膨胀土边坡防护

铁路膨胀土路堑边坡的防护处理
1 引言
铁路膨胀土路堑边坡的防护处理，直接关系到铁路施工和运输的安全，是一个非常关键的工程措施。

根据国内已建的铁路路堑膨胀土边坡经验，膨胀土路堑边坡首先应防止雨水直接冲刷，同时应减少雨水渗入土体引起强度的大幅度衰减。

因此，对待膨胀土路堑边坡应以“预防为主”，在充分掌握膨胀土滑坡发生与发展的工程地质条件的基础上，要采取有效的可靠工程措施，以保证路堑边坡的稳定，制止滑坡的发生。

现时对膨胀土路堑边坡防护与处理措施主要可以归纳为表水防护、坡面防护和支挡防护三种。

具体说包括下列基本原则：
（1）严格按设计坡度从上而下逐级进行路堑边坡开挖，严禁淘底开挖。

如遇土体软弱结构面发育，易于风化的强膨胀土，要快速施工，并在路堑边坡开挖完成时立即进行防护与处理。

（2）设置排水系统。

水是膨胀土产生胀缩变形和降低强度的重要因素，因此在膨胀土路堑边坡的开挖和防护过程中，对地表水要设置排水沟等措施截排坡面水流，减少地表水渗入土体与冲刷坡面。

（3）合理堆放弃土。

路堑开挖弃土应避免堆放在路堑坡顶处，一般应离开挖线2～5m，因为堑顶堆土会增加边坡的荷载和侧向应力，以至堑顶产生过大拉应力而开裂，使雨水沿拉开裂缝渗入路堑边坡内，导致土体强度降低。

（4）对已发生滑坡的路堑边坡，应根据滑坡的深度和范围、所处位置、受力条件和危害程度及时采取处理措施。

（5）如果路堑边坡土体已进入增速变形阶段但未发生整体滑动，应及时采取如减载，放缓坡度，采用支挡结构物等措施，防止发生滑坡现象。

（6）采用片石护坡、骨架护坡等防护措施时要做好排水措施，以利于坡面水快速排走，防止水渗入土体，引起破坏。

铁路膨胀土路堑边坡防护应根据工程的规模和等级，边坡的不同部位应力分布特征来选择方案，我们在长荆铁路建设中对膨胀土路堑边坡采取了多种形式的防护与处理，为今后膨胀土边坡防护与处理积累资料，提供参考。

2 工程概况
长荆铁路东起汉丹线长江埠站，经应城、天门、皂市、京山、钟祥，至焦柳线荆门站，运营全长176.92Km，我公司承担十二标段DK147＋000～DK156＋000段的施工任务，线路等级为：国铁Ⅱ级、单线，限制坡度：6‰。

DK147＋000～DK156＋000段位于荆门地区钟祥市冷水镇境内，地形为丘陵、垄岗，海拔100m左右，线路经过地带以丘陵、坡地为主，间有水库、稻田；本段土质以膨胀土为主，基本承载力150～200Kpa，间有基岩为志留系的页岩，灰黄色、灰色，风化严重至极严重，基本承载力自上而下为120～400Kpa。

区间路基土石方734064m3，其中填土328682 m3，填石7313 m3，挖土方390956 m3，挖石方7213 m3，路堑部分最大挖深在20m以内，路堑边坡采用了挡土墙、骨架护坡、片石护坡、土工格栅护坡等不同形式的防护工程措施。

3 路堑边坡的防护处理
我们在膨胀土路堑边坡施工中，根据膨胀土吸水膨胀软化、失水收缩开裂、反复变形及强度衰减等特性及其结构面产状、地面横坡、路堑边坡高度、降水量等来确定开挖方法和边坡的防护与处理措施，现就膨胀土路堑边坡的开挖和防护处理综述如下：
3.1 路堑开挖
根据膨胀土的特性，在开挖时要做好防排水措施，首先在堑顶开挖天沟，并进行铲草、挖树根等清表工作，以利于地表水顺利排走，然后放出路堑开挖线，根据施工段路堑宽度和深度比较大的特点，采用通道纵挖法，机械化快速施工；路堑开挖时，沿路堑纵向分层先开挖出一条纵向通道，然后开挖两旁，按照设计坡度逐层开挖，挖方边坡不一次开挖到设计线，沿边坡预留厚度30～50cm一层，待路堑挖完时，再削去边坡预留部分，并立即进行边坡防护，防止其强度过底发生坍滑。

