MAHALAKSMI ENGINEERINGCOLLEGE

TIRUCHIRAPALLI - 621213.

QUESTION WITH ANSWERS

DEPARTMENT: CIVIL –IV SEMESTER:VII

SUB.CODE/ NAME: CE 2033 / Ground Improvement Techniques

Unit II - DRAINAGE AND DEWATERING

Part – A

1. Define dewatering? Dewatering or construction dewatering are terms used to describe the action of removing groundwater or surface water from a construction site. Normally dewatering process is done by pumping or evaporation and is usually done before excavation for footings or to lower water table that might be causing problems during excavations. Dewatering can also be known as the process of removing water from soil by wet classification.

2. What is the need for drainage and dewatering?

To provide suitable working surface of the bottom of the excavation.

To stabilize the banks of the excavation thus avoiding the hazards of slides and sloughing.

To prevent disturbance of the soil at the bottom of excavation caused by boils or piping. Such disturbances may reduce the bearing power of the soil.

Lowering the water table can also be utilized to increase the effective weight of the soil and consolidate the soil layers. Reducing lateral loads on sheeting and bracing is another way of use.

3. What are the various methods of dewatering?

Surface water control like ditches, training walls, embankments. Simple methods of diverting surface water, open excavations. Simple pumping equipment.

Gravity drainage.Relatively impermeable soils. Open excavations especially on

sloping sites. Simple pumping equipment.

Sump pumping

Wellpoint systems with suction pumps.

Shallow (bored) wells with pumps.

Deep (bored) wells with pumps.

Eductor system

8. Drainage galleries. Removal of large quantities of water for dam abutments, cut-

offs, landslides etc. Large quantities of water can be drained into gallery (small

diameter tunnel) and disposed of by conventional large – scale pumps.

9. Electro-osmosis. Used in low permeability soils (silts, silty clays, some peats)

when no other method is suitable. Direct current electricity is applied from anodes

(steel rods) to cathodes (well-points, i.e. small diameter filter wells)

4. How are sumps and ditches used in dewatering?

A sump is merely a hole in the ground from which water is being pumped for the purpose

of removing water from the adjoining area .They are used with ditches leading to them in large excavations. Up to maximum of 8m below pump installation level; for greater depths a submersible pump is required.

5. What are the advantages of sumps and ditches in dewatering?

It is the most widely used and economical of all methods of ground water lowering.

This method is also more appropriate in situations where boulders or other massive obstructions are met in the ground.

6. What is a well point system? This type of dewatering system is effective in soils constituted primarily of sand fraction or other soil containing seams of such materials. In gravels spacing required may be too close and impracticable. In clays it is also not used because it is too slow.

7. What are the different types of well point systems?

single stage well point

multistage well point

well points in braced excavations

deep well drainage 8. When are deep wells used for dewatering? (AUC NOV /DEC 2012)

Deep well systems are of use in gravels to silty fine sands and in water bearing rocks. They

are priority or use with deep excavations and where artesian water is present below an impermeable stratum. If this type of installation is to be designed economically the ground permeability must be assessed from full scale pumping tests.

9. What is the principle behind vacuum dewatering?

Gravity methods, such as well points and deep wells are not much effective in the fine-grained soils with permeability in the range of 0.1 –10 x 10 -3 mm/s.

Such soils can be dewatered satisfactorily by applying a vacuum to the piping system

10. What is electro-osmotic dewatering? When an external electro motive force is applied across a soild liquid interface the movable diffuse double layer is displaced tangentially with respect to the fixed layer . this is electro osmosis. As the surface of fine grained soil particles causes negative charge, the positive ions in solution are attracted towards the soil particles and concentrate near the surfaces

11. What are the various types of drains?

open drains

closed drains

horizontal drains

foundation drains

blanket drains

interceptor drains

12. Define permeability and seepage. Permeability of soil its capacity to transmit a fluid to pass through its interconnected void spaces . K = v/I V= the discharge velocity I = hydraulic gradient Water flows through the voids in a soil which are interconnected. This flow may be called seepage, since the velocities are very small.

