By Jack Wetmore

Construction of a $20 million state-of-the-art facility — one of the first public-private partnerships in the province — was completed in little over 14 months, and marked the first attempt at large scale topsoil conservation in the region.

     Situated on a 20 ha heavily wooded site on the north side of the city, the new Leo F. Hayes High School in Fredericton, NB, with its innovative top­soil conservation initiative clearly demonstrated the economic and environmental benefits of the conservation approach, saving more than $100,000 over the costs of purchased material. As an added benefit, the 15,000 cu. m of recovered material proved to be as superior a growing medium as any commercially available soils in the area, a loam texture with 2.8 to 3.4 per cent organic matter content. Constructed by owner Diamond Construction (1961) Ltd., and with coordination and landscaping plans conducted by High Design Inc. of Fredericton, Daniel K. Glenn Landscape Architect Ltd., with the help of a team of architectural firms handling specific areas, the school underwent site clearing and grubbing in July 1998, opening on schedule on September 7, 1999.

     The topsoil material, as stripped, contained eight to 10 per cent of rocks and debris, and would previously have been discarded as fill and replaced with imported material. Conservation of the existing topsoil substantially reduced pressure on farm­land topsoil sources in the area. Working with the recovered soil was a learning experience for all parties involved. Fortunately, all were located in Fredericton. This permitted close co-ordination and communication for the duration of the project to resolve most difficulties that arose.

Screening
Screening was carried out in the fall of 1998 and spring of 1999. Most soil was stockpiled for later movement to finished subgrades.

     The construction specifications called for 50 mm maximum particle size, therefore, a vibrating screen with a two-inch mesh was utilized. After an initial learning curve, average screening rates of about 100 cu. m/hour were achieved, with relatively moist source material. Debris content averaged an estimated eight to 12 per cent, with this being moved to on-site locations that required fill.

     The 50 mm specification resulted in surface material totally unsatisfactory for finished lawns and playing fields. (The British Topsoil Specification BS3882: 1994, probably the most comprehensive in existence, calls for 20 mm maximum particle size for these applications.) Further cleanup was required, with workers employing the services of a Harley Rock Rake. An additional 300-500 cu. m. of stones were re­moved in this process. This operation proved to be equipment and labour intensive, requiring about 300 equipment hours and an additional 700 hours of labour to complete, or 30 and 70 hours per ha, respectively. Again, it was a learning experience, and future productivity levels of half these figures should be attainable.

     In the event, it seemed to be a lot of thankless extra work to correct a specification error. However, it highlighted important lessons for the future. First, this procedure will likely be the norm for topsoil reclamation, and second, it appeared cost effective.

     Three quarter inch maximum stone size is a desirable target for most topsoil applications. Screening topsoil to this dimension requires dry soil for efficient production; as soil moisture levels increase, screening production decreases sub­stantially. It is estimated that screening time on this project would have more than doubled if a 3/4-inch screen had been used.

     Total costs for soil reclamation on this project were in the range of $7-8/cu m, from initial stripping to finished, rock raked surfaces. Dry soil would probably have reduced these costs to the $6 -6.50/cu m range, about half the costs of imported soils in this region. The Harley raking operation proved unsatisfactory for the playing field surfaces — too many of the larger stones appeared at the surface by the spring of 2000. Resurfacing operations were undertaken, with a one-inch soil mixture and sodding.

     Hindsight suggests that the two-inch screening is cost-effective for much of the recovered soil, but that a small quantity of soil, sufficient for a two-inch layer over critical areas such as playing fields, should be screened to the 3/4-inch dimension.

Textural amendment
Mechanical analysis of the recovered topsoil determined a 38 to 46 per cent sand content and 13 to 21 per cent clay, placing the soil in the loam textural classification. While this is ideal for turfgrass growth, 60 to 70 per cent sand content is preferable for playing field surfaces to improve soil structural strength and playability, with reduced compaction.

     It was decided to amend the playing field topsoil surfaces to figures closer to the 60 per cent sand content range.

     There were two options to amend the soil. Additional sand could have been incorporated at the screening plant or soil could have been amended after spreading by incorporating a one-inch layer of sand. Although there was little experience with the latter approach, construction deadlines suggested it would be preferable to try it.

     The required quantity of sand was calculated, divided into the number of loads, and then the loads spotted in rows on the fields for spreading with a grader.

     This led to more lessons on the learning curve. A check after final spreading revealed a depth from ¾ inch to 3 inches, caused by two factors: improper spotting of piles and uneven subgrade. The piles were spotted midway in each block, while the grader worked in one direction spreading the piles outward. This left shallow areas at the start of each pass and thicker areas at the ends. Corrective measures involved locating and removing the surplus material, using an agricultural land leveller rather than a grader.

     The finished topsoil grade proved to be uneven — 1-1/2 inches, as a result of insufficient attention to this phase of construction. This was corrected.

     At this point, a further, and major, problem became evident: compaction. Handling the moist topsoil and spreading with heavy construction equipment packed the topsoil layer like asphalt. This layer was broken up with a York Rake scarifier with sharp teeth — new, or rebuilt every half-hour. Multiple passes in three directions (0, 45 and 90 degrees) were required. This was followed with a flex tine harrow to further loosen and mix the soil. The surfaces were then regraded and finished.

     Moist topsoil will be a concern in many cases with recovered topsoil. Imported soils have been self-protecting for moisture content. The products cannot be delivered at high moisture levels, and spring work is usually held up, waiting for dry product. Recovered topsoil material will often be stockpiled on site, and stockpiled material dries slowly. At the same time, construction deadlines encourage its use as early as possible. It may be practical to cover the stockpiles, if it has been piled dry. Otherwise, extra care will be required in placing and spreading to avoid the compaction problem. It will help to spot material in smaller piles on site, allowing it to dry for a few days. It will also help to spread the material with very small dozers (JD350 or smaller).

     Both the topsoil conservation and the textural amendment processes were innovative approaches in the region. The experience gained in both areas, in handling techniques and technical criteria, will be invaluable for future conservation and soil amendment projects. It is now clear that the topsoil from most new construction sites in the province, even in heavily wooded areas, can be recovered cost-effectively, and the resulting material will be a good quality base for a lawn.

     To recognize the initiative, the New Brunswick Horticultural Trades Association, supported by the two Provincial Federations of Agriculture, The New Brunswick Soil and Crop Improvement Association and the Eastern Canada Soil and Water Conservation Centre, presented a plaque to the High School. Certificates of appreciation were presented to the general contractor, the architectural firms involved, and the landscape contractor — Wetmore’s Landscaping, Sod and Nursery of Fredericton, NB.

     The awards were presented by Milton Sherwood, Minister of Agriculture and Peter Forbes, MLA- Fredericton North, representing The Minister of the Environment, and expressed the Province’s support for this outstanding conservation initiative.

Taking the initiative
Watching the best farmland disappear at an alarming rate and seeing the stripping away of perfectly good topsoil in highway and housing construction prompted the New Brunswick Horticultural Trades Association to explore ways to conserve this precious resource.

     After the provincial government proposed an immediate ban on topsoil stripping, the NBHTA initiated studies to explore the alternatives. As a result, the association undertook the first major recovery project in the province for which they received no subsidies.

     Following the apparent success of this initiative, the NBHTA has proposed three conservation demos on smaller (housing) sites this summer, and is now lobbying for the ban on topsoil stripping to force conservation and preserve at least a small measure of the world’s future food production resource.