Showing papers on "Settlement (structural) published in 1994"

An approximate numerical analysis of pile-raft interaction

Abstract: This paper presents an approximate method of numerical analysis of piled–raft foundations in which the raft is modelled as a thin plate and the piles as interacting springs of appropriate stiffness. Allowance is made for the development of limiting pressures below the raft and of the ultimate axial load capacity of the piles. Comparisons between this analysis and existing solutions verify that, despite the approximations involved, the analysis can provide solutions of adequate accuracy for the settlement and pile load distribution within a piled raft. Comparisons are also made with the results of a series of centrifuge tests and with measurements of the performance of a full-scale piled raft. In both cases, the analysis predicts very well the settlement and proportion of load carried by the piles.

Spatial and temporal patterns in barnacle settlement rate along a southern California rocky shore

Abstract: Barnacle settlement was monitored at 5 sites separated by 50 to 250 m a t Dike Rock, La Jolla, California, USA. Chthamalus spp. and Pollicipes polymerus settlement were spatially correlated at those sites. Within sites, settlement of the 2 species were correlated These results support the hypothesis of common onshore larval transport events for all sites and both species. Other spatiotemporal patterns were contrasting: 1 peak accounted for most of P polymerus settlement, while there were 5 peaks of similar magnitude for Chthamalus spp. At 2 sites, settlement plates were installed at 2 heights in the intertidal; Chthamalus spp. settlement was similar at the 2 heights, while P polyn~erus settlement was relatively different at the sites. Such spatial patterns may have resulted from a stronger behavioral component affecting settlement in P polymerus. These results suggest that, at scales of 100 m, temporal variability in settlement rate may be related to larval pool and physical transport processes, while spatial vanabllity may be associated with behavioral response and substrate availability. Chthamalus spp. settlement was hlgher at sites where rocks were surrounded by unsuitable sandy substrate, possibly because settlement on available substrate is intensified where total suitable settlement area is relatively scarce To test this, plates were installed at another site where suitable substrate was also scarce, as predicted, settlement was higher at both sites. At another study slte in Medio Camino, Mexico, settlement became more predictable anlong sites along the rocky shore after the shoreline had been partially inundated by sand, further supporting this hypothesis. The proportion of unmetamorphosed settlers of Chthamalus spp., relative to total settlement, appeared to peak on particular days of the lunar cycle and was spatially correlated at the 5 sites. The periodicity of the peaks was close to the 14.75 d spnng-to-neap cycle, suggesting that these peaks may be related to periodic shortimmersion hmes that did not allow the attached cypnds to metamorphose. Mortality of recently metamorphosed (5 l d ) Chthamalus spp. spat by physical damage was spatially vanable.

Settlement of Shallow Foundations on Sand

Abstract: A new method for estimating the settlement of shallow foundations on sand is presented. The method is based on elastic stress-strain theory and is designed for use with soil data from the Standard Penetration Test (SPT). The elastic modulus is assumed to vary with mean effective normal stress and strain level. The variations of modulus with depth and the effects of footing size and shape, depth of embedment and groundwater level are introduced through their effects on the mean effective normal stress. The non-linear nature of the stress-strain relation is based on a normalized version of the Seed-Idriss curves, which relate shear modulus to strain level and relative density. SPT data are used to estimate relative density. In the general solution, the zone of influence beneath the footing is divided into sublayers and the foundation load is applied in increments. Then the incremental load solution is used to develop a simplified procedure in which the total load may be applied in one increment. The proposed method is applied to data from published case histories. The results of the proposed method are shown to compare favorably with those from other currently used methods.

Fine-scale temporal variation in recruitment of a temperate demersal fish: the importance of settlement versus post-settlement loss.