13. What are the requirements of drains should be satisfy. (AUC NOV /DEC 2010) Sand drains consist of a column of pervious sand placed in a cased hole, either driven or

drilled through the soil, with the casing subsequently removed. The ca-pacity of sand drains can be significantly increased by installation of a slotted 1% or 2-inch pipe inside the sand drain to conduct the water down to the more per-vious stratum.

14. Define sensitive clay (AUC NOV /DEC 2010)

Clay whose shear strength is decreased to a fraction of its former value on remolding at constant moisture content.

15. How the dewatering carried out for the construction of the bored tunnel . (AUC MAY/JUNE 2013)

Groundwater Engineering provides complete dewatering solutions: Design of dewatering systems Well drilling and installation Pumping tests Equipment sales and rental Monitoring systems On-site operation and maintenance

16. What are the problems occurred to seepage of water (AUC MAY/JUNE 2012)

Common causes of water seepage : 1. Leakage in the drainage pipes of the upper, adjacent or your own flat. 2. Leakage in the water supply pipes of the upper, adjacent or your own flat. 3. Deteriorated waterproofing of floor slabs or bath-tub seals. 4. Seepage of waste water or rain water through roof / external wall

17. State the advantages and disadvantages of dewatering. (AUC NOV /DEC 2012)

ADVANTAGES

Reduces the amount of sediment leaving the site

Allows for a more in-depth site assessment – additional necessary erosion control measures may be identified

DISADVANTAGES o Must abide by multiple government laws and standards and obtain appropriate

permits o Requires frequent maintenance o May be costly

18. Define Cutoffs. Cutoff curtains can be used to stop or minimize seepage into an excavation where the cutoff can be installed down to an impervious formation. Such cutoffs can be constructed by driving steel sheet piling, grouting existing soil with cement or chemical grout, excavating by means of a slurry trench and backfilling with a plastic mix of betonies and ‘soil, in-stalling a concrete wall, possibly consisting of overlap-ping shafts, or freezing.

19. what are the types of drainage ?

Land Drainage: This is large scale drainage where the objective is to drain surplus water from a large area by such means as excavating large open drains, erecting dykes and levees and pumping. Such schemes are necessary in low lying areas and are mainly Civil Engineering work

ii) Field Drainage This is the drainage that concerns us in agriculture. It is the

removal of excess water fromthe root zone of crops. 20. State electro osmotic consolidation

Due to the applied electric potential the electrolysis of water occurs at the electrodes 2H2O -> O2 (g) + 4H+ +4e- oxidation (anode) 4H2O + 4e- -> 2H2 (g) + 4OH- reduction (cathode) The clay particles have a “ve charge. These “ve charge produce an electro static surface property known as the double layer which creates a net abundance of cations

Part –B

1. Explain in detail with a neat sketch the method of dewatering using sumps and ditches stating its advantages and disadvantages. (Or) How the dewatering is carried out during the construction in detail (AUC NOV /DEC 2010)(AUC MAY/JUNE 2012) (AUC MAY/JUNE 2013)

.Sumps and sump pumping: A sump is merely a hole in the ground from which water is being pumped for the purpose of removing water from the adjoining area (Fig 9.1). They are used with ditches leading to them

in large excavations. Up to maximum of 8m below pump installation level; for greater depths

a submersible pump is required. Shallow slopes may be required for unsupported

excavations in silts and fine sands. Gravels and coarse sands are more suitable. Fines

may be easily removed from ground and soils containing large percent of fines are not

suitable. If there are existing foundations in the vicinity pumping may cause settlement

of these foundations. Subsidence of adjacent ground and sloughing of the lower part of a

slope (sloped pits) may occur. The sump should be preferably lined with a filter

material which has grain size gradations in compatible with the filter rules. For

prolonged pumping the sump should be prepared by first driving sheeting around the

sump area for the full depth of the sump and installing a cage inside the sump made of

wire mesh with internal strutting or a perforating pipe filling the filter material in the space

outside the cage and at the bottom of the cage and withdrawing the sheeting. Two simple

sumping details are shown in Figures 2 and 3.

