The use of Geopier Rammed Aggregate Pier™ elements, Geopier� rigid inclusions and Rapid Impact Compaction (RIC) to support heavily loaded floor slabs (500 to 2000 psf) is a cost saving alternative to structurally supported floor slabs.  Much of the savings is the result of converting thick heavily reinforced slabs into thinner unreinforced or lightly reinforced slabs.

Design of the ground improvement system for floor slab support requires close coordination between the geotechnical engineer, structural engineer, and specialty geotechnical contractor.  The savings in concrete slab thickness and reinforcing steel for slabs supported with ground improvement often results in more savings than the difference in costs between ground improvement and deep foundations.  In order to realize all the savings from the use of ground improvement, a look at the entire system is needed (i.e., foundation support type, slab thickness, slab reinforcing, etc).

  

Ground Improvement vs. Pile Supported Slabs - Design Differences

Pile supported slabs transfer the entire slab load to the piles and require that the slab be designed as a pile cap at the pile locations.  The slabs need to have sufficient thickness and steel reinforcement because they act as a beam to transfer the loads structurally to each pile support. The RIC process takes soils with variable stiffness's and creates a uniform subgrade stiffness for support of slabs which are modeled as a series of equally stiff springs. For Geopier ground improvement the springs are stiffer near Geopier locations and less stiff as the distance from the pier increases.  This design model is close to a conventional soil supported slab, with the end result being a relatively uniform subgrade support model with limited slab deflection which results in low flexural stress within the slab.

 

You can imagine how the stiff springs in your bed's box-spring support the softer springs in your mattress, which support you uniformly.  A similar mechanism is applied here; the stiff RAP elements support the new structural fill "mattress", which supports the floor slab.  In areas where the structural soil "mattress" is thin, (higher flexural stress areas) the slab design is checked using a finite element analysis (FEA).  The final Geopier layout and structural fill thickness is determined when the flexural stress is below the acceptable levels for a given slab thickness.  Once the economical balance between Geopier spacing, fill thickness and slab thickness is determined, then the ground improvement design, slab design and control joint spacing can be finalized. 

 

 

 

 

 
    

Rapid Impact Compaction

 

Rapid Impact Compaction (RIC) is often used for slab support when the soil conditions consist of shallow loose granular soils.  RIC provides smart compaction by means of an on-board computer which records the stiffness achieved at each compaction point.  This data provides confirmation that a uniform stiffness has been achieved or alerts the operator when additional compaction is needed. Upon completion of compaction, a dense and uniform subgrade response is achieved.  The Naval Square Biddle Hall Annex and Townhomes, Philadelphia, PA  case study provides an example of a project where fill soils were improved with RIC and soft zones were identified which were ultimately improved with Geopier elements. 

 

Geopier Technology 

 

When you have soils that are quite variable, are fine-grained or contain peat or organics, which cannot be densified sufficiently in place with RIC, the use of Geopier RAP elements are the preferred technology.  The slab load is supported by relatively uniform subgrade support from a granular pad which transfers the slab loads to Geopier elements through arching.  The thickness of the granular pad and the spacing of the Geopier elements will depend on the floor loads and the capacity of the Geopier elements. (see December 2014 Newsletter - Load Transfer Platforms for Footings and Floor Slabs)

 

 

 

The modulus of subgrade reaction is highest at the pier location and reduces as you move away from the pier location. Piers are typically placed at control joints since the slab cannot transfer bending moments along joints.  The actual stresses in the slab can best be determined using finite element analyses (FEA) which results in a stress diagram as shown below. 

 

Designing the Slab and Slab Support System 

Some of the key input design variables that are needed to perform a comprehensive FEA analyses for slab support include: 

• Actual dead and live floor loads
• Presence of rack or other point loads
• Flexural strength which is a function of concrete compressive strength
• Floor slab control joint spacing
• Soil compressibility
• Type of ground improvement - RAPs vs. rigid inclusions vs. RIC
• Settlement tolerances
• Thickness of and type of new fill to be placed

Defining this data and running FEA to see which variables control costs is a key element in developing the most economical design in the pre-construction phase of a project. 

 

 

Grade Raise Fill, Organics, Urban Fill, Variable Loading conditions 

Each site has its own set of site-specific conditions.   Some special conditions include:

Organics and Peat Layers - The use of Geopier rigid inclusion elements or partially grouted Geopier elements may be needed to stiffen elements passing through peat and organic soils layers.  This was the case for the Seafrigo Cold Storage Warehouse in Elizabeth, NJ.  This 175,000 sf warehouse had floor slab loads as high as 550 psf for refrigerated space.  The geotechnical field investigation revealed a 2 to 12 ft thick layer of uncontrolled sandy fill underlain by a 2 to 12 ft thick layer of organic marsh deposits, underlain by dense sand. Geopier rigid inclusions were installed to provide floor slab support. In order to transfer the load from the rigid inclusion to the thin floor slab, a load transfer platform (LTP) was used which consists of a 2 to 3 ft thick layer of crushed recycled concrete grade raise fill with a 6-inch gravel pad.  An LTP consisting of a 12-inch thick ungrouted RAP was used for footing support.

Urban Debris Fills - Deep rigid inclusions (25 to 40 ft deep) can be used to span urban fills and organic soils to support footings and floor slabs.  This was needed at the Specfuel Waste to Energy Plant in Philadelphia, PA.  Urban fill and alluvual soils with organic silt layers overlaid dense sands along the Delaware River.  The depth to the dense sands varied up to 40 ft deep as you moved closer to the river.  For this project, the 6-inch thick slab was supported by a load transfer layer consisting of a 6-inch aggregate layer overlaying a 5 ft deep aggregate pier overlying the grouted Geopier rigid inclusion pier. 

(See January Newsletter on Use of Ground Improvement in Uncontrolled and Contaminated Fill Soils)

 Grade Raise Fills and Variable loads - The need to support both new fill and floor slab loads often requires high capacity piers to shed the high floor loads while maintaining economical pier spacing.  One of the benefits of the grade raise fill is that it can act as an excellent load transfer platform to spread loads and create a uniform soil subgrade for slab support. This approach was used on the PortSouth Bryla Warehouse Facility in Carteret, NJ to speed up construction sequencing while utilizing on-site quality fills.  The soils underlying this 460,000 sf building included variable silty sand uncontrolled fill and urban debris. Geopier Impact� System provided cost savings by supporting up to 5 ft of new structural fill in addition to the 600 psf floor slab load and enabled the slab to be designed as a standard 6-inch slab on grade.

Bottom Line

Ground improvement provides a more uniform soil support model that limits differential slab deflections and flexural stress in slabs so they respond like a slab on a relatively uniform soil subgrade.  This results in significant project cost savings by allowing thinner slabs and minimal steel reinforcement to be used even when poor soil conditions exist.