Ground Improvement: Methods & Systems

Why is ground improvement necessary? 

Simply stated, not every project site is underlain by competent bearing soils. Sites with poor soils need ground improvement to change the conditions so that it becomes suitable to deliver better performance relative to project needs. Poor soils typically consist of loose sands, soft clays, organic soils, and undocumented fill soils. Even stiff clays and dense sands can be problematic for heavily loaded structures. The Geotechnical Engineer will work with the Structural Engineer and the rest of the project team to evaluate the various foundation options for support of the proposed structure and recommend cost-effective foundation solutions.

Ground improvement has many benefits including:

• Increased bearing capacity
• Decreased settlement
• Liquefaction mitigations
• Foundation stability
• Minimal spoil
• Less invasive/disruptive

Geopier  Impact® System in action

Surface densification

This consists of using specialized machinery to impart energy into poor soils to densify the material in place. The depth and magnitude of improvement  depends on the soil type and the amount of energy imparted into the soil. The end result is a densified mass of soil with enhanced settlement characteristics. Common surface densification methods include heavy surface compaction, rapid impact compaction (RIC), and deep dynamic compaction (DDC). If schedule allows, consideration can also be given to preloading the site with a large mound of soil to induce settlement of the poor soils. 

Surface densification works by creating a densified mass of soil which enhances the settlement characteristics

Aggregate piers

Poor soils can be reinforced by replacing/displacing the weaker soils with a densified column of aggregate generically known as an aggregate pier. In this process, aggregate pier elements penetrate weak soils to create dense columns of aggregate surrounded by a soil matrix. The matrix soils adjacent to the piers are also densified during pier installation. The end result is an improved mass of soil with enhanced settlement characteristics. Common types of aggregate piers include traditional vibratory stone columns and Geopier’s proprietary Rammed Aggregate Pier® solutions. 

Aggregate piers work by creating a rammed aggregate pier system that consists of densified columns of stone surrounded by a stiffened matrix soil

Rigid inclusions

Rigid inclusions are somewhat of a hybrid between aggregate piers and deep foundations. Rigid inclusions are generally very stiff, cementitious elements that transfer structural loads through weak soil layers and into competent bearing soils (much like a deep foundation). Rigid inclusions are installed by either drilling through or displacing the existing weak soils. The cemented nature of the rigid inclusion elements provides confinement and prevents bulging of the element into weak soils. This is particularly important for very soft clay and organic soil profiles. Rigid inclusions differ from deep foundations in that they are not structurally connected to the overlying foundation. Rigid inclusions are overlain by a gravel pad or load transfer platform and the footing and/or slab is designed as a conventional spread footing and/or slab-on-grade. 

Rigid inclusions work by creating a unreinforced grout column that is installed through weak soils

Soil mixing/grouting

Poor soils can be improved in place by mixing in admixtures such as lime or cement to create a mass with improved engineering properties. This is generally done by mechanically mixing the admixture directly into the soil. Grout can also be introduced to the soil mass to improve the engineering properties, typically by means such as permeation grouting, jet grouting, or compaction grouting. 

Poor soils can be improved in place by mixing in admixtures such as lime or cement

Removal and replacement

The engineering behind removal and replacement has not really changed throughout history. If your site has poor soils, you can simply remove them and replace them with better materials. It really does not get simpler than that, or does it? Certain site conditions can make this option impractical. Poor soils can extend so deep as to make removal and replacement cost-prohibitive or difficult, even with modern equipment. Shallow groundwater can necessitate a dewatering program. Adjacent structures can require protection by underpinning or installing sheeting and shoring. Environmentally impacted soils can greatly increase the cost of haul off and disposal. All of these factors can make removal and replacement a less than favorable solution. 

Remove and replacement works by digging out the poor soils and replacing with better soils 

Deep foundations

Deep foundations mitigate the concerns associated with weak bearing soils by bypassing them. The very stiff deep foundation elements take the loads from the structure and transfer them through the weak soils and into suitable bearing soils such as dense sands, hard clays, and bedrock. Modern technology has resulted in many different types of deep foundation options; drilled shafts, driven piles, ACIP piles, and micropiles, to name a few. This geotechnical solution provides excellent settlement control, particularly for heavily loaded structures. However, deep foundations are typically expensive (at least compared to other geotechnical solutions) and require structural slabs, pile caps, and tie beams. In addition, deep foundations generally require several large pieces of equipment on site, which can be problematic on small sites or sites with existing development. 

Deep foundations works by installing a structural column to support the structure and transfer the load to competent strata