No Walk in the Park for Discovery Parks BLDG 12 Shoring Design in Slickensided Silts Tyler Trudel, B.A.Sc., EIT Stantec, Burnaby, BC, Canada Wayne Quong, M.A.Sc., P.Eng. Stantec, Burnaby, BC, Canada ABSTRACT Shored and unsupported cut-slopes were used for a large excavation in difficult soil conditions, including slickensided, heavily overconsolidated stiff silt, for the 2007 construction startup of the Discovery Parks Building 12 development. Shoring consisted of conventional shotcrete and soil anchors, commonly used for the majority of soil and bedrock conditions encountered in the Vancouver Lower Mainland. As the excavation progressed, challenges arose due to the variations in the soil conditions; the shoring design had to be adapted to localized conditions as soil properties and slickenside orientations changed across the site, as well as with depth. In some areas, remedial measures had to be implemented as survey monitoring indicated slope movement had occurred a significant time after completion of the excavation. RÉSUMÉ Des coupes de pente étayées et sans support sont employées pour une grande excavation dans des conditions difficiles de sol, y compris du sol type ‘slickenside’, en consolidant fortement la vase raide, pour le démarrage 2007 de la construction du développement du Discovery Parks Building 12. L'étayage s'est composé de béton projeté conventionnel et d’ancrages de sol, utilisées généralement pour la majorité d'états de sol et de bedrock rencontrés dans la région du Lower Mainland de Vancouver. Pendant que l'excavation progressait, les défis ont surgi en raison des variations des conditions de sol ; la conception d'étayage a dû être adaptée aux conditions localisées comme propriétés de sol et orientations filoniennes changées à travers l'emplacement, aussi bien qu'avec la profondeur. Dans quelques secteurs, des mesures réparatrices ont dû être mises en application comme la surveillance et le sondage ont indiqué que mouvement de pente s'était produite longtemps après la fin de l'excavation. 1 INTRODUCTION The Discovery Parks Building 12 (DiscoveryGreen) development, located in Burnaby, B.C., consists of a five storey office tower with three levels of underground parking on a site with sloping topography and variable soil conditions. Due to lateral constraints, a combination of engineered cut slopes and shored excavation slopes, consisting of soil anchors with a shotcrete cover, were used for the building excavation. This type of shoring system is commonly used in the Vancouver Lower Mainland.

As the excavation progressed challenges arose due to the variations in the soil conditions, and the shoring design had to be adapted to localized conditions as soil properties and slickenside orientations changed across the site, as well as with depth. An extensive surveying program was implemented to monitor slope movements during and after completion of the excavation. The survey results indicated that after the completion of the excavation and placement of the perimeter foundations, some excavation slopes were undergoing significant movement. Immediate remedial measures including cut and fill earthworks and the installation of additional soil anchors were used to stabilize these slopes.

2 PROJECT AND SITE DESCRIPTION The DiscoveryGreen Building has a footprint of 3133 m2 (33,725 ft2), and is situated on an approximately 3 acre site which has up to 13 m (42 ft) of topographic relief from the south west to the north east. As shown in Figure 1, the site is bound by residential roads (Canada Way and Gilmore Way) to the north and east, respectively, by existing office buildings to the south and by a residential development to the west.

Due to the proximity of roadways, sidewalks and neighbouring buildings; several of the excavation slopes were shored to sustain the steep grades necessary to facilitate the construction of the three level underground parkade. Based on the sloping topography and the slab elevations of the lowest level of underground parking, the vertical height of the shored excavation slopes varied from approximately 11.5 m (38 ft) to 17.1 m (56 ft).

3 SOIL AND GROUNDWATER CONDITIONS The soil conditions at the site were complex and consisted of surficial overburden underlain by till-like silt, sand and gravel over very stiff to hard slickensided silt with variable clay content.

