The Great Soil Debate: Understanding competing approaches to soil design ASLA Annual Meeting September 20, 2009

Transcription

1 The Great Soil Debate: Understanding competing approaches to soil design ASLA Annual Meeting September 20, 2009 Introduction Over the last 20 years soils design has advanced radically from using stripped loam from untested and unsustainable sources. Today landscape architects are using highly specified soils for high use turf, passive turf, planting beds, steep slopes, structural soils, etc. that need to perform in intensive environments. Yet there is no settled science in this field. Landscape architects approaching the question are faced with competing approaches to soil design. Three leaders in the field will advocate for competing design approaches giving landscape architects the tools they need to make educated and sustainable choices. For landscape architects the questions abound: How are we to know which approaches work best for our specific applications? How are we to make choices between approaches? And how are we to present these choices intelligently to clients, regulators, and design review panels? What is a sustainable soil? With the Sustainable Sites Initiative in its second draft form, the issue is front and center. Pending guidelines have the potential to direct landscape architects toward one approach or remain open to multiple systems. What are the next steps in this developing science? Through case studies, science-based evidence of long term performance, and an open discussion of cost, constructability, and sustainability, landscape architects will gain the knowledge required to make choices, lead a collaborative design process, and advocate for the soils design to owners, regulators, and contractors. Presentation Outline The first portion of the session will be the three primary panelists giving the audience a general understanding of how they approach each design problem. They will then present a case-study describing how their approach was applied in a real-world situation. The second portion of the session will be a moderated discussion focusing on assessing the comparative values of each approach in different situations. The session will close with questions and answers from the audience. 1) Panelists Presentations:

2 The Great Soil Debate: Understanding competing approaches to soil design page 2 i) Two strategies for improving compacted soil: amending existing soil with organic matter to create planting beds, and mixing uniform gravel and soil to create CU-Structural Soil for planting under pavement. Nina Bassuk, Cornell University, Ithaca NY, NLB2@cornell.edu Description of the problem When do we need blended soils? When there is no existing soil or when soil structure is damaged, soils need to be blended with materials that will allow them to support the various functions they are asked to perform (e.g. compaction resistant sports turf, shrub planting beds, street tree soils under pavement). Soil compaction is the most commonly found and most difficult soil condition to overcome. The delicate balance between micropores and macropores is disrupted reducing drainage and aeration as well as preventing roots from growing through the soil. It is important to understand that remediating compacted soil must address two issues: the high soil density that limits root growth, and a lack of adequate drainage in the root zone. Testing Root-limiting bulk density in sandy soils ranges from 1.5 g/cc in well-graded sandy soils to 1.7g/cc in coarse or uniformly graded sandy soils. For clayey soils, root-limiting soil density is approximately 1.4g/cc. You can do a field test on site to know if and where soil needs remediating. (1) It would also be wise to test drainage (percolation) on site at different depths to determine if the subgrade and surface soil layers drain. Using field perc. methods, 2-4 drainage per hour is satisfactory. (1). Anything less than this may require soil amendment and/or the placement of underdrainage to move excess water. Even after supplemental drainage is added it may be necessary to choose plants that will tolerate periods of wet soil conditions. Creating macropores where there are none using compost or bulk organic matter for planting bed soils. How much compost must be added to a soil to reduce soil density even after expected recompaction? In compacted sandy soils a minimum of 25% compost by volume (1 part compost to 3 parts sandy soil) must be mixed into the soil to reduce soil density below root limiting levels and improve drainage. If the soil is more loamy than sandy, at least 33 % compost amendment is needed (1 part compost to 2 parts loamy soil). In clayey soil 50% compost amendment is necessary to reduce the effects of compaction (1 part compost to 1 part soil). Mix organic matter and soil either on site with a deep ripper, asphalt mill or small excavator to or off site with a front-end loader. Do not use a rototiller (too shallow and it destroys soil structure). It is critical to assess the subgrade to determine if it will drain. If it doesn t drain at least 2-4 per hour, use subsurface sculpting and underdrainage, French drains or sumps to allow water at the interface of the amended soil and the existing subgrade to move away from the root zone. Always amend a site, never a hole. Calculate soil volume necessary to support design-size plants (1.) Place plants to share improved soil in a common bed. For small to medium shrubs inches useable soil depth is adequate. For large shrubs and trees, of soil depth is preferable. To replenish organic matter in these high organic matter beds, add 2 of mulch (bark chips, compost) to the beds every year.

