Analysis of settlements of bored-cast-in-place piles in collapsible soils

Conclusions 1. The proposed method of analysis makes it possible to investigate the behavior of bored-cast-in-place piles interacting with collapsible bases for different complex cases of "soaking" of the soils.2. The sequence in the "soaking" of the soil base and the application of the load to the pile exerts a determining effect on their settlement.3. The loading force of "negative" friction depends on the soil base "soaking" scheme, the "soaked" soil layer thickness, the collapsible soil properties, and other factors.4. The additional pile settlements caused by the "negative" friction forces developed as a result of hanging of the collapsing soil from the pile can be significantly larger than the pile settlements under the load in the absence of collapse.5. "Soaking" of the collapsible soil base from the bottom in parallel layers is more dangerous for bored-cast-in-place piles than "soaking" from the top.Scientific-Research Department of S. Ya. Zhuk Gidroproekt Institute. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 14–17, January–February, 1987.     

Interaction of cast-in-place piles with soil under type II collapsibility conditions

Conclusions 1. Under the given and similar hydrogeologic conditions, the settlements of piles fully cutting through the collapsible mass and sunk 2–12 m into the noncollapsible soils, when the pile bearing capacity under the external load is satisfied, are practically equal to the ground surface settlements in the pile site.2. For long-term soaking of the soils from the top, the low position of the depth at which soil compression starts under the action of the dead weight of the mass (it is in the range 14–16 m), and the development of compression deformations in the noncollapsible layers, which prevents detachment of the upper soil zone from the piles, are essential under the given conditions.3. For increase in the pile length from 24 to 31 m and support on denser soil layers (alluvial clays), the soil base resistance and the additional load caused by the soil mass action increase correspondingly, but the pile settlements remain practically unchanged.For substantial reduction of the possible absolute pile settlements with prolonged soaking of the soils under the given hydrogeologic conditions, the pile length should be increased by about 20 m. However, for establishment of the foundation dimensions it is extremely important to compare the allowable and possible differential settlements of adjacent foundations, which are comparatively small under the given conditions.Scientific-Research Institute of Bases and Underground Structures. TISI. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 2, pp. 14–17, March–April, 1986.     

特殊浸水下桩基负摩阻力试验研究

为妥善解决湿陷性黄土地区负摩阻力引起的桩基承载力下降问题,以消除桩周黄土湿陷性为出发点进行新途径和新方法的探索,采用非传统的特殊浸水方式分别对混凝土灌注桩、微型钢管砂浆复合桩进行了现场试验。结果表明:桩顶受荷时采用注水孔与试坑相结合浸水量较大,浸水段桩侧以负摩阻力为主,最大值达319.62 kPa,局部出现正摩阻力;成桩过程中采用泥浆循环浸水量较小,桩周土体在桩顶受荷前完成部分湿陷,受荷后桩侧以正摩阻力为主,局部出现负摩阻力,且数值较小,最大值为32 kPa;特殊浸水条件下桩周土体沿桩身分段湿陷,桩侧出现多个负摩阻力峰值及中性点,正、负摩阻力交错分布;桩基负摩阻力的大小受浸水方式、加载方式、浸水固结时间的综合影响,建议在桩基施工过程中采用合理的施工工艺对桩周土体预先进行微量浸水,消除部分黄土湿陷,以避免由于地下水环境改变而引发的桩基承载力下降。     

自重湿陷性黄土与单桩负摩阻力离心模型试验

 为研究和分析黄土的湿陷变形性质与桩基的负摩阻力,采用离心模型试验的方法分别对原状自重湿陷性黄土与重塑湿陷性黄土进行模拟浸水试验。试验研究结果表明：原状和重塑湿陷性黄土浸水湿陷过程主要分为3个阶段,即显著湿陷变形阶段、湿陷稳定变形阶段以及水位下降后土体的固结变形阶段。根据试验结果,对于以沉降观测为目的的试验研究中,用重塑黄土代替原状黄土进行离心模型试验模拟其湿陷变形的方法是可行的。进而分析基桩负摩阻力分布规律及中性点位置的变化规律。单桩的负摩阻力分布及中性点位置是一动态变化过程,中性点位置与桩长的比例为0.68～0.82。     

Multifunctional combined foundations on permafrost soils

The writers examine the construction of multifunctional combined foundations on permafrost soils, as approved for strengthening the foundation system of an administrative-industrial building in Norilsk. Such foundations make it possible to control the building deformations developed as a result of increase in the pile loads or in the settlements of the piles due to decrease in their bearing capacity when the soil base temperature increases.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 26–28, September–October, 1992.     