OPEN SUMPS AND DICTHES:

The essential feature of this method is a sump below the ground level of the excavation at one or more corners or sides.. a small ditch is cut around the bottom of the excavation , falling towards the sump. It is the most widely used and economical of all methods of ground water lowering. This method is also more appropriate in situations where boulders or other massive obstructions are met with the ground. There is also a disadvantage that the groundwater flows towards the excavation with a high head or a steep slope and hence there is a risk of collapse of the sides.

2. Explain in detail the well point system of dewatering. (AUC MAY/JUNE 2013)

.WellPoint systems A wellpoint is 5.0-7.5 cm diameter metal or plastic pipe 60 cm – 120 cm long which is perforated and covered with a screen. The lower end of the pipe has a driving head

with water holes for jetting (Fig 9.4.a,b). Wellpoints are connected to 5.0-7.5 cm diameter

pipes known as riser pipes and are inserted into the ground by driving or jetting. The upper

ends of the riser pipes lead to a header pipe which, in turn, connected to a pump. The

ground water is drawn by the pump into the wellpoints through the header pipe and

discharged (Fig 9.5). The wellpoints are usually installed with 0.75m – 3m spacing (See

Table 1). This type of dewatering system is effective in soils

constituted primarily of sand fraction or other soil containing seams of such materials.

In gravels spacing required may be too close

and impracticable. In clays it is also not used because it is too slow. In silts and silt –

clay mixtures the use of well points are aided by upper (0.60m – 0.90m long) compacted clay

seals and sand-filtered boreholes (20cm – 60cm diameter). Upper clay seals help to

maintain higher suction (vacuum) pressures and sand filters increase the amount of

discharge. Filtered boreholes are also functional in layered soil profiles (Figures

9.6.a,b,c,d,e)

9-5

Table 9.1 Typical spacings for some common soil types and the

approximate time required for effective drawdown

Soil Typical Spacing (m) Time (days)

Silty sand 1.5-2 7-21(Could be longer)

Clean fine to coarse sand 1.0-1.5 3-10

and sandy gravel

Fine to coarse gravel 0.5-1.0 1-2

The header pipe (15-30 cm diameter, connecting all wellpoints) is connected to a vacuum

(Suction assisted self – priming centrifugal or piston) pump. The wellpoints can lower a water

level to a maximum of 5.5 m below the centerline of the header pipe. In silty fine sands this

limit is 3-4 m. Multiple stage system of wellpoints are used for lowering water level to a

greater depth. Two or more tiers (stages) are used. (Fig 9.7). More pumps are needed and

due

to the berms required the excavation width becomes wider. A single wellpoint

handles between 4 and 0.6 m3/hr depending on soil type. For a 120 m length (40 at 3 m

centers) flow is therefore between 160 and 24 m3/hr.

Nomograms for selecting preliminary wellpoint spacing in clean uniform sand and gravel, and

stratified clean sand and gravel are shown in Figures 9.8 and 9.9.

9-7

Horizontal wellpoints are used mainly for pipeline water. They consist of perforated pipes laid horizontally in a trench and connected to a suitable pump. Shallow Wells Shallow wells comprise surface pumps which draw water through suction pipes installed in

bored wells drilled by the most appropriate well drilling and or bored piling equipment. The

limiting depth to which this method is employed is about 8 m. Because wells are

prebored, this method is used when hard or variable soil conditions preclude the use of

a wellpoint system. These wells are used in very permeable soils when wellpointing would

be expensive and often at inconveniently close centers. The shallow well can be used

to extract large quantities of water from a single hole. On congested sites use of smaller

number dewatering points is preferred (no hiderance to construction operations) hence

shallow wells may be preferred to wellpoints. Since the initial cost of installation is more

compared to wellpoints it is preferred in cases where dewatering lasts several months

or more. Another field of

application is the silty soils where correct filtering is important.

3. What is a deep well? When is it adopted? What are its merits and demerits?

(AUC NOV /DEC 2010) (AUC MAY/JUNE 2013)

Deep Wells When water has to be extracted from depths greater than 8 m and it is not feasible to lower

the type of pump and suction piping used in shallow wells to gain a few extra meters of depth

the deep wells are such and submersible pumps installed within them. A cased borehole

can be sunk using well drilling or bored piling rigs to a depth lower than the required

dewatered level. The diameter will be 150 – 200 mm larger then the well inner casing,

which in turn is sized to accept the submersible pump. The inner well casing has a

perforated screen over the depth requiring dewatering and terminates below in 1 m of

unperforated pipe which may serve as a sump for any material which passes the filter.

