Crest Settlement Research Articles

Preliminary geotechnical assessment of Nkari earth damsite, southeastern Nigeria

This paper discusses the geotechnical investigations performed for a proposed earthfill dam at Nkari in the northeastern part of Akwa Ibom State in southeastern Nigeria. The geotechnical testing included soil classification, compaction, and hydraulic-conductivity determination, direct shear tests, uniaxial and triaxial compression tests, and bearing capacity (cone penetration) tests. The tests demonstrated that the local soils were silty (MH and OH under the USCS classification), with medium to high expansion potential, poor drainage characteristics (hydraulic conductivity less than 1.50 × 10^-5 cm/sec), and compressive strength between 60 and 70 kPa. The shale at the site is heavily consolidated with an overconsolidation ratio (OCR) between 9.0 and 13.0. Despite the high potential for expansion of the soil when excavated, collapse of the soil upon loading by the dam is unlikely and settlement is likely to be minimal with maximum crest settlement estimated at 3.61cm. The site geology and the drainage characteristics of the soil at the site suggest a potentially watertight reservoir. Preliminary analyses indicate that the dam structure has adequate stability against sliding.

Seismic deformation analysis of a rockfill dam with a bituminous concrete core

This paper reports the results of seismic deformation analyses of Shur River Dam, which is an 84 m high water retaining rockfill embankment with a bituminous concrete core. The proposed Shur River Dam is located in a region of high seismicity in Kerman province, Iran. The purpose of the dam is to store raw and process water for use in the Sarcheshmeh Copper Production Complex. The maximum crest settlement of the dam under three earthquake loadings was evaluated using simplified procedures and numerical analysis. Plane strain, dynamic deformation analysis of the Shur Dam has been performed using the commercial software FLAC and its built-in Mohr-Coulomb elastic-plastic model. The main objective of these analyses has been to identify the earthquake induced movements that could lead to uncontrolled release of water. The predicted deformations are compared with the results of simplified analyses and the observed displacements in other embankment dams [1] during past earthquakes. The relative movement of the asphaltic core with respect to the rockfill material on either side of the core was also studied.

Seismic displacement analysis of embankment dams with reinforced cohesive shell

Suitable materials for use as shell of embankment dams are clean coarse-grained soils or natural rockfill. In some sites these materials may not be available at an economic distance from the dam axis. The use of in-situ cohesive soils reinforced with geotextiles as the shell is suggested in this study for such cases. Dynamic behavior of reinforced embankment dam is evaluated through fully coupled nonlinear effective stress dynamic analysis. A practical pore generation model has been employed to incorporate pore pressure build up during cyclic loading. Parametric analyses have been performed to study the effect of reinforcements on the seismic behavior of the reinforced dam. Results showed that reinforcements placed within the embankment reduce horizontal and vertical displacements of the dam as well as crest settlements. Maximum shear strains within the embankment also decreased as a result of reinforcing. Furthermore, it was observed that reinforcements cause amplification in maximum horizontal crest acceleration.

Dynamic behavior of small-scale model slopes in shaking table tests

The shaking table tests were conducted on small scale embankment slopes to study their behaviors under dynamic conditions. The model embankment slopes were built in a rigid box with a length of 1.0 m, width of 0.90 m and height of 0.48 m. Medium grained local sand with a water content of 3% was used in the study. Three slope angles, 30°, 35° and 40° were considered for the embankment. Three slope heights of 15 cms, 18 cms and 21 cms were considered for each slope to observe the effect of slope height during a ground motion. All the nine slope models were tested under the peak ground acceleration of 0.1 g. The acceleration and deformation at the base and the crest of model slopes were monitored to find the responses of embankments under dynamic condition. The failure surfaces developed during the induced motion were fairly shallow and confined within the slope surfaces. The failure surfaces were circular in the upper part of the slopes. At the toe of the slopes, bulging and translational movements were observed. The amplitude of crest acceleration was found to be larger toward the outward slope direction which implied development of a sliding surface and separation of the soil body near the crest. Amplification in acceleration response was observed with the increase in height of model slopes and slope angle. The numerical simulation of the model tests was performed using a commercial program called FLAC. The numerical analyses modeled the crest settlements reasonably well but did not predict the heaving and outward movement of soil at the toe of the slopes. No distinct slip surface was predicted by the numerical analyses. The amplification of response at the crest computed by FLAC was lower than the corresponding test value. The increase in amplitude of acceleration toward the outward slope direction, observed in shaking table tests, was also not predicted by FLAC.