大厚度自重湿陷性黄土地基处理深度和湿陷性评价试验研究

 为解决大厚度自重湿陷性黄土地区地基处理深度和湿陷性评价等难题,在湿陷性黄土厚度大于36.5 m的场地进行以下浸水试验：不同深度的挤密桩处理地基深层浸水载荷试验,不同深度的孔内深层强夯处理地基载荷浸水试验,不打注水孔、埋设TDR水分计的原位浸水试验。研究结果表明：(1) 大厚度自重湿陷性黄土地基处理6～12 m、深层浸水时,发生显著地基下沉;15～20 m时,地基沉降较小;处理深度大于20 m时,地基沉降基本可忽略。(2) 浸水试坑22.5～25.0 m以上土体含水率增加较快,甚至达到饱和,以下土体含水率增加缓慢,基本没有发生湿陷。建议22.5～25.0 m作为大厚度自重湿陷性黄土地基处理和湿陷性评价的临界深度。(3) 大厚度自重湿陷性黄土地基在采取有效的综合处理措施之后,甲类建筑可以不全部消除湿陷量,乙、丙类建筑可以根据控制建议适当放宽对剩余湿陷量的要求。(4) 不同地区、不同微结构类型土的湿陷性应当采用不同的湿陷系数 来判定,即“湿陷系数 = 0.015”在自基础底面至基底下15 m的范围内可继续使用;15 m以下适当放宽,按不同深度对 进行修正,可使大厚度自重湿陷性黄土湿陷性评价趋于合理,有效节约大量地基处理费用。     

Prediction of soil collapse from trench soaking test data

Results are presented for long-term full-scale field soaking of test trenches in the Ukraine and in the Rostov Region, for determination of the type of collapsibility soil conditions and the possible soil collapse in accordance with the SNiP 2.02.01-83 Norms. The relation between these results and the soaking regime is shown: water feed rate, number of drainage holes, soaking periodicity, engineering-geologic characteristics of the soils masses, presence and depth of occurrence of aquicludes, etc., which must be taken into account for soil collapse prediction.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 18–23, September–October, 1992.     

Structure settlements at the Atommash plant as a result of prolonged soil wetting

Conclusions 1. In the 14 years that have elapsed since the start of construction on the Atommash plant, deformation phenomena, which have not, however, lowered the strength of the structure in the production buildings, have developed as a result of nonobservance of waterproofing requirements at the site in areas where surface-water domes that had developed as a result of prolonged wetting of the stratum from above have been detected. The design solutions for these foundations have ensured reliable operation under a rising GWT. The absolute settlements, exceeding those permitted by the Construction Rules and Regulations, affect less than 1% of the total number of columns in the buildings at the Atommash plant, while the relative settlements are within allowable limits. Cracks have formed in some auxiliary buildings primarily because their superstructures were built disregarding possible nonuniform foundation settlements.2. The causes of pile-foundation settlements that had exceeded the standard values with respect to absolute magnitude are not associated with their bearing capacity, but are the result of deformations of the underlying soil layers that are not prone to slump-type settlement and to the development of deformation phenomena. This hypothesis explains the development of additional deformations in the stratum due to the compression of soils beneath the surface-water dome.3. The deformation phenomena that develop at the site of the Atommash plant as a result of the wetting of the soil stratum from above do not affect the foundation settlements induced by live loads, and should be accounted for in design as a possible displacement of the area. It is possible to eliminate the effect of probable relative settlements on a structure by taking structural measures and observing waterproofing measures.4. Under conditions favorable to the development of the deformation phenomenon, it is recommended to predict possible compressive deformations in a portion of the stratum underlying the stratum prone to slump-type settlement within the limits of the layers with a relative compression of more than 3% under a pressure equal to the natural pressure in compression devices.5. Studies directed toward the nature of the processes explaining the development of the deformation phenomenon must be continued.All-Union Scientific-Research Institute of Bases and Underground Structures. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 12–15, July–August, 1988.     

