C+S March 2021 Vol. 7 Issue 3 (web)

one of the modes of improvement, while other systems may provide two or three of the modes of improvement. Densification is used to improve granular soils such as sand or gravel and is typically achieved by imparting vibration into the ground to prompt the granular soil grains to rearrange into a denser state. Den- sification can also result from the displacement of granular soils by elements that are drilled, vibrated, or grouted in place. Densification, also referred to as compaction, reduces compressibility, and increases shear strength, leading to improved bearing properties and reduced settlement of the treated soils. Reinforcement is achieved by installing relatively stiff elements within soft, weak, or variable layers. The elements typically are installed by drilling, vibrating, or driving a casing or probe and filling the resultant void with grout or compacted stone to form a column. If the receiv- ing ground is granular, densification of the surrounding ground is a corollary benefit of the installation for elements that are installed with displacement methods or during compaction of the stone column. Soil preloads have long been used as a simple, economical method for preparing sites. In the simplest terms, a preload consists of a pile of soil that exerts stresses on the receiving ground that are similar or greater than the stresses that the ground will receive from the planned structure. As the preload remains in place, the underlying soil layers are squeezed and compressed. Where saturated clay layers are present, it can take months, years, or even decades for the consolidation to occur due to the lower permeability of clays. Keeping a preload in place for more than a few months is typically not practical. Vertical drains are commonly installed to expedite consolidation drainage in conjunction with preloads and also under fills, berms, dikes, levees, and reclaimed land. Common Ground Improvement Techniques Densification, reinforcement, and drainage enhancement can all be used to increase bearing, reduce total and differential settlement, in- crease shear resistance, and to mitigate liquefaction. Selection of the appropriate technique or combination of techniques is done on a case- by-case basis, and dependent on the geotechnical conditions, depth of problematic soils. the configuration and layout of the structure, loading conditions, settlement criteria, construction schedule and sequencing, size and location of project, and site access. Following are brief de- scriptions of some of the more commonly used techniques being used Dynamic compaction uses large crawler cranes to drop weights that typically range from 10 to 20 tons repeatedly in a predetermined grid pattern to increase density and reduce voids in granular soils, fills, and landfill materials. Most effective for large-footprint sites, the tech- nique is less effective in overly clayey or silty soils and where shal- low groundwater is present. Because of the large amount of vibration introduced into the ground, special care should be taken when using dynamic compaction near utilities, buildings, or other vibration- or settlement-sensitive structures. A related technique, Rapid Impact Compaction (RIC) is used to compact soils when it is necessary to in the United States today. Dynamic Compaction

limit vibrations. RIC typically treats a shallower depth of soil and is performed using a pile-driving hammer that is mounted to the front of an excavator to pound a steel plate seated on the ground surface. Vertical Drains Wick drains are thin, flexible drains that typically measure 1/8-inch thick by 4-inch wide. The drains consist of a channelized plastic core encased in a geosynthetic filter fabric designed to let water in and keep soil particles out. The drains, typically spaced between 3 ft and 8 ft in a grid pattern, are pushed into the ground and can be installed to depths of well over 100 ft. Wick drains are used to alleviate excessive pore pressure that would build up in fine-grained soil layers when fill is placed, and act to speed up consolidation settlement; reducing the time necessary for soil preloads to stay in place, mitigating long term settle- ment of fills, and enhancing stability of embankments as construction progresses. To perform as intended, an outlet for the water flowing from the wick drains is necessary – this is typically in the form of a granular drainage blanket installed at the ground surface prior to wick drain installation. General depth of application for various ground improvement techniques

Four drills installing Controlled Modulus Column (CMC)TM Rigid Inclusions to improve ground for a very large distribution center in South Dakota.


march 2021


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