Storm Sewer Trenchless Upgrade Alternatives and Recommendations

Transcription

1 Storm Sewer Trenchless Upgrade Alternatives and Recommendations Background Approximately 1,930 feet of the 40-inch and 42-inch CMP storm sewer pipe from manhole M22 to manhole M12 will be evaluated for an upgrade using trenchless construction methods. The existing pipe is located under the paved street from 4 th Avenue to Pine Street. At manhole M22, the incoming upstream storm water flow will be diverted around this pipe section to be rehabilitated. A new parallel storm sewer will be constructed in a new alignment. The new parallel storm pipe would convey the incoming upstream runoff while the trenchless portion is being built. With the proposed reduced design flow capacity, the existing storm pipe from M22 to M12 can be rehabilitated with a smaller diameter pipe. The existing CMP storm pipes will be reduced to an inside diameter 24-inches. The 24-inch pipe will have the capacity to convey the storm flow from the adjacent nearby drainage area. Two trenchless repair options were evaluated: Traditional sliplining with a HDPE pipe and grout in the annular space Cured in place pipe liner with grout in the annular space Existing Pipe Condition The 1,930 feet of existing storm pipe has the following characteristics: There are 9 manholes total. Five of the manholes are constructed of CMP and four are precast concrete. There is 652 feet of perforated 40-inch CMP pipe in the section along 4 th Avenue. The 40-inch CMP is perforated with ¾ holes located at the bottom of the pipe at the 4:30 and 7:30 positions. There is 1,278 feet of 42-inch CMP pipe. Portions of the existing CMP have significant metal loss in the invert. Two sanitary sewer service pipes penetrate through the existing 42-inch CMP in the 330 foot segment between MH17 and MH15. One electric or phone utility cable penetrates the CMP pipe between MH15 and MH13. A number of localized defects were observed in the existing pipe including rough joints, holes, offset joints and debris piles. The invert grade is fairly uniform. Overall the pipe is fairly round and is not collapsing at this time. Two catch basin leader pipes connect directly into the storm pipe (no manhole at these connections). 1

2 The pipe is buried in the street, under 4 feet to 8 feet of cover. The storm pipe has a constant flow of approximately 8 to 10-inches. Sliplining with a HDPE Pipe In this trenchless repair alternative, a 28-inch SDR 17 HDPE butt-fused pipe with an inside diameter of approximately 24.7-inches would be inserted inside the existing 40 and 42-inch CMP storm drain pipe. The annular space between the two pipes would be filled with a cement grout. The five existing CMP manholes would be removed and replaced with modern concrete manholes. The end result would be a strong, long-lasting HDPE storm sewer. This pipe rehabilitation method includes: Pipe cleaning and pre-construction CCTV inspection Storm flow bypassing Modify three utility crossings Install HDPE slipliner from excavated access pits Connect two catch basin leaders Install cement grout in the annular space Surface restoration Pipe cleaning and pre-construction CCTV inspection Prior to starting rehabilitation work, the pipe will be thoroughly cleaned by the construction contractor and inspected with a camera. The purpose of the pre-construction CCTV is to identify any defects that may restrict the insertion process and confirm the pipe is ready for sliplining. Storm flow bypassing A new parallel storm pipe system will be constructed from M22 to M12 prior to performing the sliplining work. All runoff that is collected upstream of M22 will be diverted through the new parallel pipe. Therefore, the only storm flow that will need to be controlled during the sliplining work will be local runoff coming from the immediate surrounding streets and drainage areas. The design flow for this section of the storm system is 6.5 cubic feet per second or 3,000 gallons per minute (gpm). The storm flow must be bypassed around the work area to prevent washout of the cement grout in the annular space and reduce complications with water flowing through the pipe when the HDPE liner is being installed. The primary bypass pump shall have a minimum capacity of 3,000 gpm and a standby pump of the same capacity shall be available at the site. It is anticipated that two 8-inch dri-prime type pumps would be needed. Bypassing would be accomplished by installing pumps and a temporary bypass pipe between manholes that are upstream and downstream of the pipe being lined. Typically, the sewage is pumped from the next manhole upstream of 2

