3.4.7 Monolithic Concrete Basement Walls These should have a minimum thickness of 6 in. Where insulation is desirable, as where the basement is used for living quarters, lightweight aggregate, such as those prepared by calcining or sintering blast-furnace slag, clay, or shale that meet the requirements of ASTM Standard C330 may be used in the concrete. The concrete should have a minimum compressive strength of 2000 psi. For the forms in which concrete for basement walls is cast, form ties of an internal-disconnecting type are preferable to twisted-wire ties. Entrance holes for the form ties should be sealed with mortar after the forms are removed. If twisted- wire ties are used, they should be cut a minimum distance of 11/2 in inside the face of the wall and the holes filled with mortar. The resistance of the wall to capillary penetration of water in temporary contact with the wall face may be increased by the use of a water-repellent admixture. The water repellent may also be used in the concrete at and just above grade to reduce the capillary rise of moisture from the ground into the superstructure wails. Where it is desirable to make the wall resistant to passage of water vapor from the outside and to increase its resistance to capillary penetration of water, the exterior wall face may be treated with an impervious coating. The continuity and the resultant effectiveness in resisting moisture penetration of such a coating is dependent on the smoothness and regularity of the concrete surface and on the skill and technique used in applying the coating to the dry concrete surface. Some bituminous coatings that may be used are listed below in increasing order of their resistance to moisture penetration: Spray- or brush-applied asphalt emulsions Spray- or brush-applied bituminous cutbacks Trowel coatings of bitumen with organic solvent, applied cold Hot-applied asphalt or coal-tar pitch, preceded by application of a suitable primer Cementitious brush-applied paints and grouts and trowel coatings of a mortar increase moisture resistance of monolithic concrete, especially if such coatings contain a water repellent. However, in properly drained soil, such coatings may not be justified unless needed to prevent leakage of water through openings in the concrete resulting from segregation of the aggregate and bad workmanship in casting the walls. The trowel coatings may also be used to level irregular wall surfaces in preparation for the application of a bituminous coating. For information on other waterproofing materials, see A Guide to the Use of Waterproofing, Dampproofing, Protective and Decorative Barrier Systems for Concrete, ACI 515.1R, American Concrete Institute. 3.4.8 Unit-Masonry Basement Walls Water-resistant basement walls of masonry units should be carefully constructed of durable materials to prevent leakage and damage due to frost and other weathering exposure. Frost action is most severe at the grade line and may result in structural damage and leakage of water. Where wetting followed by sudden severe freezing may occur, the masonry units should meet the requirements of the following specifications: Building brick (solid masonry units made from clay or shale), ASTM Standard

When concrete is placed for walls, the only practicable means to avoid segregation is to place no more than a 24-in layer in one pass. Each layer should be vibrated separately and kept nearly level. For walls deeper than 4 ft, concrete should be placed through vertical, flexible trunks or chutes located about 8 ft apart. The trunks may be flexible or rigid, and come in sections so that they can be lifted as the level of concrete in place rises. The concrete should not fall free, from the end of the trunk, more than 4 ft or segregation will occur, with the coarse aggregate ricocheting off the forms to lodge on one side. Successive layers after the initial layer should be penetrated by internal vibrators for a depth of about 4 to 6 in to ensure complete integration at the surface of each layer. Deeper penetration can be beneficial (revibration), but control under variable jobsite conditions is too uncertain for recommendation of this practice for general use. The results of poor placement in walls are frequently observed: sloping layer lines; honeycombs, leaking, if water is present; and, if cores are taken at successive heights, up to a 50% reduction in strength from bottom to top. Some precautions necessary to avoid these ill effects are: Place concrete in level layers through closely spaced trunks or chutes. Do not place concrete full depth at each placing point. Do not move concrete laterally with vibrators. For deep, long walls, reduce the slump for upper layers 2 to 3 in below the slump for the starting layer. On any delay between placing of layers, vibrate the concrete thoroughly at the

Based on the stress history of saturated cohesive soils, they are considered to be either underconsolidated, normally consolidated, or overconsolidated. The overconsolidation ratio (OCR) is used to describe the stress history of cohesive soil, and it is defined as: OCR  vm / vo, where: vm or p  maximum past pressure ( vm), also known as the preconsolidation pressure ( p ), which is equal to the highest previous vertical effective stress that the cohesive soil was subjected to and consolidated under, and vo or v  existing vertical effective stress. In terms of the stress history of a cohesive soil, there are three possible conditions: 1. Underconsolidated (OCR  1). A saturated cohesive soil is considered underconsolidated if the soil is not fully consolidated under the existing overburden pressure and excess pore water pressures (ue) exist within the soil. Underconsolidation occurs in areas where a cohesive soil is being deposited very rapidly and not enough time has elapsed for the soil to consolidate under its own weight. 2. Normally Consolidated (OCR  1). A saturated cohesive soil is considered normally consolidated if it has never been subjected to an effective stress greater than the existing overburden pressure and if the deposit is completely consolidated under the existing overburden pressure. 3. Overconsolidated or Preconsolidated (OCR  1): A saturated cohesive soil is considered overconsolidated if it has been subjected in the past to a vertical FIGURE 6.22 Example of a consolidation curve. The Casagrande construction technique for determining the maximum past pressure is also shown on this figure. effective stress greater than the existing vertical effective stress. An example of a situation that creates an overconsolidated soil is where a thick overburden layer of soil has been removed by erosion over time. Other mechanisms, such as changes in groundwater elevation and changes in soil structure, can create an overconsolidated soil. For structures constructed on top of saturated cohesive soil, determining the overconsolidation ratio of the soil is very important in the settlement analysis. For example, if the cohesive soil is underconsolidated, then considerable settlement due to continued consolidation owing to the soils own weight as well as the applied structural load would be expected. On the other hand, if the cohesive soil is highly overconsolidated, then a load can often be applied to the cohesive soil without significant settlement. The oedometer apparatus is used to determine the consolidation properties of saturated cohesive soil. The typical testing procedure consists of placing an undisturbed specimen within the apparatus, applying a vertical seating pressure to the laterally confined specimen, and then submerging the specimen in distilled water. The specimen is then subjected to an incremental increase in vertical stress, with each pressure remaining on the specimen for a period of 24 hr. Plotting the vertical stress versus void ratio of the soil often yields a plot similar to Fig. 6.22. The plot is known as the consolidation curve and consists of two important segments, the recompression curve (defined by the recompression index Cr) and the virgin consolidation curve (defined by the compression index Cc). Figure 6.22 also illustrates the Casagrande construction technique, which can be used to determine the maximum