Difficulties in Building Pumping Station
Sub-aqueous Foundation had to be Laid without a Rock Base— Sixteen Octagonal Dredging Wells Used—Special Requirements had to be Met in Design—Earthen Cofferdam Built Around Station
IN the design of any sub-aqueous foundation where rock is not to be found at a reasonable depth, the controlling feature of any design is usually determined by the character of the soil which is to be penetrated, and the kind of material upon which the structure is to be finally founded. In many of our western rivers rock is far distant and when so located is usually overlaid with a deposit of alluvial sand and gravel which in turn is covered with fine sand, clay, quicksand and mud in varying combinations and thicknesses. In the work under discussion, as originally shown before the American Water Works Association at its annual convention at Montreal, rock was so far below the surface of the river that founding upon it would have been impossible except by very difficult and expensive operation.
Test holes were drilled over the entire area involved in the work and the nature of the soil carefully tabulated. The soil brought up from all test holes was similar in character and ranged from loam on the surface to compact sand and gravel at a depth of 80 feet below the river bed.
Several Proposed Methods Discussed
The type of foundation was then fixed for the new structure and several tentative designs were made, namely: a steel sheet pile cofferdam with steel truss bracing, which contemplated open excavation into which would be driven a wood pile foundation capped with a thick mat of concrete tied together with reinforcing bars, and on this mat to place the building walls; a wooden pneumatic caisson; a combined wood and concrete open dredging well caisson; and an all concrete open dredging well caisson.
A design was sought which would present features that would be both economical and the construction of which could be kept under absolute control at all times.
Three Possible Schemes Considered
Three possible schemes then presented themselves as fulfilling the above requirements. The first was the wood pneumatic caisson, which in view of the experience with that one on which old station was built, was rejected as unworthy of much consideration on account of its being too flexible an engine foundation, its high cost, and the serious objection of its not being able to be made sufficiently water tight to fulfill the requirement of a dry pump well.
The second was the combined wood and concrete open dredging well caisson, which was rejected for some such reasons as given for the wood pneumatic caisson, although its control would be almost absolute.
The third scheme was the consideration of an open dredging well caisson built wholly of reinforced concrete with steel cutting edges. Such a design presented features that were the most attractive of any considered. All the materials of which it would be constructed could be easily obtained in the local markets. Its construction presented no more difficulties than an ordinary concrete structure of equal size. It could be constructed on the bed of the river within a cofferdam in the dry, directly over the location where it would be sunk.
The difficulties attending the building of such a foundation would be experienced in the sinking of so large a mass of reinforced concrete, to the great depth required, and to control the sinking so that it would attain its final position perfectly level and without having shifted horizontally either in one direction or another.
Final Design as Adopted
The final design was a caisson 90 feet square with a bay on the river side 22 feet by 51 feet in plan and a total depth of 33 feet in outside vertical dimension.
Sixteen octagonal dredging wells were placed in the main body and 2 rectangular wells in the river bay. The dimensions just given were determined by the outside of the upper foundation walls or substructure, the height of the main pump pit and the final depth of the caisson. It was deemed proper to carry down the caisson to the same depth as the wood caisson under the present station, or even lower. The idea was also incorporated in the design, of being able to convert the caisson from an open dredging well to a pneumatic caisson, if this became necessary from any cause whatsoever. The encountering of some kind of hidden obstruction that would prevent sinking the caisson to its final designed position would require its conversion to a pneumatic caisson so as to make the obstruction accessible and its removal certain. Therefore, the cutting edges in the interior were raised over 5 feet so as to make two working chambers, one under the large portion and one under the bay, and by sealing the dredging wells with thick diaphgrams of reinforced concrete, these chambers could be made air tight. This contingency was considered rather remote in the light of the nature of the underlying soil.
The calculations in the design of the caisson were necessarily based on several assumptions—the nature of the soil to be penetrated, and past experience of sinking similar caissons. The first assumption made was, that the caisson might be supported at any two diagonal corners, and the strength of the caisson was calculated to support its own weight neglecting skin friction and sinking weight of the caisson in its various stages of construction. As it was assumed that only about half the caisson would be constructed before the first sinking operation would be commenced, and that a fair maximum skin friction for the kind of soil found was between 500 and 700 pounds per square foot of outer surface, weight enough would have to be provided to overcome the latter and allow the caisson to sink.
The Caisson as Finally Designed
The caisson as designed contained 5,858 cubic yards of concrete, 290,000 lbs. of reinforcing steel, and 159,000 lbs. of steel cutting edges. The total sinking weight, therefore, was 23,195,000 lbs. This would overcome a maximum skin friction of 1,739 lbs. per square foot, provided there was no support under the cutting edges. As no such skin friction has ever been experienced in a purely sand and gravel soil without clay, it was concluded that the caisson would sink without any supplemental weight being employed.
The caisson was reinforced on all surfaces both interior and exterior against any surface cracks by 1-inch deformed bars placed horizontally and vertically. Horizontal bars were spaced 18-inch centers, while vertical bars were spaced about 2 feet to 3 feet centers.
Other reinforcing, 1-inch deformed bars spaced 6 inches on centers, were placed in horizontal layers at 10 feet 6 inches, 18 feet and 26 feet respectively from the bottom of the lowest cutting edges, running in all the walls between the dredging wells and in the outside walls. The office of these bars was to allow the walls to act as horizontal beams to make the caisson selfsupporting during the sinking operations. Diagonal bars of the same size were also introduced between the wells to efficiently tie the structure together in any diagonal direction. The wells were made octagonal especially for this purpose and to keep them as large as possible.