After the slotted PVC or metal well screen (casing) has been installed it is surrounded by

backfill over the unperforated pipe length and with graded filter material over the

perforated length as the outer casing progressively withdrawn (Fig 9.10). As with the

shallow wells the initial pumping may involve twice the volumes when equilibrium is

achieved.

Deep well systems are of use in gravels to silty fine sands and in water bearing rocks. They are priority or use with deep excavations and where artesian water is present below an impermeable

stratum. If this type of installation is to be designed economically the ground permeability must be

assessed from full scale pumping tests. Because of their depth and the usually longer pumping period

these installations are more likely to cause settlement of nearby structures, and the use of recharge

methods may have to be considered.

4. Explain in brief the principle, equipment used, installation and operation and

precaution adopted in electro-osmotic dewatering. (AUC NOV /DEC 2012)

Dewatering by electro – osmosis

When an external electro motive force is applied across a soild liquid interface the movable diffuse double layer is displaced tangentially with respect to the fixed layer . this is electro osmosis. As the surface of fine grained soil particles causes negative charge, the positive ions in solution are attracted towards the soil particles and concentrate near the surfaces. Upon application of the electro motive force between two electrodes in a soil medium the positive ions adjacent to the soil particles and the water molecules attached to the ions are attracted to the cathode and are repelled by the anode. The free water in the interior of the void spaces is carried along to the cathode by viscous flow. By making the cathode a well, water can be collected in the well and then pumped out.

EFFECT OF ELECTRO OSMOTIC TECHINQUE IN CONSOLIDATION OF SOIL full report EFFECT OF ELECTRO OSMOTIC TECHINQUE IN CONSOLIDATION OF SOIL INTRODUCTION Electro osmotic consolidation means the consolidation of soft clays by the application of electric current. It was studied and applied for the first time by Casagrande. It is inherent that fine grained clay particles with large interfacial surface will consolidate and generate significant settlement when loaded. The settlement creates problem in the foundation engineering. Electro osmosis was originally developed as a means of dewatering fine grained soils for the consolidation and strengthening of soft saturated clayey soils. Electro osmotic dewatering essentially involves applying a small electric potential across the sediment layer. It is the process where in positively charged ions move from anode to cathode. ie. Water moves from anode to cathode where it can be collected and pumped out of soil Electro osmotic flow depends on soil nature, water content, pH and on ionic type concentration in the pore water. ELECTRO OSMOTIC CONSOLIDATION Due to the applied electric potential the electrolysis of water occurs at the electrodes 2H2O -> O2 (g) + 4H+ +4e- oxidation (anode) 4H2O + 4e- -> 2H2 (g) + 4OH- reduction (cathode) The clay particles have a “ve charge. These “ve charge produce an electro static surface property known as the double layer which creates a net abundance of cations

in pore space. Electro osmotic transfer of water through a clay is a result of diffuse double layer cations in the clay pores being attracted to a negatively charged electrode or cathode. When electrodes are placed across a saturated clay mass and direct current is applied ,water in the clay pore space is transported towards cathode by electro osmosis. In addition frictional drag is created by the motion of ions as they move through the clay pores helping to transport additional water. The flow generated by the electric gradient is called electro osmotic flow. EVALUATION OF ELECTRO OSMOTIC CONSOLIDATION Determination of parameters Electrical operation systems for field application Materials Spacing between electrodes Cost of electrodes and installation cost CASE STUDY 1 ELECTRO OSMOTIC DEWATERING OF DREDGED SEDIMENTS :- BENCH SACLE INVESTIGATION AT INDIANA HARBOUR (USA) The Indiana Harbour has not been dredged for over 20 years due to lack of an acceptable disposal site It resulting in the accumulation of large amount of highly contaminated sediments The main problem with the disposal of sediment to the CDP was slow consolidation of sediment due to its very high water content. This study investigated the feasibility of using an electro osmotic dewatering technology to accelerate dewatering and consolidation of the sediment there by allowing more rapid disposal of sediment in to the CDP. MATERIAL (SOIL SAMPLE) Samples were obtained by dredging from the Indiana Harbour.The sediments were tested for water content, optimum moisture content and pH CASE STUDY 2 EFFECT OF USING ELECTRO CONDUCTIVE PVD IN THE CONSOLIDATION OF RECONSTITUTED ARIAKE CLAY

