Earthquake resistant building design codes and safety standards: The California experience

Seismologists and earthquake engineers have sought to understand and predict earthquakes and to develop better building designs to withstand them for well over a century. In the United States, the 1906 San Francisco earthquake provided the first real impetus for establishing building design codes and safety standards. Subsequent major California earthquakes in Santa Barbara (1925), Long Beach (1933), San Fernando (1971), Loma Prieta (1989), and Northridge (1994) each led to additional seismological understanding and engineering response in the form of enhanced building design codes. Nonetheless, the process to incorporate good seismic design and mitigation efforts has been slow, and by no means failsafe, especially in the Eastern U.S. where much of the building stock predates more recent design codes, and hence where a major earthquake could collapse large numbers of buildings. Even in the absence of catastrophe, it is still important to guard against a false sense of security.     

Index test for the degradation potential of carbonate sands during hydraulic transportation

Carbonate sands undergo degradation when transported hydraulically from a dredging vessel to the reclamation site. Risks involved in the production of fines during hydraulic transport are a deterioration of the mechanical properties of the fill in certain areas of the reclamation area where these fines concentrate. In order to assess sand degradation in the concrete and road construction industry, the Micro-Deval test for fine aggregates was developed in France and Canada. During Micro-Deval testing, aggregates degrade when tumbled in a rotating steel drum with water and steel balls. In the Canadian standard, the steel charge is lower than in the French standard and balls have the diameter of the smallest balls used in the French test. During testing, the quantity of fines produced is measured. This makes the Micro-Deval test an option for assessing the degradation of sand during hydraulic transportation of slurries in the dredging industry. Three sands are tested: two shelly carbonate sands and one quartzic river sand for comparison. The carbonate sands were sampled on reclaimed land and had been subjected to hydraulic transportation in pipeline before artificial deposition. Due to differences in particle shape, the carbonate sands and the quartz sand have a different number of contacts with the steel charge in the drum of the Micro-Deval and are not exposed to the same degradation forces during testing. It was found that the quartzic sand suffers relatively much degradation as compared to the carbonate sands, despite its greater hardness and resistance to crushability. The French test appears to create turbulence that promotes floatation of the curved shells of the carbonate sands instead of shell grinding at the bottom of the drum by contact with the steel balls. Microscopic observations of sand samples before and after testing showed that the French Micro-Deval test is too destructive. The test leads to the fragmentation by impact of the quartz sands and the destruction of certain particle types that had survived hydraulic transportation. With respect to the Canadian test, microscopic examination of sand grains before and after testing shows there is still much fragmentation of quartz grains. A lower rotational speed combined with a limited steel charge made of uniform balls of a smaller diameter, reduces turbulence, shell floatability and last but not the least, impact forces. It produces the desired result: a high Micro-Deval loss for the carbonate sands in comparison to that for the quartz sands and a very limited fragmentation of the quartz grains. Once the slurry reaches its destination, the grain size distribution of the fines fraction will influence the mechanical behaviour of mixed sands. A procedure is proposed to measure the particle size distribution of the fines fraction produced during Micro-Deval testing.Field validation is needed to validate preliminary laboratory results.     

Effect of Natural Zeolite and Cement Additive on the Strength of Sand

It is widely known and well emphasized that the cemented sand is one of economic and environmental topics in soil stabilization. In some instances, a blend of sand, cement and other materials such as fiber, glass, nano particle and zeolite can commercially be available and effectively used in soil stabilization especially in road construction. In regard to zeolite, its influence and effectiveness on the properties of cemented sands systems has not been completely explored. Hence, in this study, based on an experimental program, it has been tried to investigate the potential of a zeolite stabilizer known as additive material to improve the properties of cemented sands. A total number of 216 unconfined compression tests were carried out on cured samples in 7, 28 and 90 days. Results show unconfined compression strength and failure properties improvements of cement sand specimens when cement replaced by zeolite at optimum proportions of 30 % after 28 days due to pozzolanic reaction. The rate of strength improvement is approximately 20–78 and 20–60 % for 28 and 90 days curing times respectively. The efficiency of using zeolite has been enhanced by increasing the cement content and porosity of the compacted mixture. The replacement of cement by natural zeolite led to an increase of the pH after 14 days. Chemical oxygen demand (COD) tests demonstrate that the materials with the zeolite mixture reveal stronger adsorptive capacity of COD in compare to cemented mixture. Scanning electron microscope images show that adding zeolite in cemented sand changes the microstructure (filling large porosity and pozzolanic reaction) that results in increasing strength.     