Experience with construction on long underreamed piles in collapsible soils

Conclusions 1. The noneffectiveness of construction of underreamings in long piles resting on insufficiently dense soils is due to high resistance on their lateral surfaces and small settlements until rupture occurs, as a result of which the tip resistance is not realized.2. The increase in the settlements of bored-cast-in-place piles for long-term soaking under the above-mentioned soil conditions does not depend on the external load applied to them and on the use of underreamings, for all practical purposes, but it is detemined by the laws of development of collapse depressions.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 2–5, July–August, 1990.     

Compaction and deformation characteristics of collapsible soil in the hydroblasting method

Conclusions 1. In the accelerated hydroblasting method, the process of compaction of collapsible masses is characterized by three phases, and the total deformation amounts to 10–12% of the collapsible mass thickness. In this connection, the collapse in phase I amounts to 25–30%, in phase II to 40–45%, and in phase III to 30–35% of the total compaction deformation.2. In the compaction of collasible loess soils by the hydroblasting method, the critical factor is the degree of saturation s _r ^cr =0.75, for which there is swelling of the camouflaged hollow, and the optimal is s _r ^opt =0.85, for which the maximum effect is reached.3. In contrast with the usual soaking method, in the hydroblasting method compaction of the collapsible mass below a depth of 3–5 m takes placed quite uniformly, with increase in the dry density of the soil to 1.60–1.65 tons/m³.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 11–13, September–October, 1991.     

隧道内挤密桩改善湿陷性黄土地基

在湿陷性黄土地区,由于未能消除地基土体的湿陷性,在施工或使用中由于水的侵入导致黄土湿陷,从而导致隧道内地基失稳而遭破坏或使道路、地下管线等被毁的事故称为黄土湿陷灾害。在隧道基面上设置若干个挤密桩孔,并在挤密桩孔内铺设填料层构成挤密桩。用挤密桩进行地基处理结构简单,施工时产生震动较小,噪声污染小,不仅能够达到隧道内消除地基黄土湿陷性的目的,同时,还能够保证隧道工程内部支护结构的稳定和施工安全。     

Bored/cast-in-place piles employing soil compaction of hole bottom,for the "Atommash" Plant

Conclusions 1. Compaction of the soil at the bottom of deep holes of 1-m diameter as described, raised the bearing capacity of piles at their ends, under the Volga-Don ground conditions, by three to five times.2. In view of the fact that the toe of long (exceeding 15 m) floating bored/cast-in-place piles of usual construction contributed only to a weak extent (up to 10% of the total resistance, under Volga-Don conditions) to the performance of the pile, right up to its failure, it is expedient and economically efficient to install such piles only in holes previously rammed at their ends. Under ground conditions of type II in regard to collapsibility, this measure is particularly important in order to avoid inadmissible pile settlements resulting from negative friction following soil collapse under the weight of its own thickness. Cast-in-place piles placed in prerammed holes were introduced in the construction of Section 5 of the Volga-Don "Atommash" Plant, and are recommended for buildings of a substantial number of stories in Volga-Don.Scientific-Research Institute of Bases and Underground Structures. (NII Osnovanii) Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 6, pp. 10–13, November–December, 1980.     

