1、design and execution of ground investigation for earthworks abstractthe 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 of importing engineering fill increase. an outl

2、ine 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 incorporation with ground investigations and earth

3、works.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 and have particle sizes ranging from clay to boul

4、ders. 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 highly variable ranging from as low as 7% for t

5、he 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 this.the fine soil constituents are generally sensi

6、tive 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 been rejected at the selection stage, but good pla

7、nning 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 poor or marginal soils within fill areas and embank

8、ments. 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 low height embankments (i.e. 2 to 2.5m) but not su

9、itable 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 effectively governed by the trafficability of earthmo

10、ving 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 (srw), march 2000. the majority of current ground

11、 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, geophysical etc.) laboratory testingthe importance of phas

12、ing 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 intervals or change of strata.in many instances,

13、 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 classification.trial pits are considered more appropri

14、ate 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 the soils on a larger scale than boreholes. tri

15、al 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 of the soils during trial pit excavation should

16、 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 of its in-situ condition.3. engineering performan

17、ce testing of soilslaboratory testing is very much dictated by the proposed end-use for the soils. the engineering parameters set out in table 6.1 pf the nra srw include a combination of the following: moisture content particle size grading plastic limit cbr compaction (relating to optimum mc) remou

18、lded undrained shear strengtha number of key factors should be borne in mind when scheduling laboratory testing: compaction / cbr / mcv tests are carried out on 20mm size material. moisture content values should relate to 20mm size material to provide a valid comparison. pore pressures are not taken

19、 into account during compaction and may vary considerably between laboratory and field. preparation methods for soil testing must be clearly stipulated and agreed with the designated laboratory.great care must be taken when determining moisture content of boulder clay soils. ideally, the moisture co

20、ntent should be related to the particle size and have a corresponding grading analysis for direct comparison, although this is not always practical.in the majority of cases, the mcv when used with compaction data is considered to offer the best method of establishing (and checking) the suitability c

21、haracteristics of a boulder clay soil. mcv testing during trial pitting is strongly recommended as it provides a rapid assessment of the soil suitability directly after excavation. mcv calibration can then be carried out in the laboratory at various moisture content increments. sample disturbance ca

22、n occur during transportation to the laboratory and this can have a significant impact on the resultant mcvs.igsl has found large discrepancies when performing mcvs in the field on low plasticity boulder clays with those carried out later in the laboratory (2 to 7 days). many of the aforementioned l

23、ow plasticity boulder clay soils exhibit time dependant behaviour with significantly different mcvs recorded at a later date increased values can be due to the drainage of the material following sampling, transportation and storage while dilatancy and migration of water from granular lenses can lead

24、 to deterioration and lower values.this type of information is important to both the designer and earthworks contractor as it provides an opportunity to understand the properties of the soils when tested as outlined above. it can also illustrate the advantages of pre-draining in some instances. with

25、 mixed soils, face excavation may be necessary to accelerate drainage works.cbr testing of boulder clay soils also needs careful consideration, mainly with the preparation method employed. design engineers need to be aware of this, as it can have an order of magnitude difference in results. static c

26、ompaction of boulder clay soils is advised as compaction with the 2.5 or 4.5kg rammer often leads to high excess pore pressures being generated hence very low cbr values can result. also, curing of compacted boulder clay samples is important as this allows excess pore water pressures to dissipate.4.

27、 engineering classification of soilsin accordance with the nra srw, general cohesive fill is categorised in table 6.1 as follows: 2a wet cohesive 2b dry cohesive 2c stony cohesive 2d silty cohesivethe material properties required for acceptability are given and the design engineer then determines th

28、e upper and lower bound limits on the basis of the laboratory classification and engineering performance tests. irish boulder clay soils are predominantly class 2c.clause 612 of the srw sets out compaction methods. two procedures are available: method compaction end-product compactionend product com

29、paction is considered more practical, especially when good compaction control data becomes available during the early stages of an earthworks contract. a minimum target dry density (tdd) is considered very useful for the contractor to work with as a means of checking compaction quality. once the mat

30、erial has been approved and meets the acceptability limits, then in-situ density can be measured, preferably by nuclear gauge or sand replacement tests where the stone content is low.as placing and compaction of the fill progresses, the in-situ tdd can be checked and non-conforming areas quickly rec

31、ognised and corrective action taken. this process requires the design engineer to review the field densities with the laboratory compaction plots and evaluate actual with theoretical densities.5. supplementary ground investigation methods for earthworksthe more traditional methods and procedures hav

