Proper Construction of detention and sediment basins and their conversions

Transcription

1 Proper Construction of detention and sediment basins and their conversions By Chris Droste, CESCO Senior Erosion Control Specialist Westmoreland Conservation District

2 Discussed topics Pond embankment construction Installing the Outfall barrel Installing the risers Building fore bays and weirs Conversions and as-built surveys Problems with ponds (embankment failures)

3 Embankment construction & compaction

4 Keyway and embankment construction Location of select material for embankment needs decided early Inspections are critical at this time

5 Embankment construction Soils should be compacted according to plan specs or by an on-site geotechnical engineer. Final slopes should be to plan configuration

6 Excavation for keyway

7 Ensure the excavation is close to depth as shown on the plans

8 Inspections of keyway drains Inspectors should witness the installation of these

9 Material for the embankment should compose of clay or silty clay. Locate the soils drawing to determine about where the soils may be

10 Slow layering and moving of the earth to establish proper lifts is important to the compaction of the soil

11 The embankment should be constructed with a NONvibratory roller/ sheep foot from suitable materials using 6-9 lifts

12 The Final stage Now is the time to make sure the pond Complies with the plan design Is the embankment to correct elevation? Are the slopes 2:1, 3:1? Is the pond about the right volume?

13 Outlet barrel Cradle Detail

14 Observing the barrel installation

15 Method to install the barrel The barrel should be placed early Avoid having the contractor cut through the embankment to install the pipe. If they must cut through the embankment..

16 Outlet Barrel Installation Detail

17 Problem Seep collars should be notched Into existing embankment Compaction around sides of the pipe is important

18 Barrel and seep collar installation Bedding with 2A modified Around pipe is an acceptable Fill material around the pipe 2A modified solidifies when Moisture comes in contact. ADS HD (grey) pipe is used in this example

19 Hand work is needed to push the 2A modified into the base ADS- HD plastic pipe is acceptable To using concrete if it meets the Standards

20 Small compactors wackers can be used to tamp The 2A modified around the base of the pipe

21 Ensure your seep collar excavated area is into the excavated area of embankment

22 Forms can be used, or backing up the trench with the 2A modified, to pour the collar

23 The poured collar should be high enough above the pipe as shown in the plan details

24 Problems with using plastic pipe

25 The problems using corrugated plastic pipe as the barrel Plastic is meant to be bed with Stone to prevent collapse. The sides of the pipe will compress with compaction or weight on top unless there is sufficient support on each side of the pipe. Using stone around the pipe will act as a French drain and water will pull soil with it. This causes failure of the embankment.

26 Sinkhole Sinkholes that develop near the riser will tell you there is a problem with the barrel.

27 The Cause A joint failed under compaction Pond was draining too fast Sediment pollution occurred

28 Another example

29 Water is traveling along side the pipe in the Embankment, causing a sinkhole to develop

30 A Sign that you have a problem is a hole in the embankment resulting from Piping.

31 Pipe is elliptical due to compaction on top Pipe sections have separated allowing Water to seep through embankment

32 Riser installation

33 Riser/outlet barrel should be properly sealed

34 Outlet risers require footing and trash racks

35 Concrete footing for (Type M) outlet riser

36 Skimmers

37 Skimmers Must be attached to either a permanent Riser box or barrel Skimmer flexible hose should be secured and water tight to Type M inlet box Close off holes in the box riser Install correct orifice plate in dewatering tube.

38 Skimmers Skimmers must be set on a concrete block or perch, not a haybale! Guide posts are required to keep the skimmer moving and detaching from the flex hose A rope is recommended to be attached to skimmer and tied to embankment for access.

39 Skimmer with rebar posts and rebar perch at sediment cleanout

40 Skimmers problems Skimmer will move of block and be stuck in mud Skimmer needs guideposts or skimmer will move off block

41 Skimmers with long flex hoses Avoid Long flex hoses as they float and do not allow for dewatering of the skimmer

42 Problems with Contractor made skimmers This skimmer won t drain and may break off fittings The discharge pipe is not long enough Skimmer won t rise.

43 Problems with Contractor made skimmers Furnco Fittings do not work well!

44 Skimmer is very large and heavy and won t float

45 Skimmer problems Corrugated plastic pipe Duck taped is not a good alternative For connection

46 Skimmer Problems Guideposts are laying down Skimmer is disconnected from flexible pipe

47 Skimmer problems Guideposts and perch are good Holes in riser box are not sealed

48 Emergency spillway ** Over-excavate to Accommodate the size of rock Ensure the rock does not fill in the entire spillway

49 Cutoff walls in spillway Make sure the depth and width is correct.

50 Forebay construction

51 Wet Pond with Forebay Detail

52 Forebay weirs Correctly lined Extend the rock into the bottom of the pond or erosion WILL occur!

53 Forebay weirs with concrete.

54 Forebays with inlet overflows Install Barrel in embankment the same as the main pond

55 Aprons Discharge should exit onto a Proper rock lined apron Should be installed very soon after end wall is installed

56 Are the aprons to the correct size?

57 Use the proper size rock. Too large blocks the flow Too small: water washes out the apron.

58 - Watch using cheaper Quarry rock or on-site rock. - Shale or broken sandstone will weather and fall apart. - The Rock should be to the correct size according to the plans

59 Conversions

60 Vegetation has grown on sediment mounds. - Old Baffles need removed - Sediment needs removed to original elevation - Sediment Riser needs removed - New Apron Rock will need installed, low flow channels

61 Does the pond need to be cleaned out before conversion? Yes: if the pond was used as a sediment basin, most likely it will need cleaned out

62 - Most of the time Ponds are full of water, soils are very mucky - A Hauling material out is very time consuming - Keeping water to one side helps, create a dam and keep a - Pump handy to remove water from entering workspace. - Place a layer of 2B stone around old riser base to filter sediment

63 -Phased projects may allow you to find a spot to dig a pit to dump the mucky soil and Allow to dry. -Place orange fence around the area to keep people from entering and getting stuck.

64 - Establishing the bottom elevation (survey) - Installing the underdrains - Preparing for the low flow channels Sourcing good material For the pond bottom may Be a problem at the end Of many projects

65 Seed with the appropriate mix, slope mix And detention pond mix can be used. Embankments need to be stable with Straw/net blanket or hydro-mulch

66 Survey in the orifice elevations of the riser when converting as to ensure as-builts are correct

67 Inspection and measurement of the orifices is very important to the functionality of the pond and for the As-built survey to be correct

68 Establish the aprons and taper into the low flow channels (if required on plans)

69 Pond bottoms should be stabilized. Blankets work better than blown straw mulch. At this pond, the surveyor can come in and perform the final pond survey.

70 The end Contact Chris Droste or