S.K.Tiwari, Reader, Raghuveer Singh, Research Scholar, Sanjay Mathur, Research Scholar, Ankit Gupta, Research Scholar, Kuldeep Soni, Research Scholar, Department of Structural Engineering, (MNIT) Jaipur.
IntroductionSoil improvement is the modification of existing site foundation soils or project earth structures to provide better performance under design and/or operational loading conditions. The term "Soil improvement" is open to different interpretations. First, it is an intention or objective, not the process of achieving it, although the term is often used in that sense. Second, improvement is a relative condition as to which aspect and to what degree there is improvement. Virtually all engineering construction involves the soil. In poor soil conditions there are five options viz; to bypass the poor soil by moving to a new site or using deep foundations to stronger ground, to remove the poor soil replacing it with better material, to design the structure to allow for the behavior of the poor ground under load, to treat the poor soil to improve its properties (i.e ground improvement), to abandon the project (the promoter's decision).
Selection Criteria, Working Concept of Site ImprovementSoil improvement includes systems that use the ground or some medication of it, to transfer or support loads. Soil improvement can increase soil strength and stiffness and/or reduce permeability. In many situations, soil improvement can be used to support new foundations or increase the capacity of existing foundations in place of bypass systems, such as pilling, caissons, or remove and replace. Soil improvement techniques are used increasingly for new projects to allow utilization of sites with poor subsurface conditions and to allow design and construction of needed projects despite poor subsurface conditions which formerly would have rendered the project economically unjustifiable or technically not feasible. More importantly, such techniques are used to permit continued safe and efficient operation of existing projects.
Principal Methods for Soil ImprovementThe techniques of soil improvement have been grouped into broad categories:
- Improvement by vibration
- Improvement by adding load (or increasing the effective stresses)
- Improvement by structural reinforcement
- Improvement by structural fill
- Improvement by admixtures
- Improvement by grouting
- Improvement by thermal stabilization
- Improvement by vegetation
Methods of Soil ImprovementTable 1 contains a list of potentially applicable soil improvement methods for civil Engineering structures.
|Table 1: Potentially Applicable Soil Improvement Methods for Civil Works Structures|
|Increase resistance to liquefaction Reduce movements||Vibrocompaction, Vibrorod Stone columns Deep dynamic compaction Explosive compaction Gravel drains||Deep soil mixing Penetration grouting, Jet grouting Compaction grouting Sand and gravel compaction Piles|
|Stabilize structures that have undergone differential settlement||Compaction grouting Penetration grouting||Jet grouting Mini-piles|
|Increase resistance to cracking, deformation and / or differential settlement||Compaction grouting Penetration grouting||Jet grouting Mini-piles|
|Reduce immediate settlement||Vibrocompaction, Vibrorod Deep dynamic compaction Explosive compaction Compaction grouting||Deep Jet grouting Deep soil mixing Sand and gravel compaction Piles|
|Reduce consolidation settlement||Pre-compression Jet grouting Compaction grouting||Stone columns Deep soil mixing Electro-osmosis|
|Increase rate of consolidation settlement||Vertical Drains, with or without surcharge fills Sand and gravel compaction piles|
|Improve stability of slopes||Buttress fills Gravel drains Penetration grouting Compaction grouting||Jet grouting Deep soil mixing Soil nailing Sand and gravel Compaction Piles|
|Improve seepage barriers||Jet grouting Deep soil mixing||Penetration grouting Slurry trenches|
|Strengthen and / or seal interface between embankments / abutments foundations||Penetration grouting||Jet grouting|
|Seal leaking conduits and / or reduce piping along conduits||Penetration||Compaction grouting|
|Reduce leakage through joints or cracks||Penetration grouting|
|Increase erosion resistance||Roller compacted concrete Admixture stabilization||Biotechnical stabilization|
|Stabilize dispersive clays||Add lime or cement during construction Protective filters
For existing dams, add lime at upstream face to be conveyed into the dam by flowing water
|Stabilize expansive soils||Lime treatment Cement treatment||Soil replacement Keep water out|
|Stabilize collapsing soils||Prewetting / hydro blasting Deep dynamic compaction||Vibrocompaction Grouting|
Drainage and SurchargeDrainage and Surcharge is a very old means of ground improvement. For this system, consolidation properties of the soil and the spacing of the vertically installed drainage elements will govern the design and construction schedule. Ground stability issues may require staged loading and waiting periods to ensure safe constructions. This system offers cost advantages in soft silty and clayey soil if the project schedule permits adequate construction time.