32、e been outlined in section 2. the following are examples of methods which are believed to enhance ground investigation works for road projects: phasing the ground investigation works, particularly the laboratory testing excavation & sampling in deep trial pits large diameter high quality rotary core

33、 drilling using air-mist or polymer gel techniques 譯文： 土方工程的地基勘察與施工摘 要：當工程場地的處理面積有限且填方工程費用大量增加時，土方工程的地基勘察設計與施工已逐漸地變得重要。由于冰漬土以及含礫粘土的提出使土方工程地基勘察方法的綱要比傳統的勘察方法更詳細。 本文提出“巖土認證”觀點以及對地基勘察與土方工程相結合的優點加以概要說明。1、引 言許多愛爾蘭含礫粘土的勘察與再利用評價使巖土工程師與道路工程師感到為難。這些冰漬土或含礫粘土主要表現為低可塑性而且還含有從粘土到漂石的不同粒徑顆粒。大部分本地粘土與淤泥質土中包含不同比例的砂、礫石、

34、卵石、漂石。顆粒級配控制著土體的行為，而且淤泥使土體性質易受天氣變化影響。土體含水量隨著地區不同而不同，從都柏林硬灰黑含礫粘土的7%到中部、西南部或西北部淺灰色含礫粘土沉積物的20%-25%。含礫粘土吸附水的能力建立的較好但土方工程中計劃的不恰當常導致其擴大。一般來說，良好級配的土體對于含水量的輕微變大相當敏感，將導致強度下降或不適合用作工程回填土。許多含礫粘土（尤其中等淤泥質土或良好級配的砂）在選擇階段已經被篩除，但事實上它們能對壓縮或強度起到特定的作用。篩選過程應盡量使用本地土體或者回填區或路堤邊性質相對較差的土體，通過仔細評價應加以應用。回填材料必須保持一定的含水量，既不能太濕導致土體不

35、穩定也不能太干以致不能被充分壓縮。高含水量、低強度含礫粘土適用于低路堤回填（相當于2到2.5米的高度）但不適用于沒有使用土工織布隔離與回填層的土方回填工程。因此，土方工程承包商充分認識土體的處理特性相當重要，因為許多工程都受到挖掘設備通行能力的影響。2、傳統地基勘察方法對于道路工程來講，地基勘察最基本目標是對土體適用性進行類似表6.1的分類，該表源于國家檔案登記處2000年3月版的道路施工規范。目前大部分道路施工中的地基勘察包含以下提供有關巖土參數的試驗方法： 取樣孔 靜壓法取樣 動力探測 回轉鉆進 原位測試（標準貫入試驗，變水頭滲透試驗，巖土物理試驗等） 室內試驗評價場地工作的重要性特別是評

36、價土體深部取樣區域的適用性時不能過分強調其適用性。靜壓法取樣通常將取樣器下沉至要求深度進行取樣，并每間隔一米進行取樣。在許多情況下，靜壓法取樣由于卵石、漂石阻礙不能壓入非常堅硬的含礫粘土。土樣在鉆孔內應盡量少擾動，但級配變壞是很正常的，級配變壞將導致土樣分類不夠精確。取樣孔對于恢復適當尺寸的土樣以及觀察碎屑巖在卵石、漂石中所占比例來說應該是適當的。因此，詳盡且精確的地區描述取樣區域以及取樣空來說都相當重要，而且還為它們提供了檢查土體在鉆孔范圍以外性質的良機。取樣孔也提供了孔壁穩定性的評價以及觀察孔壁內水進入時所造成的影響。一位有經驗的巖土工程師或工程地質專家應監督取樣孔工作以及土樣的恢復。因為土樣性質為土樣敏感性提供了信息，所以取樣時土體性質應被密切關注，尤其是水從小顆粒區域遷移到良好級配區域。而且土體在開挖時的條件為其原位條件提供了一個相對精確的評價。3、土工試驗由于室內試驗的許多規定使其被建議用作土的最后試驗。土的工程參數列于表6.1，該表源于國家檔案登記處2000年3月版的道路施工規范。其中包含以下內容： 含水量 顆粒級配 塑 限 加州承載比 密實度（最優含水量） 重塑土不排水抗剪強度當進行室內試驗時，大量的關鍵因素應該被考慮。 密實度.加州承載比.mcv試驗土樣小于20mm。