Two Combined Suction and Screen Wells
Two combined suction and screen wells were incorporated in the design of the caisson. This requirement was caused by the floor of the suction well being 10 feet 6 inches below the main pump pit floor, which is the same elevation as the top of the caisson. The suction and screen wells occupy the river bay and serve as the effluent termini of the intake pipes. These pipes, four in number, are 36 inches in diameter as they enter the suction wells. They are connected to four 36-inch double disk gate valves which are electrically operated and controlled from the screen room above. The four pipes as they leave the face of the suction well bay connect in pairs to two 48-inch intake pipes coming out of the intake tower. One of these 48-inch pipes was in existence before this work commenced, it being changed somewhat in direction to accommodate the new connections.
Foundation Wall Requirements
The next step in the design of the station was that of the foundation walls, surrounding or forming the pump pit and the suction and screen well shafts. The requirements to be fulfilled were several in number: First, as the river level in its fluctuations of 40 feet would surround the substructure walls, they would have to be devoid of openings up to or above the highest known water in order to obtain a dry pump pit; Second, they would have to be designed to resist the external pressure due to the highest stage of water; Third, they would have to be similar in appearance to the corresponding portion of old station.
The first and third of these were easily satisfied, but the second requirement, that of making the new station similar in appearance to the old, made it necessary to design the walls either as retaining walls of the buttress type, or of the gravity type. In both forms of walls it was found that the amount of reinforcing steel was enormous, and the lower thickness of walls would project in too far toward the center of the building and encroach on the space for the proper placing of the pumps. The design finally adopted was that of making the pump pit walls circular within and square without, or in other words, to consider the walls as horizontal circular arch rings, fixing the arch elements 3 feet width and 1 foot thick vertically.
Reinforcement consisting of 1-inch deformed steel bars were placed both vertically and horizontally in the walls at both outer and inner surfaces and spaced 18 inches on centers. Their principal function was to efficiently tie the walls together to prevent any shrinkage or settlement cracks which might produce leaks, and to stiffen the arch ring.
The substructure walls around the screen and suction wells were designed of vertical beams and struts. A division wall 3 feet thick divides the space within into two suction and screen wells. The wall runs from the floor of these wells up to the main engine-room or screen room floor. In this wall at its lower end, and as close to the floor as possible, was placed a 48-inch double disc gate valve, which allows the intake water coming into the suction and screen wells to be distributed from the east to the west well or vice-versa. This allows either pump to draw water from either well. Also when this valve is closed and the two 36-inch valves in the intake pipes are closed, in either well, while corresponding pump is not operating, that well can be emptied of water by a sump pump and cleaned out of mud and accumulated rubbish which has passed the outer intake screens.
Earthen Cofferdam Precedes Caisson
Before the work of constructing the caisson was begun, an earthen cofferdam was built entirely around the site of the new station. It consisted of two walls of wood sheeting, supported by horizontal lines of 8xl0-inch wales and wood piles driven 12 feet apart. The two walls were tied together with 1-inch round rods and the space between the walls was then filled with puddle material dredged from the river. The height of the cofferdam was such as to withstand a rise of 15 feet in the river without flooding the enclosure and stopping the work.
The contractors’ equipment consisted of the usual river equipment including dredge boats, pile drivers: derricks, pumps, etc., and a land equipment of locomotive and dump cars, locomotive crane, stiff leg derricks, concrete mixers, etc. There was nothing unusal in the equipment to warrant further mention.
Sinking of the Caisson
The sinking of the caisson by the open dredging well method is probably the only feature of any importance in the work under discussion as no mass of this area has been attempted, to my knowledge, without the use of air. With the eighteen dredging wells available for excavation and three still leg derricks, in operation with clam shell buckets, the sinking of the caisson was fairly rapid. The bottom of the excavation in the center of the caisson was maintained at about 12 feet below the lowest cutting edge and in this condition the caisson was supported wholly around its outer periphery. When the caisson was within three feet of its final position the excavation by buckets was discontinued and preparation made to jet it down as the sinking could be controlled better and as the surface of the excavation in the interior of the caisson was so much deeper than the interior cutting edges; the removal of more material was useless. The jetting was done from the inside of all the outside dredging walls, passing the jet pipe under the outside cutting edge from one well to the next, around the whole periphery of the caisson. The total sinking of the caisson was 38.28 feet, which was accomplished in 28 days, actual digging time. The average sinking, therefore, amounted to 1.35 feet per day of 22 hours. The maximum in any one day was 2.15 feet.
Sealing the Dredging Wells
It was thought advisable to allow the caisson to rest at this elevation, as a slight settlement would take place in the next two or three days, while the dredging wells were backfilled to the underside of the interior cutting edges. This was done so that communication from one well to another would be cut off, while the concrete seal of six feet was being placed.
This was done by means of a 20-inch pipe, or “Tremie” placed in the water in the well and resting on the bottom. When the pipe was filled it was raised about three feet, allowing the concrete to flow out and deposit. In this way all the dredging wells were sealed, taking them in order around the caisson.
The thirty feet of water remaining in each well was then pumped out and the seal found to be fairly tight. Such leaks that had developed were stopped and all wells were then filled by the usual method of pouring and the caisson completed.
Considering the entire weight of the concrete caisson, the weight of the pump well and superimposed walls, the weight of the two thirty-million gallon pumping engines, it is interesting to note that the total bearing pressure on the sand and gravel upon which the caisson is founded is 4,700 lbs. per square foot.