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1、大滩口水库坝前淤泥面加坝技术初探马志坚（宁夏固原市水利勘测设计院，宁夏 固原 756000）摘 要：以大滩口水库坝前淤泥面上利用加坝自重形成堆载，对水库淤积层进行预压固结为例，针对库区淤积物这类饱和软土具有高压缩性、高灵敏度、高流变性和低强度、低渗透系数的工程特性，借助堆载预压地基处理计算理论，进行坝前淤泥面上加固设计，探求加固机理，制定加荷计划，为类似工程设计及安全施工起到了重要的指导作用。关键词：坝前淤泥；加坝；预压设计；加荷计划中图分类号：TV641.2 文献标识码：A 文章编号：16721144(2009)01007103Research on Technology of Increa
2、sing Dam Height onSludge before Dam of Datankou ReservoirMA Zhi-jian(Ningxia Guyuan City Investigation and Design Institute of Water Conservancy，Guyuan，Ningxia 756000,China)Abstract：Taking the preloading consolidate on the sludge before the dam Datankou Reservoir by using the self-weight of increasi
3、ng the dam height to form piled load for example，and aiming at the engineering characteristics of the saturated soft soil in reservoir area such as the high compression，high sensitivity，high-rheologic behavior and low strength，low permeability，etc. the reinforcement design on the surface of sludge b
4、efore dam is made by using the theoretical caculations for the piled-load preloading and subgrade treatment，so as to research on the reinforcing mechanism and loading plan，which may provide important guidance for the design and construction of other similar project.Keywords：sludge before dam；increas
5、ing dam height；preloading design；loading plan1 引言建国以来，宁夏回族自治区共修建200多座中小型水库，宁夏南部山区占86，水库的建设，对有效利用当地水资源，兴利除害，解决人畜饮水，改善群众的生产生活条件和生态环境，促进当地农业生产和经济发展，维护民族团结和地区的社会稳定，发挥了巨大的作用，其作用是无可替代的。但由于这些水库大都建于上世纪60、70年代，水库建设标准低，建筑物配套不全或无配套设施，存在着各种各样先天性安全隐患。经过30a、40a的运行，建筑物老化、失修，多数处于病险状态，加之该地区属于黄土高原区的多泥沙河流，水库淤积非常严重，据20
6、07年全区病险水库安全状况普查情况，除列入第一批规划（29座）进行改造的水库外，还有病险水库113座，其中因库容淤损导致防洪库容不足、防洪标准降低而变为险库的占到70以上。因此要对这些水库除险加固，除对存在问题的建筑物进行改造外，首先要对主坝进行加高，以增加防洪库容，鉴于库区泥面均较高，一般库区泥面与坝后沟道底高差在1020m左右，部分甚至高达30m以上，采用坝前泥面上加坝与坝后加坝工程量相差甚大。根据我区已列入一期规划（29座）及专项规划（54座）的83座已除险加固和正在除险加固改造的水库统计分析，90以上是采用坝前泥面上加坝方式。坝前淤泥属于自建库开始至除险加固时止沉积了多年的近似饱和的库
7、区淤积物，饱和软土具有高压缩性、高灵敏度、高流变性和低强度、低渗透系数的工程特性，因此在软基上施工面临着孔压过高、变形过大、抗力过小的难题。在施工期间，如果上坝速度过快，软基内的水无法及时排出，会使地基孔隙水压力升高，有效应力降低，进而导致坝体产生开裂、滑坡或者地基失稳等事故。目前适用于处理厚度较大的淤泥、淤泥质土、冲填土等饱和黏性土地基的主要方法有换填垫层法、堆载预压法、真空预压法、强夯置换法、水泥粉煤灰碎石桩法、水泥土搅拌法、石灰桩法等，这些方法中投资较小且适应于坝前淤泥面上的为堆载预压法，这里主要指利用建筑物本身重量对地基加压的经济有效的方法。此法一般应用于以地基的稳定性为控制条件，能适
8、应较大变形的建筑物，如路堤、土坝、水池等。土坝由于填土高、荷载大，地基的强度不能满足快速填筑的要求，工程上都采用严格控制加荷速率，控制上坝速度、分期施工，即采用不超过天然地基承载力的分级加荷方式，以确保地基的稳定性。本文主要介绍大滩口水库软基加固处理方案，探求坝前淤泥面上加坝技术及方法。2 工程概况大滩口水库位于宁夏中卫市海原县境内，为小型四等工程，水库总库容212104 m3，拦河坝为黄土均质土坝，坝顶高程1930.0m，最大坝高22m，淤泥面高程为1927.0m。本次除险加固采用在坝前淤泥面上加高土坝7.3m，加坝后坝顶高程1937.3m，加坝增容132104 m3。库区淤积土最大厚度19
