SOIL ENGINEERING: TESTING, DESIGN, AND REMEDIATION phần 10

REFERENCES F.H. Chen, Foundations on Expansive Soils, Elsevier Science, New York, 1988. Effects of Defects in Composite Materials, STP 836, ASTM, 1984. P. Rainger, Movement Control in Fabric of Buildings, Batsford Academic and Educational, London, 1983. R. Weingardt, All Building Moves — Design for It, Consulting Engineers, New York, 1984. ©2000 CRC Press LLC 16 Construction CONTENTS 16.1 Scope of Contractor 16.1.1 Subcontractor 16.1.2 Owner’s Responsibility 16.2 Contract and Specification 16.2.1 Differing Site Condition 16.2.2 Specification 16.3 Foundation Construction 16.3.1 Drilled Pier Foundation 16.3.2 Driven Pile Foundation 16.4 Construction Control 16.4.1 Field Testing 16.4.2 Moisture Control 16.4.3 Record-Keeping 16.5 The Technician 16.5.1 Qualifications of a Technician 16.5.2 Length of Service References The final product of an engineering project is a combined effort of the architect, the engineers, and the contractor. For an outstanding project, the architects get the honor and credit and will be remembered. We all recognize the famous architect, Frank Lloyd Wright, yet the builder is seldom mentioned. At the same time, if there is problem in the project the first one to blame is the contractor. It is the contractor who uses substandard material, abuses the specification, ignores the advice of the consultant, and causes the problems. When damage appears in a structure, foundation failure is at once suspected. The contractor must prove that the construction complies in every respect with the plan and specifications. The contractor must produce evidence that foundation quality control has been followed. Quality control begins with the geotechnical investigation of the site and continues through the construction with proper construction control. The relation between contractor and geotechnical engineer can be illustrated by Irving Youger’s reply to a plaintiff when his cabbages were eaten by the defendant’s goat. You did not have any cabbages, If you did, they were not eaten. If they were eaten, it was not by a goat. If they were eaten by a goat, it was not my goat. And if it was my goat, he was insane. 0-8493-????-?/97/$0.00+$.50 © 1997 by CRC Press LLC ©2000 CRC Press LLC Ultimately translated to a foundation failure case, the defense might be postulated as: The contractor was not negligent. If the contractor were negligent, the geotechnical engineer was comparatively negligent. Even if the contractor were negligent, the negligence did not cause the failure. The actual theories are more than a little like the crazy goat reference above. 16.1 SCOPE OF CONTRACTOR In the beginning, all the parties involved in the design and construction of a project must share the success or the failure of a project. The owner hires the architect. The architect in turn hires a variety of engineering services, including the geotechnical engineer. The architect compiles all the information and makes a recommendation to the owner concerning the design and construction of the project. After the design is accepted, the general contractor is selected to construct the project. The architect and the owner generally share in the responsibility of the selection since both have a vested interest in choosing a qualified contractor capable of completing the work. In the case of geotechnical engineering, a quality assurance program is generally accepted as part of construction. This program can include periodic or full-time observation and testing as well as documentation of adherence to the recommenda-tions set forth in the soil report. In the case of a subdivision development, the developer sometimes acts as both owner and contractor. The developer hires an in-house architect and sometimes an in-house engineer. Most of the time, the only other engineer he seeks is the geotech-nical consultant. When the owners of the residence sue the developer, the liability will be entirely on the contractor and sometimes the geotechnical engineer. 16.1.1 SUBCONTRACTOR The general contractors subcontract the project to the following specialized firms: Earth Moving Pier Drilling Pile Driving For site preparation, fill removal, and fill placement. The testing and observation of this operation is generally left in the hands of the geotechnical engineer. The placement of local fill, such as in a narrow ditch or backfill, is usually placed without control. The constructions of drilled pier foundation systems are usu-ally subcontracted to the specialized firms. The general con-tractors seldom interfere with the pier drilling operation, except when there is an overrun or when unexpected prob-lems are encountered. The performance of the pile-driving operation is observed by the geotechnical engineer. The general contractor is con-cerned about the time schedule and cost overrun. ©2000 CRC Press LLC 16.1.2 OWNER’S RESPONSIBILITY A contractor may have faithfully performed all the obligations contained in the contract, including those of the subcontractors — yet the building suffered damage after occupation. Should the contractor be held responsible? Legally, although the project has been accepted by the owner, that does not relieve the contractor of responsibility. Many buildings have suffered damage long after occupation, and the contractor as well as the engineers and architect are sued by the owner. The arguments by the