SPECIAL SPECIFICATION 5989 Settlement Gauge

Transcription

1 2004 Specifications CSJ SPECIAL SPECIFICATION 5989 Settlement Gauge Description. Install, maintain, and subsequently abandon settlement gauges as indicated for the purpose of obtaining foundation settlement data during and following placement of embankment construction. 1. General. A. Work also consists of periodically monitoring settlement plates during and following placement of embankment fill, and providing monitoring data to the Engineer. B. The Engineer may perform occasional concurrent monitoring of selected settlement gauge installations. C. The purpose for installing and monitoring these settlement gauges on a regular basis as the fill is placed is to observe the settlement of the underlying soils, compare actual to prior predictions, and to determine when sufficient settlement of the native soils has occurred so that pavements and accompanying structures can be installed. D. Not only the total settlement occurring at these settlement gauges; but also the rate of settlement is a key issue being monitored to assist with permitting the continuation of embankment placement and facility construction with the planned use of wick drains. E. The wick drains are intended to promote the rate of settlement and to minimize pore pressure increases; hence promoting embankment slope stability. The need to provide a scheduled stoppage or staged delay in embankment placement at a limited height of fill will be decided in these regions in part from the review of these time-rate of settlement observations along with significant review of pore pressure measuring devices. 2. Materials. Each settlement gauge shall consist of the following: A. A 0.5 inch thick steel plate, 36 inches square in size, placed upon a minimum of 1 inch thick sand leveling course, and with a 1-1/2 inch steel riser pipe that is welded in position perpendicular to the plate at its center. B. Sections of 1-1/2 inch diameter standard threaded galvanized steel riser pipe welded to the base plate and extended progressively upward at a vertical plumbness as embankment fill is placed and compacted. A 1-1/2 inch standard galvanized steel cap shall be attached to the threaded inner riser pipe as a survey reference member, and progressively removed and extended upward as each new section of riser pipe and external sleeve are added due to fill

2 C. Sections of 3 inch diameter standard threaded steel pipe sleeve initially placed with a 2 foot separation from the base plate and then extended progressively upward encompassing the 1-1/2 inch pipe with the internal annulus filled with grease to promote free sliding between sleeve and internal pipe. This sleeve is intended to be continuous so as to prevent embankment soils from coming in contact with the internal riser pipe over the length of sleeve to the surface as progressive lifts of fill are placed. 3. Assembly Requirements: A. Settlement gauge plates shall be furnished and installed by the Contractor at locations indicated in the contract documents. B. Benchmarks registering their coordinate locations and elevations to the nearest 0.01 feet elevation shall be established in near proximity to settlement gauges before the settlement plates are installed. C. Benchmarks shall be located sufficiently outside of the zone of stress-settlement influence from embankment fill, and other on-going construction activities. D. The method of determining alignments and elevations, along with the method of preserving control points will be subject to review and approval by the Engineer. E. Approval shall not relieve the Contractor from the responsibility for the correctness of the survey work by their subconsultant licensed surveyor. F. Plan cross-sections shall not be permitted as a means to establishing benchmarks or initial plate elevation readings for vertical or horizontal control. G. Settlement plates shall be approved by the Engineer before beginning any wick drain installation or embankment construction. H. Plate Construction. The first section of the 1-1/2 inch threaded riser pipe shall be firmly welded to the flat settlement plate. I. A 1-1/2 inch galvanized steel cap shall be placed on the top of the riser pipe, and subsequently transferred to each new riser section as progressive increments of pipe are attached to accommodate increased embankment fill. J. The distance from the top of plate to the top of pipe shall be accurately measured and recorded by the Contractor s surveyor. 4. Installation. A. Initially install settlement gauges subsequent to the installation of the vertical wick drains and prior to the placement of the embankment fill

