This article is based on the Project Administration Handbook for Civil Engineering Works, 2022 Edition

Appendix 4.25 Geotechnical Manual for Slopes - Guidance on Interpretation (Subsumed from WBTC No.13/99)

1. This Appendix provides guidance on, and clarification of, the interpretation of some aspects of the Geotechnical Manual for Slopes (2nd Edition).

2. Terminology

2.1 The use of the term "risk" in the Geotechnical Manual for Slopes (Manual) to mean "consequence in the event of failure" has led to some misunderstanding. For this reason, the term "risk" shall be replaced by "consequence". This is consistent with international usage. The two types of consequence classification of slope failure given in Tables 5.2 and 5.3 of the Manual shall be referred to as "consequence-to-life" and "economic consequence" respectively.

2.1 《斜坡岩土工程手册》（简称为"手册"）中使用“风险”一词来表示“发生破坏时的后果”，导致了一些误解。因此，“风险”一词应改为“后果”。这与国际惯例一致。该手册表5.2和表5.3中给出的两种边坡破坏后果分类应分别称为“生命后果”和“经济后果”。

2.2 The descriptive terms "high", "low" and "negligible" are intended to reflect the likely relative severity of the failure consequence, but these have also resulted in misconceptions. To avoid possible confusion, the three categories of consequence-to-life shall be denoted as Categories 1, 2 and 3 respectively instead of "high", "low" and "negligible". For the same reason, a new system is also adopted to denote the different categories of economic consequence. The three categories of economic consequence shall be denoted as Categories "A", "B" and "C" respectively instead of "high", "low" and "negligible".

2.2 描述性术语“高”、“低”和“可忽略不计”旨在反映破坏后果可能的相对严重性，但这些也导致了误解。为了避免可能的混淆，生命后果的三个类别应分别表示为类别1、2和3，而不是“高”、“低”和“可忽略不计”。出于同样的原因，还采用了一个新的系统来表示不同类别的经济后果。三类经济后果应分别表示为类别“A”、“B”和“C”，而不是“高”、“低”和“可忽略不计”。

2.3 A combined notation shall now be used to indicate both the consequence-to-life and the economic consequence of a feature. For example, a Category 2A feature refers to one having the second highest consequence-to-life and the highest economic consequence in the new three-tier classification system.

2.3 现在应使用组合符号来表示特征对生命的影响和经济影响。例如，2A类特征是指在新的三级分类系统中，对生命具有第二高后果和最高经济后果的特征。

3 Consequence-to-life Categories

3.1 The recommended minimum safety factors for slopes given in Tables 5.1 and 5.4 of the Manual are related to assessed consequence-to-life categories. Because of the change in terminologies (see section 2), these Tables shall be replaced by Tables 1 and 2 in this Appendix respectively. The consequence-to-life category reflects the severity in terms of loss of life in the event of failure. Table 5.2 of the Manual, which gives typical examples of each consequence-to-life category, shall be replaced by Table 3 in this Appendix.

3.1 手册表5.1和表5.4中给出的边坡建议最小安全系数与评估的生命后果类别有关。由于术语的变化（见第2节），这些表应分别替换为本附录中的表1和表2。生命后果类别反映了发生破坏时生命损失的严重程度。手册中的表5.2给出了每个生命后果类别的典型示例，应替换为本附录中的表3。

3.2 In determining the consequence-to-life category of a slope, the designer should use his own professional judgement in assessing the "severity in terms of loss of life in the event of failure" in each particular case, giving due consideration to the types of buildings and facilities that may be threatened, and how the buildings and facilities would be affected in the event of slope failure. In assessing the effects of a slope failure on buildings and facilities, account should be taken of such factors as possible mechanisms and scale of failure, site conditions, proximity of the buildings and facilities to the slope and their likely density of occupation and frequency of usage in the event of failure, travel distance of the landslip debris, resistance of the buildings and facilities to debris impact and vulnerability of occupants and users.