3 the segment being lined and discharged into the next manhole downstream of the segment being worked on. There are intermediate catch basins that contribute runoff flow to the storm sewer being rehabilitated. Water would be collected with individual pumps that would be placed at each one of these points as the work progresses through the site. Modify Three Utility Crossings Two sanitary sewer services and one electric/phone cable penetrate and cut through the existing CMP storm pipe in three separate places along the pipe alignment. The electrical/phone cable that is approximately 14 feet upstream of M13 will need to be protected from damage during the HDPE liner installation. This can be accomplished by having a man enter the storm pipe and temporarily raise the flexible conduit up and support it from the crown of the 42 inch CMP host pipe. An excavation will not be needed here. The existing electric/phone conduit can remain inside the CMP host pipe. After the liner is installed it will be encased and protected in the annular space grout. Excavations will be required to expose the two sanitary sewer pipes that cross through the 42 inch storm pipe between M17 and M15. From the video image, it is estimated that there is approximately 24 to 30 inches of space between the bottom of the CMP casings and the invert of the 42 inch CMP host pipe. Prior to inserting the HDPE liner, the small CMP casings will be removed and the sanitary sewer lines temporarily removed. After the HDPE liner is in place, the sanitary pipes can be reinstalled, without the CMP casings around them. The new grout in the annular space will support the sanitary sewer crossings. Therefore the CMP casing around the sewer services is not required. This will provide additional space below the existing sanitary pipes. Some slight downward deflection of the HDPE crown may be needed to make this fit. The overall impact to the new HDPE liner capacity will be minimal. Install HDPE slipliner Sliplining the existing storm pipe would involve insertion of a continuous butt-fused HDPE liner into the existing CMP host pipes. A 28-inch SDR 17 HDPE liner would provide an interior pipe diameter of approximately 24.7-inches. The 24.7-inch pipe would have adequate capacity to convey the future design flows. Sliplining with a continuous butt-fused pipe requires the excavation of insertion pits at strategic points along the storm pipe to provide access points for the pipe insertion. The insertion pits would be approximately 40 feet long and 15 feet wide at the ground surface. Four insertion pits would be required along the CMP host pipe alignment. Additional excavations would be required to reconnect the two directly connected storm pipes to the new HDPE liner. Insertion pits can be located where other excavations are already required for other work such as new manholes, utility crossings or reconnection of catch 3

4 basin leaders. The final location may be adjusted depending on the Contractor s method of construction. We anticipate that the five sliplining insertion pits would be constructed in conjunction with manhole removal and replacement work. It is assumed that the slipline pit excavations would be at the following manholes: M13 M15 M17 M19 M22 These five proposed pit locations also have adequate space behind them to layout the long butt-fused HDPE pipe during the initial insertion process. Two Direct Storm Pipe Connections Excavations will be needed to connect two existing catch basin leaders to the new HDPE liner. One is located downstream of M17 and the other is located downstream of M15. The tie-in between the new HDPE and the existing CMP leaders would be completed before the annular space grout is installed. Install cement grout in the annular space The entire annular space between the liner and host pipe will be filled with light-weight flowable cement grout. This will prevent ground water from entering the annular space, prevent the liner from floating inside the carrier pipe, prevent collapse of the liner due to hydrostatic pressure and will also provide a structurally sound storm pipe system when complete. A special light-weight flowable grout is required to prevent the HDPE liner from floating during installation and also to fill the entire space when injected from one end of the pipe run. This specialized grout is available locally and has been used successfully on other projects in Anchorage. Post-rehabilitation CCTV inspection A post-construction CCTV inspection by the contractor will be reviewed by the engineer to confirm that the work has been completed as required and that the pipe is clean and functioning properly. Surface Restoration Restoration of the surface will be required after the work is completed. Asphalt repair and landscaping repairs would be needed at the excavation areas as well as surrounding surface that is damaged during the work. 4