ReplacementReplacement is the most simple in concept and reliable technique if employed properly. Soil replacement involves excavating the soil that needs to be improved and replacing it. The excavated soil can sometimes be recompacted to a satisfactory state or it may be treated with admixtures and then be replaced in a controlled manner. It can also be replaced with a different soil with more suitable properties for the proposed application. Soft soil, mostly soft clay or highly organic clay under or near the expected structure is removed and replaced by a good quality foreign material up to the extent required to maintain stability or to avoid unfavorable settlement of the structure.
DensificationDensification of heterogeneous soils, municipal wastes, liquefiable soils, loose granular soils is quite a common practice. The purpose of densification is to increase strength and to reduce settlement of loose granular soils. Often, improvement of uniformity of the originally heterogeneous soil become the purpose of densification. Vibro-rod, vibro-flotation, sand compaction pile method, compaction grouting, and heavy tamping are well established techniques of recent days. The technologies of this category may be described into details at the separate session which devotes on the Seismic problems. After the densification process, in most cases, Density is predicted preliminary based on the accumulated experience and confirmed later by the field test.
Vibro-compaction is a technique of densification by inserting vibratory probes into in situ soil. The probes if vibrates horizontally called Vibro-flotation or if vertically called Vibro-rods to densify the soils at the probe location. The granular materials are supplied from the ground surface to fill the void created by the vibration. In some machine, the bottom feed of granular material is possible. Typically 3 to 15m is the improvement depth. Vibro Compaction is used to mitigate liquefaction potential in seismic areas. Comprehensive overview of the technology may be found in Brown (1977), Welsh (1987) and Wightman (1991).
Vibroflotation is used for compacting thick deposits of loose, sandy soils upto 30m depth. A vibroflot consists of a cylindrical tube, about 2 m diameter, fitted with water jets at the top and the bottom. It contains a rotating eccentric mass which develops a horizontal vibratory motion.
The vibroflot is sunk into the loose soil upto the desired depth using the lower water jet. As water comes out of the jet, it creates a momentary quick condition ahead of the vibroflot due to which the shear strength of the soil is reduced. The vibroflot settles due to its own mass. When the desired depth has been reached, the vibrator is activated. The vibroflot then vibrates laterally and causes the compaction of the soil in the horizontal direction to a radius of about 1.5m.
The water from the lower jet is transferred to the top jet and the pressure is reduced so that it is just enough to carry the sand poured at the top of the bottom of the hole. Vibration continues as the vibroflot is slowly raised to the surface. Additional sand is continually dropped into the space (crator) around the vibroflot. By raising the vibroflot in stages and simultaneously backfilling, the entire depth of the soil is compacted.
The spacing of the holes is usually kept between 2 to 3m on a grid pattern. The relative density (density index) achieved for the sandy soils is 70% or more. In soft, cohesive soils, vibroflotations is not effective. For cohesive soils, it can be used to form a sand pile to reinforce the deposit and to accelerate consolidation and thus improve its engineering properties. This process is referred to as Vibro-Compaction, and has been used to compact loose sands to the depths of 30m, such as in the world and Palm Island Projects off the Dubai coast and Edinburgh’s Leith Docks.
Terra probe method in many respects is similar to the vibroflotation method. The terra probe consists of an open-ended pipe, about 75 cm diameter. It is provided with a vibratory pile drive. The vibratory pile driver when activated gives vertical vibrations to the terra probe and it goes down. After reaching the desired depth, the terra probe is gradually raised upward while the vibrodriver continues to operate. Thus, the soil within and around the terra probe is densified.
The terra probe method has been successfully used upto depth of 20m. The spacing of the holes is usually kept about 1.5m. Saturated soil conditions are ideal for the success of the method. For the sites where the water table is deep, water jets are fitted to the terra probe to assist the penetration and densification of the soil.
The terra probe method is considerably faster than the vibroflotation method. As it does not require backfilling of sand, it can even be used at offshore locations. However, the method is less effective than vibroflotation method. In the terra probe, the zone of influence is considerably smaller and the relative density achieved is also lower.
The Vibro Stone Column technique is one of the most widely-used ground improvement processes in the world, although its potential for improving Irish sites has yet to be fully exploited. Historically, the system has been used to densify loose granular soils, but over the past 35 years, the system has been used increasingly, to reinforce soft cohesive soils and mixed fills.