3.2 在确定斜坡的生命后果类别时，设计师应根据自己的专业判断，评估每种特定情况下“发生破坏时生命损失的严重程度”，适当考虑可能受到威胁的建筑物和设施的类型，以及在发生斜坡破坏时建筑物和设施将如何受到影响。在评估斜坡坍塌对建筑物和设施的影响时，应考虑到以下因素：可能的坍塌机制和规模、现场条件、建筑物和设施与斜坡的接近程度及其可能的占用密度和坍塌时的使用频率、滑坡碎片的行进距离、建筑物和设备对碎片影响的抵抗力以及居住者和使用者的脆弱性。

3.3 Examples (1) and (2) of Table 3 refer to situations where the buildings or facilities lie within the expected travel distance of the landslip debris, and hence the severity in terms of loss of life is high, and the consequence-to-life category is "1". No examples are given in the Table on situations where the buildings or facilities are located further away from the slopes. Following the consequence-to-life definition given in the Manual, where the buildings or facilities lie beyound the expected travel distance of the debris and the category may be downgraded to "2". Where the buildings or facilities lie beyond the possible extreme limit of landslip debris, the consequence-to-life category may be taken as "3". Similar considerations apply to buildings and facilities located behind the slope crest with respect to the expected and the possible extreme limits of the area affected by the landslip.

3.3 表3的例子（1）和（2）指的是建筑物或设施位于滑坡碎片的预期行进距离内的情况，因此生命损失的严重程度很高，对生命的后果类别为“1”。表中没有给出建筑物或设施远离斜坡的情况示例。根据《手册》中给出的生命后果定义，如果建筑物或设施超过碎片的预期行进距离，则该类别可降级为“2”。如果建筑物或设施超出了滑坡碎片的可能极限，则对生命的影响类别可被视为“3”。类似的考虑因素适用于位于坡顶后面的建筑物和设施，涉及受山体滑坡影响区域的预期和可能的极限。

3.4 In consequence-to-life classification for the purposes of slope design and stability assessment, bus shelters or similar sheltered public waiting areas shall be regarded as occupied buildings (example 1 in Table3).

3.4 根据斜坡设计和稳定性评估的生命分类，公共汽车候车亭或类似的遮蔽公共等候区应被视为有人居住的建筑物（表3中的示例1）。

3.5 Further technical guidelines for the classification of the consequence-to-life category for slopes and retaining walls are given in GEO Technical Guidance Note No.15, which can be downloaded from the CEDD website.

3.5 土力工程处技术指导说明第15号提供了斜坡和挡土墙生命后果类别分类的进一步技术指南，该文件可从土木工程拓展署网站下载。

4 Economic Consequence Categories

4 经济后果类别

4.1 Table 5.3 of the Manual, which gives typical examples of each economic consequence category, shall be replaced by Table 4 in this Appendix.

4.1 手册表5.3给出的每种经济后果类别的典型示例，应替换为本附录中的表4。

4.2 Although Table 1 recommends the minimum safety factors for slopes for different economic consequences, the choice of safety factors against economic loss is a decision which must be made by the owner upon the advice of the designer. In advising the owners, the designer should decide for himself of degree of economic consequence and should balance the potential economic consequence in the event of a failure against the increased construction costs required to achieve a higher factor of safety.

4.2 尽管表1建议了不同经济后果下边坡的最小安全系数，但针对经济损失的安全系数的选择是业主必须根据设计师的建议做出的决定。在向业主提供建议时，设计师应自行决定经济后果的程度，并应在破坏情况下的潜在经济后果与实现更高安全系数所需增加的施工成本之间取得平衡。

5 Safety Factors for Existing Slopes

5.1 The minimum safety factors recommended in Table 2 may be used for the stability assessment of and design of modifications to any existing slope which is associated with new works, as long as rigorous geological and geotechnical investigations are conducted (which should include a thorough examination of slope maintenance history, groundwater records, rainfall records and any slope monitoring records) and there is sufficient knowledge of the geology, groundwater and performance history of the slope. Under these conditions, Table 2 can be used for stability assessment for known changes in imposed loadings, and for the design of remedial or preventive works, including slope flattening, improvements to surface and subsurface drainage, and the installation of support measures.

5.1 表2中建议的最小安全系数可用于与新工程相关的任何现有边坡的稳定性评估和监控设计，只要进行了严格的地质和岩土工程调查（应包括彻底检查边坡维护历史、地下水记录、降雨记录和任何边坡监测记录），并且对边坡的地质、地下水和性能历史有足够的了解。在这些条件下，表2可用于对施加荷载的已知变化进行稳定性评估，并用于补救或预防工程的设计，包括边坡平整、地表和地下排水的改善以及支撑措施的安装。

5.2 As Section 5.2.2 of the Manual indicates, the designer is able to adopt with confidence a lower factor of safety for an existing slope because he has the benefit of the performance history and other information that is not available for the design of a new slope. This does not imply that the standards of safety deemed to be acceptable for existing slopes are lower than those recommended for new slopes. Reference should be made on discussion of the philosophy of this approach by Malone (1985). There will often be instance, however, where particular circumstances (such as lack of adequate groundwater and rainfall records) will lead the designer to adopt, for remedial and preventive works, the standards specified for new slopes.