The site investigation, carried out in 1999, consisted of five solid stem auger holes advanced to depths of 10.6 m (35 ft) to 18.3 m (60 ft) below existing site grades

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using a track mounted drill rig. In general, the surficial overburden consisted of 0.15 to 0.3 m (0.5 to 1.0 ft) of organic topsoil over approximately 0.2 to 0.9 m (0.7 to 3.0 ft) of tan brown silt with sand and traces of rootlets and organics. The overburden was underlain by till-like material generally consisting of 2.5 to 11.5 m (8 to 37 ft) of dense or very stiff sand and silt with variable amounts of gravel. The till-like material was further underlain by heavily overconsolidated, very stiff silt which contained variable amounts of clay and sand that extended beyond the termination depths of the drill holes. In multiple drill holes slickensided zones were encountered in the underlying very stiff silt deposit. Based on the variability of the soil strata and the presence of slickensides, it is envisaged that there may have been significant shearing and distortion of some or much of the soils on-site.

Atterberg Liquid Limits of 39.1 and 41.6 and Plasticity Indices of 19.2 and 22.6 were measured on two representative samples of the slickensided silt deposit, indicating a medium plastic clayey silt / silty clay (CL), in accordance with the Unified Soils Classification System (USC) as set forth in ASTM D2487. Natural water contents were 21.4% and 24.3%.

Seven days after completion of the drill holes, the ground water was measured to be approximately 9.7 m (32 ft) below existing site grades, which corresponds to the interface between the overlying till-like silt, sand and gravel and the underlying clayey silt. It is likely that the

groundwater table encountered is a perched condition from water seepage related to the location of sandy layers (seams) present at various elevations within both the till-like material and underlying silt.

4 SOIL ANCHOR DESIGN The design of the soil anchor system used for shored excavation slopes was based on wedge stability and global slope stability analyses. The wedge stability analysis is a Limit Equilibrium approach and was applied to multiple level anchor systems in layered soils (CANFEM 2006). Wedge stability analysis assumes a linear failure mass (i.e. wedge) and is a numerical solution of the force vector diagram which represents the internal forces (i.e. weight) and external forces (i.e. soil anchor loads and sliding friction). The numerical solution resolves the calculated anchor force required for stability of the critical soil wedge. A minimum factor of safety of 1.30 for wedge and global stability was used for design of the shoring system. Drained conditions were assumed in the wedge stability analysis. Therefore, it was important that the groundwater table was lowered to below the base of the building excavation and sufficiently beyond the face of the excavation slopes to achieve site stability. The various drainage measures used for the building excavation are described below.

Figure 1: DiscoveryGreen Development Site Plan

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To prevent hydrostatic build up, temporary weep drains were installed at regular intervals along the shotcrete panels, with additional drains placed in observed seepage zones. The weep drains consisted of 25 mm (1 in) PVC pipe placed a minimum of 75 mm (3 in) into the cut-face prior to shotcrete placement, and extending a minimum of 100 mm (4 in) beyond the finished surface of the shotcrete panel. Similarly, sub-horizontal wall drains were installed at regular intervals along shotcrete panels at depth, specifically along the south and west shored excavation walls. The sub-horizontal wall drains consisted of 1.5 inch diameter perforated PVC pipe, installed in a hole drilled a minimum of 12 m (40 ft) into the cut face. Vertical gravel drains were installed at 6 m (20 ft) centers, generally adjacent to the toe of an excavation slope. The vertical gravel drains consisted of 8 inch diameter columns of pea gravel extending from project datum elevation 35 m (115 ft) to elevation 24 m (80 ft), (i.e. 15 ft above to 20 ft below the floor elevation of the lowest parkade level). A total of 30 vertical drains, configured into three rows, were installed. A row of vertical drains was installed along each of the west and south foundation walls, (i.e. the foundation walls located on the high sides of the site), approximately 3 m (10 ft) inside the building area and a third row of vertical drains along a north-south alignment through the centre of the building area. It should be noted that the vertical gravel drains were installed when the base of the excavation reached an elevation of 35 m (115 ft) to lower the groundwater table and potential hydrostatic pressure in advance of the excavation.

A spreadsheet was developed to perform the wedge stability analysis and for the design of the shoring system. The spreadsheet was also used during construction to check the adequacy of the shoring system to the actual soil conditions encountered during the excavation.

The values of the soil strength parameters used for shoring design were selected based on local experience and back-analysis of a similar shoring system at a nearby site with similar soil conditions which experienced large

slope movement (i.e. near imminent failure). Specifically, the back-analysis was used to derive an estimate of a representative value for the effective friction angle of the slickensided silt deposit. The soil strength parameters used for the wedge stability analysis are summarized in Table 1.

