Chapter 11 Site grading.
Grading is accomplished through a process called cutting and filling. Cutting is the removal of unneeded or undesirable soil from a location. Filling is the placement of appropriate soil where it is needed. While this concept seems simple, conditions exist that complicate the process. If soil with plant matter, such as turf, is reused when filling, it would later decompose and settle. The desire to save topsoil and place it in the area where it will benefit plants adds complexity to the cut and fill process. Respreading topsoil and smoothing the finish grade complicate the idea of cut and fill even further.
The amount of soil moved and the types of landforms required by the landscape plan will determine whether the project can be accomplished by the landscape contractor using typical landscape equipment, or whether the project should be subcontracted to a grading contractor with special equipment and expertise in large earth-moving projects. Determining the size of project that can be effectively graded with a skid-steer and dump truck is a matter of experience. It is not uncommon for a landscape contractor to undertake projects that require up to 50 cubic yards of earth moving (roughly enough soil to cover the floor of a two-car garage 3 feet deep). Because of the greater efficiency of heavy equipment, many landscape contractors subcontract larger projects to grading specialists. When considering the grading portion of a project, the decision should properly match a contractor's equipment and capabilities.
By reading and practicing the techniques described in this chapter, the reader should be able to successfully complete the following activities:
* Recognize the size and scope of grading required for a project
* Change the grade on a site.
RELATED INFORMATION IN OTHER CHAPTERS
Information provided in this chapter is supplemented by instructions provided elsewhere in this text. Before undertaking activities described in this chapter, read the related information in the following chapters:
* Safety in the workplace, Chapter 6
* Construction staking, Chapter 8
* Erosion control, Chapter 13
STEPS TO GRADING A SITE
Small projects offer the possibility of using a simplified grading approach of pushing the soil and sod away from a project and pushing them back when construction is completed. When major grade changes are required, a process that accommodates the orderly completion of all steps is necessary (Figure 11-1). Unless other specialized equipment is identified, each step can be accomplished by a skidsteer on a small site or by a bulldozer or scraper on a larger site.
[FIGURE 11-1 OMITTED]
Because vegetative matter will decompose over time, ground covers such as sod should be removed separately and either disposed of, composted, or stockpiled separately from topsoil for use in noncritical areas. The site should be stripped to a depth of approximately 4 inches to remove the most dense layer of vegetative matter. In some areas, this depth may be decreased or increased based on the type and maturity of the covering plant material. Any areas where tree stumps were located should be excavated with a backhoe to a depth of 24 inches in a 10 foot diameter circle centered on the stump.
Strip Topsoil and Stockpile
Following the removal of vegetative matter from the site, strip enough topsoil to cover all areas where topsoil is to be respread in the final grading step. Include an additional 15% in this stockpile to cover settling, erosion, and compaction losses. When calculating areas to receive topsoil, include all turf and ground cover areas as well as planting beds. A shortage of topsoil indicates that topsoil will need to be imported to cover planting areas. If possible, save all healthy topsoil from areas of a site that are disturbed. Due to the energy required for hauling and the damage done to harvest sites, it is preferable to recycle topsoil from the existing site rather than to import topsoil from outside sources. To achieve this goal it may be necessary to adjust the cut and fill areas to balance the soil available. Place this topsoil in a stockpile that is located away from construction areas, yet is easily accessible both for immediate needs and at the end of construction.
Excess topsoil can be stockpiled or, if low in organic matter, used to establish subgrade on the site. Most topsoils are stable enough to serve as the subgrade for berms, lawn areas, and other areas without permanent improvements. It is not recommended that soils high in organic matter be used as subgrade for paved areas or structures because of the possibility of soil settlement. Soils that are high in sand content or have poor structural qualities such as high shrink/swell clay should also be avoided as fill material.
Rough grading involves cutting and filling the site to establish proper subgrade elevations. This adjustment of grade should always be done on subsoils rather than by manipulating healthy topsoils. If topsoil still exists in an area where grades are to be changed, strip the topsoil before grading to new elevations. These elevations are obtained by subtracting the required topsoil layer, pavement and pavement base, or other nonsoil subgrade improvements from finish grade. Subgrade establishment does not require the precision of finish grading, but should be as accurate as possible to reduce grading work later in the project. Productivity can be increased in some situations by loading dump trucks for hauling rather than hauling soil in the bucket of slow earth-moving equipment. When using the skid-steer to excavate, use a bucket with teeth allowing penetration into the surface and making excavation easier.
Cutting will take place in all areas where the current grade is higher than the desired subgrade. This soil is often directly moved to areas that require fill. When filling, it is recommended that soil be placed in 6 inch lifts (layers) and compacted before more fill is placed. This provides more stability than a site that is compacted only after all fill is placed. Water may have to be sprayed lightly on lifts to assist compaction.
During this process, the contractor should also be aware of area soils that are unsuitable for construction. These areas appear wet, spongy, foul smelling, or differently colored. If such areas are encountered, the entire depth of unsuitable soils should be excavated and replaced with suitable soils. If the depth of unsuitable soils exceeds 24 inches, a soils engineer should be consulted to recommend a solution. The area should also be reviewed as to the cause of the soil problem. Tile installation may be required to remove excess water. Providing stable subsoil is important to the long-term stability of all paved areas and structures.
To minimize the amount of erosion after rough grading, terrace long slopes rather than leaving the site as a series of long exposed slopes. Grades can be adjusted to required elevations prior to respreading topsoil.
Installation of Temporary Erosion Protection
In most projects, the grading work halts after rough grading so that walls, paving, and other site improvements can be constructed. Some projects may be small enough to allow the grading process to proceed from beginning to end, but as the size and complexity of a project increases, grading must be completed in stages. If grading halts after rough grading, temporary erosion control measures (see Chapter 13) should be installed.
When the majority of heavy traffic over the site has ceased and all walls, pavement, and structures have been installed, grading operations can continue. Topsoil from the stockpile is removed and spread over the site to the required depths. Depths of topsoil vary depending on project specifications, but lawn areas typically require a minimum of 6 inches up to 12 inches. Planting beds may require a minimum of 12 inches up to 24 inches. The topsoil is laid in and roughly spread to the finish grade required by the site grading plan.
The final step prior to seedbed preparation or planting is to smooth the surface to the exact grades stated on the grading plan. This work is typically done with a grader or tractor-mounted blade. Hand work with shovels and rakes may be required in tight areas to accurately obtain desired grades. In cases in which no grading plan exists, finish grading will need to maintain proper elevations next to paved areas and permanent improvements and to achieve proper drainage direction and slope. At this stage, soil is often lightly compacted with a drum roller. Further surface manipulation, such as gilling or rough leveling, is performed as part of the preparation for turf and areas. Permanent erosion protection should be installed immediately after finish grading to protect the site.
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|Title Annotation:||Section 3 Grading, Site Drainage, and Erosion Protection|
|Publication:||Landscape Construction, 2nd ed.|
|Date:||Jan 1, 2005|
|Previous Article:||Section 3 Grading, site drainage, and erosion protection.|
|Next Article:||Chapter 12 Site drainage.|