The deposits of Ariake clay consolidate generate significant settlement when loaded which causes problems in foundation engineering. The low permeability of clay results in longer duration to achieve primary consolidation. To shorten the consolidation time vertical drains are installed to shorten the drainage path. The induced consolidation of reconstituted Ariake clay was conducted using electro conductive PVD as electrodes and the results were compared with an ordinary PVD MATERIAL (SOIL SAMPLE) The soil sample used was leached marine Ariake clay collected at Saga Plain in Saga Japan. The sample excavated at about 2m depth was generally a very soft gray silty clay (about 66% clay, 26% silt, 8% fine sand) with natural water content of about 110% “ 120% ADVANTAGES The consolidation period can be reduced by electro osmotic consolidation technique. The process is very efficient in low permeability clays in which the electro osmotic permeability is greater than the hydraulic permeability. Electro osmotic permeability was 200 to 1000 times greater than the hydraulic permeability. This method is suitable for local application on small volumes or for impermeable barrier construction. After treatment water content decreases and shear strength increases and it was more than what was expected just from dewatering. DISADVANTAGES The pH of soil will increase to as high as 11 or 12 at the cathode and decrease to almost 2 at the cathode. Metal anodes will corrode. The applied voltage and electric current generates heating. The heating effect increases power consumption. CONCLUSION Dewatering of sediment can be enhanced by the electro osmosis induced by the

application of an electric potential Incorporating electro kinetic item in the properties of PVD is very useful in dewatering highly compressible and low permeability clay Faster rate of settlement was achieved using electro conductive drains Significant rate of increase in strength of EO-treated soils was achieved as a consequence of decrease in water content formation of menisci in the soil voids and bonding of soil particles by insoluble chemical precipitates as a result of the complex chemical reactions generated and ion re-exchange which altered the plasticity of clay. Application of electric potential will result in energy consumption.

5. What are the various components, stages and methods of drainage? Explain in

detail. Drainage means the removal of excess water from a given place. Two types of drainage can be identified:

i) Land Drainage: This is large scale drainage where the objective is to drain surplus water from a large area by such means as excavating large open drains, erecting dykes and levees and pumping. Such schemes are necessary in low lying areas and are mainly Civil Engineering work ii) Field Drainage This is the drainage that concerns us in agriculture. It is the removal of excess water fromthe root zone of crops.

Water in Soil After Heavy Rain The main aims of Field drainage include:

To bring soil moisture down from saturation to field capacity. At field capacity, air is available to the soil and most soils are mesophites ie. like to grow at moisture less than saturation. ii) Drainage helps improve hydraulic conductivity: Soil structure can collapse under very wet conditions and so also engineering structures. iii) In some areas with salt disposition, especially in arid regions, drainage is used to leach

excess salt. iv) In irrigated areas, drainage is needed due to poor application efficiency

which means that a lot of water is applied.

v) Drainage can shorten the number of occasions when cultivation is held up waiting for soil to dry out.

Two types of drainage exist: Surface and Sub-surface drainage.

DESIGN OF SURFACE DRAINAGE SYSTEMS:

Surface drainage involves the removal of excess water from the surface of the soil. This is done by removing low spots where water accumulates by land forming or by excavating ditches or a combination of the two. Surface Drainage

Land forming is mechanically changing the land surface to drain surface water.

This is done by smoothing, grading, bedding or leveling.

Land smoothing is the shaping of the land to a smooth surface in order to eliminate minor differences in elevation and this is accomplished by filling shallow depressions.

There is no change in land contour. Smoothing is done using land levelers or planes

Land grading is shaping the land for drainage done by cutting, filling and smoothening to planned continuous surface grade e.g. using bulldozers or scrapers. Design of Drainage Channels or Ditches

Estimation of Peak Flows: This can be done using the Rational formula, Cook's method, Curve Number method, Soil Conservation Service method etc.