3 The Great Soil Debate: Understanding competing approaches to soil design page 3 Use of CU-Structural Soil to grow trees surrounded by pavement. The need for a load-bearing soil under pavement gave rise to the development of CU-Structural Soil, a blended soil that can be compacted to 100% dry density (Proctor density or modified Proctor density) to bear the load of a pavement while allowing tree roots to grow through it. Previously, soils compacted to meet engineering specifications for load bearing restricted tree root growth. The concept behind it CU-Structural Soil is a mixture of crushed gravel and soil with a small amount of hydrogel to prevent the soil and stone from separating during the mixing and installation process. The keys to its success are the following: the gravel should consist of crushed stone approximately one inch in diameter, with no finer particles, to provide the greatest porosity. The soil needed to make structural soil should be loam to clay loam containing at least 20% clay to maximize water and nutrient holding capacity. The proportion of soil to stone is approximately 80% stone to 20% soil by dry weight, with a small amount of hydrogel aiding in the uniform blending of the two materials. This proportion insures that each stone touches another stone, creating a rigid lattice or skeleton, while the soil fills the large pore spaces that are created by the stone. This way, when compacted, any compactive load would be borne from stone to stone, and the soil in between the stones would remain uncompacted. (2) CU-Structural soil uses the concept of uniformly graded sands and supersizes the sand to accommodate large tree roots. In our experience the use of a uniformly graded sand /soil mix cannot be compacted to 95% Proctor density without limiting tree roots. How is it used? CU- Structural Soil requires a large volume of soil under pavement, approximately 2 cubic feet of soil for every square foot of envisioned crown diameter. We recommend a 36 soil depth, although several projects have been successful using as shallow as 24. We would not recommend any less than 24. CU-Structural Soil has an available water holding capacity between 7% and 12% depending on the level of compaction. This is equivalent to a loamy sand or sandy loam. (See the table below for soil volume recommendations). Because of its well-drained nature, trees that prefer well-drained soils do best in CU- Structural Soil. Depending on the stone type used to make it, the ph of the soil may be affected (e.g. limestone vs. granite). Good tree selection practices and establishment procedures should be used with CU- Structural Soil as would be done with any tree installation. It is important to maximize the water infiltration through the pavement to replenish CU-Soil as with any soil. Unfortunately, designers often do no allow for adequate water infiltration or trunk growth when planting in a paved surface. Although CU-Structural Soil is made of readily available local crushed stone and soils, it is essential to make it correctly. To insure quality control, CU-Structural Soil is made by licensed producers who make it according to its specification all over the country (75 producers currently). Samples from the licensed producers are tested at Rutgers University for compliance. Over 1000 CU-Structural Soil projects have been installed successfully all over the US, Canada and Puerto Rico during the past 12 years.

4 The Great Soil Debate: Understanding competing approaches to soil design page 4 Figure 1. Cross-section of typical tree installation into CU-Structural Soil under pavement from curb to building face. Note where the tree pit is open, topsoil should be placed around the tree ball, but CU- Structural Soil should be placed under the ball to prevent tree ball subsidence. Soil volumes necessary to support trees in Midwest or Northeast US without irrigation after establishment Tree size Small-medium tree, Crown diameter 20 Crown projection, (square feet) Available water 8% (CU-Soil) cubic feet, (16 cu yards)* Available water 12% (Sandy loam) 400 cubic feet, (15 cu yards) Available water 15% (Loam) 325 cubic feet, (12 cu yards) Large Tree, Crown diameter cu. feet* (44 cu. yards) 910 cubic feet (34 cu. yards) 725 cubic feet (30 cu. yards) *420 and 1175 cubic feet of CU-Structural Soil assumes loam soil will be placed around the tree ball, but not under the ball in the pavement opening of 7 x7 or 5 x 10. CU-Structural Soil only be used under adjacent pavement. If CU-Structural Soil were used in the pavement opening and under the pavement, the total amount of CU-Structural Soil would have to be raised to 600 (22 cu. yards) and 1363 (50 cu. yards) cubic feet, respectively. References. 1.Trees in the Urban Landscape, 2004, Peter Trowbridge and Nina Bassuk. John Wiley 2.Cornell Urban Horticulture Institute Structural Soil website:

5 The Great Soil Debate: Understanding competing approaches to soil design page 5 ii) The Case for Blended Soils as Planting Media Robert Pine, PE, MLA The Nature of Planting Soils Natural topsoil is the product of geologic and biological processes over millennia or longer. Plants have developed in response to the characteristics of topsoil, yet many topsoil resources are not well suited to modern urban landscapes where there are intense plantings and heavy use. Quality topsoil is a rare commodity in many urban markets and, where available, sometimes comes from stripped agricultural resources. In many markets, the so-called loam is a mixture of random topsoil-like materials, Such uncontrolled mixes typically have very poor horticultural value and are extremely vulnerable to compaction. Blended soils create a means to stretch topsoil resources, thereby reducing market pressure on prime farmland soils. Blended soils improve the horticultural performance and the sustainability of soil environments and can be designed for special uses such as structural soil, high use turf, wetlands, wetland transition zones, bioretention, and rooftop plantings. Why Soils Fail While there are many reasons for failed soils, three of the most common problems are the following. Compaction is the bane of soils and leads to poor rooting, reduced infiltration and internal drainage, and a host of interrelated problems. While compaction is the result of excess forces applied to the soil, vulnerability to compaction is a function of grain size distribution. Well-graded soils, including most natural topsoils, tend to compact easily, while uniformly (poorly) graded, sandy soils tend to resist becoming over-compact. Excessive fines, or loss of soil structure, cause poor aeration, low infiltration rates, wet soils and poor plant performance. Macropore space is critical to soil performance and is created primarily by adequate sand content in sandy loams and crumb structure (peds) in soils with high clay contents. Anaerobic conditions cause ongoing saturated soils and, consequently, planting disasters. Anaerobic conditions develop when there is inadequate oxygen in the soil, usually due to extremely wet conditions or excessive compaction. Once established, anaerobic microbes exude products to change the soil environment so that it retains excess moisture and thus allows them to continue to dominate the soil behavior. Designing Blended Soils The design of blended soils involves varying the percentages of topsoil, sand and compost in a mix in order to 1) match soil performance with horticultural and use requirements, 2) prevent soils from failing, and 3) create sustainable conditions. Components of Blended Soils Natural Topsoil provides silt and clay particles, natural soil biology, and stable organics. Increasing the topsoil content in a mix improves water and nutrient holding capacities but excessive amounts make blended soils more vulnerability to compaction and decreased infiltration and aeration. Uniform Sand provides the primary physical structure for a soil blend. The uniformity of the sand particle size distribution, expressed by a coefficient of uniformity, is critical to influencing soil performance. Increasing the sand content in a mix increases macropore space, aeration, infiltration and permeability and provides resistance to compaction, but reduces water and nutrient holding capacities. Compost provides organic material for a mix when the organics in the topsoil are not adequate. Increasing the compost content in a mix increases water and nutrient holding capacities, provides beneficial soil biology, improves soil structure and therefore aeration and is a source of nutrients. Compost at high concentrations can result in settlements and can increase vulnerability to anaerobic conditions if the soil has poor aeration, especially if the compost is not adequately mature.