5.2 如手册第5.2.2节所示，设计者能够自信地对现有边坡采用较低的安全系数，因为他可以从性能历史和其他信息中获益，而这些信息对于新边坡的设计是不可用的。这并不意味着现有斜坡可接受的安全标准低于新斜坡的建议标准。应参考Malone（1985）对这种方法哲学的讨论。然而，通常情况下，特殊情况（如缺乏足够的地下水和降雨记录）会导致设计师采用新斜坡规定的标准进行补救和预防工程。

6 Safety Factors for Temporary Works

6 临时工作的安全系数

6.1 Section 5.2.4 of the Manual shall be replaced by the following. The safety factors required for the design of temporary works (i.e. works undertaken during construction which are not part of the permanent works) shall be the same as those for permanent new works (Table 1), but with due regard for the conditions which are likely to exist during the life of the temporary works. In some cases, for example, the consequence-to-life category during construction may be classified as "2" or "3", compared with consequence-to-life category "1" once the buildings are completed and occupied.

6.1 手册第5.2.4节应替换为以下内容：临时工程（即施工期间进行的不属于永久工程的工程）设计所需的安全系数应与永久性新工程（表1）的安全系数相同，但应适当考虑临时工程使用寿命期间可能存在的条件。例如，在某些情况下，施工期间的生命后果类别可能被归类为“2”或“3”，而一旦建筑物竣工并被占用，则生命后果类别为“1”。

7 Reliability of Slope Design

7 边坡设计的可靠性

7.1 The reliability of slope design is discussed in Section 5.3.6 of the Manual and should be considered in deciding on the minimum safety factor to be adopted.

7.1 手册第5.3.6节讨论了边坡设计的可靠性，在决定采用的最小安全系数时应考虑该可靠性。

7.2 Different design solutions, e.g. open-cuttings and cuttings with structural support, have different levels of uncertainties associated with the various components of investigation, design and construction. They will have different reliability indices (hence diffrent levels of safety) even if the assessed factor of safety is the same. In order to have a meaningful comparison of options, a higher minimum safety factor needs to be adopted for the solution with a lower reliability index than the solution with a higher index.

7.2 不同的设计解决方案，如直接开挖和带结构支撑的开挖，与调查、设计和施工的各个组成部分相关的不确定性程度不同。即使评估的安全系数相同，它们也会有不同的可靠性指数（因此安全水平不同）。为了对方案进行有意义的比较，可靠性指数较低的解决方案需要采用比指数较高的解决方案更高的最小安全系数。

8 Compaction of New Fill Slopes

8 新填土边坡的压实

8.1 The design and construction of new fill slopes are governed by Sections 5.5.1 and 9.5 of the Manual respectively. It should be noted that in some exceptional cases, such as fill forming a large platform that will not suppot structures, the requirement for a compacted density of 95% of maximum dry density (GEO, 2017d) can be relaxed to 90% for the interior of the platform. This may be done provided that the fill at formation level and the fill forming the peripheral slopes is compacted to 95% of maximum dry density for a vertical thickness of at least 1.5 metres and 3 metres respectively. Please note that Figure 9.1 of the Manual illustrates only one of many configurations which can be adopted. In any case, it is good engineering practice to provide adequate subsurface drainage to avoid build-up of groundwater pressure at the rear of the less permeable peripheral slopes.

8.1 新填方边坡的设计和施工分别受手册第5.5.1节和第9.5节的管辖。需要注意的是，在某些特殊情况下，例如形成不支撑结构的大型平台的填土，平台内部对最大干密度95%的压实密度（GEO，2017d）的要求可以放宽到90%。可以这样做，前提是在顶部至少1.5米和3米的垂直厚度下，水平区域填土和形成周边斜坡的填土被压实到最大干密度的95%。请注意，手册的图9.1仅说明了可采用的众多配置中的一种。在任何情况下，提供充足的地下排水是一种良好的工程实践，以避免在透水性较差的外围斜坡后部形成地下水压力。

8.2 Fill in reclamations, or behind retaining structures and in other small areas of flat land, does not generally need to meet compaction requirements for slope stability reasons. It is therefore for the designer to determine the compaction requirements based on other criteria.

