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Featured Article
Shoring for Rankin Pump Station by David Berti, J. Kumar Singh, and James Chia from Geotechnical News, September 1995
Introduction The City of San Francisco has a combined sanitary sewage and storm drain system. When it rains, heavy storm flows are carried into the system. The resultant combined flow often exceeds the sewage treatment capacity, requiring discharge of untreated sewage into the San Francisco Bay. The construction of a network of underground sewage transport and storage (T/S) structures has been undertaken by the Department of Public Works, City of San Francisco in order to minimize these storm discharges. An integral part of this Combined Sewer Overflow Control Program (CSO) is the Islais Creek System, which is located in southeast San Francisco. The primary storage structures of the Islais Creek System are 11,000 feet long, comprised of underground reinforced concrete structures and large diameter pipe line. The Rankin Pump Station at the down-stream end of the Islais Creek System is now under construction. The purpose of the Rankin Pump Station is to pump CSO flows from the Islais Creek System into the nearby Southeast Treatment Plant.
Construction of the Rankin Pump Station required a deep excavation in very soft marine clays. Support of excavation was complicated because the pump station consisted of two separate structures, which are constructed in two stages.
The first stage of excavation and construction, referred to as the Pumping Structure, is about 100 ft x 80 ft x 64 ft deep. The second stage, the Pipe Gallery, is about 100 ft x 60 ft x 26 ft deep. The walls of the pump station have long vertical spans.
Because of the well-defined soil strata and soil properties, the selected shoring system provides a model for comparing the theoretical design with instrumentation data collected during construction. The measured structural response to the earth pressures shows the limitations of using apparent loading diagrams where a soft clay stratum extends below the excavation subgrade.
Description of the Structure
Rankin Pump Station is located near the intersection of Rankin Street and Evans Avenue on the southeast side of San Francisco. See Figure 1. For reference, ground level is at elevation 0± San Francisco Datum, and mean sea level is at elevation -8.6±.
The pump station consists of three interconnected structures. The lower Pumping Structure, which channels and holds the sewage, has an inlet structure at the north end for interface with the Islais Creek Tunnel. Most of the Pumping Structure (Stage I) was excavated to elevation -60±. The maximum excavation depth is elevation -64.5± at the west end, where the Pumping Structure interfaces with the Pipe Gallery (Stage II) above. The Pipe Gallery was excavated to elevation -27±. The Pump Building sits atop the Pump Station and Pipe Gallery. Refer to Figure 2 and Figure 4 for a plan and elevation of the structure.
Subsurface Conditions The upper 15± feet of the soils are fills, with the ground water table located at approximately elevation -6. The principal stratum affecting the construction activities is Young Bay Mud, which extends to elevation ‑60±. Bay Mud is a slightly over-consolidated, soft to medium stiff, plastic clay. It has a high water content (up to 100%), unit weight of 95 pcf and is relatively impervious to water (approximately 10-7cm/sec). Because of some over-consolidation, the undrained shear strength (su) of the Bay Mud at the Pump Station is somewhat higher than other locations in the area. Field vane shear tests provided a design su of 400 psf at the top of the stratum (elevation -15±), with strength increasing at a gradient of 15 psf per foot of depth.
Below the Bay Mud stratum is a 15 to 20 foot thick layer of marine sand (SM to SP) under slight artesian pressure. This sand is very dense (standard penetration resistance of over 100 blows per foot), with little or no cohesion. Total unit weight is 132 pcf and the estimated angle of internal friction (f) for design is 40°. This sand is relatively pervious to water (approximately 5x10-4 cm/sec). Below the sand (and extending for considerable depth) is a layer of Old Bay Clay, which has an undrained shear strength of approximately 2000 psf, and a unit weight of 110 pcf.
Description of the Shoring Excavation support for the Pump Station included several design challenges because of the large excavation in deep soft clays, presence of highly pervious sand stratum under artesian pressure, and permissible limits on adjoining ground settlement. Tiebacks were not considered feasible in these soft clays. Because of a tight schedule and the difficult ground conditions, the City of San Francisco decided to include a detailed shoring design in the Contract Plans and Specifications.
Shoring walls were designed as soldier pile-tremie concrete (SPTC) slurry walls, 30 to 36 inches thick. W36X135 piles were specified for the Pumping Structure, and W30X99 piles were specified for the Pipe Gallery. The piles were to be installed at 6'-0 on center. A secant or soil-cement wall, with W27X102 and W27X84 piles spaced 4'-0 on center, was a specified acceptable alternate. The diaphragm walls were founded in the Old Bay Clay stratum to provide a hydraulic barrier between the water bearing sand stratum and the excavation. Three penetration criteria were specified for the diaphragm walls: 1) minimum of 25 feet embedment below subgrade; 2) minimum 25 feet embedment below bottom of Bay Mud stratum; and 3) minimum of 5 feet penetration into the Old Bay Clay layer.
The contractor (Amoroso/Kulchin-Condin JV) chose to install the soil-cement mix wall with the specified W27 piles. S.M.W. Seiko of Hayward, California, installed the soil-cement wall as originally designed, using a triple auger system.
Because of the long spans across the excavation, intermediate vertical pin piles were considered to support the bracing; however the final bracing system was chosen with no support. The bracing system consisted of four levels of crosslot bracing. See Figure 2 and Figure 4. All wales and struts were W36X230 or larger, and struts were interconnected, where necessary, as shown. The typical wale-strut connection is shown in Figure 3.
Bracing Level 1 was originally located above the roof at elevation ±5. The Contractor chose to lower Level 1 to elevation -3 and install an additional rebrace level during the removal stages. Levels 2, 3, and 4 were designed and installed at elevations -17, -34 and -45 respectively, with Level 4 centerline located approximately 18 inches above the bottom invert slab.