压实黄土增湿变形性质及其影响因素试验研究

作为建筑材料的黄土在工程建设中大多经过压实处理,其增湿变形特性与工程安全密切相关。已有的对黄土增湿变形性质的研究,基本上是针对原状黄土开展的,对压实黄土较少涉及。通过双线法增湿湿陷试验,对压实黄土在增湿条件下的压缩及增湿变形性质进行了较系统的研究,并进行了压缩和湿陷变形影响因素的方差分析。结果表明:①随增湿含水率的增加,压实黄土的压缩性增大,湿陷性减小,压实度越小,这种效应越明显;不同压力下,湿陷变形对增湿的敏感性不同;②随增湿含水率的增加,增湿变形起始压力减小,增湿变形终止压力增大,增湿变形压力区间增大,可用增湿变形系数反映已有增湿水平下土体湿陷性的退化程度;③方差分析表明,相同增湿含水率下,压实黄土最终变形仅受最终压力的影响,加荷路径、浸水路径及两者的耦合对其影响很小;④未浸水饱和压缩时,密实度、初始含水率和压力,以及它们的交互作用均对压缩变形有显著影响,压力的影响最大,其次是密实度和初始含水率;与未浸水情况不同的是,浸水饱和压缩时初始含水率的影响很小;⑤增湿含水率、压力和密实度,以及它们的交互作用均显著影响压实黄土的湿陷性,其中增湿起始含水率的影响最大,密实度次之,压力最小。     

Computing the settlements of foundations placed on beds with layers of weak clayey soils

Conclusions 1. In designing natural foundation beds with layers of weak clayey soils, the final settlements of the foundations and the settlements with time can be computed using the theories of elasticity and seepage consolidation in accordance with the method outlined in the paper. This is confirmed when additional (over and above the natural) pressure p₀ 5c are applied to the top of the weak clays, where c is the minimum specific cohesion of the clayey soils as determined with allowance for the possible reduction in cohesion with time. When p₀ > 5c, including pressures on the roof of the weak clays equal to the design pressures given in Section 3.62 of SNiP II-15-74, there are only isolated cases of building construction from which it is impossible to judge laws governing bed deformation.2. In conformity with Section 1, the method of computation clarifies the computation of settlements as compared with SNiP II-15-74, simplifying the design of beds with layers of weak clayey soils.Scientific-Research Institute of Bases and Underground Structures (NIIosnovanii). Estonian Institute of Industrial Design and Planning. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 15–18, July–August, 1981.     

不同含水率下黄土冻融循环对湿陷性影响探讨

黄土通常可作为公路路基主要的填筑材料,湿陷性黄土经压实后,很大程度消除了湿陷性,能够满足路基整体强度和稳定性的要求,但在季节冻土区,黄土路基运营几年后仍发生大量的不均匀沉降、塌陷等病害。为分析冻融循环作用下各级含水率对黄土湿陷性的影响,采用室内试验的方法,对湿陷性黄土进行不同的冻融次数,探究其在冻融循环过程中变形及冻融循环后湿陷情况。实验结果表明:冻融循环之后的各级含水率重塑黄土仍具有二次湿陷性;冻融循环作用下高含水率的土体结构比低含水率的土体结构破坏的较早;干密度一定时,低含水率的土体冻融循环之后的净变形量越大,湿陷系数越小,冻融循环之后的净变形量越小,湿陷系数越大。     

湿陷性黄土隧道的工程性质分析

黄土隧道往往具有特殊的湿陷性黄土围岩和干旱半干旱气候地区特殊的地形地貌及地质环境,对隧道结构构成了潜在的不利影响。在对湿陷性黄土隧道修建和运行中工程特性认识的基础上,首先,根据地形地貌、地层结构、侵蚀发育、地下水条件和黄土浸水水源,进行了隧道的岩土环境等级和浸水等级划分,以及湿陷性黄土隧道的环境等级划分。其次,给出了隧道黄土地基湿陷变形量的计算分析方法,依据黄土地基湿陷变形不均匀沉降对衬砌结构的作用影响,以及列车运行对路基沉降变形的控制标准和建筑地基湿陷变形对结构的作用影响,确定了隧道湿陷性黄土地基等级的划分标准。最后,考虑到现行《湿陷性黄土地区建筑规范》适用的局限性,结合湿陷性黄土隧道的工程特点,针对隧道施工过程中围岩稳定性和湿陷变形对衬砌结构影响的两个重要问题,相应的提出了隧道地基湿陷性变形的评价方法和围岩压力的确定方法;由竖向压缩应力系数确定湿陷压缩应力,结合试验得出的湿陷系数计算隧道下地基土在实际压缩应力条件下的湿陷变形量。在考虑黄土结构性的条件下,利用太沙基公式计算隧道围岩压力,得到了围岩压力随黄土构度的变化关系。     

