River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW

Transcription

1 River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW Richard Lane 1, Phanta Khamphounvong 2, Sisira Mabotuwana 2 1 NSW Public Works 2 Roads and Traffic Authority of NSW Abstract In June 2007, a severe storm over the Hunter Valley caused extensive damage to private and public infrastructure throughout the Hunter Valley. Moderate flooding caused damage to the abutments of the nominated bridges. The NSW RTA engaged the NSW Public Works to design the rectification works at Bowman s Crossing Bridge and to Project Manage the design and construction of restoration works at Luskintyre Bridge. The paper discusses and illustrates design, construction, cost and time considerations for the rectification to the bridge abutments for the Bowman s Crossing and Luskintyre Bridges. As part of the oral presentation, construction photographs shall be presented of the various phases of work for the Luskintyre Bridge. The restoration works at Bowman s Crossing were moderate at a cost of less than $160k (Incl. GST). The river bank and pier restoration works at Luskintyre Bridge were far more extensive and entailed realigning the river bank some 80 metres up stream of the bridge with the placement of approximately 9,000 tonnes of rock, 5,500 cubic metres of sand, 300 cu metres of top soil and 3,300 square metres of turf. Bowmans Crossing Bridge : Bridge No 1 This bridge crosses the upper reaches of the Hunter River near Jerry s Plain. The flood gradient at the site is reasonably steep where stream velocity is expected to be high. V. Ponnampalam, H. Madrio and E. Ancich 304 Sustainable Bridges: The Thread of Society AP-G90/11_081 ABC 2011

2 River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW 305 Table I. Configuration of the bridge crossing is given below. Bridge Length 180m Depth of Super Structure 900mm No of Spans 10 Design Deck Level RL Span Length 18m Design Flood 1 in 20 Bridge Type PSC Planks Year Built 1996 The bridge and its approaches are on a flat grade. The road approaches and bridge overtop simultaneously during flood. As the river level rises above the soffit level of the bridge the increased head on the upstream side causes the water velocity to accelerate allowing it to pass through the bridge opening. This flow pattern occurs until with a very large flood, such as in 1955, when the complete bridge is submerged. Fig. 1. Bowmans Bridge Crossing ( Cross Section)

3 306 Richard Lane, Phanta Khamphounvong and Sisira Mabotuwana Fig. 2. Comparison of Bed profiles between 1996 and Considerable erosion of up to 3 metres occurred in the river bed around Piers 3, 4 & 5. Original stone protection works around these piers was eroded away. The bed of the river in this area is mobile sand. Flood Event The June 2007 flood level was reported as being approximately 0.5 metres above the deck level. The flood is rated as being an average Recurrence Interval of 25 to 30 years. The data for the 2007 flood is set out in Table II. The peak discharge for the 2007 flood was calculated based on the open channel flow using Manning s formula. For flood level (m AHD) Table II. Velocities & Discharges based upon the bed slope of the river Bed Slope of the River (m/m) Total Discharge (m3/sec) Effective Water Way area ( m2) Average Velocity (m/sec)

4 River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW 307 Proposed Restoration Works It was proposed to streamline the left side of the riverbank where Piers 3, 4 and 5 had suffered severe scour. The objective of the re alignment is to provide an embankment with few irregularities and to gently guide the water flow in the required direction. This re-alignment will reduce turbulent flow conditions that increase erosion and scour effects. The channel profile was surveyed between 100m upstream and downstream of the bridge. The following works were proposed. 1. The regrading of the eastern river embankment to a slope of 1 to Construction of Rock Blanket revetment on the eastern intermediate embankment protecting Pier rows 3 and 4. The revetment is basically a blanket or layer of rock to prevent erosion around the piers. 3. Construction of isolated rock blanket around Pier 5. Rock Blanket Design Based upon the above hydraulic data, and the proposed rock blanket slopes of 1 in 5.3 and 1 in 1.33, a D50 rock size of 0.8 m was established. Fig 3 shows the recommended D50 rock size required to protect an embankment for a range of flow velocities. The graph has been based on three other projects where the side slopes of the rock berm or rock blanket were approximately 1vertical (v) to 1.5 horizontal (h). At Bowmans Crossing the slope of the rock blanket is approximately 1v to 4h. It is much more difficult to erode or dislodge rocks from an embankment with side slope of 1 in 4, rather than a steeper side slope of 1 in 1.5. Therefore in the assessment of the required rock size, the effective velocity, for interpretation from the graph, was reduced to 4.05 m/sec. The rock size was determined both on the velocity analysis (Fig 3) and a review of the size of rocks that were not eroded from the site during the 2007 flood event. The rock weight of 0.8 tonnes also corresponds to a 0.8 metre diameter rock. D50 Rock Size The rock diameter D50 represents the size of a rock from a rock sample where the combined weight of all rocks having a larger diameter than the D50, weigh 50% of the total sample weight.