Vibro Replacement was born when it was found that bucklling the probe hole with gravel sized stone particles would increase the densication effect, and leave a stiff, dense stone pier for reinforcement, expanding Vibro System’s applicability to a much wider range of soil types. Vibro Replacement can desify silty sands with silt contents up to 30% and ner-grained soil types can be reinforced by creating high modulus stone columns to reduce deformation and increase shear strength. When water jetting usage was restricted, the construction community developed equipment able to dry “bottom feed” the stone, yielding higher displacement capacity and increased densication. Although Vibro Replacement is capable of installing stone columns to depths of over 100 feet, numerous projects require reinforcement of the ground to depths of 30 feet or less.
Stone columns are installed using a process similar to Vibrocompaction, except that a gravel backfill is used, and they are usually installed in slightly cohesive soils or silty sands rather than clean sands. In the dry process, a cylindrical cavity is formed by the vibrator that is filled from the bottom up with gravel or crushed rock. Compaction is by vibration and displacement during repteated 0.5 m withdrawals and insertions of the vibrator.
Vibro Piers are capable of densifying surrounding granular soils up to a 5 foot radius. In weak or compressible soils, Vibro Piers are typically spaced at 6 to 10 foot centers under embankments, tank foundations, and poor slabs. Closer spacing is utilized under spread footings. Unlike other methods that utilize tampers, Vibro Piers are installed using the dry bottom feed method and pre-augering of soils is rarely required. As such, this method offers additional advantages in contaminated soils. In addition, the costly installation of a temporary casing is not required to maintain a stable hole when working in collapsible soils or when installation is below the water table. In very weak cohesive or organic soils that offer less than adequate connement to stone columns, concrete can be pumped through a depth vibrator creating a Vibro Concrete Column (VCC).
The pounding method is used to compact the soil deposit to a great depth. It is very effective for densifying loose sandy deposits. Recently, the method has been successfully used to compact fine-grained soil deposits as well. The depth (D) of influence is proportional to the square root of the tamper weight times the drop height, in meters can be determined from the following relations:
D = C MH (1)
Where C = coefficient (0.5 to 0.75), M = mass (Mg), H = height of drop (m)
Menard (1975) increased the impact energy tremendously, introduced systematic repetition of tamping and improved this as a modern technique of deep compaction. The recent development of Heavy Tamping may be found in Mitchell and Zoltan (1984), Mitchell and Welsh (1989), ASTM Committee on Soil and Rock (1990), Rolling and Rogers (1994), Chow et al. (1994), Loetal (1990). Oshima and Takada (1997, 1998) stimulate the compaction process of heavy tamping by geotechnical centrifuge modeling.
The equipment of the Sand Compaction Pile Method (SCP) resembles with the equipment for sand drain installation. When the equipment reached a desired depth, the equipment is withdrawn leaving a loose sand pile of predetermined length through its mandrel. Then with the aid of a vibrator at the top of the mandrel, the mandrel compresses the sand pile and expands its diameter. By the sequence of this process, compacted sand piles are created and the surrounding soils are also densified. Aboshi et al. (1990) provided a concise state-of-the-art of this technology.
ConsolidationStructures constructed on the ground will experience the stability problem and/or the long term unfavorable settlement when a foundation ground is the cohesive soils with low strength and low permeability. These soils however increase strength and improve their compressibility with time under the sustained loading. Applied external load causes the increase of total stress in the ground. The increment of the total stress is sustained by the excess pore water pressure if the soil is saturated. Then excess pore water pressure dissipates with time and results in the reduction of soil volume and increase of effective stress and increase of strength. This is the principal of consolidation. Most often preloading is done by staged construction to avoid the instability at the edge of embankment. Preloading by embankment fill is one of the oldest techniques to improve this kind of soils. As is mentioned above preloading is given to the ground with final target of increasing effective stress. The same can be achieved by alternative techniques of decreasing the pore water pressure. With increasing thickness of cohesive soil (with increasing drainage path), consolidation time becomes longer and unacceptable. The idea of accelerating the consolidation by reducing the length of drainage path was born in 1930’s in USA and in Nordic countries. Commonly used artificial drainage are vertical drainage by means of sand drain or prefabricated drains.
Apart from the application to cohesive soils, vertical drainage is recently employed to dissipate quickly the excess pore water pressure induced by the earthquake in order to tame the liquefaction.
