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1、DESIGN AND EXECUTION OF GROUND INVESTIGATION FOR EARTHWORKS PAUL QUIGLEY, FGSIrish Geotechnical Services LtdABSTRACTThe design and execution of ground investigation works for earthwork projects has become increasingly important as the availability of suitable disposal areas becomes limited and costs
2、 of importing engineering fill increase. An outline of ground investigation methods which can augment traditional investigation methods particularly for glacial till / boulder clay soils is presented. The issue of geotechnical certification is raised and recommendations outlined on its merits for in
3、corporation with ground investigations and earthworks.1. INTRODUCTIONThe investigation and re-use evaluation of many Irish boulder clay soils presents difficulties for both the geotechnical engineer and the road design engineer. These glacial till or boulder clay soils are mainly of low plasticity a
4、nd have particle sizes ranging from clay to boulders. Most of our boulder clay soils contain varying proportions of sand, gravel, cobbles and boulders in a clay or silt matrix. The amount of fines governs their behaviour and the silt content makes it very weather susceptible.Moisture contents can be
5、 highly variable ranging from as low as 7% for the hard grey black Dublin boulder clay up to 20-25% for Midland, South-West and North-West light grey boulder clay deposits. The ability of boulder clay soils to take-in free water is well established and poor planning of earthworks often amplifies thi
6、s.The fine soil constituents are generally sensitive to small increases in moisture content which often lead to loss in strength and render the soils unsuitable for re-use as engineering fill. Many of our boulder clay soils (especially those with intermediate type silts and fine sand matrix) have be
7、en rejected at the selection stage, but good planning shows that they can in fact fulfil specification requirements in terms of compaction and strength.The selection process should aim to maximise the use of locally available soils and with careful evaluation it is possible to use or incorporate poo
8、r or marginal soils within fill areas and embankments. Fill material needs to be placed at a moisture content such that it is neither too wet to be stable and trafficable or too dry to be properly compacted.High moisture content / low strength boulder clay soils can be suitable for use as fill in lo
9、w height embankments (i.e. 2 to 2.5m) but not suitable for trafficking by earthwork plant without using a geotextile separator and granular fill capping layer. Hence, it is vital that the earthworks contractor fully understands the handling properties of the soils, as for many projects this is effec
10、tively governed by the trafficability of earthmoving equipment.2. TRADITIONAL GROUND INVESTIGATION METHODS For road projects, a principal aim of the ground investigation is to classify the suitability of the soils in accordance with Table 6.1 from Series 600 of the NRA Specification for Road Works (
11、SRW), March 2000. The majority of current ground investigations for road works includes a combination of the following to give the required geotechnical data: Trial pits Cable percussion boreholes Dynamic probing Rotary core drilling In-situ testing (SPT, variable head permeability tests, geophysica
12、l etc.) Laboratory testingThe importance of phasing the fieldwork operations cannot be overstressed, particularly when assessing soil suitability from deep cut areas. Cable percussion boreholes are normally sunk to a desired depth or refusal with disturbed and undisturbed samples recovered at 1.00m
13、intervals or change of strata.In many instances, cable percussion boring is unable to penetrate through very stiff, hard boulder clay soils due to cobble, boulder obstructions. Sample disturbance in boreholes should be prevented and loss of fines is common, invariably this leads to inaccurate classi
14、fication.Trial pits are considered more appropriate for recovering appropriate size samples and for observing the proportion of clasts to matrix and sizes of cobbles, boulders. Detailed and accurate field descriptions are therefore vital for cut areas and trial pits provide an opportunity to examine