Behavior of Concrete-Faced Rockfill Dams during Initial Impoundment

Centrifuge tests to investigate the behavior during initial reservoir filling of a concrete faced rockfill dam (CFRD) with face slab stiffnesses that vary by a factor of about two are described. The two centrifuge models exhibited similar deformations at the crest and along the face slab, with crest settlements averaging 0.19H (%) and maximum face slab deformations averaging 0.88H (%). The centrifuge test results suggest that the face slab stiffness had little effect on deformations, at least for the range of stiffnesses examined here. A parametric study of transition (supporting) zone stiffness was performed using a numerical model calibrated using the centrifuge results. The numerical results indicated that face slab deformation is more influenced by transition zone stiffness than face slab stiffness, supporting the centrifuge results. Deformation measurements for 25 in-service CFRDs (including six Korean CFRDs—one of which was used as the basis for the centrifuge model dam) are presented and compared w...

Estimation of Earthquake-Induced Crest Settlements of Embankments

Problem statement: Freeboard requirement, a major consideration in the design of embankment dams, is controlled to a great extent by the crest settlements during earthquakes. Approach: The parameters that influence earthquake induced crest settlement had been studied using 152 published case histories on performance of embankment dams during earthquakes. Results: Based on the results a correlation had been proposed for obtaining preliminary estimates of earthquake-induced crest settlements. The correlation used the ratio of the peak horizontal ground acceleration and the yield acceleration as the estimator. Conclusion/Recommendations: The database analysis also indicated that crest settlements are larger where the fundamental periods of the embankment were similar to the predominant periods of the earthquake. Earthquake magnitude and the vertical component of earthquake ground motion, on the other hand, appeared to have a small influence on crest settlement.

Investigation of the parameters controlling the crest settlement of a major earthfill dam based on the threshold correlation analysis

The factors which control the crest settlements of the Kremasta Dam (Central Greece), one of the highest earthfill dams in Europe, were analyzed on the basis of the threshold correlation. The latter is a new stochastic technique which permits identification of parts of time series which are highly correlated on the basis of the optimization of the correlation coefficient between two parameters in combination with a high-pass filter of gradually increasing width. Based on a unique, >35 years long monitoring record consisting of leveling data, reservoir levels and precipitation levels we formed three time series describing the parameters which may affect the dam geometry (fluctuations of the reservoir level, of the rate of change of the reservoir level and of the rate of rainfall) and a fourth time series describing the dam settlements (settlement index fluctuations). Threshold correlation analysis permitted to identify critical levels for each of the above three parameters influencing the dam settlements. It was found that none of the three parameters alone can explain excessive dam settlements, but if the critical values of the three parameters are at the same time exceeded (i.e. under a conditional Boolean probability), the rate of the settlements exceeds its critical value. This approach may prove useful for other dams, but also in the analysis of data in other fields of science and engineering.

A case study on the post-construction deformation of concrete face rockfill dams

Concrete face rockfill dams (CFRDs) are considered to be fundamentally safe, especially against leakage, and to be a low-cost alternative at many sites. Their design is based on empirical experiences rather than theory. The present paper describes post-construction deformation and leakage using 27 CFRD case studies. The analysis shows that deformation normal to the face slab is larger than the post-construction crest settlement when the dam height exceeds 100 m and that the long-term leakage rate increases rapidly when the dam height exceeds 125 m.

Predictions of total deformations in Jebba main dam by finite element analysis technique

This paper examined the deformations of the Jebba Main Dam, Jebba Nigeria using the finite element method. The study also evaluated the predicted deformations and compared them with the actual deformations in the dam to identify possible causes of the observed longitudinal crack at the dam crest. The Jebba dam is a sloping earth core rockfill. The study methodology consists of the modeling of the dam with the utilization of the actual dam section, development of Finite Element mathematical formulations using plain strain conditions and total stress approach, and simulation of such model using a computer program developed in Visual Basic Language. The 3-node isoparametric triangular elements were used to analyze the model dam. The dam was simulated for end of construction (no water) and full reservoir loadings. The results of the study showed that the actual crest settlement as at 2001 was 184 mm and the predicted total settlements were 280 mm and 307 mm for the two models adopted. Generally, the deformations in the dam body were higher than deformations in the dam foundation. This may have resulted from the adequate foundation compaction during construction. The cracks development criteria were based on the criteria developed by Thomas. From these criteria, the dam was found to have developed cracks due to settlements and hydraulic fracturing. Journal of the Nigerian Association of Mathematical Physics Vol. 9 2005: pp. 479-492