defendants are: The contractor cannot control the maintenance of the building, and if the owner chose to abuse the building, the responsibility should lie with the owner. The engineer, especially the geotechnical engineer, can only give recommenda-tions but not insurance; it is up to the insurance company to guarantee the integrity of the structure. The most common maintenance negligence is described in Chapter 15. As an example: a municipal building is founded with drilled pier foundations in an expansive soil area. The front part of the building suffered damage from pier uplifting. The owner sues the contractor for poor pier placement and the geotectnical engineer for assigning insufficient dead load on the pier. After many months of investigation, it is suspected that water from excessive lawn watering caused the pier uplift. The owner argues against the allegation. It is decided that the owner and the geotechnical engineer should inspect the subsoil conditions under the crawl space. Upon entering the limited entrance into the crawl space, to the surprise of the owner the ground at the front portion in the crawl space was flooded. The owner agrees to drop all claims. Concrete slabs, placed directly on the ground, are much less expensive than structure slabs with no direct contact between slab and ground. In an expansive soil area, geotechnical engineers recommend the use of structural slab, unless the owner assumes the risk of floor movement. In such a case, all slab-bearing partition walls should be provided with a void at the bottom, so that uplifting of the slab will not affect the upper structures. The engineer provides the contractor with detailed plans. At a later date, the owner decides to add partitions in the basement. The partitions are placed directly on the floor with no void space underneath. Consequently, the lifting of the partition walls causes severe distortion of the upper structure. The owner blames the contractor for such damage. The geotechnical engineer is able to determine that the fault lay with the owner and not the contractor. 16.2 CONTRACT AND SPECIFICATION The American Institute of Architects publishes “General Conditions of the Contract for Construction” for use by the architect. The contents of this document increase each year; it is now over 20 pages long in closely typed form. With this document, the owner and the architect sign the “Standard Form of Agreement Between Owner and Architect.” Before signing the contract, the architect will provide the contractor with all designs and drawings concerning the project. These include a soil report with logs ©2000 CRC Press LLC of exploratory holes. Most construction contract bid documents require a site exam-ination before the contractor bids on the project. In such cases, the contractor must make a prebid examination as a prerequisite to recovery for a changed condition. This duty of the bidder must involve a site inspection, but generally would not require an independent boring or test pit investigation. However, if the contractor does not agree with the subsoil conditions depicted by the soil engineer, he should at his own expense conduct a separate subsoil investigation. In almost every case, the contractor accepts the soil report provided by the owner. Construction is a risky business. Between signing of the construction contract and the ultimate completion, a number of factors invariably impact the project. The contractor assesses the risks of such factors and includes a contingency factor in the bid to protect himself. In such cases where the unanticipated subsoil conditions do not materialize, the contingency factor increases the contractor’s profit. If the risk actually encountered exceeds those assumed, the contractor will generally seek to obtain relief by the filing of claims. In order to reduce the number of disputes and claims, owners have included in their construction contracts provisions for risk-sharing by the owner and the con-tractor. These risk-sharing clauses include “Changes,” “Differing Site Conditions,” and “Suspension of Works,” among others. 16.2.1 DIFFERING SITE CONDITIONS The contractor shall promptly, and before such conditions are discovered, notify the contracting officer in writing of subsoil conditions at the site differing materially from those indicated in the contract. Most contracts allow two types of differing site conditions. Type I applies where the conditions actually encountered differ from the conditions indicated on the contract documents. This includes the following: Groundwater Rock and boulder Bedrock Man-made objects Utilities Soils Discussions on Type I differing site conditions are given in Chapter 17. The Type II differing site conditions are those conditions which are unknown and unusual when compared with conditions customarily encountered in the partic-ular type of work. This category of differing site conditions is less frequently alleged and considerably more difficult to prove because the contractor must prove he has encountered something materially different from the “known” and the usual. In one case, the site for the building of a bridge was flooded because of a diversion of the river by another contractor who was constructing a highway upstream. The court held that the diversion was a changed condition even though it had occurred after contract award. ©2000 CRC Press LLC ... - tailieumienphi.vn