DEEP SOIL MIXING SHORING SYSTEM TO CONSTRUCT A 60 MGD, 40 FOOT DEEP WASTEWATER PUMPING STATION

Transcription

1 DEEP SOIL MIXING SHORING SYSTEM TO CONSTRUCT A 60 MGD, 40 FOOT DEEP WASTEWATER PUMPING STATION BY DARLA REAMS 1, JEFFREY B. GLOVER 2 AND DAVID J. REARDON 3 HDR ENGINEERING 5175 HILLSDALE CIRCLE EL DORADO HILLS, CA (916) Senior Engineer, City of San Mateo, CA 2 Project Manager, HDR Engineering, El Dorado Hills, CA 3 Project Manager, HDR Engineering, El Dorado Hills, CA

2 The City of San Mateo, CA operates three wastewater pumping stations on a 80 wide lot located in a residential area near San Francisco Bay. The age of the stations range from 12 to 50 years. the stations have difficulty pumping peak wet weather flows, and have structural and maintenance problems. For these reasons, a new station was proposed to replace the existing stations. The site layout is shown in Figure 1. Flows through the existing stations are highly variable, averaging 10 MGD dry weather and approximately 50 MGD peak wet weather. At night flows drop to as low as 1 MGD. the new station was designed to pump peak flows of 60 MGD. The new Dale Avenue Pumping Station that replaces the three exiting stations was originally conceived as a circular caisson type. However, a geotechnical investigation by Kaldveer Associates, Oakland CA, recommended against a caisson type structure because the stiff soils could cause difficulties in sinking caisson structure. This prompted an investigation of possible shoring methods for the proposed structure. This paper describes the shoring methods investigated, and the selection and fundamentals of the deep soil mixing (DSM) shoring system as well as construction considerations. SHORING OPTIONS The desired shoring system for the station was deemed to have the following characteristics: Low vibration levels during installation Suitable for soils with cobbles and gravel No tiebacks under existing homes Low permeability Minimum effect on adjacent groundwater levels Systems investigated included steel sheeting, soldier beams and lagging, tieback systems, caisson structures, open excavation and others. However, each had disadvantages that prevented their use for this Project. Steel sheeting necessitated driving the steel members through stiff clays and cobble gravels, a technique that could have caused settlement and structural damage to adjacent homes due to vibration during driving. Drilled soldier beams are lagging would have produced dewatering problems. Tieback systems would have required drilling the adjacent homes, which was undesirable. A caisson type structure would have eliminated a shoring system. However, as previously stated, it was rejected as too risky. because theses conventional techniques were unacceptable, attention was turned to deep soil mixing to serve as a diaphragm wall during construction of the pumping station. The anticipated advantages were impermeability, fast and efficient installation, adaptability to subsurface conditions ranging from soft ground to stiff clay to gravels and a lack of pile driving requirements. To the author s knowledge, this is the first application of

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4 the DSM method for use as a diaphragm wall in the United States. The DSM diaphragm wall technique has been used extensively in Japan since A description of the DSM system follows: DESCRIPTION OF DSM SYSTEM The DSM system uses a crane supported set of leads that guide up to four 36- inch diameter auger/mixing shafts. As the shafts advance vertically into the soil, a cement/bentonite grout is injected through the stems of the mixing shafts and discharged at the head. By combining auger flights and mixing blades along the shafts, soil is lifted and blended with the grout in pugmill fashion. When the design depth is reached, the mixing shaft rotation is reversed and the mixing process continues as the shafts are brought to the surface, leaving a thoroughly mixed column of soil, cement and bentonite. Continuity is achieved in two ways, first auger flights and mixing blades overlap with adjacent shafts. Secondly, the DSM machine penetrates with a multiple set of shafts that overlap one of the previously drilled columns, while simultaneously creating three new columns with the other shafts. Construction of the DSM installation at the Dale Avenue Pumping Station is shown in Figure 2 (Parts 1-4). Figure 3 shows the proximity of the pumping station to nearby homes and the DSM assembly in action. the bottom of the auger assembly and guide trench and the cement batch plant are shown in Figure 4. Because the resulting soil columns cannot provide unbraced support for deep excavations, a structural system must be provided. Immediately after mixing is completed, H-beams are dropped and vibrated into the columns. during curing and subsequent excavation, the wall material is chipped away to expose the H- beams bear directly on the walers. The walers are designed as structural members braced in the interior of the excavation, or they can be restrained by a tieback system that extends outside the excavation. Figure 5 shows the completed DSM wall. DSM SYSTEM FOR THE DALE AVENUE PUMPING STATION The Dale Avenue Pumping Station has a depth of 40 feet. The DSM wall was designed to penetrate 10 feet lower than the bottom of the structure as shown in Figure 6. After the placement of the H- beams in the DSM column, the structure was excavated and two sets of internal support walers were placed to support the excavation. See Figure 2 (Parts 5 and 6). A design summary of the pumping station and diaphragm wall is presented in Table 1. As the structure was constructed, the walers were used as convenient walkways and staging platforms. Construction of the 2 foot thick concrete walls was accomplished by pouring directly against the diaphragm wall; thus no exterior forms were used. Interior wall forms were supported by ties welded directly to