Performance of heterogeneous earthfill dams under earthquakes: optimal location of the impervious core

Abstract. Earthfill dams are man-made geostructures which may be especially damaged by seismic loadings, because the soil skeleton they are made of suffers remarkable modifications in its mechanical properties, as well as changes of pore water pressure and flow of this water inside their pores, when subjected to vibrations. The most extreme situation is the dam failure due to soil liquefaction. Coupled finite element numerical codes are a useful tool to assess the safety of these dams. In this paper the application of a fully coupled numerical model, previously developed and validated by the authors, to a set of theoretical cross sections of earthfill dams with impervious core, is presented. All these dams are same height and have the same volume of impervious material at the core. The influence of the core location inside the dam on its response against seismic loading is numerically explored. The dams are designed as strictly stable under static loads. As a result of this research, a design recommendation on the location of the impervious core is obtained for this type of earth dams, on the basis of the criteria of minor liquefaction risk, minor soil degradation during the earthquake and minor crest settlement.

Prediction of relative crest settlement of concrete-faced rockfill dams analyzed using an artificial neural network model

A neural network model has been developed for the prediction of relative crest settlement (RCS) of concrete-faced rockfill dams (CFRDs) using 30 databases of field data from seven countries (of which 21 were used for training and 9 for testing). The settlement values predicted using the optimum artificial neural network (ANN) model are in good agreement with these field data. A database prepared from reported crest settlement values of CFRDs after construction was used to train the ANN model to predict the RCS. It is demonstrated here that the model is capable of predicting accurately the relative crest settlement of CFRDs and is potentially applicable for general usage with knowledge of the three basic properties of a dam (void ratio, e; height, H; and vertical deformation modulus, E V). The performance of the new ANN model is compared with that of conventional methods based on the Clements theory and also with that of a proposed equation derived from the field data. The comparison indicates that the ANN model has strong potential and offers better performance than conventional methods when used as a quick interpolation and extrapolation tool. The conventional calculation model was proposed based on the fixed connection weights and bias factors of the optimum ANN structure. This method can support the dam engineer in predicting the relative crest settlement of a CFRD after impounding.

지진하중을 받는 CFRD 정상부 변위에 영향을 미치는 사력재료 입력물성에 대한 민감도분석

The purpose of this study is to carry out the quantitative sensitivity analysis on rockfill material influencing the dam crest displacement of CFRD(Concrete-Faced Rockfill Dam) subjected to earthquake loading. The total 105 dynamic numerical analyses (2 input earthquake, 2 magnitudes for each earthquake. 27 rockfill material property combinations obtained from large triaxial tests) on CFR type D dam in operation were conducted. The global sensitivity analysis was carried out using the results of numerical analysis. From the results of sensitivity analysis, It was found that the crest settlement of the CFR type dam subjected to earthquake was absolutely affected by the shear modulus of rockfill material irrespective of the input earthquakes and the maximum acceleration of each earthquake. Also, it was found that the horizontal displacement of the dam crest was highly affected by the shear modulus of rockfill material though the extent of effect on that was smaller than the settlement and the extent of effect depended on the input earthquakes and the maximum acceleration of each earthquake. On the contrary, it was found that the effect of friction angle was negligible.

