the rear wall of theHeadmasters House at Aldenham.

Minor ongoing movement (8mm or so) hasbeen recorded to the rear right hand cornerwhen viewed from the rear garden, whichimplicates another Willow tree growingnearby.

The interesting points are (a) precise levelshave been successful in indicating whichitems of vegetation were contributing mostwhen there were several trees and shrubsnearby and (b) the amount of movement(42mm) that the Wisteria had caused and (c)the seasonal influence of the Willow whichmay not, at this stage, have caused anydamage.

It appears that the Aldenham Willow hasbeen entirely innocent of any involvement.

In contrast, the boreholes and various soiltests (oedometers shown below) simplyindicated the soil was dry in every location.It was impossible to distinguish which ofthe shrubs and trees were causing most ofthe damage, or exactly how muchmovement had already taken place, andwhat was happening seasonally.

Soil testing confirmed desiccation in all 4bores, but gave little information about whichshrub or tree was implicated. Precise levels

were far more useful.