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6 DSM Assembly in Action Location to Nearby Homes Figure 3

7 the vertical H-beams. As the wall construction proceeded upwards, the lower waler was removed and the 4 foot thick concrete floor of the pumping station served as the internal bracing system. After the mezzanine floor of the station had achieved strength, it served as the upper bracing system for the walls, and the upper walers were then removed. No waterproofing or damp proofing was applied to the exterior walls of the station below the groundwater level. TABLE 1. DESIGN SUMMARY OF DALE AVENUE PUMPING STATION STRUCTURE AND DIAPHRAGM WALL SIZE, CAPACITY OR COMPONENT DESCRIPTION Pumping Station Lot Width 80 ft. Station Dimensions (outside) LxWxD Type of Shoring System 76 ft x 49 ft x 40 ft DSM diaphragm wall Diaphragm Wall Number of shafts for DSM equipment 4 Effective width of diaphragm wall Auger diameter Depth of wall 24 in 36 in 50 ft Spacing of vertical H-beams 4 Size of H-beams Specified permeability of finished soil/cement column Strength of soil/cement column W24x76 5x10 6 cm/sec Approx. 100 psi Bracing system Upper Walers Lower Waler Groundwater Elevation W27x84 through W36x135 W36x135 through W36x ft above bottom of structure

8 DSM Assembly Bottom and Guide Trench Grout Batch Plant Figure 4

9 Figure 5 Excavation Showing Vertical H-Beams and Internal Bracing

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11 DSM SYSTEM FOR THE DALE AVENUE PUMPING STATION The DSM diaphragm wall was intended to provide an impervious barrier to the groundwater that existed outside the station. It was anticipated that the wall would virtually eliminate dewatering during the construction and preclude wall seepage after the station was constructed. Unfortunately, the DSM wall did not act as a perfect barrier; small leaks occurred at two locations during construction. These leaks (approximately 10 gpm) provided more a nuisance than a severe construction problem. Water was contained at the leaks and discharged from the excavation with small submersible pumps. the DSM wall was effective enough to prevent any significant water from entering the excavation during the construction process. Because the groundwater surface was over 25 feet above the bottom of the excavation, some leaks appeared on the walls of the dry well of the station. Most leaks occurred at locations where form ties penetrated the wall and were welded to the vertical H-beam. These leaks were stopped by epoxy grouting. Installation of the vertical H-beams into the mixed soil columns was accomplished by lifting the H-beams into place and allowing them to sink into the liquid mixture of soil, cement and water. Where resistance was encountered, the H-beams were vibrated into place. However, not all H-beams cooperated; some could not be sunk completely. Where difficulties were encountered, the H-beams were cut off at ground level and the bottom of the H-beam was cross welded to adjacent H-beams after excavation was complete. The cost of the deep soil mixing system, which consisted of the drilling and mixing of the diaphragm wall was approximately $350,000 for approximately 10,000 square feet. It was provided by a subcontractor. The $350,000 figure includes approximately $100,000 for mobilization/demobilization, $50,000 for installation of H-beams, and $200,000 for construction off the DSM wall. The general contractor purchased the H-beams, and provided and installed walers and all internal bracing. The cost of these items was approximately$198,000. A summary of costs for the DSM wall system is shown in Table 2. The total cost of this system $548,000 or approximately $55/sf. The construction of the Dale Avenue Pumping Station, San Mateo, CA, was begun in June 1989 on a narrow lot in a residential neighborhood. To prevent potential damage to adjacent homes, driving of soldier beams and sheet piling were eliminated as shoring options. Tieback systems were also undesirable because they would penetrate below homes adjacent to the station. The need for a system with relatively low vibration and the ability to retain the high groundwater led to the use of the deep soil mixing system as an internally braced shoring system. The installation was rapid and allowed the placement of a complete diaphragm wall before excavation commenced. An added bonus of the

12 system was that the DM wall served as the back form to pour the cast-in-place concrete walls for the pumping station. Parties involved in the project included: Owner Designer Geotechnical Engineers General Contractor DSM Wall Subcontractor City of San Mateo, CA HDR Engineering, Inc., El Dorado Hills, CA Kaldveer Associates, Oakland, CA Dan Caputo, San Jose, CA Geo-Con Inc., Pittsburgh

13 TABLE 2. SUMMARY OF DSM SYSTEM COSTS AT DALE AVENUE PUMPING STATION COMPONENT COST DSM WALL (Subcontractor) Mobilization $100,000 Construct DSM Wall 200,000 Place Vertical H-beams, Miscellaneous 50,000 Subtotal $350,000 Internal Bracing, Miscellaneous (General Contractor) Purchase Vertical H-beams 65,000 Internal Bracing Purchase Steel 55,000 Fabricate 25,000 Install/Remove 30,000 Structural Design 10,000 Assistance with DSM Wall Construction 5,000 Haul Spoils from DSM Wall Construction 8,000 Subtotal $198,000 Total $548,000