(An unsolicited report submitted to the Limestone District School Board)

Unsafe pea gravel in play areas

By Bruce WilsonA parent of a child who attends Rideau Public School

June, 2010

Pea gravel around climbersPea gravel, which consists of smooth rounded pebbles, surrounds climbers in play areas.

The pea gravel is supposed to cushion the impact of a child who falls from a climber. I suspectthat the pea gravel cushions the impact of a child because as the child hits the ground, therounded pebbles slide past each other. I have been told that to meet safety standards, the peagravel is supposed to be twelve inches deep within six feet of climbers.

A hard layer in the pea gravelIf you dig into the pea gravel in a play area you may find a layer of pea gravel that

contains little or no sand and clay, then a hard layer, and then a layer of pea gravel that containsconsiderable sand and clay; in short, you may find a surface layer of clean pea gravel, then ahard layer, and then a layer of dirty pea gravel. The hard layer is composed of a mixture of peagravel, sand, and clay, cemented by the clay. It is commonly one to four inches thick, and it iscommonly buried under one to four inches of clean pea gravel.

In June of 2009, I examined the pea gravel in play areas at twelve Limestone DistrictSchool Board elementary schools, and at three City of Kingston parks. In every play area, thesurface layer of clean pea gravel was underlain by a hard layer.

Current means of dealing with the hard layerThe presence of the hard layer is known to the Limestone District School Board. To

mitigate the problem, new pea gravel is added or the pea gravel is roto-tilled.In May of 2009, new pea gravel was delivered to Rideau Public School. Up to about

twelve inches of new pea gravel was added to part of the eastern (kindergarten) play area, and upto about eight inches of new pea gravel was added to part of the northern play area and part ofthe western play area. After the next rain the surface layer of new pea gravel was still clean butin all three play areas the surface layer of new pea gravel was underlain by patches of hard layer.Since May of 2009, the area underlain by a hard layer has grown. Today, in the eastern andwestern play areas most of the surface layer of new pea gravel is underlain by a hard layer, andin the northern play area all of the surface layer of new pea gravel is underlain by a hard layer.

Since May of 2009, the pea gravel at Rideau Public School has been roto-tilled severaltimes. Each time the result was the same: roto-tilling broke up the hard layer; and after the nextrain, the surface layer of pea gravel was clean but places that had been underlain by a hard layerbefore roto-tilling were once again underlain by a hard layer.

Thus adding new pea gravel leaves the hard layer farther below the surface of the groundonly until the next rain, and roto-tilling the pea gravel removes the hard layer only until the nextrain.

After the pea gravel has been roto-tilled, the surface layer of pea gravel is dirty. I suspect

that because the rounded pebbles may not slide past each other easily, dirty pea gravel may notmeet safety standards.

The most dangerous placesIn most parts of most play areas the hard layer is commonly buried under three or four

inches of clean pea gravel. Under a set of monkey bars or at the bottom of a slide, however, thehard layer may be buried under only an inch or two of clean pea gravel, or may even be exposedat the surface.

Children often do flips over the monkey bars, go up or down slides on their feet, or godown slides head first. Falls may not be common, but they are a possibility.

Where there is a hard layer a child who falls to the ground certainly would not beadequately protected, and where the pea gravel is dirty a child who falls to the ground may notbe adequately protected.

I have been told that at Rideau Public School, several children have suffered brokenbones. Could a fall from monkey bars onto an exposed hard layer cause a more serious injury?

A possible cause of the problemToday at Rideau Public School, the pea gravel in the eastern and western play areas

contains a lot of sand and clay, and the pea gravel in the northern play area contains even moresand and clay. Some people believe that the sand and clay came with the pea gravel, that thesand and clay were blown in from surrounding areas, or that the sand and clay were tracked infrom surrounding areas. When the pea gravel was delivered to Rideau Public School in May of2009, I examined the pea gravel and found very little sand or clay. I doubt that large amounts ofsand and clay could be blown in or tracked in from surrounding areas.

I’m a geologist, and I have watched the development of the hard layer at Rideau PublicSchool for many years. I believe that the sand and clay came from below the pea gravel. A civilengineer who discussed the problem with me in one of the play areas agreed with me.

About ten thousand years ago, as the last glaciers receded from the Kingston area, theshoreline of Lake Ontario was many miles north of Kingston, and the Kingston area wassubmerged under several hundred feet of water. Rivers that entered the lake brought sand andclay that was deposited on the limestone bedrock. The resulting widespread layer of sand andclay is commonly up to six feet thick, and commonly contains a lot of clay.