9、m，其物理力学指标见表1。表1 库区淤积土物理力学指标表项 目指 标项 目指 标含水量/%28.5凝聚力c/kPa10.0湿容重/（gcm-3）1.85内摩擦角/（o）13.0干容重d/（gcm-3）1.4塑限Wp 17.5孔隙比e0.951液限 WL26.7压缩系数12/MPa-10.47塑性指数 Ip9.2压缩模量Es/MPa4.2液性指数 IL1.2渗透系数k （cm s-1）1.7610-6加坝土料为左岸的第四系全新统壤土，经取样击实试验，最优含水率为14.8%，最大干容重为1.85g/cm3。3 坝前淤泥面上加固设计3.1 加固机理堆载预压法2是指在建筑物或构筑物建造前，先在拟建场地
10、上用堆土或其他荷重，施加或分级施加与其相当的荷载，对地基土进行预压，使土体中孔隙水排出，孔隙体积变小，地基土压密，以增长土体的抗剪强度，提高地基承载力和稳定性；同时可减小土体的压缩性，消除沉降量以便在使用期间不致产生有害的沉降和沉降差。由于软土的渗透性很小，土中水排出速率很慢，为了加速土的固结，缩短预压时间，常在土中打设砂井，作为土中水从土中排出的通道，使土中水排出的路径大大缩短，然后进行堆载预压，使软土中空隙水压力得以较快地消散，这种方法称为砂井堆载预压法。有时，也在土中插入排水塑料带，代替砂井。由于该工程投资较小，若采用打砂井或排水塑料带的方法势必会加大投资，通过以往工程实践经验并借鉴砂井
11、堆载预压法，通过加坝坝体本身重量5及采用重型机械碾压的方式分级对地基进行加压，达到提高地基承载力，最终完成加高坝体的目的。3.2 提高抗剪强度预压设计3.2.1 荷载大小及范围预压荷载（）大小应根据设计要求确定，一般采用等载预压（取，为基底压力），该工程中加坝高度为7.3m，从淤泥面起算为10.3m，加坝壤土试验室最大干密度平均值 =1.85g/cm3，压实度按0.98控制，则现场控制干密度为=1.813g/cm3，设计最优含水率为14.8，碾压后的湿容重为= (1+)=2.08 g/cm3(2.08104N/m3)，则建基面的基底压力为214.2kPa。对沉降有严格限制的建筑或为了减少预压时
12、间，应采用超载预压法处理（取），如采用重型机械施工。3.2.2 加荷速率及分级加荷速率应根据地基土的强度确定。当天然地基土的强度满足预压荷载下地基的稳定性要求时，可一次性加载，否则应分级加载，待前期预压荷载下地基土的强度增长满足下一级荷载下地基的稳定性要求时方可加载。3.2.2.1 加载速率式中：为第级荷载加载速率，初步设计时可取48kPa/d；为第级荷载加载增量（kPa）；为第级荷载历时（d），；、分别为第级荷载加载的起始和终止时间（d）（从零点算起）。3.2.2.2 分级荷载估算(1)估计第一级容许施加的荷载，一般可按斯开普敦极限承载力半经验公式确定，对饱和软黏土可近似按下式计算：式中：为
13、天然地基不排水抗剪强度（kPa），成层土可采用加权平均值（按十字板剪切试验室测或依土的自重应力及三轴固结不排水试验指标、计算）3。根据试验指标，采用黏性土的抗剪强度公式推求4，在初始未加外荷的情况下，=10kPa；为安全系数，建议取=1.31.5。(2)计算作用下，达到固结度时，土的抗剪强度：式中：为原地基土三轴固结不排水压缩试验求得的内摩擦角（）；为第一级预压荷载作用下，时间地基平均固结度，设计时可初步设定，或依加荷历时反求。(3)计算第二级容许施加的荷载：同样，如果第二级荷载尚未达到预压荷载，可按上述原理，重复计算第三、四3.2.3 制定加荷计划3.2.3.1 确定计算参数根据建筑物在地基
14、中造成的附加应力与自重应力的比值确定受压土层厚度10m。库区淤积土天然土层竖向渗透系数=1.7610-6cm/s，=0.951，天然土的压缩系数=0.47 Mpa-1=0.47，=。土的竖向排水固结系数=0.075cm2/s。固结度计算参数、分别为=8/=0.81，=0.016（）。3.2.3.2 第一级荷载作用计算(1)求天然地基可能承受的荷载（第一级容许施加的荷载），为安全系数，取=1.3，即=42.5kPa，取=40kPa。(2)确定第一级荷载作用下的固结度取加荷速率=5kPa/d，=60Kpa，加载持续时间=/=8d。这里取加荷速率=5kPa/d是基于以下考虑的，加坝壤土碾压后的湿容重
15、为=2.08104N/m3，即当=5103N/(m2d)时，相当于坝体加高速率=0.24m/d，8d共计1.92m。设为30d(第一次加荷开始至下一次加荷开始的时间)，计算=30d时对应于=40kPa的固结度：=0.4651。(3)计算施加第一级荷载后土的抗剪强度10+400.465tan13 =14.3kPa。3.2.3.3 第二级荷载作用计算(1)计算二级容许施加的荷载将=14.3kPa，=1.3代入公式=60.7kPa，取=60kPa。(2)确定第二级荷载作用下的固结度第二级荷载加到=60kPa，则=60-40=20KkPa。取加荷速率=5kPa/d，加载持续时间4d。取60d,计算=6