8.2 出于边坡稳定性的原因，填海区、挡土结构后面和其他小块平地的填土通常不需要满足压实要求。因此，应由设计者根据其他标准确定压实要求。

9 Treatment of Existing Fill Slopes

9 既有边坡的处理

9.1 Section 5.5.2 of the Manual defines the standard treatment of existing loose fill slopes by recompaction of the surface fill to a vertical depth of 3metres. This treatment does not need to be prescribed, however, if:

(a) a fill slope is of consequence-to-life category "3",

(b) the fill slope is judged to be too small to pose a significant hazard, or,

(c) the fill slope has a cover of mature vegetation which is beneficial to the stability of the slope, and where there is a reasonable alternative engineering solution.

(Note: The size of a fill slope which may be regarded as "too small to pose a significant hazard" would depend on the distance to and the type of the facilities being affected, the topography of the ground below the slope, the liquefaction potential of the fill body, etc. Professional judgement should be exercised in individual cases in determining the size of a fill slope which could be considered as such.)

9.1 手册第5.5.2节规定了通过将表面填土重新压实至3米的垂直深度来对现有松散填土边坡进行标准处理。但是，如果出现以下情况，则无需按此要求进行：

（a） 填方边坡是生命类别“3”的后果

（b） 填土坡度被判断为太小而不会造成重大危险

（c） 填土边坡覆盖着成熟的植被，这有利于边坡的稳定性，并且有合理的替代工程方案

（注：可能被视为“太小而不会造成重大危险”的填方边坡的大小取决于与受影响设施的距离和类型、边坡下方的地面地形以及填方体的地震液化可能性等。在确定可被视为填方边坡的填方边坡大小时，应针对每种情况进行专业判断。）

9.2 Reference should be made to the Report on the Slope Failures at Sau Mau Ping (Government of Hong Kong, 1977) for background on the standard treatment of existing fill slopes as recommended in the Manual.

9.2 有关手册所建议的现有填土斜坡标准处理的背景资料，请参阅《秀茂坪斜坡破坏报告》（香港政府，1977年）。

10 Superseded Chapters/Sections and Other Relevant Technical Guidance

10 被取代的章节和其他相关技术指南

10.1 The following Chapters/Sections of the Manual are no longer applicable, having been superseded by later publications:

(a) Chapter 1 and Section 2.3.3 are superseded by the Geological Survey Maps and Memoirs and Geoguide 3 (GEO, 2017c).

(b) Chapter 2 (except Section 2.3.3), Section 3.5 and 10.2 are superseded by Geoguide 2 (GEO, 2017b).

(c) Section 4.6 is superseded by GEO Publication No. 1/93 (GEO, 1993).

(d) Chapter 7 (except the parts relevant to the design of remedial or preventive works to existing gravity retaining walls as given in Section 7.3.3) is usperseded by Geoguide 1 (GEO, 2017a), (GCO Publication No. 1/90 (GCO, 1990) and GEO Circular No. 33 (GEO, 2018a).

(e) Chapter 11 is superseded by Geoguide 5 (GEO, 2018b).

(f) References to BS 1377:1975 concerning Phase 1 tests described in Works Branch Technical Circular 6/94 are replaced by Geospec 3 (GEO, 2017d).

(g) "The Hong Kong Bibliography" referred to in the Manual is Bibliography on Geology and Geotechnical Engineering of Hong Kong (Brand, 1984). Please refer to the updated bibliography online at the Hong Kong Slope Safety website.

10.2 Please also read the latest updates promulgated in the following GEO Technical Guidance Notes:

(a) GEO Technical Guidance Note No. 30 - Updated Intensity-Duration-Frequency Curves with Provision for Climate Change for Slope Drainage Design

(b) GEO Technical Guidance Note No. 39 - Guidelines for Estimation of Surface Runoff from Natural Terrain Catchments for Drainage Design Purposes

(c) GEO Technical Guidance Note No. 40 - Guidelines on Temporary Drainage Provisions and Precautionary Measures against Severe Rainfall during Site Formation Works and Construction of Reinforced Fill Structures

(d) GEO Technical Guidance Note No. 41 - Amendments to British Standards References in Technical Guidance Documents for Migration to Eurocodes

(e) GEO Technical Guidance Note No. 43 - Guidelines on Hydraulic Design of U-shaped and Half-round Channels on Slopes