6 The Great Soil Debate: Understanding competing approaches to soil design page 6 Soil Design Criteria For any given soil design, there are a variety of interrelated criteria that must be considered. The primary criteria are normally the following. Soil physical structure is governed by particle size distribution. The presence of uniformly graded sand particles, or well-developed crumb structure (peds) in clay soils, improves many horticultural properties, including aeration and drainage. Sand provides resistance to compaction. Plant available water holding capacity increases most rapidly with increasing organic content, but also with increasing silt and clay content. Nutrients are available from certain mineral components and from organics. Nutrient holding capacity increases with increasing clay content and organic content. Quality Control and Placement For most projects approximate mix ratios are suggested but specifications control final grain size distributions, horticultural properties and saturated hydraulic conductivities at anticipated field compaction levels. Specified compaction ranges for planting soils are typically from 82 to 86 percent Standard Proctor with a minimum saturated hydraulic conductivity of 2 or 3 inches per hour and from 86 to 88 percent for high use lawns with a minimum rate of 4 inches per hour. Sustainability Natural Topsoil is a limited, non-renewable resource. Although excess topsoil may occur in some markets, when project requirements call for the use of large quantities of topsoil, stripping of agricultural resources, including prime farmland, can result. Typical soil blends contain 25 to 33 percent topsoil, and therefore reduce pressure on topsoil resources. Sand is generally an unlimited resource. Energy can be required for screening, if bank run sources re not available, and for trucking. Compost is produced from recycled waste organics. Maintenance requirements are reduced because blended soils are designed to meet targeted horticultural requirements. Water requirements are reduced by increasing infiltration rates and by designing water holding capacity to meet plant needs. Stormwater runoff is reduced due to increased infiltration rates. Plant lifetimes are increased by designing soils and planting environments to support mature plant development. Designing Lawn Soils Compaction resistance and deep rooting are primary design criteria for lawn soils, especially for high use areas. Lawn soils are therefore generally designed with relatively high sand contents. High sand contents also create good aeration, which tends to encourage deep rooting for grasses. Increasing the amount of compost in a mix will improve water holding capacity, but must be balanced against creating soils which remain wet after rain or irrigation and result in rapid wear if then subjected to moderate or high use. A typical mix ratio for a high use lawn is 3 parts sand to 1.5 parts topsoil to 1 part compost. A typical mix ratio for a passive use lawn is 2 parts sand to 1.5 parts topsoil to 1 part compost. Designing Planting Bed Soils Optimal horticultural support for plantings is typically the highest priority criterion in designing soils for planting beds. Most planting beds are designed with a two-layer soil system with a higher organic soil in the upper foot and lower organic, horticultural subsoil for deeper soils. A typical mix ratio for the upper, planting bed soil is equal parts of sand, topsoil and compost. A typical mix ratio for horticultural subsoil is equal parts sand and topsoil. On-site mineral soils can sometimes be blended with compost to create horticultural subsoil. However, if the mineral soil is well-graded it will be vulnerable to becoming over-compact. If it is too fine-grained, it is likely to be poorly drained.

7 The Great Soil Debate: Understanding competing approaches to soil design page 7 Designing Sand-Based Structural Soil Sand-based structural soil is a system for providing an appropriate rooting medium for trees which can also support pavements. A typical profile consists of pavement, underlain by several inches of crushed stone, underlain by two to three feet of sand based structural soil, underlain by a drainage system. Aeration pipes are placed within the crushed stone layer to create an air/soil interface at the top of the structural soil. An irrigation system, typically drip irrigation within the aeration pipes, or harvested stormwater distributed though the aeration pipes, provides moisture and nutrients as needed. The structural soil medium consists of very uniform medium to coarse sand, mixed with lesser quantities of compost and topsoil. The uniformity of the mix gradation allows a high level of compaction but which also permits root penetration. Sand-based structural soils can be compacted to 95 percent proctor and typically have a minimum saturated hydraulic conductivity of six inches per hour at that density. A typical mix ratio is 4 parts sand to 1 part topsoil to 1 part compost. Root growth of eighteen inches per year into compacted structural soil has been documented. Successful street tree plantings on numerous projects, including the Boston Central Artery, have repeatedly demonstrated the success, flexibility and cost effectiveness of the system.