Geotechnical Properties of Sandy Seafloors and the Consequences for Dynamic Penetrometer Interpretations: Quartz Sand Versus Carbonate Sand

The societal usage of coastal zones (including offshore wind energy plants, waterway deepening, beach conservation and restoration) is of emerging importance. Sediment dynamics in these areas result in sandy deposits due to strong tidal and wave action, which is difficult to simulate in laboratory geotechnical tests. Here, we present data from in situ penetrometer tests using the lightweight, free-fall Nimrod penetrometer and complementary laboratory experiments to characterize the key physical properties of sandy seafloors in areas dominated by quartzose (North Sea, Germany) and calcareous (Hawaii, USA) mineralogy. The carbonate sands have higher friction angles (carbonate: 31–37°; quartz: 31–32°) and higher void ratios (carbonate: 1.10–1.40; quartz: 0.81–0.93) than their siliceous counterparts, which have partly been attributed to the higher angularity of the coral-derived particles. During the in situ tests, we consistently found higher sediment strength (expressed in deceleration as well as in estimated quasi-static bearing capacity) in the carbonate sand (carbonate: 68–210 g; quartz: 25–85 g), which also showed a greater compressibility. Values were additionally affected by seafloor inclination (e.g., along a sub-aqueous dune or a channel), or layering in areas of sediment mobilization (by tides, shorebreak or currents). The study shows that the differences in in situ measured penetration profiles between carbonate sands and quartz sands are supported by the laboratory results and provide crucial information on mobile layers overlying sands of various physical properties.     

Seismically induced rock slope failures resulting from topographic amplification of strong ground motions: The case of Pacoima Canyon, California

The 1994 Northridge earthquake (M_w = 6.7) triggered extensive rock slope failures in Pacoima Canyon, immediately north of Los Angeles, California. Pacoima Canyon is a narrow and steep canyon incised in gneissic and granitic rocks. Peak accelerations of nearly 1.6 g were recorded at a ridge that forms the left abutment of Pacoima Dam; peak accelerations at the bottom of the canyon were less than 0.5 g, suggesting the occurrence of topographic amplification. Topographic effects have been previously suggested to explain similarly high ground motions at the site during the 1971 (M_w = 6.7) San Fernando earthquake. Furthermore, high landslide concentrations observed in the area have been attributed to unusually strong ground motions rather than higher susceptibility to sliding compared with nearby zones. We conducted field investigations and slope stability back-analyses to confirm the impact of topographic amplification on the triggering of landslides during the 1994 earthquake. Our results suggest that the observed extensive rock sliding and falling would have not been possible under unamplified seismic conditions, which would have generated a significantly lower number of areas affected by landslides. In contrast, modelling slope stability using amplified ground shaking predicts slope failure distributions matching what occurred in 1994. This observation confirms a significant role for topographic amplification on the triggering of landslides at the site, and emphasises the need to select carefully the inputs for seismic slope stability analyses.     

钙质砂抗剪强度特性的环剪试验

珊瑚礁沉积的钙质砂与石英砂的物理力学性质有较大差别。对取自南海岛礁的钙质砂进行了单次往返环剪试验以分析钙质砂的抗剪强度特性,试验中考虑了相对密实度和竖向应力对结果的影响,并与相同级配和试验条件下的石英砂进行对比分析。结果表明：钙质砂正向剪切时应力-位移曲线为软化型,具有明显的残余强度特性,而反向剪切时则表现为硬化型,正向和反向剪切强度基本一致;石英砂正向剪切和反向剪切均表现为软化型。钙质砂正向剪切和反向剪切残余强度与峰值强度的比值在0.75～0.93之间;石英砂正向剪切和反向剪切残余强度与对应峰值强度的比值在0.89～0.96之间。相同级配和试验条件下,钙质砂残余强度均大于石英砂,且强度比值基本保持在1.05～1.3之间。在100、200 kPa竖向荷载作用下,钙质砂0.5～2.0 mm的颗粒发生了破碎,破碎率分别为4%和6%。     

Sand dunes as a major proximal dust source for late Pleistocene loess in the Negev Desert, Israel

Grain size analyses of three hilltop, primary eolian loess sequences in the Negev desert, southern Israel, show a bimodal grain-size distribution at 50-60 μm and 3-8 μm. Using analyses of mineralogy and OSL ages we demonstrate that the coarse mode is composed mostly of quartz grains and its relative magnitude increases regionally with time, suggesting an enhancement of a time-transgressive proximal dust source compared to a distal, Saharan fine-grain dust. The only proximal dust source for large amount of coarse silt quartz grains is the sands that advanced into Sinai and the Negev concurrently with the loess accretion during the late Pleistocene as a result of the exposure of the Mediterranean shelf. We therefore propose that the coarse silt quartz grains were formed through eolian abrasion within the margins of an advancing sand sea. This relationship between desert sand seas as a source for proximal coarse dust and desert margin loess deposits can be applicable to other worldwide deserts such as Northern Africa, China and Australia.     