A few years ago in the eastern play area, far from the climbers, two sand and clay domesappeared at the surface of the ground. Each dome was about four feet in diameter, and wascovered by only a few scattered pebbles.

Last year in the eastern and western play areas, after new pea gravel had been added, ithad rained, and the ground had dried, the buried patches of hard layer consisted of sand and claydomes, or of overlapping sand and clay domes. Each dome was about one foot in diameter.

Today, close to the eastern play area, the northern play area, and the asphalt pad, there isa sand and clay dome (a little hill) that is about thirty feet in diameter. It was not always there,and it was not built by anyone. After new pea gravel was added to the eastern and northern playareas the dome seemed to be higher, and a few months later after the asphalt pad was built thedome seemed to be even higher.

When it rains, water combines with sand and clay to form mud. I suspect, with someconfidence, that because the mud is fluid, because the weight of overlying pea gravel or asphalt

pressurizes the mud, and because the mud is lighter than the pea gravel, the mud rises incolumns toward the surface of the ground, producing domes. After new pea gravel was added tothe eastern play area the first domes were small and isolated, but over the last year the domeshave grown and coalesced to form a continuous hard layer.

In desert areas, groundwater that contains dissolved minerals rises toward the surface ofthe ground. As rising water gets close to the surface of the ground the water evaporates, and asthe water evaporates the minerals come out of solution and cement the sand, producing a hardlayer called hard pan below the surface of the ground. I suspect, with some trepidation, that inthe play areas as rising mud gets close to the surface of the ground the water evaporates, and aswater evaporates the clay cements the sand and gravel, producing a hard layer called clay panbelow the surface of the ground.

A possible solution to the problemIn June of 2009 I discussed the formation of the hard layer with a professor of Civil

Engineering at Queen’s. He said that although the way in which the hard layer forms below thesurface of the ground may be debatable (hence my trepidation), the solution to the problem issimple. What is needed to prevent the formation of a hard layer is a barrier between the peagravel and the underlying sand and clay. In the past, civil engineers used a layer of sand. Today,they use a layer of cloth that they would call a filter cloth or geotextile, but that we might call alandscape cloth.

In some of the play areas that I examined, cloth had been placed between the pea graveland the underlying sand and clay. In each of those play areas, the cloth did not prevent theformation of a hard layer.

Filter cloth contains holes. To prevent mud from moving though the filter cloth, the holesmust be the correct size. Determining the correct hole size involves analyzing the sand and clayto determine the grain sizes, and the proportion of each grain size. Once the correct hole size hasbeen determined, the proper cloth must be purchased from a specialized dealer. There may notbe an appropriate dealer in the Kingston area.

I used to live a few blocks from Rideau Public School. In 2002, I installed pea gravel in aplay area in my back yard. Because I was aware of the problem at Rideau Public School, I placedlocally purchased landscape cloth between the pea gravel and the underlying sand and clay.Seven years later the pea gravel was still clean right down to the cloth, even under the swingsand at the bottom of the slide. Although I had been unaware of the need to choose the propercloth, by dumb luck I had purchased a cloth that had the correct hole size. Unfortunately, thecloth that I purchased is no longer available.

Rehabilitating pea gravelRehabilitating the pea gravel would involve digging up the pea gravel, cleaning the pea

gravel, covering the sand and clay with the proper cloth, and putting the clean pea gravel back.The school board may have the equipment to dig up, transport, and put back the pea

gravel, but it may not have the equipment to clean the pea gravel. I have been told that a localcompany that supplies pea gravel cleans it with a large machine. Thus to rehabilitate the peagravel, the school board may have to contract cleaning to a local supplier, or lease or purchase acleaning machine.

It’s time to do somethingI suspect that the cost of rehabilitating the pea gravel would be very large. I also suspect,

however, that the amount of money the school board has invested in the pea gravel is very large.In July of 2009 I spoke with Glen Carson, Manager of Facility Services for the Limestone

District School Board. I talked to him about the hard layer, about how adding new pea gravel orroto-tilling the pea gravel only works until the next rain, and about how to choose the propercloth.

If a child were to be seriously injured by the hard layer, would the school board be liablefor damages? The risk may be very small, but the cost could be very large.

Because adding new pea gravel or roto-tilling the pea gravel only works until the nextrain, there are many days when the pea gravel is not safe. Liability or no liability, we should dosomething now to protect our children.

