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HomeMy WebLinkAbout6618; AGUA HEDIONDA STREAMBANK RESTORATION; Construction Observation Summary Memo; 2016-01-09 216127 Construction Observation Summary Memo.docx 1 BALANCE HYDROLOGICS, Inc. MEMO To: Sherri Howard From: Dana Jepsen Date: January 9, 2016 Subject: Construction Observation Summary Memo Introduction Balance Hydrologics assisted the City of Carlsbad as an observer at the Agua Hedionda Creek Streambank Restoration Project. Our first site visit was in the afternoon of November 2nd, 2016. Our final site visit occurred on the morning of December 3rd, 2016. During our time onsite, we were able to observe the installation and construction of a number of project elements. We have outlined our observations of a number of these elements below. We did not include a discussion of the Fabric Encapsulated Soil because this was a Dudek design element and these have been previously summarized by others. Keyway and Foundation Boulder Placement With the diversion and dewatering activities normalized, boulder placement within the keyway began in earnest on November 4th. Despite the difficulties of removing all of the water from the excavations, the contractor appeared to achieve the placing of these boulders on top of the approved geotextile. Note that this geotextile was not shown in the drawings but, as I understand, it was approved by the City as an acceptable substitute for the bedding material as drawn. This substitution of geotextile for well grading bedding occurred in additional areas as well: below the channel boulders including the boulders that form the drops, below the outfall boulders, and below the stilling pool. The contractor worked diligently and effectively to assure that the top of boulder elevations within the keyway were as indicated in the drawings, and the size of the boulders used in construction appeared to meet the specifications. It was discovered mid-way through keyway boulder placement that the entire keyway would be inundated once the diversion pump was removed. This posed a problem for two reasons. First, the design intent was that base flows within the creek be below the vegetated crib wall and the fabric encapsulated soil (FES). Additionally, the work plan was to remove the diversion pump once the channel and keyway boulders were in place, and to construct the remaining items from outside of the channel. The contractor and the City agreed that the best solution was to raise the elevation of the keyway by adding an additional course of boulders. It was agreed that the elevations for the cribwalls and FES shown on the drawings would be adjusted in the field to fit the revised elevation at the 216127 Construction Observation Summary Memo.docx 2 foundation boulders. This solution had the benefit of decreasing the amount of elevation that needed to be made up later through the labor intensive construction of the vegetated cribwalls. In summary, the construction of the keyway and the foundation for the cribwalls and FES appeared to meet the design intent and specifications even if the elevations were altered in the field. The contractor had sufficient large material available, and placed and compacted the boulders per the specifications. Channel Boulders The contractor worked from downstream to upstream when constructing the channel. Prior to completing the keyway and foundation described above, the boulder placement began within the channel itself. As indicated earlier, the bedding material shown on the drawings was replaced with City approval by a geotextile. We do not have experience in the construction of channel bed forms lined with geotextile, but the contractor appeared to place the material as per its intended use. From my observations and spot measurements, the thickness of the channel material met or exceeded the 4.3’ thickness called for in the drawings. The contractor was able to reserve sufficient large material to construct the two steps shown. The downstream step was constructed in line with the drawings, but a change was made during construction to the elevations at the upper drop. The plug of sandy soil between the diversion pump and the constructed channel could not be entirely removed without flooding the worksite. As opposed to using undersized material to construct the drop, it was decided that the top of the drop would be a foot higher than shown on the drawings. Another change to the design was the addition boulders both upstream and along the side of the channel at the upstream end. The boulders upstream were placed to provide additional stability for the channel and drop structure in anticipation of the sandy soil plug eroding over time and the boulders along the side of the channel were placed to arrest erosion observed after a rain event during construction. In summary, the rock placement to create the channel appeared to meet the design intent and specifications. The only change we note is the placement of additional large material at the upstream end of the project. Outfall and Stilling Pool The rock slope protection at the storm drain outfall and the associated stilling pool were constructed on November 18th, after the completion of the upstream cribwall. Once again, geotextile was used in place of the well graded bedding material shown on the drawings. We observed the contractor placing smaller material in the voids left after the placement of the boulders, but we should note that some of this smaller material was washed out during the storm event that occurred on November 20th and 21st. It is our understanding that the City’s approval of the geotextile is intended to mitigate for loss of some of the smaller bedding material. Although it appears to us that these elements were constructed per plan, we also believe that it is important that these elements be monitored in the future. This monitoring is discussed below in the section on recommendations. 216127 Construction Observation Summary Memo.docx 3 In summary, boulders associated with the stilling pool appear to have been placed per plan and should work to assist in dissipating the energy associated with the flow out of the outfall. In addition, the boulders serve to assist in the containment of cribwall backfill. Vegetated Cribwalls The upstream vegetated cribwall was constructed such that the top header log was set at the elevations indicated on the drawings. While the log diameters are generally smaller than were shown, the contractor did not need to add additional lifts because of the higher starting elevation at the foundation boulders. We worked alongside the city inspector to observe and document that the log connections were done per specifications and that the fill within the crib cells was compacted appropriately. We do not note any deviations or irregularities in the upstream cribwall. There are two items worth noting regarding the downstream cribwall. First, a decision was made jointly by the contractor and the City that installing an additional lift (two layers of logs) would be beneficial to the project from both a performance and cost perspective. We agree that the slope will be more stable because this decision was taken. The second item we noted is that the alignment of the fourth layer of logs at the downstream conform is not ideal. In this case, a decision was taken in the field in consultation with the City that corrective action was not needed and that the FES and plantings allow for sufficient flexibility that the design intent could be achieved without dismantling and re-constructing the cribwall. In summary, the vegetated cribwalls appear to have been constructed in compliance with the design intent and we have reasonable confidence that they will perform as expected. Engineered Fill This project benefited from the almost continuous presence of a soils technician who took responsibility for confirming compliance with the specifications in this area. We do not have anything to add in this regard. Final Thoughts and Recommendations This project was constructed later in the season than anticipated. This late start necessitated that the team move quickly and continuously evaluate the benefits and risks associated with making field adjustments in the anticipation of coming rain events. Given this time pressure, we are impressed with the final product and feel that it should perform well so long as appropriate adaptive management procedures are put in place and followed. In our opinion, the main focus of adaptive management should be on plantings. For the vegetated cribwall and the FES to perform as desired, the plantings called for must not only survive, but thrive. It is not possible to overstate the importance of planting survival both within the engineered reach and at the upstream and downstream transition zones. The second adaptive management item to prepare for is at the outfall. The rock slope protection between the outfall and the stilling pool should be monitored regularly during the rainy season, and the City should be prepared to place material in this area if any becomes dislodged. The 216127 Construction Observation Summary Memo.docx 4 geotextile provides a certain amount of protection against erosion, but will not function unless it is covered with the appropriate sized rock. Per our observations the constructed project was completed in general conformance with the plans and specifications.