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Control and Inspection of Earthwork Construction

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Control and Inspection of Earthwork Construction PDHonline Course C284 (8 PDH) Control and Inspection of Earthwork Construction Instructor: George E. Thomas, PE 2012 PDH Online | PDH Center 5272 Meadow Estates Drive Fairfax, VA 22030-6658 Phone & Fax: 703-988-0088 www.PDHonline.org www.PDHcenter.com An Approved Continuing Education Provider www.PDHcenter.com PDH Course C284 www.PDHonline.org Control and Inspection of Structural Earthwork Construction George E. Thomas, PE A. Principles of Construction Control 1. General. In many types of engineering work, structural materials are manufactured to obtain certain characteristics; their use is prescribed by building codes, handbooks, and codes of practice established by various engineering organizations. However, for earth construction, the common practice is to use material that is available locally rather than specifying that a particular type of material of specific properties be secured. A variety of procedures exists by which earth materials may be satisfactorily incorporated into a structure. When earth is the construction material, personnel in charge of construction control must become familiar with design requirements and must verify that the finished product meets the requirements. Design of earth structures must allow for an inherent range of earth material properties. For maximum economy, tolerance ranges will vary according to available materials, conditions of use, and anticipated methods of construction. A closer relationship is required among the operations of inspection, design, and construction for earthwork than is needed in other engineering disciplines. Construction control of earth structures involves not only practices similar to those normally required for structures using manufactured materials, but also the supervision and inspection normally performed at the manufacturing plant. In earthwork construction, the processes by which an acceptable material is produced are performed in the field. Large earth structures are usually built by a contractor. The basis of the contract is a set of specifications that has a schedule of items of work. Through information provided in specifications, the contractor proposes prices for performing the items of work that, when accepted, become a part of the agreement between the owner and the contractor. The primary functions of the construction control organization are to: • Ensure the structure is built according to specifications. • Report and document any changed conditions. • Certify to what extent the items of work have been completed. • Test for compliance with requirements. • Determine payment due the contractor. Although Engineers prepare plans and specifications defining work to be performed, once a contract is signed, the Engineers or there representatives have no right to change contract requirements, and the contractor has no right to change unit prices. A condition may exist in the field that is different from what was anticipated to exist during preparation of the specifications. If changes to designs are necessary, a contract modification is agreed upon by both the contractor and the owner. Specifications requirements for earth structure construction may be grouped into two types: those requirements based on performance and those requirements based on procedures. This distinction must be clearly recognized. The requirement for earth fill in a dam embankment is usually based on both a minimum procedure and on performance. If the two requirements prove to be incompatible, the performance is adjusted to obtain compatibility. The desirability of a performance-type requirement is recognized; however, the present state of knowledge of soil behavior, the complexity of specifications required, and the extensive testing requirement make this type of specification very expensive for some types of noncritical earth structure construction. When developing specifications, it is very important to include the assumptions, confidence, and uncertainties inherent in the design. This allows both the contractor and owner a clearer understanding of the expectations of the designer and the inherent risks involved in the contract. It has been common practice to make all existing data available to bidders either by incorporation of the data into specifications or by referencing reports and making them available for review. In the past, it was practice to develop a separate document called "design considerations" which included the designers’ assumptions, uncertainties, and requirements for successful construction. Engineers should attempt to include some of these considerations directly in the specifications. An example would be to include the reasoning for excavations shown on certain levels in the foundation. Acceptable foundation conditions should be given along with the reasoning and the geologic interpretations providing the basis for the excavation limits. Also, it is recommended that the owner, with support for the engineer, provide a prebid briefing to © George E. Thomas Page 2 of 87 www.PDHcenter.com PDH Course C284 www.PDHonline.org prospective contractors. During these briefings, it is helpful to show preconstruction excavations or to actually perform excavations such as test pitting for the prospective bidders. Often, engineers do not perform extensive design investigations; however, design investigations are performed to collect design data related to constructability. Given the nature of the earth and its complex geology, it is not economical to attempt to perform extensive investigations to alleviate any change of condition. Contractors are encouraged to perform prebid investigations and investigations as construction progresses, as necessary, to avoid surprises during construction. Specifications requirements also may be divided into two groups: definite requirements and those qualified by the phrase "as directed." The undesirability of the latter requirement is recognized and is avoided whenever possible. However, the "as directed" type of requirement is used in establishing minor dimensions in areas where investigations sufficient to establish such dimensions are not justified and when new conditions are encountered for which requirements have not been established. Where the "as directed" requirement refers to dimensioning, either maximum and minimum dimensions or an average dimension is noted on the design drawings. If maximum and minimum dimensions are stated, the usual practice is to excavate to the minimum dimension; and where visual examination indicates the excavation is still within inferior material, excavation up to the maximum dimension is continued, if required, to reach a satisfactory foundation. Where the "as directed" requirement refers to a new condition, the contractor and construction engineer should exercise joint effort to establish practical performance limits or to establish procedures that will produce satisfactory results where performance cannot readily be defined. Some experimentation is desirable, but this aspect should be small compared to the total requirement. 2. Inspection. The adequacy of construction can only be determined by visual examination, by measurements, and by testing. Inspection determines whether the requirements of plans and specifications are being satisfied, it does not determine what the requirements should be. An inspector, commonly the design engineer or his representative, are familiar or should become familiar with the specified requirements for work to be inspected. In determining whether work satisfies the requirements, an inspector needs to be familiar with how the requirement is defined. For earth structure construction, dimensional requirements and quality requirements are evaluated. A requirement may be defined as a condition to be achieved from which certain deviations (plus or minus) are tolerable, or it may be defined as a limiting condition from which deviation is allowable in only one specified direction. Both methods are in common use. Standard practice is to allow a 10% plus or minus variation before a change order is required Although a procedure is specified for some types of work (notably, earth fill in dam embankments) inspection may be made on the basis of satisfying a performance requirement. Basically, inspection determines only whether work is acceptable or not acceptable, but it also is desirable to determine the extent at what level the work is acceptable or unacceptable. An inspector should be familiar with the various safety and health regulations for construction activities. Inspectors should stay fully informed on progress of work and on future programmed work. To facilitate construction control, relationships between engineering properties test results and index test results are established wherever possible. In addition, visual observations of soil characteristics are correlated with both index and engineering properties. When noticeable differences exist between acceptable and unacceptable work or materials, sufficient testing is required to confirm that differences persist. As these differences become less obvious, the amount of testing should be increased. Testing should be sufficient to provide adequate quality control and to furnish the necessary permanent records. It is impractical to completely test all the work performed. The usual procedure is to select samples of work or materials for testing that are representative of some unit of work or material. Accuracy of such procedures depends on: • the relationship of sample size to size of unit it represents, • procedures
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