5 308 Richard Lane, Phanta Khamphounvong and Sisira Mabotuwana Fig. 3 Assessment of D50 Rock Size Note: The above Graph is a basic design tool giving approximate results for relatively small projects. The Graph was prepared by Author 1 based upon the review of other rock embankment works carried out at Blandford [1] via Murrurundi, Singleton [2] and Maitland [3], also in the Hunter Valley. Where larger works are being considered, reference should be made to appropriately experienced waterway design engineers. Rock Blankets Used Main Blanket Piers 3 & 4 Blanket size; approximate plan dimension 31m by 26 m. he extent of the rock blanket was based upon practical considerations. Length 31m; If the sand erodes at the edge of the blanket, the edges of the blanket will collapse until a stable situation is achieved. It is unlikely that a scour hole of more than 3 metres shall occur. This still leaves 5 metres of rock blanket in this direction beyond the piles.

6 River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW 309 Width 26m; On the low side the rock blanket depth is 1.5 metres allowing for a considerable quantity of rock to be lost in a collapsing toe situation without adverse affect on the pile. The top of the blanket was taken beyond the extent of erosion during the 2007 flood event. Pier 5 Isolated Rock blanket; Blanket size 7m by 15m. The isolated rock blanket around the piles of pier 5 was based on similar considerations with the blanket being taken beyond the extent of scour of the 2007 flood event. Blanket Thicknesses The main blanket Toe Section (3 metres wide) thickness = 1.5 metres (Approximately equal to 2*D50 allowing for the inter positioning of the top layer of rocks on the base layer). The double layer of rock was considered to be adequate to allow for any collapsing toe situations the may occur through scouring. Normal Blanket thickness = 1.2 metres ( 1.5*D50) The thickness was based upon other rock embankment works carried out at Murrurundi [1], Singleton [2] and Maitland [3], also in the Hunter Valley. Also upon the practical consideration that if the D50 size is 0.8 metres, some of the delivered rocks are likely to be in the order of 1.0 to 1.2 metres in size. These can be accommodated in the full depth of 1.2 metres The sourcing of rocks of this size and required strength quality is difficult. Care must be taken to ensure the constructed blanket meets the required minimum dimensions. The total quantity of rock placed was approximately 1850 tonnes.

7 310 Richard Lane, Phanta Khamphounvong and Sisira Mabotuwana Geotextile: A heavy duty Class 1 Geotextile similar to Bidim A64 was selected as part of the design and was designated below all of the rock blankets. This is a heavy duty Geotextile and construction experience has shown that if this grade of Geotextile is not used, tears and rips occur when the large rocks are being placed. For further details of the rock blankets refer to Figures 6 and 7. Figure 6 Plan of Constructed Rock Mattresses Figure 7 Drawing of Blanket Sections Project Timing and Funding 1. 1/12/2008; NSW Public Works engaged by RTA to prepare concept sketches 2. 5/2/2009; NSW Public Works, Newcastle Regional Office, prepares and forwards concept sketches to the RTA. 3. 1/6/2009; RTA (Newcastle Office) uses concept sketches to gain competitive Tenders and awards contract /6/2009 Contractor; The JDS Group Pty Ltd completed the work. This project was Funded from the Natural Disaster Relief Assistance Program. The Program is funded by both the Australian Federal Government and the NSW Government. The construction cost was in the order of $160,000 (incl. GST).