5 Estimated Cost Sliplining in conjunction with grouting of the annular space will provide a structural rehabilitation of the pipe. The overall construction cost to complete this rehabilitation method is estimated to be $1,020,000. A more detailed description of the cost is contained in the following table. PROJECT: Pine Street Storm Sewer Upgrade OPTION: HDPE Sliplining NO. DESCRIPTION QTY UNIT UNIT TOTAL COST 1 Type II & II-A classified fill and backfill 1400 Ton $20 $28,000 2 Bedding material 100 Ton $25 $2,500 3 Disposal of unusable or surplus material 800 CY $18 $14,400 4 Adjust Sanitary Sewer Crossing 2 EA $10,000 $20,000 5 Adjust Electric/Phone Utility Crossing 1 EA $3,000 $3,000 6 F&I Type I Storm Manhole 5 EA $7,000 $35,000 7 Furnish and Install 28" SDR 17 HDPE Slipliner 1930 LF $180 $347,400 8 Storm Sewer Flow Control 1 LS $35,000 $35,000 9 Connect Existing Catch Basin Leaders 2 EA $5,000 $10, Annular Space Grouting 380 CY $600 $228, Pre-rehabilitation Sewer Pipe Television Inspection 1930 LF $7 $13, Post-rehabilitation Sewer Pipe Television Inspection 1930 LF $4 $7, Remove and Replace Existing Pavement 900 SY $60 $54, Construction Survey Measurement 1 LS $5,000 $5, Traffic Maintenance 1 LS $15,000 $15,000 SUBTOTAL $818,530 CONTINGENCY 25% $204,633 TOTAL ESTIMATED CONSTRUCTION COST $1,020,000 Lining with a Cured in Place (CIPP) Pipe 5

6 In this trenchless repair alternative, a 24-inch CIPP liner pipe would be inserted inside the existing 40 and 42-inch CMP storm drain pipe. The annular space between the new liner and the host pipe would be filled with a cement grout. The existing CMP manholes would be removed and replaced with modern concrete manholes. The end result would be a strong, long-lasting storm sewer. This pipe rehabilitation method includes: Pipe cleaning and pre-construction CCTV inspection Storm flow bypassing Install CIPP from existing manholes Connect existing catch basin leaders Install cement grout in the annular space Surface restoration Pipe cleaning and pre-construction CCTV inspection Prior to starting rehabilitation work, the pipe will be thoroughly cleaned by the construction contractor and inspected with a camera. The purpose of the pre-construction CCTV is to identify any defects that may restrict the insertion process and confirm the pipe is clean and ready for sliplining. Storm flow bypassing A new parallel storm pipe system will be constructed from M22 to M12 prior to performing the sliplining work. All runoff that is collected upstream of MH22 will be diverted through the new parallel pipe. Therefore, only local storm flow coming from the immediately surrounding streets and drainage areas will need to be controlled during the lining work. The design flow for this section of the storm system is 6.5 cubic feet per second or 3,000 gallons per minute (gpm). The storm flow must be bypassed around the work area to prevent washout of the cement grout in the annular space and reduce complications with water flowing through the pipe when the CIPP liner is being installed. The primary bypass pump shall have a minimum capacity of 3,000 gpm and a standby pump of the same capacity will be available at the site. It is anticipated that two 8-inch dri-prime type pumps would be needed. Typically, the sewage is pumped from the next manhole upstream of the segment being lined and discharged into the next manhole downstream of the segment being worked on. There are two intermediate catch basins that contribute runoff flow to the storm sewer being rehabilitated. Water would be collected with individual pumps that would be placed at each one of these points as the work progresses through the site. Three Utility Crossings 6