Drainage coefficients (to be treated later) are at times used in the tropics used in the tropics especially in flat areas and where peak storm runoff would require excessively large channels and culverts.

This may not apply locally because of high slopes.

The Rational FormulaIt

states that:

qp = (CIA)/360

where qp is the peak flow (m3 /s);

C is dimensionless runoff coefficient;

I is the rainfall intensity for a given return period. Return period is the average number of years within which a given rainfall event will be expected to occur at least once.

A is the area of catchment (ha). Using the Rational Method

Obtain area of catchment by surveying or from maps or aerial photographs.

ii) Estimate intensity using the curve in Hudson's Field Engineering, page 42.

iii) The runoff coefficient C is a measure of the rain which becomes runoff. On a corrugated iron roof, almost all the rain would runoff so C = 1, while in a well drained soil, nine-tenths of the rain may soak in and so C = 0.10. The table (see handout) from Hudson's Field Engineering can be used to obtain C value. Where the catchment has several different kinds of characteristics, the different values should be combined in proportion to the area of each.

b) Cook's Method

Three factors are considered:

Vegetation,

Soil permeability and

Slope.

These are the catchment characteristics.

For each catchment, these are assessed and compared with Table 3.4 of Hudson's Field Engineering

A catchment may be heavy grass (10) on shallow soils with impeded drainage(30) and moderate slope(10).

Catchment characteristics (CC) is then the sum of the three ie. 50.

The area of the catchment is then measured, and using the Area, A and the CC, the maximum runoff can be read from Table 3.5 (Field Engineering, pp. 45).

Surface Drainage Channels

The drainage channels are normally designed using the Manning formula (see Chapter 6). The required capacity of a drainage channel is calculated from the summation of the inflowing streams (See Note)

The bed level of an open drain collecting flow from field pipe drains should be such as to allow free fall from the pipe drain outlets under maximum flow conditions, with an allowance for siltation and weed growth. 300 mm is a reasonable general figure

Surface Ditch Arrangements

The ditch arrangement can be random, parallel or cross- slope.

Random ditch system: Used where only scattered wet lands require drainage.

Parallel ditch system: Used in flat topography. Ditches are parallel and perpendicular to the slope. Laterals, which run in the direction of the flow, collect water from ditches.

DESIGN OF SUB-SURFACE DRAINAGE SYSTEMS

Sub-surface drainage is the removal of excess groundwater below the soil surface.

It aims at increasing the rate at which water will drain from the soil, and so lowering the water table, thus increasing the depth of drier soil above the water table.

Sub-surface drainage can be done by open ditches or buried drains.

Sub-Surface Drainage Using Ditches Sub-Surface Drainage Using Ditches

Ditches have lower initial cost than buried drains;

There is ease of inspection and ditches are applicable in some organic soils where drains are unsuitable.

Ditches, however, reduce the land available for cropping and require more maintenance that drains due to weed growth and erosion.

Sub-Surface Drains Using Buried

DrainsSub-Surface Drainage Using Buried DrainsBuried drains refer to any type of buried

conduits having open joints or perforations, which collect and convey drainage water.

They can be fabricated from clay, concrete, corrugated plastic tubes or any other suitable material.

The drains can be arranged in a parallel, herringbone, double main or random fashion.

Sub-Surface Drainage Designs

The Major Considerations in Sub-surface Drainage Design Include:

Drainage Coefficient;

Drain Depth and Spacing;

Drain Diameters and Gradient;

Drainage Filters.

Drainage Coefficient

This is the rate of water removal used in drainage design to obtain the desired protection of crops from excess surface or sub-surface water and can be expressed in mm/day , m/day etc.