Geochemical behavior of rare-earth elements and other major and minor elements in soundproducing and silent beach sands in Japan

The major element composition of sound-producing sand is reported together with rare-earth elements (REE) and other selected elements for the first time. Rare-earth element concentrations in beach sands from Miyagi and Tottori in Japan were determined by induction-coupled, argon-plasma spectrometry (ICP-MS) to characterize the REE of sound-producing and silent sands relative to the parental rocks. Sound-producing sand beaches are very common and all over in Japan: five beaches in Miyagi and 2 in Tottori are selected with other silent sand beaches in the areas. Both sound-producing sand and silent sand samples from Miyagi and Tottori contain more than 60wt% of SiO2 and are composed mainly of quartz and feldspar. Miyagi sand samples are characterized by light REE enrichment and flat chondrite-normalized patterns that are similar to those of local source sandstone. However, all sand samples from Miyatojima in Miyagi show positive Eu anomalies, a characteristic feature not shown in other sand samples from Miyagi. Tottori sand samples also are characterized by high REE contents and remarkable positive Eu anomalies. The sands containing lower REE contents are due to high quartz and feldspar contents. Miyatojima sand samples and Tottori sand samples have high REE contents and show remarkable positive Eu anomalies due to the presence of feldspar. The best results are obtained using all of the geological methods and the Principal Component Analysis (PCA) as a measure of the similarity between sound-producing sand and silent sand. The difference between sound-producing sand and silent sand is obtained from the PCA results.     

Influence of a permeable sand layer on the mechanism of backward erosion piping using 3D pipe depth measurements

Backward erosion piping (BEP) poses a threat to the stability of water-retaining structures. This can lead to severe erosion and collapse of embankments. A novel economically appealing measure against BEP is the coarse sand barrier (CSB). The CSB is a trench filled with coarse sand that is placed below the blanket layer on the landward side of the embankment, which prevents the pipe from developing upstream when it encounters the CSB. Inclusion of a CSB creates a vertically layered sand, which is the situation that can also exist in practice but is different from traditional BEP tests with one homogeneous sand. This paper presents new observations and measurements in medium-scale laboratory tests. 3D measurements of the pipe depth and dimensions are presented and analysed. This analysis indicates how the pipe dimensions evolve during the piping process and shows the erosion mechanism for BEP in vertically layered sands. The findings demonstrate the significance of three-dimensional study of the pipe rather than two dimensions. The pipe depth, width and depth-to-width ratios at the pipe tip in critical erosion stages are measured and presented. In the presented tests, two different erosion behaviours (stepwise pipe progression until failure and straight failure) are found and analysed with respect to possible influential parameters. Higher head drops and flow rates are found in tests with straight failure at the stage before progression. A linear relationship between the hydraulic conductivity contrast (k_c) and the critical head drops (h_c) is found and observations are used to investigate deviations from the line.

     

钙质砂与钢界面循环剪切刚度与阻尼比的试验研究

钙质砂与钢界面的动力响应对岛礁地质条件下结构物基础的安全与稳定具有重要意义。基于界面环剪仪,开展了一系列钙质砂?钢界面循环剪切试验,探究了法向应力、剪切位移幅值和粒径大小对界面剪切刚度和阻尼比的影响,并与石英砂进行了对比。研究结果表明：法向应力和剪切位移幅值对界面剪切刚度和阻尼比起控制作用;法向应力的增大导致界面剪切刚度增大、阻尼比降低;剪切位移幅值的增大,则导致剪切刚度近似呈反比降低,而阻尼比近似呈对数增大;对于均一粒径的钙质砂,存在着分界粒径,使其剪切刚度和阻尼比的变化规律呈现显著区别;石英砂粒径较大时,其界面剪切刚度和阻尼比与钙质砂明显不同,而粒径较小时两类砂表现出类似的剪切刚度和阻尼比特性。     