大厚度湿陷性黄土地层的现场砂井浸水试验研究

针对大厚度黄土湿陷变形室内试验评价不准确,现场原位浸水试坑试验评价方法周期长、费用高且难以适应线性工程技术要求等不足,提出了一种新的现场试验评价方法:砂井浸水试验方法。其核心是利用湿陷变形土体与未湿陷变形土体之间产生相对沉降差及地裂缝,通过湿陷性黄土场地设置砂井,将水直接导入某一深层湿陷性黄土地层及砂井圆周边土体,以此来测定砂井井底下地层和砂井孔深范围内黄土的湿陷变形量。该方法具有操作简便、花费小、周期短和灵活性高等特点。依托宝兰客专建设项目,在具有代表性的大埋深自重湿陷性黄土场地开展了4个不同深度的砂井浸水试验,测试了砂井场地的沉降变形及井底湿陷性土层的沉降变形,同时配合井底土层含水量的量测,分析了井底黄土的湿陷性变形特征。参考现有规范中建议的该地区自重湿陷量修正系数,对比砂井浸水试验结果与室内试验结果,初步论证了砂井浸水试验的合理性,及其在大厚度湿陷性黄土线性工程上运用的优势。     

Results of field tests of piles in permanently frozen ground

Conclusions 1. To estimate the bearing capacity of piles and to determine the deformation characteristics of soils it is necessary to conduct comprehensive pile tests on plots with the same frozen-ground conditions.2. Pile tests should be conducted before the appearance of the second region of continuous deformations. For structures allowing considerable deformations this will permit increasing the bearing capacity, taking it to be equal to the value at which the jog of the soil consolidation under the pile point ends.3. Calculation of pile foundations with respect to the second limiting state permits a substantial increase of the design bearing capacity of piles, which ultimately leads to a reduction of construction costs.4. The use of the accelerated test method with the obtainment of the experimental values of the conversion factor to long-time strength permits a considerable reduction of the test period.5. The data obtained in pile tests permits developing more economical designs.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 5, pp. 35–36, September–October, 1977.     

Actual and calculated loads on piles and results of observing settlements of a large-panel building

Conclusions 1. During the construction period the loads on extreme piles along the axes of transverse bearing walls are considerably greater than on the middle ones.2. Under the effect of rigidity of the building a redistribution of the loads on the piles during its operation occurs even in the absence of a grillage, i.e., the more loaded piles are unloaded and the less loaded ones are additionally loaded.3. The actual loads on the piles in the operating period are considerably less than the calculated, which indicates the need to refine the recommendations of SNiP II-6-74 for the purpose of taking into account the useful loads in calculating the foundations of residential buildings.4. The factor =0.2, concerning conversion from settlements of single piles in static tests to settlements of piles in foundations under long-term load, given in the draft of the new edition of SNiP II-17-77, is acceptable.5. To accumulate data on the actual loads on pile foundations it is necessary to continue their measurements under various soil conditions.Deceased.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 17–19, May–June, 1977.     

湿陷性黄土区复杂地基上高层建筑岩土勘察设计要点浅析

高层建筑基底压力大,对沉降敏感,破坏后果很严重,相较于其他一般建筑,对地基和基础的稳定性要求更高,而湿陷性黄土在未受水浸湿时强度较高,浸水后,土结构迅速破坏,进而产生不均匀沉降,造成基础破裂。为了解决在湿陷性黄土峁、梁地区高层建筑建设过程中碰到的岩土工程技术问题,在查阅文献资料的基础上,结合工程实际经验,对湿陷性黄土峁、梁地区高层建筑工程勘察、设计及施工中主要考虑的问题进行分析,采用严格分阶段进行勘察、地基处理消除湿陷、按设计填筑土方并对坡面防护、桩基无水分段施工然后快速浇筑并扩大检测范围等手段,使得湿陷性黄土区上的高层建筑基础得以安全可靠投入使用。由此可知,详尽的勘察、针对性设计、良好的施工加之可靠的检测是湿陷性黄土区复杂地基上进行高层建筑所必不可少的。