Drainage is different in Rain-Fed Areas and Irrigated Areas

6. Compare the various dewatering systems suitability, uses, merits and demerits.

1. Open sump ditches 2. Well point systems

Deep well systems with answers …. 7. Explain in brief the various steps for designing a dewatering system. (AUC NOV /DEC 2010)

Objectives In this section you will learn the following

Subsoil investigation

Source And Water Table Details

Distance of well points from the source of seepage

Effective Wall Radius r u

Discharge computations Design steps for dewatering systems

Design of a dewatering system requires the determination of the number, size, spacing, and penetration of wells or well points and the rate at which water must be removed from the pervious strata to achieve the required groundwater lowering or pressure relief. The size and capacity of pumps and collectors also depend on the required discharge and drawdown. The essential steps involved in the designing of the dewatering system are given below:

Subsoil investigation The characteristics of the soils adjacent and beneath the excavation should be investigated well. Grain size distribution and permeability are the two parameters to be determined. Indian Standard recommends a field pumping test for this case.

Source And Water Table Details Source of seepage and knowledge of the water table at a particular site are the most important factors to be considered while designing a dewatering system. The source of seepage depends on the geological features of the area, nearby streams or water bodies and amount of drawdown. A flow may be from an aquifer being drained the distance to which is known as the radius of influence. It can be estimated from the draw down curve established from a field pumping test.

Distance of well points from the source of seepage - If the radius of influence R is large compared to the radius of the well, only an approximate estimation of R may be sufficient since the discharge is not much sensitive to the value of R - An accurate estimation of the distance L from the well to the river should be made for a particular dewatering system, since the discharge is inversely proportional to the value of L.

Effective Wall Radius r u The effective wall radius is decided based on the installation of the wells with or without filter. If a well is installed without gravel or a sand filter the effective radius can be taken as one half the outer diameter of the well screen. If a filter is used, the well radius is taken as one half the outside diameter of the filter.

Discharge computation

The discharge Q of the well is then calculated using the formula given below:

where Q is the discharge, k is the permeability, H is the depth of strata, h is the height of water in the well, r is the radius of well, R is the radius of influence. 8. Explain the vacuum dewatering systems in neat sketch. (AUC NOV /DEC 2010)

Gravity methods, such as well points and deep wells are not much effective in the fine-grained soils with permeability in the range of 0.1 –10 x 10 -3 mm/s.

Such soils can be dewatered satisfactorily by applying a vacuum to the piping system.

A vacuum dewatering system requires that the well-point screens, and rise a pipe be surrounded with filter sand extending to within a few metres of the ground surface

This method is most suitable in layered or stratified soils with coefficient of permeability of the range 0.11 - 0 x 10 -4 cm/s.

A typical vacuum dewatering system in a stratified is shown in fig.

In this system the well points should be placed closer than the conventional system . it is common to use suction pump in the system and practical maximum height off lift is about 3 to 6m .

9. Explain the properties and application of flow net in detai. (AUC MAY/JUNE 2013)\ Flow of Water in Soil Permeability and Seepage Water flows through the voids in a soil which are interconnected. This flow may be called seepage, since the velocities are very small Water flows from a higher energy to a lower energy and behaves according to the principles of fluid mechanics

An Energy Equation from Fluid Mechanics

An Energy Equation for Water Flow in Soils The velocities of water flowing through the voids in a soil are very small, and the velocity head in the previous equation may be neglected. Therefore, for flow of water in soil the equation is:

Calculation of Heads Basic Principles •Total Head = Pressure Head + Elevation Head •The pressure head is zero at a water surface. •The head loss in the water is assumed to be zero. •All head loss occurs in the soil Elevation, pressure and total head Pore pressure at a given point (e.g. point A in the diagram) can be measured by the height of water in a standpipe located at that point. The height of the water column is the pressure head (hw)

hw = u

w or

hw = P w

The elevation head (hz) of a point is its height above

the datum line. • The height above the datum of the water level in

the standpipe is the total head (h).

h = hz + hw

Energy Loss Equation Water is flowing in the direction indicated in the figure. Energy Loss Equation

Darcy's law

• The rate of flow of water q (volume/time) through cross‐sectional area A is found to be proportional to hydraulic gradient i according to Darcy's law:

Darcy's law v = q = k.ii = Dh Ads q=vA • The value of the coefficient of permeability k depends on the average size of the pores and is related to the distribution of particle sizes, particle shape and soil structure. Coefficient of Permeability The notation for coefficient of permeability is k. • It is sometimes called hydraulic conductivity. Typical Values

10. Define filter and discuss filter requirements in detail. (AUC NOV /DEC 2012)