Table 1. Soil Strength Parameters Characteristics Gravelly Sand & Silt Clayey Silt

Unit Weight (γ) 20 kN/m3 18 kN/m3

Effective Friction Angle (φ’) 37 o 22 o

Soil cohesion (c’)1 0 kPa 0 kPa

Bond Resistance 430 kPa 75 kPa 1 The till-like soils conservatively assumed to have no cohesion.

The soil anchors used for the shored excavation

slopes consisted of a combination of the three Dywidag Threadbar®, or equivalent, anchor types presented in Table 2. A typical soil anchor profile used for the west excavation slope is shown in Figure 2.

Table 2. Soil Anchor Specifications Specifications #7 Bar #8 Bar #11 Bar

Design Load (kN) 118 160 340

Anchor Length (m) 10 18 24

Anchor Diameter (mm)1 19 22 25

Minimum Ultimate Tensile Strength (MPa)

690 690 690

Free Bond Length (m)2 1.5 1.5 1.5 1All anchors installed in 100 mm diameter drill holes. 2Free bond length of 1.5 m used unless otherwise specified.

It should be noted that the stability of the excavation

slopes is greatly influenced by the complex interaction

Figure 2. Typical Section of Shored Excavation Wall 413

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between the extent and the orientation of the slickensides, and the residual friction angle along the slickensided surface(s) undergoing movement along potential failure surfaces. As the properties of the slickensided zone(s) are unknown beyond the face of the excavated slopes, an extensive anchor testing and surveying program was carried out during and after building excavation to ensure site stability. In addition, careful sequencing of the excavation, drilling, installation and grouting of the soil anchors, placement of the shotcrete panels and tensioning of the anchors including staging of the excavation and support system was used to maintain excavation stability.

In order to provide a cost efficient shoring design, individual anchor profiles were developed for the south, west and east shored excavation slopes, based on the auger hole inferred soil conditions, specifically, the thickness of the slickensided silt deposit above the base of the building excavation.

The shotcrete and grout specifications used for the building excavation support are presented in Table 3.

Table 3. Grout & Shotcrete Specifications Characteristics Grout Shotcrete

24 Hr Minimum Compressive Strength (MPa) 21 21

28 Day Minimum Compressive Strength (MPa) 34.5 27.5

The shotcrete cover was reinforced with a

4 in x 4 in x 8 gauge (Imperial) welded wire mesh, with a minimum yield strength of 414 MPa (60 ksi), covering the full height of the shotcrete slope. The welded wire mesh had a minimum shotcrete cover of 50 mm (2 in) on both sides. In many cases, shotcrete anchor pads exceeded 200 mm (8 in) in thickness and were reinforced with 19 mm (¾ in) rebar.

Engineered cut slopes were utilized where space permitted. Engineered cut slopes consisted of 1H:1V (Horizontal to Vertical) slopes which were covered with poly sheeting to protect against wet weather conditions. In some cut slope areas where slickensided silt was encountered, passive dowels (soil nails) and a shotcrete cover were installed.

5 ANCHOR INSTALLATION & TESTING To assess the performance of anchor installation and confirm the adequacy of the installed shoring systems, creep and proof testing of individual soil anchors were carried out. Creep testing consisted of loading the anchor in approximately 22 kN (5 kip) increments to 125% of the design working load. Each load increment was maintained for five minutes, and the increase in length (creep) of the anchor was measured at the start and end of each load increment. This information was utilized to assess the mobilized anchor length, bond strength and creep characteristics of the anchor. Anchors which crept more that 2 mm (0.08 inches) per log cycle of time were not accepted, and replacement or supplementary anchors

were installed to compensate for any reduced available anchor capacity. The majority of creep testing was conducted on soil anchors installed in the slickensided silt deposit located along the west excavation slope, where the silt deposits are the thickest, and the excavation slopes were the highest.

Proof testing consisted of loading the anchor to 122.5% of the design working load for two minutes while the applied load was monitored. If the reduction on the load was less than 2.5% of the proof load, the load was reduced to the design working load and locked off. If the anchor did not hold 122.5% of the working load, it was locked off at a reduced load and replacement or supplementary anchors were installed.

