Soil Stabilization/Ground Improvement

In Situ soil mixing utilizes hollow stem augers mounted on cranes or hydraulic excavators to mix dry or slurried reagents with soft subgrade soils for ground improvement to support foundations or other structures. The single auger soil mixing technique is capable of mixing 3 to 12 foot columns of material and is most often utilized for large area ground stabilization. Other applications, typically utilizing multiple auger systems, include deep cutoff walls and excavation shoring or diaphragm walls. As the augers rotate and advance into the soil, reagents are injected through the hollow drill shaft and mixed thoroughly with the soil to produce structurally improved or stabilized material. Forgen can assist clients with the development of specific column mixing patterns and grout mix designs (utilizing a variety of additives such as cement, CKD, fly ash, bentonite, hydrated lime, etc.) to achieve strength and other design parameters required for any given project.

Santa Ana, California

  • 650 lf of Sheet Pile Coffer Dam in Santa Ana River
  • Work at active plant amidst overhead and underground utilities
  • 9,000 cy of In Situ Stabilization (ISS) in rocky subgrade conditions
  • 700 ISS columns at 13 cy each
  • 5,000 cy of Jet Grouting
  • 699 Compaction Grout at 21 ft bgs
  • Installation of a 1,200 gpm de-watering system 
  • 5,000 cy of spoils disposal
  • 5,000 cy of imported fill

The Orange County Sanitation District is the fifth largest wastewater management agency in the country. Recent upgrades and improvements allow it to treat to secondary treatment standards. More than one-third of the secondary treated water is recycled by Orange County Water District, with the remainder discharged through the Pacific Ocean outfall.

This Phase 2 project was designed to complete repairs of the east and west Santa Ana River levees that were damaged during the original project and to install a pipeline between an existing pump station and the Huntington Beach Wastewater Treatment Plant in order to improve the reliability of the Sanitation District’s force main system in the City of Newport Beach. A portion of that work included tunneling under the Santa Ana River. Post-construction monitoring indicated the soil above the tunneled section of the pipeline settled more than anticipated necessitating stabilization efforts including compression grouting, jet grouting, and In Situ stabilization (ISS) on the plant site and on both Santa Ana River levees.

Prior to stabilization of the east and west levees, a 650 lf sheet pile coffer dam was constructed to facilitate sufficient dewatering to allow construction of a suitable working surface. Cofferdam sheet piles were installed to within one foot of the micro tunnel. Upon completion of the cofferdam installation, a dewatering system and treatment system capable of pumping and treating 1,200 gpm was installed. Discharge water from the dewatering operation was treated to a Total Dissolved Solids (TDS) limit of less than 20% above background. Once sufficient dewatering of the coffer dam enclosure was completed, Forgen imported and placed 5,000 cy of fill material to create a stable working platform for jet grouting operations required in the Santa Ana River.

Forgen performed the In Situ Deep Soil Mixing (DSM) using an ABI 18/22 drill rig with a 6 foot auger, a fully automated MPS 510 grout plant, a 50 ton low profile silo, a 150-ton reagent guppy, and an intricate computerized monitoring system to complete the scope of work. Our management used modern drafting and GPS technology to precisely layout the columns achieving 100% effective treatment. The computerized monitoring system, combined with GPS technology, allowed treatment of areas to within 1 foot of live utilities as well as real time quality control monitoring of pressure, verticality, flow, and auger speed (revolutions per minute). Upon completion of daily activities, data from the batch plant and the ABI rig were downloaded and utilized to generate required quality control (QC) reports.

The ISS specifications required a minimum of 250 psi unconfined compressive strength (UCS) at 28 days. Forgen developed a mix design that achieved this requirement with a minimum 85% core recovery. The In Situ stabilization effort was performed using a two-step process that began with the addition of 4% bentonite slurry on the initial pass to enhance cohesion between native soil and the cement solidification reagent and was followed by the addition of 18% cement by dry weight of soil during the second pass for each column. In several cases, the presence of rip-rap and other debris ranging between 12 and 33 inches in diameter at depths of up to 18 foot below grade, required pre-drilling or Forgen utilized specialty 6 foot mixing augers with a unique “egg beater” design that better facilitated mixing in this rocky subgrade. In total, 700 columns consisting of 9,000 cy of contaminated soil was completed using In Situ solidification.

Compaction Grouting was performed within the active wastewater treatment plant on a 10 foot grid pattern. In total, this effort required the construction of 699 holes up to 40 feet deep totaling 15,000 lf. Prior to installation of the compaction grout holes, the top 7 feet of the subgrade was surveyed and cleared for utilities and other obstructions using ground penetrating radar (GPR) and air knifing. During grouting operations, ground pressure, ground movement, and heave were monitored to ensure there was no damage to multiple utility corridors that were present in the work area.

Jet Grouting was performed to complete the tie-in of Phase 1 sections along the east and west levees adjacent to the micro tunnel and in areas where there were overhead utilities. A YBM GL-50CII grout machine was used for this activity, with jet grouting columns ranging from 10 to 50 ft below grade and totaling 5,000+ cy. Jet grouting specifications required a minimum of 250 psi unconfined compressive strengths at 28 days.

Northern Sacramento County, California

In 2010, Sacramento Regional County Sanitation District (SRCSD) completed the installation of 12 foot diameter sewer lines that would improve the sewer system in the Natomas area of Northern Sacramento County. The construction method used to install this sewer line was by micro-tunneling, also known as horizontal drilling. In locations where manholes were installed, the horizontal drill remained idle, loosening the surrounding soils due to vibration.

Particular locations that saw the greatest impact were at the Zone 3 and 18 manholes which began to form substantial subsidence sinkholes that could pose a significant public safety hazard. In 2012, SRCSD contracted Forgen for remediation services covering 100% of the area of concern using Deep Soil Mixing (DSM), or In Situ Stabilization (ISS), to accomplish the necessary ground improvement.

The Upper Northwest Interceptor Ground Improvement Project included In Situ Stabilization of approximately 650 cy of soil. Forgen developed a mix design that was to achieve a minimum of 20 psi in 28 days with a minimum 90% core recovery. All deep soil mixing was performed within one foot of live underground utilities including a pressurized 36 inch water line and a 12 foot diameter sewer line. Forgen performed the In Situ DSM using an ABI 14/17 drill rig with a 4 foot diameter auger, a fully automated MPS 100 grout plant, a low profile silo, and an intricate computerized monitoring system to complete the scope of work. Forgen management used modern drafting and GPS technology to precisely layout the columns achieving 100% effective treatment. The computerized monitoring system combined with GPS technology allowed treatment of areas to within one foot of live utilities as well as real time quality control monitoring of pressure, verticality, flow, and auger speed (revolutions per minute).

Due to environmental concerns related to the Giant Garter Snake, Forgen completed the base contract, along with a total of seven different field directives, within a very narrow work window. The difficulty of completing the work within a compressed time frame was made more difficult by the fact that Zones 3 and 18 are approximately five miles apart, requiring duplicate field efforts implementing best management practices to mobilize, protect areas adjacent to the work, conduct DSM operations, and perform final levee restoration. Despite the narrow work window and a five mile distance between the two work locations, the project was completed ahead of schedule and under budget. As a result of this success, SRCSD contacted additional phases utilizing this same method of treatment.

The second phase of the project was awarded in 2014 and involved remediation of an additional 19 zones, and included excavation and replacement of approximately 1,000 cy of soil and In Situ Stabilization of approximately 6,700 cy of soil.