(End of the unsolicited report)

Additions and updates by Bruce Wilson, May 2013

MeI have a BSc and an MSc in geology, from Queen’s. I have studied the pea gravel

problem for many hours over many years.I am confident that the clay domes formed from muddy water that rose toward the

surface, and that the hard layer consists of coalesced clay domes. Some evidence is obvious inthe photographs that are presented on following pages.

May, 2009I discussed the pea gravel problem with a Trustee. The Trustee discussed the problem

with staff at the School Board.

July, 2009I spoke in person with a manager for the Limestone District School Board, and he gave

me permission to dig in play areas at schools.I spoke over the phone with a manager for the City of Kingston, and he gave me

permission to dig in play areas in city parks.

My June 2010 reportThere is at least one mistake in my June 2010 report.I examined play areas at thirteen schools, not twelve schools. They are: Bayridge Public

School, Centennial Public School, Central Public School, First Avenue Public School, FrontenacPublic School, J.G. Simcoe Public School, Lord Strathcona Public School, Polson Park PublicSchool, Rideau Public School, Rideau Heights Public School, Sydenham Public School,Truedell Public School, and Winston Churchill Public School.

Summer, 2010The pea gravel at Rideau Public School was replaced by wood chips.

May, 2012I examined play areas at Rideau Public School and noticed that the wood chips were

reduced in size and dirty.

June, 2012I heard that they were going to install new play equipment at Polson Park Public School.

In an e-mail to the Trustee for the school I included a copy of my 2010 report, I pointed out thatthe pea gravel problem still existed at the school, and I pointed out that if the pea gravel werenot rehabilitated then they would probably end up with fine equipment surrounded by unsafe peagravel.

December, 2012I examined play areas at Polson Park Public School and Rideau Public School, before the

ground was covered with snow.At Rideau Public School the wood chips were smaller and dirtier than they were in May.At Polson Park Public School, the pea gravel problem had clearly begun.

April, 2013I examined play areas at First Avenue Public School, Polson Park Public School, Rideau

Heights Public School, Rideau Public School, and Winston Churchill Public School.First Avenue Public School, Polson Park Public School, Rideau Heights Public School,

and Winston Churchill Public School still had pea gravel, and the pea gravel problem.At Rideau Public School, the wood chips were smaller and dirtier than they were in

December. Where the chips are very small, if you try to dig into the wood chips with your handthen you will find that it’s not easy, and if you tap on the wood chips with your hand then thesound that you will produce will seem to indicate that there is a hard layer that is underlain by aless dense layer. The hard layer is not as hard as a hard layer in pea gravel, but it probably doesnot meet safety standards.

PLEASE NOTEIn the following photographs, contrast has been enhanced to reveal texture.

Rideau Public School, June 8, 2009

Small clay domes in pea gravel that was installed on May 26, 2009. Each dome was buried undera few inches of clean pea gravel, and surrounded by about 8 inches of clean pea gravel. (Ibrushed away and then replaced the overlying pea gravel.)

One of the small clay domes. Bumps on the surface of the dome seem to indicate that it formedfrom several columns of rising mud.

Rideau Public School, July 19, 2009

A large clay dome in pea gravel that was installed on May 26, 2009. The dome was buried undera few inches of pea gravel, and surrounded by about 12 inches of clean or slightly dirty peagravel. Bumps on the surface of the dome seem to indicate that it formed from many columns ofrising mud. (I brushed away and then replaced the overlying pea gravel.)

A cross-section of a hard layer that formed in pea gravel.

Rideau Public School, February 18, 2010

By chance, I found ice domes in a puddle at Rideau Public School, not far from a climber. Eachdome is higher than the ice that surrounds it, and dirtier than the ice that surrounds it. Bumps onthe surface of the large dome seem to indicate that it formed from several columns of risingmuddy water.

Rideau Public School, December 16, 2012

Wood chips under and beside a low platform. The chips that are under the platform, and thathave not been stepped on, are probably about their original sizes. The chips that are beside theplatform, and that have been trampled, are clearly reduced in size.

Rideau Public School, April 6, 2013

Wood chips close to monkey bars. The wood chips are a bit dirty. They are brown because theyare damp.

Wood chips under monkey bars. The wood chips are very dirty. Perhaps they are gray becausethey are coated with clay.

Winston Churchill Public School, April 6, 2013

Monkey bars.

Pea gravel under the monkey bars. The hard layer was exposed at the surface.

Polson Park Public School, April 6, 2013

A slide that seemed to be surrounded by safe pea gravel. A hole dug in the pea gravel.

Pea gravel at the bottom of the slide. The hard layer was buried under a few inches of clean peagravel. (I brushed away and then replaced the overlying pea gravel.)