Geosynthetics reinforced MSE wall design and construction on the Southeast Stoney Trail PPP project, Calgary AB Hardwin Zhanhong Zhen Nilex Civil Environmental Group, Calgary, Alberta, Canada ABSTRACT Geosynthetics were used to reinforce 12,000m2 of MSE walls for Southeast Stoney Trail (SEST), Alberta's largest single highway project and PPP project. From Fall of 2010 to late 2012, Nilex Inc. (Nilex) successfully completed the design, panel fabrication and the site assistance of this MSE project. Challenges such as low bearing capacity, insufficient coverage ratio, and top panel misalignment were mitigated through staged design, special construction techniques, multi-discipline team work and proper project management. The anti-chemical characteristics of geosynthetics saved time and cost to prevent the de-icing water from seeping along the wall. The maximum design height was 9.31m for the abutment wall with CSP-cased piles on each abutment spaced at 1500 mm which generally left an effective coverage ratio of 50% behind the abutment. In addition, special custom- fabricated panels were used to increase the 34% efficient coverage. RÉSUMÉ Des géosynthétiques ont été utilisés pour renforcer 12.000 m2 de murs de terre stabilisée mécaniquement (MSE selon son sigle en anglais) pour le projet Sud-est Stoney Trial (SEST), le plus grand projet d'autoroute de l'Alberta et des projets du type PPP. De l'automne 2010 jusqu’à la fin de 2012, Nilex Inc. (Nilex) a complété avec succès la conception, ainsi que la fabrication de panneaux et l'assistance sur place pour ce projet de type MSE. Des défis tels que la faible capacité portante, le taux de couverture insuffisante, le désalignement du haut des panneaux ont été atténués grâce à une conception innovante à techniques de construction particulières, ainsi qu’au travail d’une équipe multidisciplinaire et à la bonne gestion du projet. La resistance chimique des géosynthétiques a permis d’économiser du temps et des coûts en empêchant l’eau de s’infiltrer le long du mur. La hauteur maximale etait 9.31m pour le mur de la culée du pont avec des pieux de type CSP espacée a 1500 mm dans chaque culée, généralement en utilisant un taux de couverture effective de 50% derrière les culées. De plus, un panneau spécial fabriqué sur mesure a permis d'augmenter la couverture efficace de 34%. 1 INTRODUCTION venture between SNC-Lavalin Inc. and Acciona Canada Inc., who designed, built, partially financed and will As the largest single highway project and the largest P3 maintain it over a 33-year term. Chinook Infrastructures road infrastructure project in Alberta’s history (before (CI) worked as the general contractor on this project. 2010), Southeast Stoney Trail (SEST, Figure 1) is the Southeast leg of the Calgary ring road, which will extend from the south side of the current Stoney Trail / 17 Avenue SE intersection south along the east perimeter of the City to Highway 22X, then west to the east side of the Highway 22X / MacLeod Trail interchange. 1.1 Project Description SEST, the $769 million (2010 dollars) project, was constructed from Spring 2010 to Fall 2013, followed by another 30 years of maintenance. The project consists mainly of 25 km of six-lane roadway, 9 interchanges, 3 flyovers, and 27 bridge structures. A total of about 25,000 m2 of MSE wall was split between Nilex and another wall supplier for scheduling purposes. 1.2 Project Team The Alberta Ministry of Transportation (AT) awarded the SEST Project to Chinook Roads Partnership, a joint- Figure 1 SEST Project Nilex, in conjunction with Tensar International Corp (Tensar) and Lockwood Bros Concrete Products 3.1 Key Design Criteria (Lockwood), supplied the MSE wall and related services from 17 Ave SE to the intersection of 22X (inclusive) with Following the CSA specified load and resistance factors, a total area (mainly abutment wall) of more than 12,000 this project was designed using the AASHTO LFRD m2. method, under the specifications in sequence: Retained by CI, the geotechnical investigation for the Canadian Highway Bridge Design Code (CSA site was carried out by Golder Associates Ltd and Standard CAN/ CSA-S6-06); McIntosh Lalani Engineering Ltd; the QA/QC was carried Government of Alberta Ministry of Transportation out by LVM|DESSAU. Surveying was carried out by MMM Southeast Stoney Trail Schedule 18; Geomatics Alberta Ltd. G+C Gravel Inc, Wilco AASHTO LRFD Bridge Design Specifications. Contractors Southwest Inc, Rino Roadbuilding Ltd., and The design life for all MSE wall components was 100 Keystone Excavating Ltd installed the Nilex MSE walls in years. twelve months over the winter period from 2011 to 2012. 