8 The Great Soil Debate: Understanding competing approaches to soil design page 8 iii) Natural soils as planting medium or the primary component in a planting mix James Urban, FASLA, ISA Some very general guidelines What are natural soils? Existing site soils particularly upper ), A and B horizon soils but can include lower horizons soils that do not have excessively high or low ph or extreme clay, silt or sand or rock content. Collected soils harvested from development sites. Are natural soils more sustainable than high sand soil mixes? Higher water and nutrient holding capacity Low to no irrigation requirements Better supports xeriscape plant pallets, less fertilizer. Less damage to the land as often soil is harvested from a site already being developed. Less embodied energy to harvest and bring components together. Sustainable sites initiative credit for reuse of existing soil. Availability of soil? Usable natural soils are an available resource in most markets. LA needs to understand regional differences and soil building processes, talk to soil experts and soil suppliers in the area to understand available soil types Lower cost as more natural soil is added to mix Differences between natural soils and sand based blended soils? Natural soils and soil blends usually have Lower compaction resistance: develop compaction resistant designs! Higher water and nutrient holding capacity: less irrigation dependant Lower drainage rates: increase slopes on lawns and beds to improve drainage. Requirement for greater understanding of constructability limitations such as wet weather and over mixing Design compaction resistant landscapes rather than compaction resistant soils. Solve problems with sustainable design rather than sustainable products or specifications. Less lawn. Mound or depress planting beds, and use curbs to reduce pedestrian intrusion into beds. Assure adequate surface drainage. Avoid paving close to trees or use suspended pavements. Plant trees and shrubs on wider spacing and in larger planting beds. Calculate soil volumes at the preliminary design stage. Maintain macro pore space with soil ped retention in soil types with strong ped structure (sandy clay loam, clay loam and loam soils). Stop fine screening and over mixing of soil that breakdown peds. Accept sticks and stones 2mm 3 in soil up to 10% by volume. Natural soil and soil based mix types Soil with only surface compost addition. Often the best choice, but assure adequate drainage minimum 2% on lawns and 5-10% on beds to compensate for slower drainage within the soil. Soil and compost mixes plus additional surface compost application (max 15% compost by volume). Soil, sand and compost. (See mix ratio chart) Use as a last resort in soils with high levels of silt and or fine sands and weak ped structure. When adding sand, final coarse to medium size sand content in mix (sand in soil and added sand combined, must exceed 55%, higher in lawns.

9 The Great Soil Debate: Understanding competing approaches to soil design page 9 Compost Compost is not 100% organic matter! Humus organic matter in natural soils is much more stable that the organic matter in compost. Compost does not replace soil organic matter. Do not add large amounts of compost to the soil mix. 15% is a lot of compost. Save the compost for Natural soils as planting medium or the primary component in a planting mix the top 6 (up to 50% compost) installed after installation of the soil mix. Anticipate soil settlement after installation at least 10% of the total soil depth. Submittal and construction administration LA must be a part of the submittal and CA process. You have to be there and understand what you specified. LA s are in the construction business. Soil mixes where sand or compost is added to make a mineral soil submittal meet a topsoil spec must not be accepted. Require that base soils be harvested soils not mixes. Soils is an imprecise natural product, do not overly rely on testing data. Always look at the sample when evaluating soil and compost. Hydraulic conductivity at 80% compaction is the most critical data point, followed by ph and then organic matter. Other test data look for plant limiting extremes. Uses and mix ratios Soil Use Lawns - light pedestrian use Minimum Hydraulic conductivity at 80% of Mix components ratios (parts by volume) adjust so total equals 10 parts maximum dry density Soil with Coarse Compost (Proctor) Peds Sand 2 inches per hour Planting beds.75-1 inch per hour Bioretention beds including within suspended paving Soil within suspended paving 3-4 inches per hour inch per hour Planting bed soil over inches per hour structure Notes: 1. If soil is screened through less than a 2 mesh these numbers and ratio s are not applicable. 2. High aggregate stability (higher clay content) or soils with high amounts of combined medium to coarse sand in base soil (45% or greater) - Use higher soil numbers, 3. Higher silt or higher fine sand amounts in soil - Use higher sand numbers, 4. Smaller projects where soil can be placed without needing to pass over soil during installation - Use higher soil numbers. 5. Soil less than 18 deep - Use higher compost and soil numbers. Soils greater than 24 deep - Use no more than 1 part compost. 6. Organic matter as tested in final mix does not need to be above 5%. You are adding the important organic matter at the top of the soil section. 7. Mix loosely with loader bucked not a blending machine. 8. Assure subgrade layers drain. Break up subgrade compaction and soil interfaces before adding soil mixes. 9. Do not use a soil-blowing machine to install these soils 10. Soil designs for all applications except lawns requires tilling additional compost, 40-50% by volume, into the top 6 inches of the soil after installation. 11. Soil compaction not to exceed 80% maximum dry density. Set finished grades to anticipate approximately 10% of the soil depth additional settlement. Read Up By Roots: Healthy Soils and Trees in the Built Environment Amazon.com

