Numerical On Surface Drainage Sub Surface Drainage System Seepage and Capillary Cut Off

 

Lecture Hour-51

10CV755

UNIT 8

HGD NUMERICALS

1)  The distance between the farthest point in the turf covered drainage area(with an average slope of 1.5% towards the drain ) and point entry to side drain is 200m .the weighted value of runoff coefficient is 0.25. the length of longitudinal open drain in a sandy clay soil from the inlet point to the cross drainage is 540m. The velocity of flow in side drain may be assumed as 0.6m/sec so that silting and erosion are prevented .estimate the design quantity of flow on the side drain for years period of frequency of occurrence of the storm. Solution: C=0.25

Inlet time T1 for an average turf with 1.5% slope corresponding to 200m distance = 33 min ( from graph ) Time T2 for water to flow through 540m length drain at 0.6m/sec =540/(0.6*60) = 15 min Duration are time time of concentration T =33+15=4 =33+15=48min 8min Drainage area =540*200=1 =540*200=108000m 08000m2 Ad= 108000/1000=108 108000/1000=108 Design value of rain fall intensity for 10yeras frequency of occurrence and corresponding to 48min. duration =70mm/hr(from figure) therefore ,i=70/3600m ,i=70/3600mm/ m/ sec. Design quantity flow, =0.25*(70/3600)*108 =0.25*(70/3600 )*108

= 0.525m3/sec

 

2)  The maximum quantity of water expected in one f open longitudinal drains on clayey soil is 0.9m3/sec. design the cross section and longitudinal Slope of trapezoidal drain assuming the bottom width of trapezoidal section to  be1.0m and cross slope to be 1.0 vertical to 1.5 horizontal. Allowable velocity of flow in the drain is 1.2m/sec and manning’s manning’s roughness co -efficient is 0.02. Solution: 1)  Cross section: The allowable velocity of flow =1.2m/sec From the discharge equation A= Q/V = (0.9/1.2) = 0.75m2  For the trapezoidal trapezoidal sec section tion with bottom width 1.0m and side slope 1.0 vertica verticall to 1.5 horizontal, horizontal, when the depth of flow is d m meter, eter, the top width = ((1+3d) 1+3d) and cross sectional area , A= (1+1+3d)*d/ (1+1+3d)*d/2 2 0.75 = d +3d2 /2 1.5d2 + d-0.75 =0

solving the equation

d = 0.45m actual depth of drain =0.45m Free board =0.15m Depth of side drain =0.45+0.15 =0.6 m 2)  Longitudinal slo pe may be found from from manning’s manning’s formula  V=1/n (R 2/3 S1/2 ) Where R = (A/P) P= [√ (.452 + (1.5*0.45)2 ]*2 +1.0 = 2.62m R = A/P = (0.75/2.62) =0.286m

 

S1/2  = Vn/R 2/3 = 1.2*0.02/( 0.2862/3) =0.0553 Slope S = 1 in 320.

SUB SURFACE DRAINAGE SYSTEM:

Changes in moisture content of subgrade are caused by fluctuations in ground water table seepage flow , percolation of rain water and movement of capillary water and even water vapour. In sub surface drainage of highways, it is attempted to keep the variation of moisture in subgrade soil to a minimum. However only the gravitational water is drained by usual drainage system.  

Lowering of water table:

The highest level of water table should be fairly below the sub grade in order that, the sub grade and pavement layer are not subjected excessive moisture. From  practical considerations it is suggested that the water table should be at least 1.2m  below the subgrade. And places where water table is high the best remedy is to take the road formation on embankment of height not less than 1 to 1.2m when the formation It is to be at or below the general ground level it would be necessary to lower the water table. If the soil is relatively permeable it may be possible to lower the high water table  by merely construction of longitudinal drainage trenches with drain pipe and filter sand the depth of the trench wou would ld on the requir required ed lowering of water table the distance between the drainage trenches and soil type. Show in figure.

 

 

Lowering high water table in permeable soil.

If the soil is relatives less permeable the lowering of ground water level may not be adequate at the center of the pavement and in between the two longitudinal drain trenches. Hence in addition the transvers drains may have to be provided in order to the effectively drain off the water and thus to lower the water table up to the level of transvers drains.

 

  Sub surface drainage system with transvers drains.

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CONTROL OF SEEPAGE FLOW: When the general ground as well as the impervious strata below or sloping seepage flow is likely to exist if the seepage zone is at the depth less than 0.6 to 0.9m from the sub grade level longitudinal pipe drain trench filled with filter material and clay seal may be constructed to intercept the seepage flow.

Control of seepage flow.

 

    CONTROL OF CAPILLARY RISE: If the water reaching the sub grade due to capillary rise is like to be detrimental it is possible to solve problem by arresting the capillary rise instead of lowering the water table. The capillary rise may check either by capillary cut off of any one of the following two types: 1)  A layer of granular material of suitable thickness is provided during the construction of embankment between the subgrade and the highest level of sub surface water table. The thickness of the granular capillary cut off layer should be sufficiently higher than the anticipated capillary rise with in the granular layer so that the capillary water cannot rise above the cut off layer.

Granular capillary cut off

2)  Another method of providing capillary cut off Is by inserting an impermeable or an bituminous layer in place of granular blanket.

 

  Impermeable capillary cut off