9b- ground improvement techniques

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APPLICATION OF GEOTECHNICAL ENGINEERINGGEOTECHNICAL ENGINEERING

CE451A Dr. Rajesh Sathiyamoorthy, IIT Kanpur

GROUND IMPROVEMENT TECHNIQUES


Preloading and Drains


• When highly compressible, normally consolidated clayey soil layers exist at limited/large depths, large consolidation settlements are expected as the result of the loads from large buildings, highway embankments, or earth dams etc.

• Pre‐compression and provision of vertical drains in soft soil may be

Significance



used to minimize post‐construction settlement.

Methods• Precompression• Sand drains• Pre‐fabricated vertical drains• Vaccum consolidation• Electro‐osmosis consolidation

This technique is applicable to poorly draining soil (fine grained soil) such as plastic silts and soft clays

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Precompression

• Preload can be of a soil itself or any suitable material. It is generally carried out in stages to allow gradual dissipation of pore pressure and hence gradual increase in soil strength enabling it to safely support further stage of preload.



• The total settlement of Sc(p) will occur at time t2, which is much shorter than t1.

• Hence, if a temporary surcharge of Δσ’(p) + Δσ’(f) is applied on the ground surface for time t2, the settlement will be equal to Sc(p).

• At that time, if the surcharge is removed and a structure with a permanent load per unit area Δσ’(p) is built and no appreciable settlement will occur.

Precompression


The maximum primary consolidationsettlement caused by the structural load

If a surcharge of is placed on the ground, the primary consolidation settlement will be


The degree of consolidation at time t2after the application of the load is

The degree of consolidation U referred in the equation is actually the average degree of consolidation at time t2

Precompression


• The actual measured settlement in the field may not be same as computed by assuming average degree of consolidation because after the removal of the surcharge and placement of the structural load, the portion of


structural load, the portion of clay close to the drainage surface will continue to swell, and the soil close to the mid‐plane will continue to settle. In some cases, net continuous settlement might result.

• A conservative approach will be to assume U in the equation as the mid‐plane degree of consolidation inplace of average degree of consolidation.

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Precompression


Plot of U against

Plot of mid plane degree of consolidation against Tv


Precompression


Two problems may be encountered by engineers during precompression work in the field:• The value of Δσf is known, but t2 must be obtained.

• In such a case, obtain σo’, Δσp, and solve for U, using Equation or chart

(U against ) .

Procedure for Obtaining Precompression Parameters


• For this value of U, obtain Tv from plot (U vs Tv). Then

• For a specified value of t2, Δσf must be obtained.

• In such a case, calculate Tv,

• Use the plot (U vs Tv) to obtain the mid‐plane degree of consolidation, U.

• With the estimated value of U, get the required value of from the plot

and then calculate Δσf


Precompression



Precompression


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Sand drains

• The sand drains are used to accelerate the consolidation settlement of soft, normally consolidated clay layers and achieve pre‐compression before the construction of a desired foundation.


• Sand drains are constructed by drilling holes through the clay layer(s) in the field at regular


through the clay layer(s) in the field at regular intervals. The holes are then backfilled with sand.

• After backfilling the drill holes with sand, a surcharge is applied at the ground surface.

• The surcharge will increase the pore water pressure in the clay.

• The excess pore water pressure in the clay will be dissipated by drainage—both vertically and radially to the sand drains—thereby accelerating settlement of the clay layer.

Sand drains can be made using:• rotary drilling and then backfilling with sand

• drilling by continuous‐flight auger with a hollow stem and backfilling with sand and

• driving hollow steel piles.

Sand drainsThe effective zone from which the radial drainage will be directed toward a given sand drain is approximately cylindrical, with a diameter of de.


To determine the surcharge that needs to be applied at the ground surface and the length of time that it has to be maintained use the pre‐


time that it has to be maintained, use the precompression charts and equations.

Both radial and vertical drainage contribute to the average degree of consolidation.