Abstract: In order to understand variability in recruitment to populations of benthic and demersal marine species, it is critical to distinguish between the contributions due to variations in larval settlement versus those caused by post-settlement mortality. In this study, fine-scale (1–2 days) temporal changes in recruit abundance were followed through an entire settlement season in a temperate demersal fish in order to determine 1) how dynamic the process of recruitment is on a daily scale, 2) whether settlement and post-settlement mortality are influenced by habitat structure and conspecific density, and 3) how the relationship between settlement and recruitment changes over time. “Settlement” is considered to be the arrival of new individuals from the pelagic habitat, and “recruitment” is defined as the number of individuals surviving arbitrary periods of time after settlement. Replicate standardized habitat units were placed in 2 spatial configurations (clumped and randomly dispersed) and monitored visually for cunner (Tautogolabrus adspersus) settlement and recruitment every 1–2 days throughout the settlement season. The process of recruitment in T. adspersus was highly variable at a fine temporal scale. Changes in the numbers of recruits present on habitat units were due to both settlement of new individuals and mortality of animals previously recruited. The relative importance of these two processes appeared to change from day to day. The magnitude of the change in recruit number did not differ between the clumped and random habitats. However, post-settlement loss was significantly greater on randomly dispersed than clumped habitats. During several sampling dates, the extent of the change in recruit abundance was correlated with the density of resident conspecifics; however, on other dates no such relationship appeared to exist. Despite the presence of significant relationships between the change in recruit number and density, there was no evidence of either density-dependent mortality or settlement. Initially, there was a strong relationship between settlement and recruitment; however, this relationship weakened over time. Within 2 months after the cessation of settlement, post-settlement loss was greater than 99%, and no correlation remained between recruitment and the initial pattern of settlement. The results of this study demonstrate that the spatial arrangement of the habitat affects the rate and intensity of post-settlement loss. Counter to much current thinking, this study suggests that in order to understand the population ecology of reef fishes, knowledge of what habitats new recruits use and how mortality varies with structural aspects of the habitats is essential.

Variability in age and size at settlement of the tropicai goatfish Upeneus trayula (Mullidae) in the northern Great Barrier Reef lagoon

Abstract: Age, size and body mass at settlement for the reef fish Upeneus tragula (Mullidae) were examined among 5 sampling stations across the northern Great Barrier Reef, over time periods ranging from days to 3 yr. Increments on the sagittal otoliths were validated as daily usiny tetracycline treatment. Fish were caught in their late pelagic stage, brought into the laboratory, and those that mctamorphosed and settled to the bottom of the tank overnight were used In the study. Twenty-five samples of newly settled fish (n = 6 to 27, mean = 14 , N = 345) were compared Larval durat~ons ranged from 25 to 37 d, and standard length varied similarly, ranglng from 19 to 31 mm at settlement. Wet weight ranged from 0.09 to 0.61 g. Signifrant differences In age, length and weight at settlement were found at all spatial and temporal scales. For instance, no consistent pattern in settlement characteristics was found among stations across the northern shelf, with one sample of settled fish often being significantly different in attributes from another sample from the same station a month or two later. Growth rate averaged over the whole larval period ranged from 0.55 to 1.0 mm d-' Flsh that settled larger tended to have lower growth rates than smaller fish. Patterns of increment widths throughout the larval phase were remarkably consistent among samples. However, the relationship between otolith length and fish length was found to vary significantly among samples collected within 2 mon from 4 locations. Results suggest the cautious use of back-calculation of larval sizes from otolith increment widths, and stress the underestimated importance of small-scale variation in larval growth dynamics. No evidence was found that U. tragula has the ability to delay settlement once developmental competence was attained.

Modelling some effects of behavior on larval settlement in a turbulent boundary layer

Abstract: A one-dimensional model of larval concentration and settlement flux in a turbulent boundary layer was used to consider how some aspects of larval behavior in the plankton or on the bottom might affect settlement rates. The focus was on behavioral modifications of two terms in the model—the vertical (fall) velocity ( w f ), a composite measure of swimming and gravitational sinking, and the probability of settlement per unit time ( p ) for larvae interacting with the bottom. Depth-independent changes in vertical speed can increase the settlement rate by up to an order of magnitude. Such changes might be produced by negative phototactic responses that induce passive sinking of larvae throughout the water column during daylight, by photonegative swimming responses that may occur in relatively shallow or clear water (where swimming responses are fairly uniform throughout the water column), or by photonegative swimming responses that are of similar magnitude over a broad range of light intensities. In addition to increasing the long-term average settlement rate, negative phototactic responses should increase temporal variability in settlement at time scales of ≦24 h, as long as larvae are responding to changes in light intensity at depth. This prediction, coupled with results from Gross et al . [(1992) Journal of Marine Research , 50 , 611–642], suggests that for larvae that respond to light there may be two sources of substantial (order-of-magnitude) temporal variability in settlement that operate at time scales of 24 h or less— diel periodicity in light intensity and semi-diurnal or diurnal tidal periodicity in boundary shear stress and turbulence intensity. The additional effect of phase shift between tidal and light cycles introduces substantially less variability to the average (>24 h) settlement rate than do individual effects of the two forcings. Behavioral responses that produce a depth dependence in vertical speed can affect the settlement rate only if w f varies roughly by a factor of 2 or more very close to the bottom (i.e. within a few percent of total boundary layer thickness for tidal boundary layers). Phototactic or barokinetic responses of larvae are not likely to produce depth gradients in vertical speed strong enough to affect settlement rates. However, such gradients might be produced by a recent contact with a potential settlement site, by effects of velocity gradients near the bottom on larval orientation, or by larval responses to chemical cues associated with the bottom. Behavioral responses to fluid forces exerted on larvae on the bottom can alter the settlement rate by an order of magnitude or more. However, the settlement rate is predicted to be most sensitive to effects of weak, rather than strong, flow on the settlement probability. Thus, understanding larval responses to weak flows may be especially important to predicting settlement rates.