8 River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW Fig. 4 Debris on Top of Bridge Deck ; June 2007 Fig. 5 No : 4/8/2009 Rock Blanket around Piers 3 &

9 312 Richard Lane, Phanta Khamphounvong and Sisira Mabotuwana Fig. 6 Plan of Rock Blankets : Bowmans Crossing

10 River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW 313 Section No.2 Cross Section 5 Sheet 8 Sheet 9 Fig. 7 Section for Rock Blankets; Bowmans Crossing

11 314 Richard Lane, Phanta Khamphounvong and Sisira Mabotuwana Luskintyre Bridge : Bridge No 2 This Bridge is on a local road (Luskintyre Road) over the Hunter River and is approx 3.5 kilometres from Lockinvar on the New England Highway and some 50 kilometres downstream of Bowmans Crossing Bridge. This bridge has two major steel truss spans and when constructed in 1901 was one of the largest road bridges in NSW. The Bridge is listed as an Item of state significance under the RTA s heritage and conservation register under Section 170 of the NSW Heritage Act 1977, for its historical, aesthetic and social significance. Erosion at the north western abutment has been occurring over many flood events and the concern was being shown over the structural integrity of the two round steel caissons (pier 10) on this embankment Fig. 8 No ; 25/2/2011: Portion of Luskintyre Bridge with work on the rock berm having commenced.

12 River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW 315 Table III: Bridge Configurations Total Bridge Length 292m Pier Length 15m Number of Span 17 Exposed Pile Length 5m Year Built 1901 Pile Embedment Length Before Flood 10m Pier No. 10 Pile Embedment Length After Flood 7.4m Fig. 9 Erosion that had taken place: June 2007 Hydraulic Data After consideration of water flow velocities associated with river heights of RL 21 to 24 metres an average velocity in the main channel of 2.5 m/sec was adopted for the new embankment design.

13 316 Richard Lane, Phanta Khamphounvong and Sisira Mabotuwana Proposed Solutions The following stabilisation solutions were considered by the RTA. 1. Th RTA requested NSW Public Works (Newcastle Regional Office) to prepare concept sketches for a bank stabilisation solution. Est. $960, In house, the RTA Bridge Design Section worked out a structural solution to reinforce the two main piles on the North Western abutment. Est. $800,000. The RTA chose to implement Solution1 because even if the structural solution (Solution 2) was implemented, some form of bank stabilisation works around the bridge piers would have to take place to prevent further erosion from occurring. If this were not done the timbers piles supporting the timber spans on the landward side of the steel piers would ultimately be undermined. Embankment Stability Analysis and Rock Sizing All embankment stability analysis work was carried out by Dams and Civil, Water Solutions, NSW Public Works. SLOPE/W (Geo Slope International, 2007) computer program was used for the SS (Steady State) and RDD (Rapid Draw Down) computer models to check the pseudo static slope stability for the designed slopes. The computer models calculate Factors of Safety for the embankment failing in the slip circle mode. The Steady State condition simulates the situation when the embankment is inundated with flood waters and the level of the water is in a Steady State. The Rapid Draw condition simulates the situation when the peak of the flood waters have passed and the flood waters are receding. Pore pressures can still be present in the embankment material which reduce the shear strength of the materials and reduce the Factor of Safety against slip circle failure. The following sized rock rip rap was adopted in the design of the rock berm and rock blanket. Rock Berm: Graded rock with D50 = 350mm with minimum thickness 1500mm. Rock Blanket: Graded rock with D50 = 350mm with minimum thickness 750 mm.

14 River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW 317 In making the final decision for the size of this rock recognition was given to the size of the rocks used at Singleton (upstream) and Maitland (downstream) over the past years. Rock Berm. The other important property of the rock berm is the total mass of the berm to assist in the achievement of the required stability factors of safety for the embankment. The cost per tonne of specified rock of D50 = 900 mm is only marginally dearer (5 to 10%) than the smaller rock specified in the design. It was considered prudent to use the larger rock for the rock berm at minimal additional cost and considerably more extensive resistance to erosion. Rock Blanket: Again it was decide to use rock marginally larger and thicker than that specified by Dams and Civil to allow for possible construction abnormalities. The blanket rock was specified as D50 = 500 at a thickness of 900 mm. Design Cross Section and Bank Alignment As part of the early concept designs the proposed bank stabilisation works were based on the philosophy of trying to establish a new embankment alignment which would lead to more streamlined flow patterns for the flood waters passing under the bridge. Therefore the new embankment was designed with a very slightly curved section upstream of the bridge and then a 27 metre transition section back to the embankment. This was in contrast to the almost nodal bumps that are seen around some bridge abutments. Such nodes form intrusions into the flow paths of flood waters which cause turbulent flow that result in erosion and scouring. The final design incorporated; 1. A rock berm with an effective crest width of 6.0 metres (2 metres of this was covered by the sloped embankment). Rock size D50 = 0.9M 2. Toe cut off trench, 1 m deep and 2 metres wide. 3. Sand was transported from an upstream accretion shoal on the opposite side of the river and placed to form the earth embankment. 4. A heavy duty layer of Geotextile similar to Bidim A64 was placed on top of the sand embankment prior to the rock blanket being placed. 5. Full height Rock Blanket 0.9m thick with D50 = 0.5M 6. Sand was then placed in between the rocks of the rock blanket. 7. A layer of topsoil 100 to 200mm thick was then placed above the rock blanket. 8. Horizontal 500 mm wide layers of turf were then placed on a 50 % basis with grass seeds being sown in between the rows of turf.

