International Journal of Civil Engineering
Volume:7 Issue: 4, Dec2009

This paper presents the results of pullout tests on uniaxial geogrid embedded in silica sand under monotonic and cyclic pullout forces. The new testing device as a recently developed automated pullout test device for soil-geogrid strength and deformation behavior investigation is capable of applying load/displacement controlled monotonic/cyclic forces at different rates/frequencies and wave shapes, through a computer closed-loop system. Two grades of extruded HDPE uniaxial geogrids and uniform silica sand are used throughout the experiments. The effects of vertical surcharge, sand relative density, extensibility of reinforcement and cyclic pullout loads are investigated on the pullout resistance, nodal displacement distributions, post-cyclic pullout resistance and cyclic accumulated displacement of the geogrid. Tell-tale type transducers are implemented along the geogrid at several points to measure the relative displacements along the geogrid embedded length. In monotonic tests, decrease in relative displacement between soil and geogrid by increase of vertical stress and sand relative density are the main conclusions; structural stiffness of geogrid has a direct effect on pullout resistance in different surcharges. In cyclic tests it is observed that the variation of post-cyclic strength ranges from minus 10% to plus 20% of monotonic strength values and cyclic accumulated displacements are increased as normal pressure increase, but no practical specific comment can be made at this stage on the post-cyclic strength of geogrids embedded in silica sand. It is also observed that in loose sand condition, the cyclic accumulated displacements are considerably smaller as compared to dense sand condition.

Large size direct shear tests (i.e.300 x 300mm) were conducted to investigate the interaction between clay reinforced with geogrids embedded in thin layers of sand. Test results for the clay, sand, clay-sand, clay-geogrid, sandgeogrid and clay-sand-geogrid are discussed. Thin layers of sand including 4, 6, 8, 10, 12 and 14mm were used to increase the interaction between the clay and the geogrids. Effects of sand layer thickness, normal pressure and transverse geogrid members were studied. All tests were conducted on saturated clay under unconsolidated-undrained (UU) conditions. Test results indicate that provision of thin layers of high strength sand on both sides of the geogrid is very effective in improving the strength and deformation behaviour of reinforced clay under UU loading conditions. Using geogrids embedded in thin layers of sand not only can improve performance of clay backfills but also it can provide drainage paths preventing pore water pressure generations. For the soil, geogrid and the normal pressures used, an optimum sand layer thickness of 10mm was determined which proved to be independent of the magnitude of the normal pressure used. Effect of sand layers combined with the geogrid reinforcement increased with increase in normal pressures. The improvement was more pronounced at higher normal pressures. Total shear resistance provided by the geogrids with transverse members removed was approximately 10% lower than shear resistance of geogrids with transverse members.

Although there is enough knowledge indicating on the influence of frequency content of input motion on the deformation demand of structures, state-of-the-practice seismic studies use the intensity measures such as peak ground acceleration (PGA) which are not frequency dependent. The v max/a max ratio of strong ground motions can be used in seismic hazard studies as the representative of frequency content of the motions. This ratio can be indirectly estimated by the attenuation models of PGA and PGV which are functions of earthquake magnitude, source to site distance, faulting mechanism, and local site conditions. This paper presents new predictive equations for v max/a max ratio based on genetic programming (GP) approach. The predictive equations are established using a reliable database released by Pacific Earthquake Engineering Research Center (PEER) for three types of faulting mechanisms including strikeslip, normal and reverse. The proposed models provide reasonable accuracy to estimate the frequency content of site ground motions in practical projects. The results of parametric study demonstrate that v max/a max increases through increasing earthquake moment magnitude and source to site distance while it decreases with increasing the average shear-wave velocity over the top 30m of the site.

Future design procedures for civil structures, especially those to be protected from extreme and blast related loads, will need to account for temporal evolution of their frequency content. There are, however, several instances where acceleration time histories are required as seismic input. For example, to determine the ultimate resistance and to identify modes of structures’ failure, a nonlinear time history analysis is needed. In other cases, acceleration time histories are required for linear analyses. Many seismic codes require this type of analysis for buildings which have irregularities. The process of time-frequency analysis made possible by the wavelet transform provides insight into the character of transient signals through time-frequency maps of the time variant spectral decomposition that traditional approaches miss. In this paper an approach is examined and a new method for processing the ground motion which is modeled as a non-stationary process (both in amplitude and frequency), is proposed. This method uses the best basis search algorithm with wavelet packets. In this approach, the signal is expressed as a linear combination of timefrequency atoms which are obtained by dilations of the analyzing functions, and are organized into dictionaries as wavelet packets. Several numerical examples are given to verify the developed models.

Cash flow forecasting is an indispensable tool for construction companies, and is essential for the survival of any contractor at all stages of the work. The time available for a detailed pre-tender cash flow forecast is often limited. Therefore, contractors require simpler and quicker techniques which would enable them to forecast cash flow with reasonable accuracy. Forecasting S-curves in construction in developing countries like Iran in compare with developed countries has many difficulties. It is because of uncertainty and unknown situation in nature of construction industry of these countries. Based on knowledge of authors there is a little attempt for cash flow forecasting in construction industry of Iran. As a result authors produced An S-curve equation for construction project from historical data which has reasonable accuracy. A sample of 20 completed projects was collected and classified in to the three different groups. In order to model S-curves for each group, a simple and reliable method of S curve fitting has been used. S-curves were fitted into each group by using different techniques. Errors incurred when fitting these curves were measured and compared with those associates in fitting individual projects. At the end, accuracy of each model has been calculated and an equation has been proposed to forecast S-curves.

Induced total pressure by flow, including mean and fluctuating components, around a selected chute block in SAF stilling basins downstream of an ogee spillway was studied. Several pressure holes were selected on various faces of a selected chute block to get enough information regarding the total pressure field. This paper reports the results of an experimental work and measurement of mean and fluctuation pressures around chute blocks of SAF stilling basins. The observations showed that the maximum total pressure varies inversely with Froude number of incoming flow while its position of occurrence follows a quadratic polynomial relationship. Statistical analysis also showed that the peak instantaneous pressure fluctuations could be as large as ±4.5times the RMS value.It is concluded that pressure fluctuation around the chute blocks may double the magnitude of pressure field around the chute blocks and can not be overlooked in designing such appurtenances.

Pullback test has no scrupulous theoretical establishment. It is based on the hypothesis that the response of the structure can be related to the response of an equivalent single degree-of-freedom (SDOF) system. This implies that the response is controlled by a single mode. In fact، the steel frame of each safe room، which is introduced within the unreinforced masonry buildings for protecting the lives of residents in catastrophic earthquake failures، contains a SDOF structural system. In pullback test، the steel frame carries its gravity load first، and then it will be pushed under an incremental lateral roof displacement pattern، which is imposed to its center of mass. This paper expresses the results of 13 pullback tests executed by the authors on the steel frames of safe rooms. The results show that pullback test is a practical method for seismic performance evaluation of safe rooms. Also the performance of these frames located in a collapsing three storey masonry building is presented with favorable conclusions. In fact، the results of pullback test of the safe room located at the ground-floor level were compared with the requirements of Iranian code for seismic resistant design and it was concluded that the steel frame had an acceptable performance against seismic effects.