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ABSTRACT: A variety of weak rock types are found along the alignment of the proposed Caldecott 4th Bore highway tunnel in Oakland, California. One of the rock types, locally known as the First Shale, consisting of highly fractured silty-shale will be encountered in steep portal cuts that are up to 21 m (69 ft) high and within a low cover reach of the tunnel which will have an excavated width of 15 m (49 ft). The strength assessment of this material is critical due to its location along the tunnel alignment as the shale will be the first material to be encountered during turnunder and because the tunnel will be subjected to slope loading. Numerous shear zones with significant clay content within the First Shale will intersect both the portal cuts and the tunnel. The orientation, frequency and thickness of these weak zones and rock mass scale effects were considered in the development of rock mass strength. The strength of this material was assessed using consolidated undrained triaxial tests with pore pressure measurements, point load tests and empirical methods (Hoek-Brown). Hoek-Brown strength envelopes of the overall rock mass were then developed for effective stress design analyses of tunnel and portal support systems. This paper describes the strength characterization approach used on the Caldecott 4th Bore project which validates the Hoek-Brown strength envelopes for crushed shale with a clay binder. INTRODUCTION Performing meaningful laboratory or in situ testing of highly fractured rock masses is difficult because of the difficulties associated with disturbance of the rock mass during sampling or test preparation. Therefore, in many instances empirical methods have been used to predict the strength characteristics of fractured rock. Wherever possible it is desirable to verify empirical estimates of rock mass strength for engineering design with actual testing. In addition to providing a potentially more accurate strength estimate, rock mass strength tests allow the engineer to develop professional judgement as to the level of accuracy of empirical predictions. However, field testing of a rock mass is not usually possible at the scale of interest. One exception to this is for a very closely fractured, or crushed, rock mass which can be tested in the laboratory provided sample disturbance can be minimized. The proposed Caldecott 4th Bore, a highway tunnel which will transect the Oakland Hills in northern California, will encounter such a rock mass. This paper describes the strength characterization of a highly fractured shale unit that utilized laboratory testing which allowed for a comparison to an empirical assessment of strength using the Geological Strength Index (GSI) developed and advanced over the past decade by Evert Hoek and others [1, 2, 3].
- North America > Canada > Ontario > Toronto (0.29)
- North America > United States > California > Alameda County > Oakland (0.24)
- North America > United States > California > San Francisco County > San Francisco (0.16)
ABSTRACT: A large span underground facility, adjacent to the underground Palmaz winery in Napa, California, has a plan area of approximately 1000 square m (10,800 square feet), with dimensions of 20 by 50 m (66 by 164 feet). One of the design objectives was to maximize the available open space to facilitate the storage and display of an exotic car collection. The facility was constructed in a lahar formation comprised of fresh, angular andesite clasts in a matrix of highly weathered rhyolite. Cover over the facility ranges from 2 to 14 m (6 to 46 feet), with an average thickness of 8 m (26 feet). The design was predicated on the use of the Sequential Excavation Method (SEM) and involved the development of ground support details and a comprehensive sequence of excavation and support installation. Numerical modeling was used to develop an optimal geometry and construction sequence and to ensure minimal construction impact on the previously constructed adjacent underground fermentation dome located approximately 9 m (30 feet) from the proposed opening. Six horseshoe-shaped tunnels, 4 m (13 feet) wide by 5 m (16 feet) high and approximately 20 m (66 feet) long, were excavated to install the main steel arch support frames and associated foundations. The ground in between these drifts was then excavated in rounds and supported by steel ribs and shotcrete. The final lining consisted of steel members encased in reinforced concrete or shotcrete, including post-tensioned foundations, designed to reduce lateral load transfer into the surrounding ground. INTRODUCTION The heart of the winemaking industry in northern California is located in the Napa Valley and the adjacent Sonoma Valley. This region has seen tremendous growth in the last 40 years, and especially in the field of viticulture, which has put a premium on land available for vineyards. The winemaking process also requires significant cellar space for fermentation and then maturing of wine in barrels. Cellar facilities take up valuable land and the operational costs can be significant. These considerations have resulted in increased use of underground space for wineries and experience has shown that the construction cost of a typical wine cave can be comparable to that of an above ground facility. However, the advantages of underground wine caves include naturally occurring low temperatures and high humidity with very small seasonal variations resulting in reduced utility costs and a substantial reduction of wine loss due to evaporation from the barrels. These cost savings can pay for the cost of wine cave construction in 6 to 8 years, and thus wine caves can be considered to pay for themselves over time. In addition, the environmental and permitting process has made the construction of new conventional wine cellars a very tedious and costly undertaking, while very few constraints are placed on underground construction. The above-mentioned factors and the economic benefits of fermenting and storing wine underground have resulted in the construction of more than 50 wine caves in Napa Valley, most of which have been built in the last 30 years.