6 EXCAVATION SLOPE MOVEMENT An extensive surveying program was implemented to monitor all slope movements during excavation as well as after completion of the excavation. Once the excavation proceeded, fourteen survey targets were set up along the crest of the excavation. The survey targets are shown in Figure 1. Eight targets are located along the crest of the shored west excavation slope, three along the crest of the shored and unsupported south excavation slopes, two along the shored east excavation slope and one along the north cut-slope. Horizontal and vertical survey measurements were taken either weekly, twice weekly, or daily depending on the stage of the excavation and the measured crest movements. As the excavation progressed additional survey points included select soil anchors to monitor movement on the face of the excavation slopes.

Shoring support systems are typically designed to mitigate excessive or undesirable deformation, particularly where a displacement sensitive structure, such as a building, is situated in close proximity to the excavation. However, an adequately designed and installed shoring support system will not eliminate deformations completely.

7 SHORED WEST SLOPE Near the completion of the building parkade excavation, a highly slickensided zone was encountered in the silt deposit exposed at the base of the north end of the shored west slope. The area, an approximately 20 m long and 2 m high 1H:1V (Horizontal to Vertical) cut slope failed almost immediately after excavation and before soil anchors could be installed. This was an indication of the unfavourable slickensided zones in the silt deposit located along the west excavation slope.

Based on on-going review of the survey data of the crest and face of the west slope, it was concluded that for the majority of the soil anchors, the portion of the Dywidag Threadbar® beyond the critical soil wedge had undergone elongation beyond its elastic range. In addition, the survey data indicated that the rate of movement along the crest of the shored slope was not slowing.

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Steps to increase the stability of the west shored excavation slope included a temporary stabilizing soil berm placed along the base of the exposed cut slope and several 27 m (90 ft) long soil anchors installed along the north portion of the west excavation slope between the bottom rows of anchors. The free bond length of these anchors was increased from 1.5 m (5 ft) to 6.0 m (20 ft) in order to reduce the time and slope movement required to mobilize the bond resistance of the anchors. Based on the survey results subsequent to anchor installation, as shown in Figure 3, the rate of deformation in this area was reduced to acceptable levels.

Figure 3. Plot of West Excavation Wall Movement

8 SOUTH CUT SLOPE The south cut-slope was originally designed to be an approximately 15 to 17 m high 1H:1V (Horizontal to Vertical) slope. An engineered cut slope was feasible in this area of the site due to the absence of lateral constraints and favourable soil conditions consisting of predominantly till-like soils, inferred from the geotechnical investigation. The extensive surveying program included several surveying targets along the crest of the south cut-slope as well as target(s) on the face of the cut-slope.

When the south cut slope reached a height of approximately 10 m, surficial slides occurred due to the presence of near surface subparallel slickensides. To prevent further surficial slides, the lower portion of the 10 m high south cut slope was reinforced with a combination of soil nails and anchors, with a shotcrete cover. The soil nails consisted of 4.5 m (15 ft) long 19 mm (¾ in) diameter rebar grouted into a 100 mm (4 in) diameter drill hole at an approximate 90o angle to the face of the cut-slope. The soil anchors in this area consisted of 6.0 m (20 ft) long #7 Bar (¾ in diameter) anchors with a 1.5 m (5 ft) free bond length grouted into a 100 mm (4 in) diameter drill hole. The anchors for the south cut-slope had horizontal and vertical spacings of 2.1 and 3 m (7 and 10 ft), respectively, and a design working load of 44 kN (10 kips). The soil nails had horizontal and vertical spacings of 1.5 and 3 m (5 and 10 ft), respectively, and were not tensioned.

After a period of heavy rain in early December, 2007 sloughing occurred within the west portion of the south cut slope; however, the majority of the loose material was

being supported by the poly covering and wire mesh above the soil nails. Based on a review of the failure mode, it was concluded that the sloughing was likely a result of surface water migrating beneath the poly covering during the recent period of heavy rain and the unfavourable orientation of slickensides in this portion of the building excavation. It was concluded that unloading the crest of this portion of the cut-slope via benching and flattening to be the most viable remediation option. The upper portion of the cut slope was reduced to a 1.5H:1V (Horizontal to Vertical) grade and included an approximately 3 m wide bench at the toe of the remedied slope to catch any future debris. The remedied slope was covered with approximately 50 mm (2 in) of shotcrete reinforced with welded wire mesh to protect against surface water infiltration; weep drains were included to ensure that hydrostatic pressures did not build up behind the shotcrete.