Earthquake-Induced Deformation Analysis of a Bridge Approach Embankment in Missouri

Earthquake-induced deformations for a bridge approach earth embankment are predicted using a calibrated numerical model. The constitutive soil model is a modified hyperbolic model that uses Masing rules and incremental pore pressure relations. The model was calibrated using both laboratory and field data. A shaking table physical model was used to verify the numerical simulation. Additionally, the upper San Fernando dam was modeled to reproduce the deformations in the 1971 earthquake. The subsurface and embankment soil conditions were characterized using field and laboratory methods. The model developed was used to predict the earthquake-induced deformations of an approach embankment to Bridge A1466 in the NMSZ near Hayti, Missouri, where strong earthquakes M > 7.0 are anticipated in the next 50 years.     

波浪荷载作用下饱和钙质砂孔压特性及累积损失能量试验研究

钙质砂是一种海洋生物成因土,为我国南海岛礁吹填的主要材料。研究其动力特性对开发南海资源意义重大。实际工程中,钙质砂主要受到幅值和主应力方向均变化的波浪荷载作用,其作用效果与一般定轴剪切试验有较大不同。因此,利用空心圆柱扭剪仪模拟波浪荷载,开展一系列饱和钙质砂的不排水试验,重点研究循环应力比CSR和相对密实度Dr对钙质砂孔压特性的影响,并与石英砂对比分析,发现颗粒形态差异导致了两类砂土不同的孔压发展模式。同时,引入能量法,初步探究钙质砂累积损失能量变化规律。在此基础上,建立了考虑不同相对密实度的钙质砂孔压-累积损失能量模式。     

多维地震荷载等效循环周数计算模型

地震波是一种多维的随机荷载,一般在参数测定中应将其转化为等效循环荷载。采用有限元模拟多向地震荷载作用下的砂土动力响应,介绍了弹塑性边界面模型及其参数的含义,阐述了确定参数的方法,选取涵盖大震、中震、近场、远场及不同土质条件的155组多向地震输入时程进行单向、多向加载。利用混合效应回归分析法,建立了多向地震荷载作用下等效循环周数计算模型,分析验证了该模型的准确性。提出了基于等效循环周数比的多维地震荷载作用下等效循环周数计算方法。研究表明：模型预测值能较好地反映等效循环周数实际值的变化趋势;震级和震中距对等效循环周数比影响不明显;砂土特性对等效循环周数比的影响具有主导作用, 在不同震级范围内,等效循环周数比均随着相对密度的增大而增大,对应于相对密度为45%、60%、80%、100%的砂土,等效循环周数比分别近似为1.60、1.85、1.90和2.05。     

Nine unusually large tsunami deposits from the past 4000 years at Kiritappu marsh along the southern Kuril Trench

Large earthquakes along the Kuril subduction zone in northern Japan are known to have caused damaging tsunami, although there is a little information on historical earthquakes and tsunami in this area because no documents exist before the 19th century that might refer to tsunami events. To determine the likely timing and size of future events we need information on their recurrence intervals and to do this for the prehistoric past we have investigated sediments located in the Kiritappu marsh in eastern Hokaido that we interpret as laid down by tsunami. Using reliable multiple lines of evidence from sedimentological, geomorphological, micropaleontological, and chronological results, we identify 13 tsunami sands. Two of these lie within a peat bed above a historical tephra, Ta-a (AD 1739); the upper one probably corresponds to the AD 1843 Tempo Tokachi-oki earthquake (M 8.2) tsunami, and the lower to either the AD 1952 Tokachi-oki earthquake (M 8.2) tsunami or the AD 1960 Chilean earthquake (M 9.5) tsunami. Underlying are 11 prehistoric tsunami sand beds (nine large sand beds and two smaller sand beds) deposited during the past 4000 years. Because of the wide spatial distribution of the large sand beds, and inundation distances inland of between 1200 to 3000 m, we suggest that they record unusually large tsunamis along the Kuril subduction zone. According to our analyses, these tsunami sands were derived from the coastal area and, although they do not show clear graded bedding, they commonly have gradational upper boundaries and erosional bases and include internal sedimentary structures such as plane beds, dunes, and current ripples, reflecting bedload transportation. Based on our results we calculate the recurrence interval of unusually large earthquakes (probably M 8.6) along the Kuril subduction zone as about 365–553 years and estimate the youngest large event to have occurred in the 17th century.     