Mitigation Measures for Small Structures on Collapsible Alluvial Soils

Abstract: The effectiveness of treatment methods for collapsible soils was evaluated under field conditions using six full‐scale load tests performed on 1.5 m square footings. Treatments included conventional methods such as prewetting with water and partial replacement with compacted fill along with more innovative procedures such as prewetting with a sodium silicate solution and dynamic compaction under dry and wet conditions. A load test was also performed on untreated soil for comparison purposes. Soil improvement was evaluated using double oedometer testing on “undisturbed” samples along with cone penetration tests and pressuremeter tests. Settlements were monitored as a function of moisture penetration and compared with predictions based on oedometer testing. Settlement of the footing on untreated soil exceeded 400 mm and differential settlement was substantial. Prewetting prior to loading did not significantly improve footing performance. Use of a compacted fill mat delayed the onset of settlement and reduce...

Settlement of Structures Supported on Marginal or Inadequate Soils Stiffened With Short Aggregate Piers

Abstract: A short aggregate pier system, which was developed to provide an economical alternative to the overexcavation/replacement technique, has been used since 1988 to control settlement of structures located at sites with near-surface deposits consisting of marginal or inadequate soils. In this system, highly densified aggregate piers are incorporated within the marginal or inadequate soils, which results in a composite bearing material that is substantially stiffer than the unimproved soil, and on which shallow foundations can be supported with tolerable settlements. Three case histories are described in which the viability and effectiveness of the aggregate pier system in reducing settlements of shallow foundations bearing on the composite material are illustrated. Methods for analyzing and predicting settlements of footings supported on aggregate pier-reinforced soils are also discussed.

Methods to Reduce the Settlement of Embankments on Soft Clay: A Review

Abstract: New trends have been observed during the past ten years in the practice of settlement control for embankments on soft clays: they include increased use of rigid (piles) or semi-rigid (stone columns, compaction sand piles, jet-grouted columns) inclusions, increased use of prefabricated bandshaped drains, revival of the vacuum preloading technique, development of the construction of ultralightweight embankments. This paper reviews the present state of the practice of these techniques, based on recent experiments, construction works and literature reviews.

Effects of salinity and adult extract on settlement of the oligohaline barnacle Balanus subalbidus

Abstract: Balanus subalbidus (Henry) has the most oligohaline distribution of three congeneric barnacles in Chesapeake Bay and tolerates prolonged exposure to fresh water. We studied larval settlement (i.e., permanent attachment and metamorphosis) of B. subaldius in the laboratory, over a 3 yr period, May 1989 to March 1992, under the following conditions: (1) across an array of salinities at 25°C in the presence and absence of settlement factor consisting of adult B. subalbidus extract; (2) in the presence of conspecific or congeneric settlement factors; and (3) cyprids which were, and were not, induced to delay metamorphosis were compared in their capacities to settle in a range of salinities. Discrepancies between salinity profiles of larval settlement in the laboratory and adult oligohaline distribution in the estuary were striking, and there was a significant interaction between salinity and settlement factor. Averaging results of four different batches of larvae, although peak settlement (87±9%) of B. subalbidus occurred at 2 ppt salinity in the presence of adult cue, substantial settlement also occurred at higher salinities: >70% at 5, 10 and 15 ppt; and 47% at 20 and 25 ppt. In addition, settlement in the absence of settlement factor was relatively high (>50%) and peaked at mid-salinity ranges (e.g. 56±10% at 15 ppt). Variation observed in settlement among larval batches reflected detailed differences in settlement between adjacent test salinities. No difference in settlement occurred between replicate aliquots of cyprids within a batch. Cyprids of B. subalbidus settled most abundantly in the presence of settlement factor extracted from conspecifics, followed in decreasing order by settlement factor extracted from B. improvisus and B. eburneus. Delay of metamorphosis produced by keeping B. subalbidus cyprids for 8 d at 5°C resulted in a decreased level of settlement, but settlement frequency patterns of delayed and non-delayed cyprids were indistinguishable relative to salinity. These results indicate that the oligohaline distribution of adult B. subalbidus is probably not determined by larval behavior at settlement. We suggest that pre-settlement behavior, resulting in larval retention in low saline waters, could be an important factor in determining distribution of this species.