3 B. Prior to placement of the embankment fill, place a minimum 1-inch thick sand leveling course upon the native in-place soils. C. Place the 0.5-inch thick steel plate upon the leveling sand. D. Assure that the leveling course sand extends a minimum of 1-inch in all directions beyond the edges of the 36-inch by 36-inch square steel settlement plate. E. Position and weld the initial 1-1/2 inch diameter threaded galvanized steel riser pipe perpendicular to the steel settlement plate with a fillet weld as indicated. F. Place end cap, consisting of 1-1/2 inch standard galvanized steel, at the top of the riser pipe for purposes as a survey reference point. G. Obtain the first measured readings of the settlement plate and end cap. H. Place embankment fill as indicated. I. As soon as embankment soils achieve 2 feet of cover over the steel settlement plate, position a 3-inch diameter steel sleeve loosely around the smaller diameter riser pipe to isolate and protect the inner pipe for subsequent readings. J. Fill the inner annulus between steel pipes with sufficient lubricant grease to prevent rust from occurring and resulting in binding of the inner pipe to the outer sleeve. K. Progressively add both inner riser pipe and outer sleeve pipe in section increments of 5.0 feet (or other calibrated and measured increments) as embankment fill is continued to be placed, always transferring the end cap to the newest riser pipe top, and always obtaining new elevation readings at each time of extension addition. L. The Contractor shall notify the Engineer a minimum of one day in advance to and immediately upon completing each installation. M. Provide updated elevation readings at the end of each day s activities to the Engineer. N. No embankment fill shall be placed around settlement gauges until the elevation of the top of the new riser section has been determined by the Contractor s surveyor. O. Contractor shall at all times take precautions to prevent damage to gauges. In the event settlement gauges become damaged or blocked by material entering the hole through failure on the part of the Contractor to use care in his operations, the settlement gauges shall be replaced, cleaned out, or reset as required, without any additional cost to the Department. 5. Construction Operations. A. The Contractor shall observe precautions necessary for protection of settlement gauges

4 After settlement gauge installation has begun, the Contractor shall replace, at his cost, any gauge equipment that is damaged or becomes unreliable as a result of his operations. B. Settlement Readings: i. Once each settlement gauge is installed at its specific location, Engineer will prepare a location-specific predicted time-rate of settlement curve based upon the actual embankment fill at each settlement gauge location. ii. Contractor s surveyor shall obtain settlement readings each week during fill placement of embankment. iii. iv. Contractor s surveyor shall adjust frequency to obtain settlement readings to once every one to two weeks after all embankment components have achieved finish grade elevation. It will be necessary that the Engineer evaluate the settlement readings as they are taken to compare to the site-specific predicted settlement curves in order to determine whether the settlement rate has slowed to an acceptable rate such that post-construction settlement would be within predicted and tolerable limits. v. Readings shall continue at this frequency until the Engineer advises Contractor that sufficient time has occurred for settlement compared to settlement curves. vi. Contractor shall plan for a minimum construction delay as indicated following completion of fill being placed to finished grade elevation for the rate of settlement to dissipate. vii. Once the determination is made that the rate of settlement has decreased, then the Contractor will be advised that installation of the roadway pavements and appurtenant facilities may commence. viii. Settlement readings at settlement gauges shall subsequently continue at a frequency of once every month for a period of six additional months, or until advised by Engineer to suspend. ix. Engineer will confirm when settlement readings may be discontinued and specific settlement gauges may be respectively abandoned. 6. Abandonment of settlement gauges. A. Upon completion of monitoring and when advised by the Engineer, abandon all settlement gauges in-place. Cut off and remove all steel pipe to a minimum of 3 feet below finished grade and then inject under pressure a cement-based grout into both the inner steel riser pipe and the outer steel protective sleeve until completely filled. 7. Measurement. Settlement gauges installed as prescribed by this specification will be measured by each settlement gauge complete and installed in place

5 8. Payment. Work performed and materials furnished as prescribed by this specification and measured under "Measurement" will be paid for at the unit price bid per each "Settlement Gauges", which price shall be full compensation for furnishing and installing all settlement gauges and for all manipulation, labor, tools, equipment and incidentals necessary to complete the work, except as hereinafter specified