Surface flow - above ground

Transcription

1 SAICE (R&H) - Short Course: Drainage II Surface Flow May 2011 Surface flow - above ground 1

2 A Proper Surface Drain System Should: 1. Prevent or restrict water from entering substructure 2. Discharge water draining out of the substructure Surface Water Control Surface runoff from drainage basin Runoff from cut face Cut-off ditch Ditch next to track Lateral discharge out of the track 2

3 kilometers Calculating Runoff Q=CIA with I = Rainfall Intensity (mm/hr) C = Runoff coefficient A = Area (m 2 ) 3

4 Sizing a Ditch Q = R 2/3 S n 1/2 A R= wetted perimeter S = ditch longitudinal slope n = ditch roughness factor A = ditch cross-section. Track Side Drainage Earth Ditch Concrete Ditch Embankment 4

5 Track Ditch with no Grade! Open Ditch 10 5

6 Ditch Design Procedures Evaluate area Cut Track Profile Ditch outlet Ditch bottom Watershed Ditch outlet Distance Along Track Watershed Point 6

7 Minimum ditch depth Ballast Ditch Design Procedures 1.Determine from walking inspection/survey the following: Ditch placement and outlets Survey rail profile and/or ditch profile Survey elevation of outlets Check compatibility of ditch with existing or planned railroad structures Soil conditions Groundwater conditions 7

8 Ditch Design Procedures (cont.) 2. Select ditch parameters Type Minimum and maximum ditch grade Cross-section The minimum depth below track to accept lateral discharge 3. Find the ditch profile that allows minimum soil excavation Ditch Design Procedures (cont.) Evaluate area Cut Track Profile Ditch outlet Ditch bottom Watershed Ditch outlet Distance Along Track 8

9 Ditch Design Procedures (cont.) 4. Calculate flow quantities 5. Check ditch feasibility. If not feasible return to Step 2. Earth to be removed Ditch bottom follows designed ditch grade 9

10 Ditching Machine Cutting Head on Ditching Machine 10

11 Spot Ditching Concrete Lined Ditch 11

12 SAICE (R&H) - Short Course: Drainage III Subsurface Flow May 2011 Subsurface water forms Moisture content- water molecules bound to the surface of the soil particles. Capillary moisture in the voids. Gravitational or free water in the voids

13 Subsurface Drainage Defined as: Underground means to remove gravitational or free water (water that can move under gravitational force) from the track substructure and includes: 1. Rainfall on the track 2. Ground water flow 25 Flow Directed Toward a Water Table with Permeable Subgrade: No problems! Original Ground Surface Water table 26 13

14 Diversion by Subgrade/Subballast Rainfall 27 Lateral drainage impeded Often a proper lateral slope does not exist and/or the subballast lateral drainage is blocked. Examples include the following: Subballast keyed into the subgrade. Major depressions caused by soft subgrade are known as ballast pockets. Foul shoulder impeding lateral drainage. Drainage layers with long flow paths 28 14

15 Subballast Keyed Into Subgrade 29 Ballast Pocket Original Subgrade Surface Soft Clay Subgrade Trapped Water 30 15

16 Shoulder Ballast Contamination Traps Water 31 Long Flow Path Restricts Drainage Flow Flow path length Q= AkH L flow 32, 16

17 Case Study: Pueblo Colorado 0.3m thick k = 10m/day 2m Imported impermeable clay subgrade Natural subgrade : silty sand

18 Subsurface Solutions Examples 35 Draining Ballast Pocket Trench Pipe 36 18

19 Draining Ballast Pocket 37 Longitudinal Drain Between Tracks Perforated Pipe 38 19

20 Longitudinal Drain Between Tracks Prefabricated unit 39 Drawdown Drains Drain 40 20

21 Blanket and Drawdown Drain Blanket 41 Pipe Drain Examples Low Permeability Cap Broadly Graded Aggregate Perforated Pipe Fine Aggregate Coarse Aggregate Filter fabric 42 21

22 Perforated pipe 43 Geocomposite Edge Drain Filter Fabric Drainage Core Slot 44 Trench 22

23 Combination Drain Concrete Subsurface Drain 45 23