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Hydraulic conductivity and strength of pervious concrete for deep trench drains
No full textPervious concrete for deep trench drains, used to stabilise slopes, must meet many requirements, namely, adequate hydraulic conductivity, adequate shear strength a few days after pouring, capacity to act as a protective filter for soils in which the drain is installed, good resistance to clogging and adequate residual hydraulic conductivity. In current engineering practice, the composition of pervious concrete for drains is improperly selected according to criteria used for no-fines concrete for road pavements. To detect more suitable and specific criteria, a laboratory investigation has been conducted aimed at identifying the composition and the properties of pervious concrete satisfying the above requirements. The results of these tests are reported in the paper. They show that it is possible to devise mix-designs of pervious concrete that satisfy all the abovementioned requirements. Special attention has been paid to the hydraulic conductivity and the strength and to their relationship to the water/cement and the aggregate/cement ratios and to the granulometric composition of the aggregates
Il calcestruzzo poroso per il miglioramento delle condizioni di stabilità dei pendii sede di falda idrica
No full textReduction of pore water pressures is a useful strate-gy to improve the stability conditions of slopes. Deep draining trenches can be used for this scope. For the realisation of deep trenches, the usual con-ventional construction techniques are not adequate and the use of adjacent vertical panels, built by means of the methods well established for dia-phragm walls, is necessary. However, unbonded ma-terials (i.e. gravels) cannot be used, since the excava-tion of a panel adjacent to already built ones will cause instability. For this scope a bonded material such as the pervious concrete can be used. It must have high permeability, filtering capacity in order to prevent the internal erosion of the soil in which the trench drain is installed, sufficient shear strength af-ter a short curing time so to avoid the instability of adjacent previously built panels or piles. Results of an extensive laboratory experimental research on the mix-design, the filter capacity and residual hydraulic conductivity prove that proper mix-design can be devised meeting the above requirements. Experi-mental results show, in particular, that the shear strength of the concrete after a short curing time permits to excavate intermediate panels tens of me-ters deep without jeopardising the stability of previ-ously built ones. Moreover the hydraulic conductivi-ty of pervious concrete subjected to vertical stress level corresponding to the weight of a column of concrete of about 40 metres deep drops to a third of its value at low stress level. Nevertheless, it is still acceptable for draining trenches installed in many soils including fine-grained ones (fine sands, silts, clays)
Aspetti geotecnici e marittimi di due interventi di difesa costiera
No full textTwo case-histories of the design of submerged breakwaters for coast protection are shortly reported in the paper. The first one refers to the stabilisation and protection works of the sand beach of S.Alessio Siculo village, in Sicily, that suffered severe erosion triggered, some forty years ago, by the construction along the shoreline of a vertical seawall in lieu of the pre-existing sand-dune system. As a consequence, long stretches of the seawall were undermined and collapsed, and the beach almost disappeared. A submerged breakwater was built to reduce the inten-sity of wave attack, the seawall was rebuilt where necessary and provided with a rock revetment at the toe, and then a “perched” beach was formed. The submerged breakwater option was necessary to pre-serve the aptitude for bathing activities of the vil-lage.
The second case-history refers to the submerged breakwater built in front of a marine cliff in order to protect it against erosion and undercutting. The cliff is located on the eastern narrow promontory of Or-tigia Isle (Syracuse), on which rests the medieval Maniace Castle dating back to the age of Fedrerik II. The cliff as well as the nearby sea bottom consist of yellowish soft calcarenitic layers interbedded with weak or poorly cemented yellow calcarenitic sand lenses.
The breakwater consists of two layers of unrein-forced concrete blocks connected to the rocky sea bottom by means of active steel bar anchorages. Blocks are spaced 1 m so to form a network of channels permitting water and fish fauna to pass through the breakwater.
During construction some basal concrete blocks, not yet anchored to the rocky sea bottom, were dis-placed or overturned by heavy sea. Therefore it was necessary to modify the design of the damaged stretches of the breakwater and revert to a mixed so-lution composed of the displaced concrete blocks, rockfill and large stones. The behaviour of the breakwater has been fully satisfactory during its 28 years life span.
The design of both breakwaters has been assisted by physical model tests carried out at Wallingford and at Delft Hydraulics for S.Alessio Siculo and Or-tigia breakwaters respectively.
Finally it must be mentioned that the S. Alessio Siculo project explicity contemplated the observa-tional approach as far as the seaside rock armour of the barrier (i.e.”sacrificial layer”) was concerned. On the contrary, the initial design of the defense system of Ortigia cliff did not foresee the use of the obser-vational approach. Therefore the designer had to think all over again a new solution when heavy sea damaged the uncompleted sections of the breakwa-ter. Luckily, a valid and economically viable solution was found
An experimental investigation into the permeability and filter properties of pervious concrete for deep draining trenches
Full text linkThe reduction of pore water pressures is one of the most effective measures that can be taken to stabilise landslides or to improve the stability conditions of marginally stable water-bearing slopes. To this end, draining trenches have been used long since. When deep trenches are needed, the usual conventional construction techniques fail and recourse must be made to secant piles or to adjacent vertical panels built by means of the methods well established for diaphragm walls. However, unbonded materials cannot be used, since the excavation of a panel adjacent to previously built ones will instabilise these latter. The problem can be solved using pervious concrete rather unbonded material. It must meet the following requirements: relatively high hydraulic conductivity, filtering capacity in order to prevent the internal erosion of the soil in which the trench drain is installed, sufficient residual hydraulic conductivity after possible clogging, sufficient shear strength after a short curing time to avoid the instabilisation of adjacent previously built panels or piles. Results of a laboratory experimental research on the mix-design, the filter capacity, residual permeability and strength of pervious concrete are reported in the paper, proving that proper mix-design can be devised meeting the above requirements; in particular, it is demonstrated that the shear strength of the concrete after a short curing time permits to excavate intermediate panels deep tens of meters without jeopardising the stability of previously built ones
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Centrifuge tests on strip footings on sand with a weak layer
Full text linkTests on small-scale physical models of a strip footing resting on a dense sand bed containing a thin horizontal weak soil layer were carried out at normal gravity (1g). The results, reported in a companion paper, point out that the weak layer plays an important role in the failure mechanism and the ultimate bearing capacity of the footing if it falls within the ground volume relevant to the behaviour of the sand–footing system. The same problem was also investigated by means of centrifuge tests on reduced-scale models at 25g and 40g. The results of these tests, reported and discussed in this paper, confirm that failure mechanisms are governed substantially by the presence of the weak layer if its depth does not exceed a critical value and highlight marked scale effects involving the ultimate bearing capacity related essentially to the mean equivalent stress level in the soil beneath and around the footing. Equivalent bearing capacity factors, N f , for footings on a dense sand bed containing a thin weak layer are derived from experimental results and are proposed in the paper
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