16、0d时对应于=60kPa的固结度：+=0.606。(3)计算施加第二级荷载后土的抗剪强度=18.4kPa。3.2.3.4 计算60d后容许施加的荷载将=18.4kPa，=1.3代入公式=78.1kPa，取=75Kpa。重复计算第三、四，直至满足建基面的基底压力要求为止，加荷计划详见下表。加荷计划表项目单位一级二级三级四级五级六级七级八级九级十级加荷开始时间d0306090120150180210240270加荷结束时间d8346393123153184213243273加荷历时d8433334333加荷速率KPa/d5555555555加荷荷载KPa4060759010512014015517
17、0185荷载增量KPa40201515151520151515固结历时d306090120150180210240270300固结度0.465 0.606 0.702 0.761 0.800 0.827 0.834 0.858 0.874 0.887 加坝速率m/d0.240 0.240 0.240 0.240 0.240 0.240 0.240 0.240 0.240 0.240 加坝高度m1.922 0.961 0.721 0.721 0.721 0.721 0.961 0.721 0.721 0.721 累计加坝高度m1.9222.8833.6044.3255.0465.7676.728
18、7.4498.178.891抗剪强度KPa14.318.422.225.829.432.937.040.744.347.9容许荷载KPa60.778.194.1109.6124.8139.8157.0172.8188.2203.3经计算在不设排水砂井依靠加坝坝体本身重量分级对地基进行加压，在满足脱水固结及基底应力的情况下，加荷级数共需11级，固结历时在300d以上，这显然不满足此类小工程对工期的要求，因此必须采取措施进行调整。根据对类似工程的处理经验，通过在建基面上加设水平排水垫层来解决，一般情况下砂垫层厚度不应小于50cm，当地基土表层含水量较大时可增加到80100cm，由于排水垫层兼作持力
19、层，则还应满足承载力的要求，砂垫层宽度等于铺设场地的宽度。第一层砂垫层的施工（砂料不足时，也有用砂壤土代替砂垫层的，但砂壤土含水量要小，须小于35）是形成建基面的关键，应根据实际情况通过局部铺筑碾压试验后确定第一层铺砂（土）的厚度，防止翻浆或形成橡皮土（有时可铺设砾石或块石，通过重型机械强行挤压）。经计算在铺筑50cm砂垫层后，再辅以100kPa（相当于10t/m2）机械碾压，天然地基抗剪强度可由10kPa提高至33.1kPa，第一级容许施加的荷载由60.7kPa提高至140.5kPa，考虑到第一级加荷过高易导致地基失稳，加荷速率按10kPa/d控制，加荷历时为10d，荷载增量为100kPa，
20、30d后固结度为0.456，施加该级荷载后土的抗剪强度达到43.6kPa，容许施加的荷载达到185.2kPa，相应的加坝速率为0.48m/d，10d的加坝高度为4.8m。30d开始第二级加荷，加荷速率不变，加荷历时为8d，荷载增量为80Kpa，加荷荷载至180kPa，60d后固结度为0.575，8d加坝3.84m，总加坝高度至8.64m，60d后再开始第三级加荷，本次加荷至214.2kPa，即再加1.66m至设计的10.3m，加荷历时为4d，加荷速率8.55kPa/d。调整后的加坝总历时为64d是未调整前的1/5，大大缩短了工期，经现场实际施工验证可完全满足要求。4 结 语(1)利用建筑物本身
21、重量对地基加压是一种经济而有效的方法，但该部分施工的分级荷重不能超过天然地基承载力，在该级荷载作用下，天然软土地基得到加固，土体强度得以提高，然后建造下一部分建筑物，依此进行。随着建筑物的建造，天然地基土的强度不断提高，最终达到设计承载力。(2)鉴于饱和软土抗剪强度较低，在受压初期易翻浆或形成橡皮土，因此一般情况下需铺筑厚度不应小于50cm的砂垫层1，必要时可铺设砾石或块石，一次性形成建基面。对部分库内泥面有硬壳层的要充分加以利用5，此时可直接铺筑含水量较小、厚度50cm以上的土层进行碾压，第一级荷载易用轻型机械慢速、多压，之后再改用重型机械。由于该建基面是坝前淤泥面上加坝成败的关键，施工中必
22、须认真对待。(3)坝前淤泥面上加坝，切忌急于求成，设计中要明确加荷速率，加荷量及分期施工时间等。施工过程中应进行竖向变形、边桩水平位移及孔隙水压力等项目的观测，及时反馈至设计部门对加荷计划进行校核和修正。(4)根据山区张湾、马莲、寺口子等中型水库及一批小型水库的加坝经验证明，在库内淤泥面上加坝，既节省了工程量和投资，同时坝体又是稳定和安全可靠的。但目前尚无较为完善的理论支持，实际应用当中甚感淤泥勘察资料不足，对分析坝前淤泥面上加坝机理带来困难。今后应加强这方面的工作，以期逐步形成完善的理论体系，指导工程建设。参考文献：1 JGJ792002.建筑地基处理技术规范S. 北京：中国建筑工业出版社，
23、2002:19-26.2 叶书麟，等.地基处理工程实例应用手册M.北京：中国建筑工业出版社，1998:313-323.3 GB500212002.岩土工程勘察规范S.北京：中国建筑工业出版社，2002:46-47，73-74.4 吴湘兴.土力学及地基基础M.武昌：武汉大学出版社，1992:96-103.5 GB500072002.建筑地基基础设计规范S.北京：中国建筑工业出版社，2002:48-53.Editors note: Judson Jones is a meteorologist, journalist and photographer. He has freelanced with