Large-Scale Shaking Table Tests on Modular-Block Reinforced Soil Retaining Walls

This paper presents an experimental study of the earthquake performance of modular-block reinforced soil retaining walls which were backfilled with sand using large-scale benchmark shaking table tests. The reinforcements used were polymeric geogrids, which were frictionally connected to the facing blocks having a front lip. In addition to observing the seismic performance, the purpose of testing was to generate quality data for future validation of numerical procedures. Three large-scale 2.8 m high modular-block geosynthetic-reinforced soil walls were subjected to significant shaking using the Kobe earthquake motions. Each wall was excited with a one-dimensional horizontal maximum acceleration of 0.4g followed by 0.86g. Vertical acceleration was superimposed on the horizontal one in the third wall. The walls were instrumented intensively using over 100 transducers to measure lateral and vertical earth pressures, wall facing displacement, crest settlement, reinforcement strains, and accelerations within the soil and the facing blocks. The material properties, instrumentations, and construction procedures are described. The test results indicated that the walls deformed very little with negligible horizontal acceleration amplification when subjected to the first shaking load. Deformation and horizontal acceleration amplification were reasonably small under the second shaking load. Part of the lateral deflection, earth pressure and tensile force in the reinforcement were recovered when shaking ceased. Amplification ratio of 1.35 indicated that the particular wall system performed better than conventional walls that had been tested for earthquake loading.

Prediction Method for Liquefaction-Induced Settlement of Embankment with Remedial Measure by Deep Mixing Method

In order to mitigate liquefaction-induced embankment settlement, remedial measures are often implemented for existing embankments. This paper describes a practical prediction method for embankment settlement due to foundation liquefaction, which are remedied with the deep mixing method. Based on a number of centrifuge shaking tests of embankment, an empirical relation between crest settlement and displacement of the remedied zone is established. Then, a calculation method for displacement of the remedied zone is developed. In the method, a macroscopic failure envelope and a plastic displacement potential in the general load space are considered to evaluate the subgrade reaction forces on the base of the remedied zone. The method is capable of calculating not only horizontal, vertical or rotational displacement alone, but their combined effect. The method is validated through comparisons of calculated results with centrifuge test observations. The calculated displacement components compare quite well with those measured. This method can be a useful tool for design of countermeasures based on a specified limit settlement.

Centrifuge modeling of loose fill embankment subjected to uni-axial and bi-axial earthquakes

The seismic risk is fairly high in Hong Kong even though it is located in an intreplate area with low to moderate seismicity. This is because of its high seismic vulnerability due to the presence of many steep loose fill slopes with a marginal static factor of safety, and a high consequence ‘value’ as a result of the dense population and intense economic activity in Hong Kong. In order to investigate the seismic stability and potential flow liquefaction of loose fill slopes, dynamic centrifuge tests in uni-axial and bi-axial directions were performed on saturated model embankments made of loose completely decomposed granite (CDG). Three windowed sinusoidal waves with peak shaking amplitudes ranging from 0.08 g to 0.3 g (prototype scale) were adopted. During the strong uni-axial shaking of 0.3 g, the measured maximum excess pore pressure ratios ranged from 0.70 to 0.85 and a relatively small crest settlement of 5.8 mm (0.22 m prototype) was measured. No soil liquefaction or flow slides were observed. Comparing the results between the strong uni-axial and bi-axial shaking, the maximum pore pressure ratios measured from the bi-axial test varied from 0.75 to 0.87, which were marginally larger than those obtained from the uni-axial test. Although the measured crest settlement during the bi-axial shaking was about 27% larger than that of the uni-axial test, soil liquefaction and flow slide did not occur. These test results suggest that loose CDG fill slopes are likely to be stable under the proposed design PGA ranging from 0.08 to 0.11 g in Hong Kong.

Rockfill Modulus and Settlement of Concrete Face Rockfill Dams

A method is presented for estimating the modulus of compacted rockfill in dams based on the particle size, unconfined compressive strength of the rock, compaction layer thickness, compactive effort, and the applied vertical stress. Also presented are methods for predicting the crest settlement and face slab deformation of concrete face rockfill dams during first filling and in the long term. It is demonstrated that the modulus is stress dependent and guidance is provided on how to assess this, as well as effects of construction in narrow valleys where arching may affect the stresses in the dam. These methods are based on analysis of the 35 dams with good quality information on construction materials and placement methods, and good quality internal and surface settlement monitoring records.