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11 The Great Soil Debate: Understanding competing approaches to soil design page 11 2) Moderated Discussion a) Testing, evidence, and science b) Constructability c) Sustainability 3) Questions and Answers Speaker Bios Panelists: Nina Bassuk, Cornell University Nina Bassuk is currently a professor and program leader of the Urban Horticulture Institute at Cornell University. She is also a member of the Sustainable Sites Initiative vegetation committee and sits on the executive board of the New York State Urban Forestry Council. Along with co- author, Peter Trowbridge she wrote 'Trees in the Urban Landscape,' a text for landscape architects and horticultural practitioners on establishing trees in disturbed and urban landscapes. A native New Yorker, Nina's current work focuses on the physiological problems of plants growing in urban environments, including improved plant selections for difficult sites, soil modification including the development of 'CU-Structural Soil' and improved transplanting technology. Robert Pine, ASLA Bob Pine is a professional engineer and a landscape architect. He holds an MS in geotechnical engineering from Cornell University and an MLA from Harvard s GSD. As a principal of Pine and Swallow Environmental he has provided consulting services in soil and drainage design, horticulture and landscape construction for thirty years. Recent projects include: Asian University for Women in Bangladesh; The Highline, Brooklyn Bridge Park and Hudson River Park in New York City; Don River Park in Toronto; Crystal Bridges Museum in Bentonville AR; and sustainable streetscape projects at Harvard University and MIT. James Urban, FASLA James Urban, FASLA specializes in the design of trees and soils in urban spaces. He has written and lectured extensively on the subject of urban tree planting and has been responsible for the introduction of many innovations including most of the current standards relating to urban tree plantings. His 2008 book Up By Roots: Healthy Trees and Soils in the Built Environment, is becoming one of the principle references on tree and soil issues. In 2007 he was awarded the ASLA Medal of Excellence for his contributions to the profession and knowledge of trees and soils. Recent ASLA lectures include: 2007 Alternatives to Structural Soil for Urban Trees and Rain Water, and Successful Landscape Planting Techniques in Difficult Clayey Soils: Soil Amendments & Fertility; 2008 Healthy Trees and Soils, and Sustainable Sites Initiative - Vegetation and Soils, Draft Standards and Guidelines Moderator: Chris Moyles, ASLA Chris Moyles is an associate and senior designer at Reed Hilderbrand Associates Inc., with eighteen years of experience. He is a graduate of the University of Virginia with a Master in Landscape Architecture. Chris has taught at the Arnold Arboretum of Harvard University and the Boston Architectural Center. Chris has designed and implemented projects for many institutions including Massachusetts Institute of Technology, the Phoenix Art Museum, Cornell University, and numerous residential projects. Chris is a member of the American Society of Landscape Architects and the Boston Society of Landscape Architects. He is a registered Landscape Architect in the State of Massachusetts. Current projects include: The Parrish Art Museum in Southampton NY; The Chazen Museum of Art in Madison WI; The National Foundation for Poetry in Chicago; and Long Dock Park in Beacon NY.