Construction Engineering Services
As we enter fall, project managers find themselves scrambling to get projects to a point where they don't need to perform a lot of grading in the winter. Earthwork tends to be more expensive and generally takes more time due to wet soils. Soil drying is hampered by the atmosphere’s reduced ability to absorb moisture at cooler temperatures, less intense sunlight, shorter days, and typically more often precipitation. Below are some practices to facilitate winter grading.
Typical summer practice is to grade a building pad to subgrade elevation. But during winter construction this practice can spell disaster. If construction equipment is permitted to operate on wet soils, the subgrade can become disturbed and softened, requiring undercut to prepare for slab construction. Grading a building pad with a crown 1-2 feet above the proposed subgrade helps to protect the slab subgrade from excessive moisture and disturbance. When the time comes to construct the slab, the excess wet/disturbed soils can be cut to the subgrade elevation often with little or no undercut required. This technique is especially cost effective if the building pad is “in cut”.
An alternative is to grade the pad to subgrade and immediately place a dense well-graded stone base such as “crusher run”, commonly referred to as “21A” in many areas. Compacted crusher run tends to shed water, whereas open-graded stone allows water to saturate the subgrade soils. However, compacted crusher run can hold water with prolonged precipitation.
It is good practice to establish a network of haul/construction roads. Without defined haul roads, construction equipment tends to run all over the site, softening soils and forming ruts that collect water. In order to protect compacted soils, the contractor should restrict construction traffic to defined haul roads.
Frequently soils are wasted because of frost, frozen soil, and excessive moisture content. Strategies to reduce the impact of weather during winter grading include: 1) Placement of soil over a large area to allow more time to dry the soil before the next lift is added. It is important that the soils are sealed with a smooth drum roller each day, and prior to precipitation. If not, a large area is exposed to excess moisture, thereby defeating the strategy. 2) Work high moisture content soils with lightweight placement and compaction equipment. Soil in the upper end of its compactable moisture range can sometimes be compacted more successfully using lightweight compaction equipment. Heavy equipment such as dump trucks, pans, and large compaction equipment can cause the soil to start “pumping” (moving) even if it was compacted and stable previously. Keep in mind that the project specifications for compaction must still be achieved regardless of equipment used.
Winter is a time when chemical stabilization of wet soils is often used. Lime can aid the drying of soils with fines, and can be used with some clays to reduce the plasticity of the clay. Portland cement can also be used to “dry” soil, and acts as a weak cementing/stabilizing agent. Properly employed, both lime and cement can be used to create nearly “weatherproof” subgrades.
Two cautions: 1) Sometimes fly ash is proposed as fill materials or as a stabilizing agent. Fly ash should not be used since there is no test that can reliably determine if the fly ash will swell over time. We have seen parts of large shopping centers demolished because of the swelling of the fly ash over time. 2) Sometimes contractors will propose thicker lifts of soil to “bridge over” softer soils. This practice results in poorly compacted soil at the bottom of the lift, and should not be permitted in structural fill areas.
Finally, there is no substitute for “good housekeeping”. The site should be sloped to promote drainage away from work areas. Uneven areas and wheel ruts should be filled in regularly to facilitate drainage.
In closing, winter grading can be more expensive and more time-consuming, but with careful planning and proper execution, costs and time can be reduced. We hope this Lessons Learned will help you manage earthwork performed in the winter.
ECS Corporate Services, LLC