Backfill Height Research Articles

Field performance of thin wall foundations

The feasibility of building residential basement foundation walls of unreinforced concrete thinner than the conventional 200 mm thick wall is investigated. An optimum thickness of 150 mm was determined for an unreinforced 2400 mm high foundation wall based on the use of equivalent fluid pressures with sand and gravel backfill material. For walls backfilled with other than clean sand and gravel, or with a submerged condition, it was found that the theoretical maximum backfill heights for both 150 and 200 mm walls are substantially less than those presently specified by the Alberta Building Code. The primary purpose of the study described in this paper was to test under actual field conditions the performance of a 150 mm unreinforced concrete wall under varying lateral loads imposed by different soil types, and to compare measured and calculated lateral loads with the equivalent fluid pressure specified as a criterion in the Alberta Building Code. The lateral earth pressures resulting from the backfilling of 150 mm thick concrete foundation walls were investigated for two backfills, namely sandy lean clay (clay till) and sand. The findings indicate that for longer term placement conditions, the resulting pressure distribution supports the hypothesis of a triangular soil pressure distribution and can be described using traditional earth pressure theory. Two design methods based on the concept of equivalent fluid density were evaluated and compared with the field measurements. Key words: thin wall foundations, earth pressure, field measurement.

Thin unreinforced concrete house foundation walls

The objective of this study was to determine the feasibility of building residential basement foundation walls of unreinforced concrete thinner than the conventional 200 mm thick wall. The thin-wall foundation should require no special engineering attention, and should be acceptable as a standard to the industry.The study comprised a literature search, building code review, and a feasibility analysis which included technical and cost considerations. Additionally, an industry perspective was obtained through interviews with house builders and contractors specializing in residential basement work.Key issues were determined to be strength, quality control, construction methods and tolerances, cost savings, and market acceptance. As thickness decreases, strength is sacrificed, and the negative effect of axial eccentricity increases, particularly for foundation walls supporting brick veneer. Quality control, particularly in respect of concrete strength and water-to-cement ratios, is important from the perspective of maintaining bending capacities. Some construction methods, for example, cast-in-place joist or the use of nailing ladders, make pouring concrete more difficult in thinner walls. Cost savings and market acceptance are linked, and though costs can be reduced, it may be difficult to influence buyers to break from the traditional 200 mm wall in favour of a thinner one.Consideration of the issues led to the determination of 150 mm as the optimum reduced thickness for an unreinforced 2400 mm high foundation wall. A basement wall of 150 mm thickness and 20 MPa concrete strength supporting a house with brick veneer would be capable of retaining 1500 mm of backfill material. The same wall supporting a house without brick veneer would be capable of retaining 1800 mm of backfill. The cost of the 150 mm thick wall would be in the order of $600 less than that of the conventional 200 mm wall, and those savings could be passed on to the consumer.Technical analysis was based on equivalent fluid pressures and indicated that the 150 mm foundation wall would be suitable for use with clean sand and gravel backfill material. For walls backfilled with soils other than clean sand and gravel, it was found that the theoretical maximum backfill heights for both 150 and 200 mm walls are substantially less than those presently specified in Table 9.15.4.A of the National Building Code. The practice of backfilling with stable native soils (which are characterized by significantly higher equivalent fluid pressures) is common and house foundations have performed well under those conditions. The study therefore concluded that either the accepted values for equivalent fluid pressures or the assumed triangular distribution of lateral soil pressures are in question. Key words: foundations, concrete, residential buildings, houses, soil pressures, lateral pressures, unreinforced concrete walls.

Centrifuge model tests on backfill stability

The 6 m diameter, 30 g-tonne geotechnical centrifuge at Queen's University is described. Results from eight model tests, carried out on plain cemented sand samples representing mine backfills, are presented. These data show that the stable prototype backfill heights obtained from centrifuge tests exceed the failure heights predicted from unconfined compression testing by factors averaging about 1.8. This factor is explained by a combination of geometrical and behavioural effects. Still photographs of typical backfill failures in the centrifuge are included and these indicate that unacceptable ore dilution and recovery costs would be associated with the prototype failures in plain cemented tailings backfills. Key words: geotechnical centrifuge, mine backfill, model tests, cemented sand.

Load Reduction Factors for Buried Clay Pipes

Current ASTM installation design of vitrified clay pipes involves the determination of backfill loads and load factors. Trench backfill loads are commonly computed using the Marston load theory, which is conservative because it ignores the load-bearing capacity of sidefill and the existence of lateral earth pressures. The load factors used were originally determined experimentally. Due to the great variability in the soils and installation methods, the field data available for determining the load factors are limited. Finite element analyses were performed to investigate the backfill loads and the distribution of earth pressures around buried vitrified clay pipes under various trench bedding conditions. The load reduction factors were calculated as the ratio of the Marston load to the finite element load. The results showed that the load reduction factor is affected by the backfill height, the pipe diameter, and the trench width.

Experimental investigation of energy absorption in vertical vibrations of foundations

1. The most accurate damping characteristic is the coefficient of relative damping in analyses using the first and second soil base models. 2. The damping coefficient for steady vibrations depends for all practical purposes on the soil base type and the backfill height, the actual energy absorption being smaller than the value recommended in SNiP II-19-79.

Metal Versus Nonwoven Fiber Fabric Earth Reinforcement in Dry Sands: A Comparative Statistical Analysis of Model Tests

Abstract An experimental model study to compare the efficiency of metal and nonwoven fiber fabric reinforcement in mobilizing “sand-tie interaction” was developed. Model retaining walls were constructed in a sample box, sand was pluvially deposited at predetermined relative densities by a specially designed stationary depositor, and reinforcements were placed during deposition at varying levels and concentrations to meet the requirements of a statistical experimental setup. Two phases of tests were performed. Pull-out tests for measuring the lateral reaction on the face of the retaining wall, which consisted of three strain-gage instrumented aluminum plates, were conducted in the first phase. From the results obtained in these tests a comparative statistical analysis was made. During the second phase, the reinforcements in models of reinforced-soil retaining walls were overstressed to failure by surcharging of the backfill, and a comparison of the height of backfill at failure versus the length of the reinforcement was made. It was concluded that fiber fabric has advantages over the metal reinforcement used in the construction of Reinforced Earth® structures.

Experimental investigations of earth pressure on a retaining wall

1. The large-scale investigations showed that when a retaining wall can yield away from the backfill the magnitude of lateral earth pressure is determined with sufficient accuracy by Coulomb's equations. The intensity of the vertical stress in soil near the walls is, according to the experimental results, by a factor of 1.5–2 less than that determined by the formula σ=γh. In these conditions the coefficients K1 and K2 vary within comparatively small limits over the wall height. 2. The development of earth pressure on more stable walls differs from that described above. At a backfill height of 4.25 m the intensity of the lateral pressure near the base is close to that determined by the equation for the active state of soil behind the wall. However, in the upper and middle parts of the backfill the stresses appreciably exceed those calculated. Just as in the first case, the intensity of the vertical component of pressure is below that calculated. The coefficient K1 and K2 in this case are variables: they are smaller in the lower layers of the backfill (0.4–0.6) and larger (1.2–1.6) in the upper parts.