Behaviour of a trial embankment on hydraulically placed pfa

The paper describes the performance of a 5.3-m trial embankment constructed on approximately 45 m of hydraulically placed pulverised fuel ash (pfa). It is planned to redevelop the 17-ha lagoon containing the pfa as a landfill. There is little variation in the particle size distribution of the uniformly graded silt sized pfa over the lagoon. However, the density of the pfa varies with depth with loose material underlying a denser surface layer, in a pattern that probably results from the water level in the lagoon during pfa deposition. Settlement under the trial embankment was apparently largely complete by the end of the construction period (17 days), with approximately 300 mm of settlement under the crest of the embankment. The embankment settlement is significantly affected by compression of the loose layers within the deposit. Analysis of the problem using the conventional one-dimensional settlement method, and an m v profile determined by CPT calibrated against laboratory tests gave a reasonable prediction of the embankment crest settlement.

SH-Wave Seismic Reflection Images of Anomalous Foundation Conditions at the Mississinewa Dam, Indiana

Geologic features (i.e., solution cavities, active∕inactive faults, fractures, and filled∕open joints) often go undetected by traditional geotechnical exploration methods, but can have long-term adverse affects on the foundation conditions of large engineered structures. These geologic features can be relatively small and nevertheless have significant consequences for foundation integrity when high hydraulic heads are present. A noninvasive, horizontally polarized shear-wave, seismic reflection survey was performed at a large flood-control facility in northern Indiana, U.S.A., in order to identify a possible geologic cause for irregular settlement along the crest of a compacted earth-filled dam. The objectives included acquisition of a high-resolution bedrock surface image beneath the structure for remedial design considerations, as well as a search for signal anomalies that may be indicative of unfavorable geologic foundation conditions. The resultant SH-wave reflection profile collected coincident with die dam’s centerline successfully imaged a coherent, relatively continuous signal consistent with known points of top-of-rock elevation. Depth to bedrock along the profile varied from [Formula: see text] near the right abutment, to [Formula: see text] near the outlet works at the survey’s western terminus. A distinct, isolated amplitude anomaly in the bedrock reflector directly below the crest settlement area was imaged and interpreted as an enlarged (open) joint or collapsed karst feature. Subsequent geotechnical drilling of the anomaly found a [Formula: see text] discrepancy in the expected top-of-rock elevation, as well as soft foundation fill, thus confirming the geophysical interpretation.

Effective stress analyses of liquefaction-induced deformation in river dikes

A series of effective stress analyses is carried out on the seismic performance of river dikes based on the case histories during the 1993 Hokkaido-Nansei-oki and 1995 Hyogoken-Nambu earthquakes in Japan. Seven case histories selected for the analyses involve a crest settlement ranging from none to 2.7 m in the dikes 3–6 m high with evidence of liquefaction at foundation soil. The effective stress model used is based on a multiple shear mechanism and was developed by one of the authors. The soil parameters are evaluated based on the site investigation and laboratory test results. The results of the analyses are basically consistent with the observed performance of the river dikes. In particular, the effective stress model shows a reasonable capability to reproduce the varying degree of settlements depending on the geotechnical conditions of foundation soils beneath the dikes. The analyses also indicate that the effect of a cohesive soil layer mixed with the liquefiable sand layers beneath the dikes can be a primary factor for reducing the liquefaction-induced deformation of dikes.

Effects of remedial measures for mitigating embankment settlement due to foundation liquefaction

In Japan, after the Hyogoken-nambu earthquake remedial measures have often been constructed for river dikes supported on liquefiable sand beds. In this study a series of dynamic centrifuge tests was conducted to assess seismic performances of embankments with and without countermeasures and to provide a basis for establishing conventional methods to evaluate crest settlement. Three types of countermeasures used in the current practice in Japan (the solidification by the deep mixing method, the densification by the sand compaction piles and the sheet pile enclosure) were tested. It was observed that the excess pore pressure under the embankment toes tends to be higher for models with countermeasures than benchmark models without countermeasures. The horizontal displacement constraint imposed by the remedial zone is considered to be responsible for this. It was also found that the countermeasure techniques could reduce lateral deformation of the liquefied foundation sand and shear deformation of the embankment significantly, however, some embankment crest settlement was inevitable. The higher excess pore pressure under the embankment due to the existence of the remedial countermeasures resulting in an increase in contractive volume change of sand under embankment was responsible for the inevitable crest settlement. It is concluded that displacement predictions of the remediation zone as well as a volumetric strain of the liquefied sand beneath the embankment are key aspects to assess crest settlements.