Machine Base Chemical Grouting

Chemical Grouting for Machine Base Stabilization, Chesapeake, Virginia

Structural Engineer: Gary W. Funaiock, PE, G. G. Cornwell & Company, Newport News, VA www.cornwellco.com

Geotechnical Engineer: Jon W. Ebbert, PE, McCallum Testing, Chesapeake, VA

General Contractor: Crofton Industries, Portsmouth, VA, www.croftondiving.com

Problem:

Three gas compressors sitting on a 24 inch thick slab inside of a metal building were vibrating violently while operating.  The structural engineer identified multiple potential causes of the vibration; the slab is considered inadequately thick by the compressor manufacturer, the mechanical connections between the battered pilings supporting the slab were deteriorated, there were no isolators between the unbalanced compressors and slab, and the soil below the slab had settled, creating a void under the slab of up to four inches deep, which was completely full of water.

The band of soil identified as problematic was approximately two feet thick, immediately under the slab, with DCP blow count of 0.  The band of soil starting at two feet below the slab bottom had DCP blow counts of 10-20 per 1 3/4 -inch increment.  The soil was classified as “silty fine sand with traces of clay, wet, very loose, SM”

The slab, which was 17 feet by 35 feet, had not settled, indicating that the pilings were sufficient to bear the load of the slab and compressors.  However, because the top two feet of pilings were surrounded by loose, saturated soil, they lacked lateral support to dampen any vibration from the compressor operation.

When one compressor was running, the slab would vibrate.  When two compressors were running, the building would vibrate, and when all three compressors were running at the same time, the entire facility would vibrate to the point that computer screens in buildings more than 50 feet away were unreadable.

Constraints:

The compressors area used for refrigeration compression of propane for large storage tanks at a ship-to-ground-transfer facility.  It was possible to place the compressors out of service, but chemical grouting work had to facilitate immediate use of compressors as needed in the case of an emergency

Work had to be completed using minimally-disruptive equipment in order to minimize disruption to operations of other facility components, and interior work required a fire watch.

Due to the extensive water below the slab and sensitive nature of equipment inside of the building, dewatering had to be performed in order to help control the movement of water being displaced by material installation.

Solution:

Two different repairs were proposed in conjunction to stabilize the slab and reduce the vibration; high density polyurethane grouting and soil modification (chemical grouting).

After dewatering the void under the slab through four-inch diameter holes which from soil study/geotechnical engineering investigation,  hydroinsensitive high density polyurethane foam was injected into the void under the floor.  Placement was done  through 5/8” holes, in a sequence designed to displace water towards the dewatering points.   The hydroinsensitive foam was able to displace water out of the void as it expanded under the slab.

After void filling was completed, single component polyurethane resin chemical grout was injected into the band of loose soil.  Polyurethane resin chemical grout is designed to react with existing moisture in the soil, while also displacing water from completely saturated soils.  Injection pipes were placed through the existing 5/8-inch holes drilled through the slab for void filling, to a depth of 15 inches below the slab bottom.   The layer of soil below the treated band was considered to be considerably less pervious due to its much higher DCP blow count, so was treated as a the limiting depth for calculating material volume for each injection point.

Chemical grout installation was started on one end of the compressor slab and progressed across the long dimension of the slab.  Chemical grout injection displaced considerable amounts of water from the saturated layer of soil.  The displaced water vented out from the adjacent injection holes and from under the sides of the building (turned down slab) sometimes up to 40 feet away from the injection points.

Results:

Less than 24 hours after chemical grout injection was completed, the compressors were returned to normal service, and the operator indicated a 70 percent reduction in vibration.  The layer of soil was impenetrable by a 5/8-inch carbide tip hammer drill.