Since most building sites start out as raw land, the first step in site construction work usually involves the grading of the site. Grading basically consists of the cutting or filling of the ground in order to create a level building pad upon which the foundation and structure can be built. The three types of level building pads that are created by the grading operations are cut lots, cut-fill transition lots, and fill lots as illustrated in Fig. 1.
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Figure 1 Three types of building pads created during the grading operation.
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Grading Operation
The typical steps in a grading operation are as follows:
1. Easements. The first step in the grading operation is to determine the location of any on-site utilities and easements. The on-site utilities and easements often need protection so that they are not damaged during the grading operation.
FIGURE 2 A large mechanical grinder has been brought to the site and the trees and brush
are being ground-up into wood chips. The wood-chips will be removed from the site and then recycled.
2. Clearing, brushing, and grubbing. Clearing, brushing, and grubbing are defined as the removal of vegetation (grass, brush, trees, and similar plant types) by mechanical means. This debris is often stockpiled at the site and it is important that this debris be removed from the site and not accidentally placed within the structural fill mass. Figure 2 shows one method of dealing with vegetation, where a large mechanical grinder has been brought to the site and the trees and brush are being ground-up into wood chips. The wood chips will be removed from the site and then recycled.
3. Cleanouts. Once the site has been cleared of undesirable material, the next step is the removal of unsuitable bearing material at the site, such as loose or porous alluvium, colluvium, and uncompacted fill.
4. Benching (hillside areas). Benching is defined as the excavation of relatively level steps into earth material on which fill is to be placed. The benches provide favorable (i.e., not out-of-slope) frictional contact between the structural fill mass and the horizontal portion of the bench.
5. Canyon subdrain. A subdrain is defined as a pipe and gravel or similar drainage system placed in the alignment of canyons or former drainage channels. The purpose of a canyon subdrain is to intercept groundwater and to not allow it to build up within the fill mass.
6. Scarifying and recompaction. In flat areas that have not been benched, scarifying and recompaction of the ground surface is performed by compaction equipment in order to get a good bond between the in-place material and compacted fill.
7. Cut and fill rough grading operations. Rough grading operations involve the cutting or excavation of earth materials and the compaction of this material as fill in conformance with the grading plans. The location of the excavated earth material is often referred to as the borrow area. During the rough grading operation, fill is placed in horizontal lifts and then each lift of fill is compacted to create a uniformly compacted material such as shown in Fig. 3.
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FIGURE 3 A lift of fill has been placed and compacted into a dense state. |
Other activities that could be performed during rough grading operations are as follows:
a. Ripping or blasting of rock. Large rock fragments can be removed from the site or disposed of in windrows. Seismic wave velocity can be used to determine if rock is rippable or nonrippable. Figure 4 shows a Caterpillar D10 tractor/ripper that can be used to excavate rock.
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| FIGURE 4 A Caterpillar D10 tractor/ripper that can be used to excavate rock. |
b. Removal of rock fragments. Large rock size fragments interfere with the compaction process and are usually an undesirable material in structural fill. The large rock size fragments may become nested, creating open voids within the fill mass. Figures 5 and 6 show one method used to remove large rock size fragments. A screen is set up as shown in Fig. 5 and then a loader is used to dump the material on top of the screen. As shown in Fig. 6, the large rock size fragments roll off of the screen while the material that passes through the screen is used as structural fill.
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FIGURE 5 A screen has been set up in order to remove large-size rock fragments from the soil.
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FIGURE 6 A loader is in the process of depositing material on top of the screen in order to
separate the large size rock fragments.
c. Cut-fill transition. Figure 1 illustrates a cut-fill transition. It is the location in a building pad where on one side the ground surface has been cut down exposing natural or rock material, while on the other side, fill has been placed.
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| FIGURE 7 Utility trench excavation for a storm-drain line. |
d. Slope stabilization. Examples of slope stabilization using earth materials include stabilization fill, buttress fill, drainage buttress, and shear keys. Such devices should be equipped with back drain systems.
e. Fill slopes. During the grading process, fill slopes can be created out of earth materials. Figure 1 shows the construction of fill and cut slopes.
f. Revision of grading operations. Every grading job is different and there could be a change in grading operations based on field conditions.
8. Fine grading. Fine grading is also known as precise grading. At the completion of the rough grading operations, fine grading is performed in order to obtain the finish elevations that are in accordance with the precise grading plan.
9. Slope protection. Upon completion of the fine grading, slope protection and permanent erosion control devices are installed.
10. Trench excavations. Utility trenches are excavated in the proposed road alignments and building pads for the installation of the on-site utilities. The excavation and compaction of utility trenches is often part of the grading process. Once the utility lines are installed, scarifying and recompaction of the road subgrade is performed and base material is placed and compacted. Figures 7 and 8 show trench excavations for the installation of storm drainage systems. The trenches should be either sloped or shored in order to prevent a cave-in.
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| FIGURE 8 Another example of a utility trench excavation for a storm-drain line. |
11. Foundation construction. Although usually not a part of the grading operation, the footing and foundation elements can be excavated at the completion of grading in accordance with the foundation plans.
Most projects involve grading and it is an essential part of geotechnical engineering. For many projects, it is usually necessary to prepare a set of grading specifications. These specifications are often used to develop the grading plans, which are basically a series of maps that indicate the type an
extent of grading work to be performed at the site. Often the grading specifications will be included as an appendix in the preliminary or feasibility report prepared by the geotechnical engineer.