15 318 Richard Lane, Phanta Khamphounvong and Sisira Mabotuwana A diagrammatic representation of the embankment cross section is shown in Figure 10. Fig. 10. Design Section of Embankment used for Construction

16 River Bank & Bed Restoration Works for Bridge Pier Protection Bowmans Crossing & Luskintyre Bridges NSW 319 Table V: The following quantities of materials were incorporated the works Material Class A rock, Berm Rock (D50 = 900 mm) Class 2A rock. Rock Blanket (D50 = 500 mm) Embankment sand Sand in Blanket Class 1 Geotextile Turf and seed area Quantity 5,529 tonnes 3,745 tonnes 4,560 cu m 1,021 cu m 3,260 Sq m 3,300 Sq m Due to an error in the contract document the rock berm was constructed to a height of RL 9.0 rather than RL 10.0 intended in the design. The main consequences of this change are as follows. 1. The constructed gradient of the embankment is 1 V : 2 H 2. The preferred design had a bank gradient of 1 V: 2.2 H 3. The preferred design with rock berm at RL 10.0 AHD had higher Factors of Safety for the embankment in relation to slip failure. In July/August 2011 NSW Dams & Civil, Water Solutions, re- assessed the stability analysis for the constructed embankment based upon new permeability test results for the sand material used in the embankment construction and a more appropriate shear capacity factor (Ø = 40 degrees) for the berm rock material. The re- assessed factors of safety for the embankment are as follows; Table V: Re- Assessed Factors of Safety August 2011 Stability Condition Actual F of S Required F of S Steady State with Rock Berm at RL Rapid Draw with Rock Berm at RL The re- assessed Factors of Safety for the constructed embankment confirmed that there are no adverse stability issues with the embankment. Design Modifications during the Construction Phase As work progressed on the rock berm at the base of the embankment it became evident that it may be possible to extend the main section of the embankment a

17 320 Richard Lane, Phanta Khamphounvong and Sisira Mabotuwana further 10 metres upstream and therefore the transition section also became straighter and less pronounced. Thus the completed embankment works appear to be a continuation of the river bank and not just another isolated bump on the river bank. The final Contract amount to carry out the work was $680,000. Incl. GST. The Contractor for the works was the JDS Group P/L from Cooma NSW. The restoration works were again funded by the Natural Disaster Relief Assistance program. Outcomes of the Project 1. The completion of the detailed investigation, design and construction in 15 months is an excellent result and would not have occurred without the cooperation and effort on the part of all people involved including the RTA, Landowners, NSW Public Works and the Contractor. 2. In designing concept drawings for future works, where the waterway area permits, embankment slopes of 1v to 2.25h or 1v to 2.5h should be considered. 3. These slopes shall make the construction and maintenance much easier and assist in maintaining turf lawn/ grass ground cover. 4. A final rock berm width of 5.0 metres should be allowed which would give more adequate room for a 25 to 30 tonne excavator to operate. 5. The lower gradient slopes of Point 2 above and the berm width of 5.0 metres shall also assist in more readily attaining the required Factors of Safety for embankment stability. 6. The cost of additional materials for the above recommendations would be minimal in comparison to the gains. References [1] Design of Blandford Bridge Abutment Protection Works, NSW: Author 1 and Dams and Civil, Water Solutions, NSW Public Works: 2003; for RTA (NSW). [2] Design of River Bank restoration works between the two Road Bridges at Singleton, NSW; Author 1and Dams and Civil, Water Solutions, NSW Public Works: 2008 for Dept of Environment and Climate Change (NSW). [3] Design of Maitland Levee Rehabilitation Works, NSW; Patterson Britton and Partners Pty Ltd. 2000; for Dept of Land & Water Conservation (NSW).