GroutingGrouting is also used to compensate the unfavorable displacement of existing structures. When grout, a pumpable material is injected into a soil or rock formation, it may permeate into the natural openings such as void space of the soil and fissures in the rocks, or create an opening by fracturing the soil mass, or displace the surrounding soil. The final location of the grout and the maximum distance of travel from the injection point are determined by a number of factors; viscosity of the grout, get time, size of the particles in relation to the openings, injection pressure, rate of injection, properties of soil and rock to be grouted, and so forth. As a consequence, the completed grouted zone usually has an irregular shape and imhomogeneity.
Jet GroutingIt is an erosion-based system that relies on kinetic energy of high velocity jetted to break down the soil structure and remove a portion of it while mixing it with cementitious grout slurry. The resulting soil cement mixture is referred to as soilcrete thus the jet grouting is composed of a combined process of cutting soil with high pressure and filling the space created by cutting with grout.
Comprehensive information on Grouting of recent decade may be found in ASCE Committee on Grouting (1995), Borden et al. (1992), Yonekura, et al. (1996) and Grouting Committee of Geo-Institute (1998). Burke and Welsh (1991) and Xanthaks et al. (1994) can be consulted for additional information regarding jet grouting.
Deep Soil MixingDeep Soil Mixing is a soil improvement technology used to cutoff or retaining walls and to treat contaminated soils, in-situ. This is accomplished with a series of overlapping stabilized soil columns (typically 24 to 56 inches in diameter and greater than 40-feet in depth). The stabilized soil columns are formed by a series of mixing shafts (2 to 4), guided by a crane-supported set of leads. As the mixing shafts are advanced into the soil, (grout or slurry) is pumped through the hollow stem of the shaft and injected into the soil at the tip. The auger flights and mixing blades on the shafts blend the soil with the (grout or slurry) in pugmill fashion. The treatment modifies the engineering properties of the soil by increasing strength, decreasing compressibility and decreasing permeability. Typical admixtures are cement and lime, but slag or other additives can also be used. The mix-in-place columns can be used alone, in groups to form piers, in lines to form walls, or in patterns to form cells. The process can be used to form soil-cement or soil-bentonite cutoff walls in coarse-grained soils, to construct excavation support walls, and to stabilize liquefiable ground. A detailed discussion of deep mixing is presented in ASCE (1997).
Admixture StabilizationAdmixture stabilization consists of mixing or injecting admixtures such as cement, lime, flyash or bentonite into a soil to improve its properties. Admixtures cant be used to increase the strength, consistency, deformation characteristics decrease the permeability or improve the workability of a soil. These improvements become possible by the ion exchange at the surface of clay minerals, bonding of soil particles and / or filling of void space by chemical reaction products. Although a variety of chemical additives has been developed and used, most frequently used additives nowadays are lime and cement due to its availability and lower cost. Admixtures can fill voids, bind particles, or break down soil particles and form cement. Admixture stabilization is discussed in detail in Hausmann (1990).
Thermal Stabilization (Heating and Freezing)Thermal stabilization is divided into two groups of heating and freezing. Event at the ordinary temperature under the sunshine, properties of fine grained soils are improved by desiccation. This is often found as a dry crust formed at the surface of reclaimed sludge. When the reclamation process is very slow, the thickness of desiccated layer becomes several meters (Katagiri et al, 1996). The artificial heating is naturally much more effective and the application of heating the soil up to 300 to 1,000 degree Celsius has been reported. As far as the improvement of the mechanical properties are concerned Mitchell (1981) is still a good source of information with comprehensive references.
Wet Soil MixingIt is a mechanical mixing system whereby insitu soils are sheared and mixed with cementitious slurry to create soilcrete columns similar to those produced by jet Grouting. Mixing may be performed by single or multiple axis mixing tools. Single axis tooling uses more mechanical mixing energy to insure thorough mixing, but offers reduced cost for mass mixing by virtue of high productivity.
Dry Soil MixingIt is a bottom-up mechanical mixing system, shearing precut soil and combining it with pneumatically injected power binders (usually cement and lime). Dry soil mixing appropriate for very wet and soft soils untreatable by other methods.
