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Maintaining Slab Subgrade

Methods to ensure a proper subgrade prior to concrete floor slab construction

By Ronald S. Lech, PE

During the construction of large slabs on grade, construction teams are faced with weather and construction traffic issues that can deteriorate the exposed subgrade prior to concrete floor slab construction. This is a particular concern where the sub-grade is a low-permeability clay or other fine-grained soil that may be sensitive to disturbance. There are several options available to owners, developers, and design and construction teams to maintain, or improve, the exposed sub-grade and subbase and keep your project on track.
A ticking time bomb

The construction of any large floor slab generally requires that the proposed building area be graded flat. This creates a situation of poor drainage that can lead to deterioration of the soil sub-grade, and even the aggregate base course, if these materials are exposed during wet weather. Likewise, construction traffic on the exposed sub-grade and aggregate base course can cause pumping and rutting of soils that are sensitive to disturbance from repeated rubber-tired traffic. This can happen even after the soil subgrade has been properly prepared and compacted. These problems can show up on any size project but are usually more pronounced on large warehouse-type jobs where the floor slab size requires multiple slab pours that may take weeks to complete. Time is often the enemy for exposed soils—the longer the soil and/or aggregate base course is left exposed, the higher the chance that undercuts or additional conditioning will be required prior to slab placement.
Wet subgrade during tilt-up panel construction. Photos: Terracon Co.

Because the extra undercut or conditioning work may cause delays that could ultimately end up being costly in terms of construction dollars and schedule, it is important to have a plan in place to protect these materials prior to achieving subgrade elevation during the mass earthwork phase. The following are several options to consider that maintain the integrity of the subgrade and keep your project on schedule.

* Chemical stabilization often is used to strengthen or make the upper subgrade soils more resistant to weather and traffic. Depending on the type of soil at your site, this could mean mixing lime, cement, or other additives with the upper 8 to 16 inches of soil below subgrade elevation prior to placement of the aggregate base course. If the process is done properly, it may be possible to achieve a higher modulus of subgrade reaction in the stabilized soil that could reduce the aggregate sub-base and/or slab thickness to offset some of the additional cost. It is important to work with the project geotechnical engineer to understand how the proposed chemical stabilization will affect the subgrade soils. During cold weather, lime can temporarily make the soil more permeable, thus creating even greater risk of subgrade deterioration during wet weather. Cement often sets up quickly and requires the contractor to be prepared to immediately fine grade the stabilized building pad. It should be noted that successful chemical stabilization can create a hard crust making excavations for utilities and foundations more difficult.
* Geosynthetics can be incorporated at the subgrade/subbase interface to provide additional strength and separation of the aggregate subbase and soil. Although both geogrids and woven textiles can be used, this is often a good application for a high-strength woven geosynthetic.
* Drainage can be improved by crowning the subgrade or installing finger drains across the building pad. Due to the large size of many warehouse-type slabs, the option of crowning the subgrade may be unattractive because this can require several inches of additional aggregate at the edges of the building pad to make the subbase level. Consideration also can be given to creating a network of shallow drains to control rainwater and prevent ponding. One source of drainage can be any interior utility trenches that are backfilled with free-draining aggregate and sloped for positive drainage. Additional finger drains can be constructed between planned utilities to provide adequate drainage. Often, it is necessary to have drains at a maximum 50 feet center-to-center spacing to provide sufficient drainage for a large, flat building pad.
* Staged construction can be used by only preparing as much subgrade at a time as the contractor can readily follow behind with slab pours. It also may be advantageous to cut or fill the sub-grade several inches higher than proposed grade in areas that will sit exposed. The high areas can later be cut to grade when slab preparations are imminent. These options minimize the exposure time for the subgrade soils but can create scheduling conflicts if multiple contractors are working on the site.
* Thickened aggregate base course can be considered above the soil sub-grade to resist weather and construction traffic. When using a thicker aggregate subbase, it is important to watch the fines content in the aggregate. Even though some state transportation departments allow up to 15% fines for typical dense-graded aggregate, many aggregates become sensitive to moisture and traffic when the fines content is above 10%. There are instances when a contractor thought he was buying additional weather “insurance” by increasing the subbase thickness with additional dense-graded aggregate—only to find that the high fines content resulted in a subbase that was as much (or more) sensitive to wet weather and construction traffic as the soil he was trying to protect.
* Frozen subgrade can be a concern for projects in northern climates. If the project area is too large to enclose and heat, it can be difficult to keep the subgrade from freezing when temperatures consistently dip below the freezing mark. In cold weather, it may be advantageous for the earthwork contractor to work 24 hours a day to avoid having to strip a layer of frozen soil at the beginning of the next day. Consideration can be given to placing a “sacrificial lift” of material over the prepared subgrade. This sacrificial material is allowed to freeze at the end of each work day and is then stripped to reveal unfrozen material. Concrete curing blankets can be used to protect the subgrade, and some blankets contain heating mechanisms that can thaw frozen subgrade.
Right: Shallow sump pits to remove water from aggregate base course.
Having a plan B

When all else fails, or the construction budget and schedule do not allow for the above listed preemptive strikes to be used, you still have options to repair a wet or disturbed building pad. If the soil subgrade is exposed and the aggregate subbase has not been placed, the typical methods include discing and aerating, undercutting and replacement, or chemical stabilization. If the aggregate base course has been placed, there are a few techniques that have been successfully implemented to save the pad without completely undercutting and starting over. If wet weather is the culprit, it may be possible to excavate a series of shallow sump holes and pump the water out of the aggregate base before it has a chance to soak into the soil subgrade. The sump holes usually are excavated adjacent to any interior column pads and at midpoints between the columns with the sump holes at maximum 20-foot spacings. This can be labor intensive, but if done immediately after a large rain event, it can get enough water out of the pad to save the sub-base.

Other techniques of drying the aggregate, such as windrowing and aerating the stone, can be considered if the aggregate subbase is thick enough but this could result in excessive mixing with the underlying soil subgrade. Sometimes the wet aggregate subbase and the uppermost soil subgrade can be mixed together and chemically stabilized in-place to avoid wasting the wet stone. This can create a firm subgrade but often at the loss of the aggregate subbase’s ability to provide drainage, cushion, and a capillary break. It also becomes very critical that the stabilized material be accurately fine graded to avoid variations in slab thickness.

If there is a light to moderate rain event, it is good practice to dig several shallow test holes with a shovel or hand auger to determine the wetness of the aggregate subbase and soil subgrade. Many times the rainwater has only wetted the stone and the upper fraction of an inch of the soil subgrade. Although this amount of moisture may lead to excessive rutting if the concrete is directly end-dumped with concrete truck traffic on the subbase, it may be possible to use a pump truck and successfully complete the slab pour by keeping traffic off of the pad.

Hopefully these techniques can provide food for thought for your next large floor slab preparation. It is important to discuss your game plan with the project geotechnical engineer, as well as the design and construction teams, to confirm that the planned approach is consistent with the project constraints.