24、 CNN for four years, covering severe weather from tornadoes to typhoons. Follow him on Twitter: jnjonesjr (CNN) - I will always wonder what it was like to huddle around a shortwave radio and through the crackling static from space hear the faint beeps of the worlds first satellite - Sputnik. I also
25、missed watching Neil Armstrong step foot on the moon and the first space shuttle take off for the stars. Those events were way before my time.As a kid, I was fascinated with what goes on in the sky, and when NASA pulled the plug on the shuttle program I was heartbroken. Yet the privatized space race
26、 has renewed my childhood dreams to reach for the stars.As a meteorologist, Ive still seen many important weather and space events, but right now, if you were sitting next to me, youd hear my foot tapping rapidly under my desk. Im anxious for the next one: a space capsule hanging from a crane in the
27、 New Mexico desert.Its like the set for a George Lucas movie floating to the edge of space.You and I will have the chance to watch a man take a leap into an unimaginable free fall from the edge of space - live.The (lack of) air up there Watch man jump from 96,000 feet Tuesday, I sat at work glued to
28、 the live stream of the Red Bull Stratos Mission. I watched the balloons positioned at different altitudes in the sky to test the winds, knowing that if they would just line up in a vertical straight line we would be go for launch.I feel this mission was created for me because I am also a journalist
29、 and a photographer, but above all I live for taking a leap of faith - the feeling of pushing the envelope into uncharted territory.The guy who is going to do this, Felix Baumgartner, must have that same feeling, at a level I will never reach. However, it did not stop me from feeling his pain when a
30、 gust of swirling wind kicked up and twisted the partially filled balloon that would take him to the upper end of our atmosphere. As soon as the 40-acre balloon, with skin no thicker than a dry cleaning bag, scraped the ground I knew it was over.How claustrophobia almost grounded supersonic skydiver
31、With each twist, you could see the wrinkles of disappointment on the face of the current record holder and capcom (capsule communications), Col. Joe Kittinger. He hung his head low in mission control as he told Baumgartner the disappointing news: Mission aborted.The supersonic descent could happen a