细颗粒组分含量对钙质砂抗剪强度的影响

由于吹填过程的水力分选作用钙质砂土地基颗粒分布不均匀,容易形成分布不均、形态各异的以不同细粒含量的粉细砂层为主的细粒汇集层,引起钙质砂土地基承载力和不均匀沉降问题。开展不同细粒含量的粉细砂三轴固结排水剪切试验,分析细粒含量对钙质砂土力学特性的影响。研究结果表明,（1）当细粒含量增加时相同围压下土样剪胀性逐渐减弱,且峰值偏应力也逐渐减小;（2）各组试样干密度为1.40 g/cm3状态下各含量粉细砂均表现出应变软化特征,当细粒含量为10%时粉细砂土体强度稳定性最差,之后稳定性随细粒含量增加而逐渐增大;（3）钙质细砂由于颗粒咬合作用,具有表观黏聚力,当细粒含量小于40%时,随着细粒含量增加,颗粒之间的咬合作用显著降低,表观黏聚力线性减小。研究成果可为粉细砂层的地基处理及边坡稳定分析提供参考。     

Experimental study of the grading characteristic effect on the liquefaction resistance of various graded sands and gravelly sands

The liquefaction vulnerability of fine- to coarse-graded saturated sand and gravelly sand is evaluated by stress control cyclic triaxial laboratory tests. Cyclic triaxial test on reconstituted specimens is carried out with relative density of 45 %. Cyclic triaxial tests performed by sinusoidal wave form with 1-Hz frequency and with the effective consolidation pressure equal to 100 kPa. Cyclic stress ratio which caused initial liquefaction in 15 cycles recognized as the liquefaction resistance in cyclic triaxial test of present study. The result had been used to find a relationship between liquefaction resistance and the grading characteristics (e.g., coefficient of curvature and coefficient of uniformity) of various graded sands and gravelly sands. Then, it is realized that a relationship between liquefaction resistance and any of the sizes (i.e., D ₁₀, D ₃₀, D ₅₀, D ₆₀) appears to be more practical.     

Single-particle crushing behaviour of carbonate sands studied by X-ray microtomography and a combined finite–discrete element method

This paper investigates the particle breakage behaviour of a carbonate sand based on single-particle compression experiments with in situ X-ray microtomography scanning (μCT) and a combined finite–discrete element method (FDEM). Specifically, X-ray μCT is applied to extract the information on grain morphology and intra-particle pores of carbonate sand particles to establish an FDEM model. The model is first calibrated by comparing the simulation results of two carbonate sand grains with the corresponding single-particle compression experiment results and then applied to model the stress evolution, cracking propagation and failure of other carbonate sand particles under single-particle compression. To study the influence of intra-particle pores, FDEM modelling of carbonate sands with completely filled intra-particle pores is also performed. The particle strength of carbonate sands both with and without pore filling is found to follow a Weibull distribution, with that of the sand with pore filling being considerably higher. This behaviour is associated with lower stress concentration, resulting in later crack development in the pore-filled sand than in the sand without pore filling. The cracks are found to usually pass through the intra-particle pores. Consequently, a larger proportion of particles fail in the fragmentation mode in the sand without pore filling.

     

Stabilization of desert sands using municipal solid waste incinerator ash

This paper presents experimental results on the use of incinerator ash in stabilizing desert sands for possible use in geotechnical engineering applications. The incinerator ash was added in percentages of 2, 4, 8, 10 and 12%, by dry weight of sand. Laboratory tests such as compaction, unconfined compression, shear box and hydraulic conductivity were performed to measure the engineering characteristics of the stabilized material. The results showed substantial improvements in unconfined compressive strength and shear strength parameters (c and φ). Thus, incinerator ash can be used to improve the shear strength characteristics of desert sands. The permeability of the sand–incinerator ash mixture was relatively low.     

钙质砂一维冲击响应及吸能特性试验

钙质砂为海相沉积的多孔介质,是防护工程分配层的理想填充材料。利用改进的铝制分离式霍普金森压杆（split Hopkinson pressure bar,简称SHPB）,分别对颗粒级配以及相对密实度相同的南海钙质砂和福建石英砂进行了83组一维冲击试验,得到了应力波透射率、冲击响应和吸能效率曲线,分析了应变率、相对密实度和含水率对两种砂冲击特性的影响。结果表明,相同的荷载和边界条件下,钙质砂的刚度仅为石英砂的1/10。由于内孔隙的存在,钙质砂对冲击波的衰减作用大于石英砂,在承载性能允许的情况下,适当减小相对密实度、增加含水率可有效提高分配层消能效果。