Ur and Uv are the degree of consolidation with radial drainage only and vertical drainage only respectively

Sand drains


Procedure for obtaining average degree of consolidation due to radial drainage only

• It is also important to realize that, during the installation of sand drains, a certain zone of clay surrounding them is smeared, thereby changing the hydraulic conductivity of the clay.


Two cases may arise that relate to the nature of the application of surcharge

• the entire surcharge is applied instantaneously or

• the surcharge is applied in the form of a ramp load

• rw = radius of the sand drain and • re = de/2 is the radius of the effective zone

of drainage.• rs = radial distance from the center of the

sand drain to the farthest point of the smeared zone.

• The average‐degree‐of‐consolidation relationship can be derived from the theory of equal strain.

Sand drains



When the entire surcharge is applied instantaneously

Considering smear zone Considering no‐smear case


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Sand drains



When the surcharge is applied in the form of a ramp

Considering no smear zone


Sand drains



Sand drains


Procedure for obtaining average degree of consolidation due to vertical drainage only



Sand drains





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Sand drains





Prefabricated vertical drains (PVD)

• A large amount of consolidation is usually experienced in swamp deposits, hence extremely flexible vertical drains should be used. In addition to flexibility, continual functioning of vertcal drains is very important.

• Problems of conventional sand drains: mixing,



clogging, installation disturbance.

• Prefabricated vertical drains (PVDs) also referred to as wick or strip drains are commonly used because of its easy prefabrication, easy quality control, economy and small disturbance to the surrounding soil during installation.

• The main advantage of PVDs over sand drains is that they do not require drilling; thus, installation is much faster.



• PVDs consist of a flat or cylindrical plastic central core, whose function is primarily to act as a free drainage channel, wrapped in a geosynthetic fabric (non‐woven filter jacket), which prevents the soil surrounding the drain from entering the central core but allows water to flow, thereby allowing water to drain up through the centre of the drain


Characteristics of PVD:• Ability to permit pore water in the soil to seep into the drain.

• A means by which the collected pore water can be transmitted along the length of the drain.

• The plastic core serves two vital functions, namely: to support the filter jacket and to provide longitudinal flow paths along drain even at large lateral pressure.

water to drain up through the centre of the drain.




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Cofra Techfab



For a given surcharge and duration t2, the average degree of consolidation due to drainage in the vertical and radial directions is





The theory of consolidation with radial drainage assumes that the soil is drained by a vertical drain with circular cross‐section. The equivalent diameter of a pre‐fabricated PVC drain (dw) is defined as the diameter of a circular drain which has the same theoretical radial drainage performance as the

f b i d PVC d i


pre‐fabricated PVC drain.

Hansbo expression for dw:




PVDs are usually placed in a square / triangular configuration, the pattern size i.e. diameter of the effective zone of drainage (de or Ds) being converted into the equivalent drainage spacing (d or D)


q g p g ( )determined from the diameter of the ground cylinders around a drain.

Choosing a drain spacing is a major design parameterThe relationship between d and de is as follows: d = 1.13 x de for a square pattern d = 1.05 x de for a triangular pattern

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Plot of α’ versus n: (a) S = 2, (b) S = 3Following tables and charts can be used to estimate α’ and Tr’


r


Procedure for the design of prefabricated vertical drains:

• Determine time t2 available for the consolidation process and the Uvr required

• Determine Uv at time t2 due to vertical drainage.• For the PVD that is to be used, calculate dw• Determine (Tr’)1 (from Table or equation)



Determine (Tr )1 (from Table or equation)• Determine Tr’• Determine α’• Determine n (chart α’ vs n vs kh / ks, S )• Determine de = ndw• Choose the drain spacing (d) by knowing the pattern

Case study (PVD)



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Vacuum Consolidation

• The steadily increasing direct and indirect costs of placing and removing surcharge fill and the advent of technology for sealing landfills with impervious membranes for landfill gas extraction systems



landfill gas extraction systems have now made vacuum consolidation an economically viable method as a replacement for or supplement to surcharge fill.