Prior to the sloughing observed in early December of 2007, the south cut slope had stood with minimal deformation for approximately 3½ months, nearly 2 months of which was after completion of the bulk excavation in this area. However, in the weeks after the slope reconfiguration, survey monitoring indicated that the slope had started to deform. Based on the magnitude and rates of displacement observed along the crest of the south cut slope, it was concluded that the slope was undergoing deep seated global deformation, and that mitigative measures must be implemented quickly to stabilize the slope. Damage observed on a concrete foundation wall along the toe of the south cut-slope also indicated that global movement had occurred. The foundation wall had cracked and bowed with horizontal movement up to 150 mm (6 in).

Immediate remedial measures included the placement of a gravel berm along the toe of the cut-slope, as well as unloading of the crest of the slope via benching and flattening. Unloading of the slope via benching and flattening consisted of excavating the upper 3 m portion of the crest of the slope. The gravel berm was approximately 3 m (10 ft) wide and 1.5 m (5 ft) high and was placed within the building area between the foundation wall and the first row of interior columns. The first row of interior columns had not undergone any movement, indicating that the deep seated global rotational movement was day-lighting between the foundation wall and the adjacent columns. Survey monitoring of lateral and vertical displacement of the south cut slope continued during and following the completion of the remedial measures.

Based on the results of the survey monitoring program, it was concluded that the magnitude and rates of the south cut slope deformation had not reduced enough to warrant the removal of the gravel berm. As removal of the gravel berm was necessary to facilitate the building construction in that area, an additional 2 m of the crest was excavated, similar to the initial stage of unloading and flattening.

Based on subsequent survey measurements, the second stage of unloading the crest of the south cut slope via benching and flattening was successful in reducing the rates of displacement to acceptable levels such that the gravel berm could be removed to facilitate the building slab-on-grade preparation.

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Figure 4. Plot of South Cut Slope Movement

9 CONCLUSIONS

The Discovery Parks DiscoveryGreen development site was underlain by surficial overburden followed by very dense till-like silts, sands and gravels further underlain by heavily overconsolidated stiff slickensided clayey silt. The orientation of the slickensides often changed with not only depth, but also within localized areas on-site. The Limit Equilibrium wedge analysis made use of back-calculated soil parameters, and the subsequent shoring design was appropriate as the DiscoveryGreen Building excavation was successfully completed despite the challenging soil conditions encountered.

The DiscoveryGreen development demonstrates that an observational approach during building excavation can be very advantageous in optimizing a successful, cost effective shoring design in difficult soil conditions. The observational approach provides greater insight into the performance testing of soil anchors, as the localized soil properties are considered prior to anchor installation. Having an experienced field engineer on-site during excavation allows the shoring design, including anchor and shotcrete specifications as well as installation techniques, to be adapted if problematic soil conditions are encountered. Similarly, when soil conditions encountered are more favourable than anticipated, having a field engineer on-site that understands the shoring design allows for the anchor lengths to be reduced or horizontal and vertical anchor spacings to be increased to provide a cost effective design.

Survey monitoring of deformation (vertical and horizontal displacement) along the crest of excavation slopes and on the excavated slope is critical in implementation of a successful shoring design, especially in challenging soil conditions. It is very difficult to gauge the performance of the shoring design without regular, accurate survey results. Without regular, accurate survey data it is not known if the shoring design is not performing as expected until a critical slope failure occurs. At this point it is likely too late to implement mitigative measures and/or modify the excavation design.

ACKNOWLEDGEMENTS The authors would like to express gratitude to Discovery Parks Incorporated and in particular Mr. Murray Rankin for permission to publish this paper.

REFERENCES CANFEM 2006. Canadian Foundation Engineering

Manual, Fourth Edition, Canadian Geotechnical Society, 488 p.

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