15、 the soils on a larger scale than boreholes. Trial pits also provide an insight on trench stability and to observe water ingress and its effects.A suitably experienced geotechnical engineer or engineering geologist should supervise the trial pitting works and recovery of samples. The characteristics
16、 of the soils during trial pit excavation should be closely observed as this provides information on soil sensitivity, especially if water from granular zones migrates into the fine matrix material. Very often, the condition of soil on the sides of an excavation provides a more accurate assessment o
17、f its in-situ condition.3. SOIL CLASSIFICATIONSoil description and classification should be undertaken in accordance with BS 5930 (1999) and tested in accordance with BS 1377 (1990). The engineering description of a soil is based on its particle size grading, supplemented by plasticity for fine soil
18、s. For many of our glacial till, boulder clay soils (i.e. mixed soils) difficulties arise with descriptions and assessing engineering performance tests.As outlined previously, Irish boulder clays usually comprise highly variable proportions of sands, gravels and cobbles in a silt or clay matrix. Low
19、 plasticity soils with fines contents of around 10 to 15% often present the most difficulties. BS 5930 (1999) now recognises these difficulties in describing mixed soils the fine soil constituents which govern the engineering behaviour now takes priority over particle size.A key parameter (which is
20、often underestimated) in classifying and understanding these soils is permeability (K). Inspection of the particle size gradings will indicate magnitude of permeability. Where possible, triaxial cell tests should be carried out on either undisturbed samples (U100s) or good quality core samples to ev
21、aluate the drainage characteristics of the soils accurately.Low plasticity boulder clay soils of intermediate permeability (i.e. K of the order of 10-5 to 10-7 m/s) can often be conditioned by drainage measures. This usually entails the installation of perimeter drains and sumps at cut areas or borr
22、ow pits so as to reduce the moisture content. Hence, with small reduction in moisture content, difficult glacial till soils can become suitable as engineering fill.土方工程的地基勘察与施工保罗圭格利爱尔兰岩土工程服务有限公司摘 要：当工程场地的处理面积有限且填方工程费用大量增加时，土方工程的地基勘察设计与施工已逐渐地变得重要。由于冰渍土以及含砾粘土的提出使土方工程地基勘察方法的纲要比传统的勘察方法更详细。 本文提出“岩土认证”观点以
23、及对地基勘察与土方工程相结合的优点加以概要说明。1、引 言许多爱尔兰含砾粘土的勘察与再利用评价使岩土工程师与道路工程师感到为难。这些冰渍土或含砾粘土主要表现为低可塑性而且还含有从粘土到漂石的不同粒径颗粒。大部分本地粘土与淤泥质土中包含不同比例的砂、砾石、卵石、漂石。颗粒级配控制着土体的行为，而且淤泥使土体性质易受天气变化影响。土体含水量随着地区不同而不同，从都柏林硬灰黑含砾粘土的7%到中部、西南部或西北部浅灰色含砾粘土沉积物的20%-25%。含砾粘土吸附水的能力建立的较好但土方工程中计划的不恰当常导致其扩大。一般来说，良好级配的土体对于含水量的轻微变大相当敏感，将导致强度下降或不适合用作工程回
24、填土。许多含砾粘土（尤其中等淤泥质土或良好级配的砂）在选择阶段已经被筛除，但事实上它们能对压缩或强度起到特定的作用。筛选过程应尽量使用本地土体或者回填区或路堤边性质相对较差的土体，通过仔细评价应加以应用。回填材料必须保持一定的含水量，既不能太湿导致土体不稳定也不能太干以致不能被充分压缩。高含水量、低强度含砾粘土适用于低路堤回填（相当于2到2.5米的高度）但不适用于没有使用土工织布隔离与回填层的土方回填工程。因此，土方工程承包商充分认识土体的处理特性相当重要，因为许多工程都受到挖掘设备通行能力的影响。2、传统地基勘察方法对于道路工程来讲，地基勘察最基本目标是对土体适用性进行类似表6.1的分类，该
25、表源于国家档案登记处2000年3月版的道路施工规范。目前大部分道路施工中的地基勘察包含以下提供有关岩土参数的试验方法： 取样孔 静压法取样 动力探测 回转钻进. 原位测试（标准贯入试验，变水头渗透试验，岩土物理试验等） 室内试验评价场地工作的重要性特别是评价土体深部取样区域的适用性时不能过分强调其适用性。静压法取样通常将取样器下沉至要求深度进行取样，并每间隔一米进行取样。在许多情况下，静压法取样由于卵石、漂石阻碍不能压入非常坚硬的含砾粘土。土样在钻孔内应尽量少扰动，但级配变坏是很正常的，级配变坏将导致土样分类不够精确。取样孔对于恢复适当尺寸的土样以及观察碎屑岩在卵石、漂石中所占比例来说应该是适
26、当的。因此，详尽且精确的地区描述取样区域以及取样空来说都相当重要，而且还为它们提供了检查土体在钻孔范围以外性质的良机。取样孔也提供了孔壁稳定性的评价以及观察孔壁内水进入时所造成的影响。一位有经验的岩土工程师或工程地质专家应监督取样孔工作以及土样的恢复。因为土样性质为土样敏感性提供了信息，所以取样时土体性质应被密切关注，尤其是水从小颗粒区域迁移到良好级配区域。而且土体在开挖时的条件为其原位条件提供了一个相对精确的评价。3、土的分类土的描述与分类应该依照英国标准5930（1999）进行并依照英国标准1337（1990）进行测试。土的工程描述应基于按粒径大小分级并依照良好级配土的可塑性进行补充。对于
27、许多冰渍土或含砾粘土（混合土）的难点在于其描述与工程性质测试的评价。关于以前的地基勘察纲要，爱尔兰含砾粘土的粘土与淤泥质土中常由易变比例的砂、砾石、卵石组成。良好级配且含水量为10%-15%的低可塑性土最难进行描述与分类。现在英国标准5930（1999）已认识到描述“混合土”所存在的难点土的良好级配较之颗粒尺寸对于控制着土的工程性质更优越。一个关键参数在土分类以及理解过程中经常被低估，该参数就是渗透系数K。检查土的颗粒级配将间接说明土的渗透系数的大小。假如可能，为了准确评价土体的排水特性，三轴单元试验将采用无扰动原状土样或高质量土样进行试验。低可塑性的中等渗透性含砾粘土(K大约在10-5到10-7米.秒范围内)能经常通过不同排水条件进行“模拟”。其必须在取样区域安装排水边界以及水坑边界或借用钻孔以减少土样的含水量。因此，由于含水量的小量减少，工程性质复杂的冰渍土也能当作合适的工程填土加以应用。