• Vacuum consolidation is an effective means for accelerating the improvement of saturated soft soils.

• The soil site is covered with an airtight membrane and a vacuum is created underneath it by using a dual Venturi and vacuum pump.

• The technology can provide an equivalent pre‐loading of about 4.5 m high as compared with a conventional surcharging fill.




• An air/water pumping system is installed and creates a vacuum in the soil below the impervious membrane equivalent to a depression of between 60‐80 KPa, depending on the global efficiency of the system.

• This pressure is equivalent to that exerted by a 3‐4m high surcharge embankment. This preloading through the application of an atmospheric pressure creates an isotropic accelerated consolidation of the compressible soils. Settlement is then achieved without a surcharge load, in a greatly reduced time, than is normal.



Cofra





• While the area is under vacuum consolidation, normally 4‐6 months, no activity that can puncture the membrane is allowed on the vacuum area without proper protection. However heavy plant and filling can usually proceed in the adjacent areas.

• A monitoring system is usually installed and operated during the consolidation period to record settlement, displacement & pressures.

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Electro‐osmosis consolidation

• Vacuum preloading is the most popular technique for consolidating hydraulic‐filled areas. However, due to extremely low hydraulic permeability of fine‐grained materials, the vacuum preloading consolidation is too slow and its effect is very poor for deep hydraulic‐filled soft ground.



• Electro‐osmotic consolidation (Electrokinetic stabilisation) can be an alternative solution for soft ground improvement. Electro‐osmotic consolidation is much quicker than preloading method. However, corrosion of electrode and relatively high electrical energy consumption are two aspects that restrain its application.

The development of electrokinetic geosynthetic materials (EKG) eliminates these problems.


• When an electrical potential difference is applied across a soil mass, cations and anions are attracted to the cathode and anode respectively, whereas neutral particles are attracted to neither.

• This forced migration occurs most readily by the ions with the greatest mobility.



• Within a soil mass these ions are to be found in the pore water existing between soil particles. The movement of ions within the pore water causes a transfer of momentum to the pore water.

• The corresponding direction and rate of pore water movement are determined by the net transfer of momentum by both cations and anions within the pore water.

Because of the nature of clay formation, which usually results in significantly negatively charged clay particles, the predominant ions within the pore fluid are cationic and the water therefore moves from the anode to the cathode.



Damp proof



• EKG materials are geosynthetics which can be enhanced by electrokinetic techniques for the transport of water and chemical species within fine grained low permeability soils and wastes, which are otherwise difficult or impossible to deal with.

• The EKG can take the form of a singlematerial which is electrically conductive, or a composite material, in which at least one element is electrically conductive.

• As shown in the figure, there are two bumps of 6 mm wide and 2.5 mm thick on both sides of the EKG. Two copper wires of 1 mm are embedded inside the bumps.



of the EKG. Two copper wires of 1 mm are embedded inside the bumps.

During soil treatment, electro‐osmotic flow is independent of hydraulic permeability and the degree of negative pore water pressure or suction that builds up is proportional to the ratio of the coefficients of electro‐osmotic and hydraulic permeabilities. Therefore, electroosmosis is most effective in fine grained soils such as clays and silts.

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General features of EKG technology• Active water flow through clays• Long term (post – EK treatment) drainage• Long term (post EK‐treatment) reinforcement• Enhanced soil –reinforcement bond• Reduction in shrink swell behaviour• Rapid increase in undrained shear strength


Benefits of EKG technology• No requirement for large plant• Greatly reduced access considerations• No need for large volume material haulage

• Require minimal levels of manning and supervision


Rapid increase in undrained shear strength• Permanent increase in drained shear strength• Improvement conditioning of clay minerals by electrokinetic ion migration, which achieves much higher transportation rates than either conventional hydraulic or electro‐osmotic flow mechanisms

and supervision• Rapid deployment• Reduced overall project time• Permits use of conventionally unacceptable material

• Lower the environmental footprint• Reduced overall project costs

9b- ground improvement techniques

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