Soil Reinforcement TechniquesSoil reinforcement consists of creating in-situ a composite reinforced soil system by inserting inclusions in predetermined directions to improve the shear strength characteristics and bearing capacity of the existing ground. The recent ASCE State of Practice Review on Ground Improvement, Reinforcement and Treatment (ASCE, 1977) covers 12 techniques including bio-technical inclusions, deep mixing, fiber reinforcement, geosynthetics, ground anchors, jet grouting, lime-cement columns, mechanically stabilized earth structures, micropiles, soil nailing, vibro concrete columns and vibro-replacement (vibro-stone columns). Engineering applications of these technologies cover an increasing range of geotechnical construction projects including soil foundation with vertical micropiles, or jet grouted, vibro-stone or lime-cement columns, as well as slope stabilization and retaining structures with sub–horizontal passive soil nails, horizontal metallic or geosynthetic reinforcement, or prestressed ground anchors. Among the major issues to be addressed (Juran, 1997) one can identify (i) quality control of innovative materials such as 3-D fiber reinforcement, (ii) long-term performance assessment of various reinforcement types and particularly geosynthetics (i.e chemical and mechanical durability and in soil confined creep), (iii) seismic performance of reinforced soil systems, (iv) reinforcement of fine grain soils, and (v) innovative construction technologies, specifically for in-situ ground reinforcement. These technologies as well as the current research and State of Practice were discussed by Professor F. Schlosser in a session of conference on sustainable concrete infrastructure developed held in May at Jaipur india.
Geotextiles are porous to water flow across their manufactured plane and also within their plane, but to a widely varying degree. It is a permeable, polymeric, (synthetic or natural) textile material, in the form of manufactured sheet (which may be woven, nonwoven or knitted) used in geotechnical, environmental, hydraulic and transportation engineering applications. These synthetic fibers are made into flexible, porous fabrics by standard weaving machinery or they are matted together in a random nonwoven manner. Some are also knitted. The major point is that geotextiles are porous to liquid flow across their manufactured plane and also within their thickness, but to widely varying degree. There are at least 100 specific application areas for geotextiles that have been developed, however, the fabric always performs at least one of four discrete functions; separations, reinforcement, filtration and/or drainage.
They are formed by a regular network of tensile elements with apertures of sufficient size to interlock with surrounding fill material, a polymeric structure, unidirectional or bidirectional, in the form of manufactured sheet, consisting of a regular network of integrally connected elements, which may be linked by extrusion, bonding or interlacing, whose openings are larger than the constituents, used in geotechnical, environmental, hydraulic and transportation engineering applications.
Geonets, called “geospacers” by some. It constitute another specialized segment of the geosynthetics area. They are usually formed by a continuous extrusion of polymeric ribs at acute angels to one another. When the ribs are opened, relatively large apertures are formed in a netlike configuration. Their design function is completely within the drainage area where they have been used to convey fluids of all types.
Geomembranes are relatively thin impervious sheets of polymeric materials used primarily for linings and covers of liquid-or-solid-storage facilities. This includes all types of landfills, reservoirs, canals and other containment facilities. Thus the primary function is always containment functioning as a liquid and / or vapor barrier. The range of applications is very great, and in addition to the geo-environmental area, applications are rapidly growing in geotechnical, transportation, hydraulic, and private development engineering.
A geocomposite consists of a combination of geotextile, geogrids, geonets and / or geomembranes in a factory fabricated unit. Also, any one of these four materials can be combined with another synthetic material (e.g., deformed plastic sheets or steel cables) or even with soil. As examples, a geonet with geotextiles on both surfaces and a GCL consisting of a geotextile/bentonite/geotextile sandwich are both geocomposites.
It is a general area of geosynthetics that has exhibited such innovation that many systems defy categorization. For want of a better pharse, geo-others describes items, such as threaded soil masses, polymeric anchors, and encapsulated soil cells. As with geocomposites, their primary function is produce-dependent and can be any of the five major functions of geosynthetics.
ConclusionThe ground improvement technique may be further developed in the future along with the emerging new needs from the construction site and more generally from the public. With increasing needs of accurate prediction and precise control of the ground deformation, the necessary development is not only within the specialized ground improvement technology. The need of the hour is to grasp the accurate ground condition before the ground improvement, we need to employ more sophisticated numerical prediction to supplement the design based on the traditional design codes and carry out monitoring during and after the construction with the aid of new instrumentation and computer. In the near future, new equipment judges the difference in the actual ground condition from those information used in the design stage and propose us to change the pre-determined specification. Due to the increased level of requirement rely, one should not rely always on single ground improvement technology. The combined use of different ground improvement may become a common practices and further more the combined use of ground improvement with the innovative idea of structure’s side. The key words for the further development of the ground improvement may be; increased accuracy of prediction and back-analyses during the ground improvement work, Quality Control and Quality assurance aided by means of computer and GPS, Synergy of ground improvement and innovative structure, Environmental concern.
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