Stress and Settlement of Footings in Sand

Abstract: In current engineering practice, the magnitude of the settlement of a footing in sand, as compared to the settlement of a different size footing in the same sand, is considered to be a non-linear function of the footing width. Further, the settlement is considered to be proportional to the density of the sand. Results of finite element analysis of settlement for footings of three sizes placed in two different sand types show that the settlement in sand is a direct function of neither footing size nor soil density. Instead, the settlement should be related to the steady state line of the sand and to the upsilon distance of the sand, that is, the initial void ratio distance to the steady state line at equal mean stress and at homologous points. This requirement imposes scaling-rules for model tests and it limits the range of application of a small scale test to a prototype behavior. Moreover, it imposes boundaries on the geometric scale, because a model test can not realistically be carried out in a sand that is looser than the maximum void ratio, and it is meaningless if performed in a sand close to the minimum void ratio, because it would then not be representative for any prototype.

An estimation of ground settlement due to shield tunnelling by the Peck–Fujita method

Abstract: In this paper, an empirical method based on Peck and Fujita's study is proposed to estimate the magnitude and extent of surface settlement associated with shield tunnelling. It is found that the gr...

Load Settlement curve Method for Spread Footings of Sand

Abstract: A newly developed method is presented to predict the complete load settlement curve for square spread footings on sand loaded at their center with a vertical load and with an embedment from 0.25 B to 0.75 B where B is the footing width. This method also allows one to obtain the settlement of the footing as a function of time for a given load. The basis for this new prediction technique is a complete preboring pressuremeter curve. The method is developed after performing finite element simulations and after performing 5 full-scale footing load tests pushed to 0.15 m of vertical displacements. Footing widths vary from 1 to 3 m. The new method satisfies observations from the experimental results including the fact that there is no scale effect on the load settlement curves for these footings if those curves are plotted as pressure versus settlement over width.

Predicted and measured behavior of five spread footings on sand : proceedings of a prediction symposium sponsored by the Federal Highway Administration at the occasion of the settlement '94 ASCE conference at Texas A&M University, June 16-18, 1994

Abstract: This publication contains the results of a full scale load testing program used to predict and measure the behaviour of five spread footings on sand. These results were presented at the Prediction Symposium sponsored by the Federal Highway Administration at the occasion of the Settlement "94 ASCE Conference at Texas AM present the data upon which all predictions are based; describe methodologies and judgments used to make the predictions; and summarize the predictions and compare predicted responses.

Settlement of Compacted Fills Caused by Wetting

Abstract: Post-construction wetting of fills can cause swell or compression, depending on soil composition and stress conditions just before wetting. The various factors affecting swell/compression of compacted cohesive soils are studied, and the steps in analyzing one-dimensional heave/settlement of compacted fills are described. Limitations of the analysis, and recommendations for reducing fill settlement are presented.

Failure and deformation of four reinforced soil walls in eastern Tennessee

Abstract: Fourteen reinforced soil walls were built along two sections of a highway in eastern Tennessee. They were built across steep ravines to support high roadway embankments. One wall collapsed during construction and another eight months after completion; the remaining walls suffered varying levels of distortion and damage. This Paper describes the design and subsequent investigation of the walls. The probable failure mechanism has been identified. The investigations show a number of issues that are either not considered in current design practice or are worthy of review, including the capacity of high adherence strips in poorly compacted, coarse backfill; the three-dimensional deformation of a reinforced soil mass; stress induced in reinforcement undergoing large deflection, as a result of either backfill settlement or wall movement; the stability of discrete facing elements; the validity of applying empirical analytical methods to reinforced soil structures with unprecedented features; the validity of separ...