Because of concerns about ground settlement resulting from the consolidation of Bay Mud, it was originally specified that dewatering not be allowed. This requirement resulted in high water pressures on the cofferdam toe. The limited depth of the sand layer below excavation subgrade, combined with the relatively low shear strength of the lower Old Bay Clay, resulted in the necessity to provide a "kicker" reaction slab for toe stability. For this reason it was specified that the sand stratum (located at or below subgrade) be chemically grouted in order to increase its unconfined compressive strength to at least 40 psi (6000 psf), and thereby provide a reaction against the high active pressures acting on the soil-cement wall.
From the higher values found for su of the Bay Mud at the Pump Station, it was postulated that some pre-consolidation of the mud had occurred sometime in the past from deep well pumping from the sand stratum below. It was therefore inferred that a relatively short term dewatering program would not result in significant additional consolidation of the Bay Mud. Dewatering of the sand stratum was therefore permitted in place of the chemical grouted kicker slab. The assessment was later shown to be correct, since ground settlements due to the dewatering have not been significant. Dewatering was effective because it reduced the hydrostatic load acting on the support system, and significantly increased the passive support available to embedded portion of the piles.
Excavation and installation of the bracing was straightforward, with excavation proceeding approximately 4 feet below each bracing level before installation of the struts and wales. Construction of the Pumping Structure and connecting Pipe Gallery is more complex, entailing long vertical removal spans, careful sequencing, and rebracing across newly constructed concrete walls. Aside from the dewatering, only minor changes in the removal sequencing were made from the original design. Schematic sketches of the critical installation stages are on Figure 4.
Instrumentation consisted of strain gages (on the struts) and inclinometers, which were attached to certain soldier piles within the soil-cement wall. Instrumentation indicated that the wall movements and strut loads were reasonably close to the design values.
Behavior of the Shoring Wall in Soft Clays As noted above, the excavation for the Rankin Pump Station is mostly in soft marine clays (Bay Mud), which overlie a competent sand stratum at about the subgrade level. Distances between shoring walls are generally large, such that "narrow trench" effects (as used in heave considerations) are not significant.
The shoring was designed for both apparent and Rankine soil loadings. The Rankine soil loads proved to be critical for various excavation stages. In Rankine loading the shoring walls span from the lowest bracing level to the sand stratum below. For design, an assumption was made that the sand layer provides fixity, and a point of inflection was assumed at the top of the sand stratum. The shape of the deflected wall (at various excavation stages) confirms that significant loads acted on the wall in the Bay Mud below the excavation subgrade. The deflected shape also confirms that no effective (passive) support from the soil was available below subgrade until the sand layer was encountered at elevation -60±.
Figure 5 shows data from an inclinometer at various excavation stages The deflected wall shapes show that the piles are spanning vertically between the lowest bracing level and the sand, as noted above. As predicted by the analysis, inward wall movements occurred below subgrade, and the maximum wall deflection was approximately 1.5 inches, which is near the value which was computed from Rankine loading.
Behavior of the Bracing Bracing loads were computed using Rankine loading during the various excavation stages. An apparent trapezoidal pressure diagram was used for the full depth condition. The maximum computed bracing unit load was 50 kips per linear foot, with the highest load occurring on the lower two levels during installation. The 50 kip/ft unit load produces a calculated maximum strut load of 1300 kips (650 tons), which is within 10% of the actual maximum strut load measured by the strain gages (for a W36X300 strut).
The bracing has behaved as expected. No distress has been observed from ground loading, although the loads were high, as noted above.
Summary The successful excavation for the Rankin Pump Station indicated that wide and deep excavations can be made in soft clays when certain beneficial ground conditions are present. On this project the primary beneficial condition was the existence of the very dense sand layer located near subgrade of the excavation. During excavation stages, the sand layer acted as a point of toe support for the diaphragm walls. The walls spanned from the lowest bracing level to the sand later, with the soft Bay Mud providing little or no support to the walls.
In these cases with deep excavations in soft clays, adequate toe support for the wall is essential. If in-situ soils provide insufficient passive pressure (for needed toe support), then remedial methods can be taken to increase the available toe support. In sands or gravels, methods such as grouting or dewatering can be used successfully to support large horizontal pressures (including hydrostatic pressures), and provide effective toe support.
With a known soil stratigraphy, well defined soil properties, and the existence of a competent soil stratum for toe support at subgrade, wall deflection and bracing loads can be predicted accurately. Predictions can be made from commonly-used geotechnical theory and elastic beam analysis, without the necessity of using sophisticated computer modeling such as the finite element method.
This project shows the limitations and potential dangers in the use of equivalent trapezoidal soil diagrams where the soft clays extend to large depths below the bottom of excavation. Many designers model active soil loads in the shape of a trapezoid which ends at the subgrade; in this model the soils below subgrade are assumed to provide a reaction to the shoring wall. In this case with deep and soft clays, the use of such a model is inappropriate, and can lead to adverse consequences (including failure of the wall and shoring).
Notes David Berti is a Principal and Kumar Singh is an Associate at Jacobs Associates, San Francisco. They designed the shoring and performed construction related activities for the City of San Francisco. James Chia is Project Manager of the Rankin Pump Station for the Department of Public Works, City and County of San Francisco.
The Contractor is a joint venture of Amoroso and Kulchin-Condon & Associates. Mike Webster is Project Manager and Jerry Brown is Project Superintendent. Bill Bydewell and C. John Daughrity were in charge of shoring fabrication and installation. John Barsky is the Resident Engineer for the Bureau of Engineering, the project Construction Manager.
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