32、s early as Sunday.The weather plays an important role in this mission. Starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. The balloon, with capsule attached, will move through the lower level of the atmosphere (
33、the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mount Everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. As he crosses the boundary layer (called
34、 the tropopause), he can expect a lot of turbulence.The balloon will slowly drift to the edge of space at 120,000 feet (22.7 miles/36.53 kilometers). Here, Fearless Felix will unclip. He will roll back the door.Then, I would assume, he will slowly step out onto something resembling an Olympic diving
35、 platform.Below, the Earth becomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, hell be traveling fast, so despite the distance, it will not be like diving into the deep end of a pool. It will be like he is diving into the shallow end.Skydiver preps for t
36、he big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph (1,110 kph) - in less than 40 seconds. Like hitting the top of the water, he will begin to slow as he approaches the more dense air closer to Earth. But this will not be enough to stop him completely.If he goes too fast o
37、r spins out of control, he has a stabilization parachute that can be deployed to slow him down. His team hopes its not needed. Instead, he plans to deploy his 270-square-foot (25-square-meter) main chute at an altitude of around 5,000 feet (1,524 meters).In order to deploy this chute successfully, h
38、e will have to slow to 172 mph (277 kph). He will have a reserve parachute that will open automatically if he loses consciousness at mach speeds.Even if everything goes as planned, it wont. Baumgartner still will free fall at a speed that would cause you and me to pass out, and no parachute is guara
39、nteed to work higher than 25,000 feet (7,620 meters).It might not be the moon, but Kittinger free fell from 102,800 feet in 1960 - at the dawn of an infamous space race that captured the hearts of many. Baumgartner will attempt to break that record, a feat that boggles the mind. This is one of those monumental moments I will always remember, because there is no way Id miss this.