7 Two sanitary sewer services and one electric/phone cable penetrate and cut through the existing CMP storm pipe in three separate places along the pipe alignment. The electrical/phone cable that is approximately 14 feet upstream of M13 will need to be protected from damage during the CIPP liner installation. This can be accomplished by having a man enter the storm pipe and temporarily raise the flexible conduit up and support it from the crown of the 42 inch CMP host pipe. An excavation will not be needed. The existing electric/phone conduit can remain inside the CMP host pipe. After the liner is installed, the utility would be encased in the grout that is pumped into the annular space. Two sanitary sewer pipes cross through the 42 inch storm pipe between M17 and M15. From the video image, it is estimated that there is approximately 24 to 30 inches of space between the bottom of the CMP casings and the invert of the 42 inch CMP host pipe. The small CMP casings would be left in place during the CIPP liner installation work and during the grouting work. The new grout in the annular space will provide additional support to the sanitary sewer crossings. Install CIPP liner Rehabilitating the existing storm pipe with a cured in place liner would involve inverting a resin impregnated CIPP liner into the existing CMP host pipes and filling the annular space with grout after the liner has cured and is rigid. The 24-inch CIPP liner would provide an interior pipe diameter of approximately 24-inches. The CIPP liner wall is relatively thin in comparison to the HDPE pipe option. The CIPP liner would be supported by the surrounding low strength grout. The CIPP liner has a smooth interior wall and would provide excellent flow characteristics and would have adequate capacity to convey the future design flows. This proposed trenchless rehabilitation method is unique. The CIPP lining process itself is standard and is proven. The annular space grouting is a common construction practice. The combination of the two is unique. The advantage to this process is the ability to reduce the number and size of the excavation areas, no need for a long layout area for the insertion pipe and a reduced impact to the public due to the smaller project footprint. The challenge with this process is inflating the liner bag and curing it into a round shape. One option is to add a rigid liner bag on the outside of the resin impregnated CIPP liner. The rigid bag would prevent the liner from stretching and help it maintain a round pipe shape. Two Direct Storm Pipe Connections Excavations will be needed to connect two existing catch basin leaders to the new CIPP liner. One is located downstream of M17 and the other is located downstream of M15. 7

8 The tie-in between the new CIPP and the existing CMP leaders would be completed before the annular space grout is installed. Install cement grout in the annular space The entire annular space between the liner and host pipe will be filled with cement grout. This will prevent ground water from entering the annular space, prevent the liner from floating inside the carrier pipe, prevent collapse of the liner due to hydrostatic pressure and will also provide a structurally sound pipe system when complete. A special lightweight flowable grout is required to prevent the CIPP liner from floating. It will also to fill the entire space when injected from one end of the pipe run. This unique grout is available locally and has been used successfully on other projects in Anchorage. The CIPP liner will be filled with water during the grout process to keep it from floating. The new CIPP liner will rest on the CMP host pipe invert when the grout work is complete and cured. Post-rehabilitation CCTV inspection A post-construction CCTV inspection by the contractor will be reviewed by the engineer to confirm that the work has been completed as required and that the pipe is clean and functioning properly. Surface Restoration Restoration of the surface will be required after the work is completed. Asphalt repair and landscaping repairs would be needed at the excavation areas as well as surrounding surface that is damaged during the work. Estimated Cost CIPP lining in conjunction with grouting of the annular space will provide a structural rehabilitation of the pipe. The overall construction cost to complete this rehabilitation method is estimated to be $1,320,000. A more detailed description of the cost is contained in the following table. 8

9 PROJECT: Pine Street Storm Sewer Upgrade OPTION: Cured In Place Pipe and Grouting NO. DESCRIPTION QTY UNIT UNIT TOTAL COST 1 Type II & II-A classified fill and backfill 1100 Ton $20 $22,000 2 Bedding material 70 Ton $25 $1,750 3 Disposal of unusable or surplus material 600 CY $18 $10,800 4 Adjust Electric/Phone Utility Crossing 1 EA $3,000 $3,000 5 F&I Type I Storm Manhole 5 EA $7,000 $35,000 6 Furnish and Install 24" CIPP Liner 1930 LF $250 $482,500 7 Storm Sewer Flow Control 1 LS $35,000 $35,000 8 Connect Existing Catch Basin Leaders 2 EA $5,000 $10,000 9 Annular Space Grouting 470 CY $800 $376, Pre-rehabilitation Sewer Pipe Television Inspection 1930 LF $7 $13, Post-rehabilitation Sewer Pipe Television Inspection 1930 LF $4 $7, Remove and Replace Existing Pavement 700 SY $60 $42, Construction Survey Measurement 1 LS $5,000 $5, Traffic Maintenance 1 LS $15,000 $15,000 SUBTOTAL $1,059,280 CONTINGENCY 25% $264,820 TOTAL ESTIMATED CONSTRUCTION COST $1,320,000 Storm Sewer Trenchless Repair Recommendations The traditional HDPE sliplining process is the recommended trenchless option. This is the lowest cost trenchless method and is also a proven process that can be constructed by any number of construction contractors in Anchorage. A trenchless upgrade of the existing storm system from manholes M22 to M12 using HDPE sliplining is estimated to cost $1,020,000 to construct. 9