2 FOUNDATION AND GEOTECHNICAL ASPECTS A dry humid continental climate with long, cold, dry, but highly variable winters, Calgary is a city of extreme temperatures ranging from historical lows of -45°C to highs of 36°C. Tensar Geogrid construction proceeded during the cold winter months with outdoor construction work being limited when temperatures plummeted below - 29°C. A superficial layer of topsoil comprised of organic silty clay approximately 150mm thick was underlain by silty clay till to depths between 6.3m and 11m below existing ground surface. The silty clay till is inferred to be firm to stiff. Below this 4m depth is inferred to be very stiff to hard in consistency. Groundwater on the project varied from 2.0m to 1.8m below existing elevations, as measured on July 7th 2010. st Figure 2. Typical cross section It was measured 4.2m and 3.1m below on Nov 1 2010 at the south end of the project. The ground water As described in AASHTO LFRD, “WSD establishes levels generally vary seasonally depending on the allowable stresses as a fraction or percentage of a given precipitation and run off conditions. material’s load-carrying capacity, and requires that calculated design stresses not exceed those allowable stresses”. For example, Resisting Force divided by 3 DESIGN OF MECHANICALLY STABILIZED EARTH Driving Force might be required to be more than 1.5 as a WALLS safety factor. A corresponding LFRD example could be TM that a Factored Resisting Force (factor say of 0.8) divided The 3mX3m modular ARES panel complete with by a Factored Driving Force (say 1.25) would have to shiplap joints was chosen for the SEST project. The exceed 1.0 as a safety factor. Actual factors are specified project consisted of engineering design, project in both the Canadian and American codes. In reality most management, and site assistance for 22 ARES walls of 12 designers use both methods. CSA requires an LFRD structures, with a total of over 12000 m² for this MSE wall design also be checked by WSD (if a WSD design is project. The wall system consists of High Density applicable). Polyethylene (HDPE) structural geogrids mechanically attached as tie back anchors to the precast concrete face. 3.2 Internal Design A typical cross section is shown in Figure 2. The SierraScape® system was used to redirect the Within the reinforced mass, stability was achieved using traffic along the existing 52 Street SE, and light weight the strength of the soil being reinforced in conjunction with concrete was used to reduce the superstructure load and the tensile force and anchorage characteristics of the lateral transfer load. geogrid. The Geosynthetic reinforced MSE wall was designed so that the reinforced backfill material was free of T-available = T-ult / (RFd x RFid x RFc) [1] subsurface drainage of water (seepage) and care was taken not to contaminate the geotextile with fine-grained T-allowable = T-available x Rc / Fs [2] soils or other deleterious materials. A minimum of 75mm of granular materials was required between any overlap of two layers of geogrids. T-ult: Ultimate strength (kN/m) RFd: Durability 4 SOIL REINFORCING MATERIAL RFid: Installation Damage RFc: Creep The AT Schedule 18 required that: in addition to satisfying Rc: Coverage Ratio a series of GG tests (Standard Test Method for Geogrid) for rib tensile and junction strength, short term and long Internal stability to ensure that the geogrids were term creep, Geosynthetic reinforcing materials shall also strong and long enough not to pull out of the fill behind the contain stabilizers or inhibitors to prevent degradation of Rankine failure plane. The above formula was used to properties due to ultraviolet light exposure. calculate the effective reinforcement strength, connection, pullout and applied bearing pressure. During 4.1 Coverage Ratio & Geogrid Cutting Around Piles construction, the geogrid was evenly distributed within the reinforced mass and that there was sufficient tensile Effective width of geogrid left after cutting holes and gaps stress to preclude rupture (either short or long term). had been subtracted to calculate the coverage ratio, Geogrids used on the project were from 70.0KN/m to which was used to design the wall with acceptable safety 175.0kN/m in Tult. factors. In actual construction, special attention was required to stabilize the panel against rotation horizontally and vertically. In many cases, the end or top modular panels were either too narrow or too low to be balanced. The geogrid running around the piles was another challenge. In order to maximize the coverage ratio, the holes were cut flush with any obstacles. In addition, the specific layer of the reinforcement was recalculated independently wherever more than 10% of efficient grid fingers were accidentally cut. A reduction of the coverage ratio or enhanced addition needed to be considered wherever the left geogrid width after cutting is narrower than 300mm wide. 4.2 Competitive Advantages of Geosynthetics De-icing salts are deleterious to steel reinforcement, including the galvanized or epoxy coated steel. Therefore, Alberta Infrastructures requested that CI add a Figure 3.