3.2 支挡结构防护
为了防止边坡的坍塌失稳，确保边坡稳定，采用倒坡基础挡墙。

根据边坡的高度和土体的压力来选择挡墙的高度和厚度，以充分利用被动土压力，减少倾覆力矩，增加挡墙稳定性，挡墙基础深入滑动面以下约1.0m，墙背后设反滤层，以利排水，挡土墙在基础开挖及施工过程中要进行分段跳挖施工，确保边坡稳定；每隔10～
15m设置一道伸缩缝，并做好伸缩缝和泄水孔。

挡土墙一般与其他坡面防护结合使用，挡土墙以上坡面若高度较底就用自然土坡，土坡的坡度在1：1.5～1：1.75之间；挡土墙以上坡面高度大于6m时采用片石骨架护坡，骨架内种植草皮，防止雨水冲刷，保护坡面稳定；当路堑邻近有水塘时采用干砌片石护坡，干砌片石下面设碎石垫层，并做好排水措施，防止膨胀土经水浸泡发生变形影响边坡稳定。

3.3 土工格栅护坡
土工合成物在国内外作为路基加固等方面应用很广泛，并可与土、石、混凝土等结合，覆盖于坡面或河底，构成抗冲刷护坡。

在长荆铁路建设中，我们在部分区段利用土工格栅对边坡进行加固处理，首先按照设计要求沿边坡挖去2m厚度，然后每50cm一层加设土工格栅进行压实处理，最后将坡面按照要求坡度刷坡进行喷播植草，减少坡面雨水冲刷。

此种方法能够使膨胀土边坡坡率降低，从而节约土地的占用，并可以有效地增加边坡的稳定性。

3.4 膨胀土掺石灰固化处理
近年来，在铁路和水利建设中曾运用石灰、水泥等材料进行化学固化，取得了许多经验。

石灰固化作用是由于盐基交换、胶结性、粘土颗粒与石灰的相互作用而显现出来；掺石灰的主要作用是使膨胀土的液限量降低，增大强度。

我们在长荆铁路施工中对部分边坡区段进行了试验，防护效果很好。

首先按照设计坡度要求沿边坡挖去1m，然后将土粉碎后根据以往经验按6%～8%比例加入石灰拌匀每30cm一层进行压实处理，在堑顶将覆盖面延伸天沟，这样就在膨胀土表面形成了灰土封面，它有效地降低了膨胀土的膨胀势，提高了强度，具有良好的抗冲蚀能力，增强了边坡的稳定性。

4 结语
铁路膨胀土路堑边坡的防护与处理工程措施多种多样，关键是要把握膨胀土的特性，合理地利用当地的资源，因地制宜采取有效的工程措施予以实施。

就以上所述几种防护措施而言各有利弊，支挡结构防护对于膨胀土边坡防护效果比较明显，能有效地控制边坡的稳定性，但是其材料投入大，适宜于石材比较丰富的地区采用；土工格栅与石灰固化防护能够有效地降低膨胀土的坡率，节约土地，并改良膨胀土
边坡的稳定性，增加边坡的抗冲刷能力，但其施工受天气影响比较严重，必须抓住有利时机快速施工。

另外，铁路膨胀土路堑边坡防护与处理过程中要仔细观察、核实地质情况，注意地质的变化，防止由于路堑边坡是膨胀土，而其附近后面是别的土层（风化岩层或其它坚硬性土质），这样容易在分界处产生裂缝，导致雨水渗入使膨胀土边坡发生变形，从而使边坡失稳滑坡，影响铁路施工安全和为以后运输安全留下隐患。

遇有此种情况要及时处理好分界面，防止产生裂缝导致雨水灌入产生危害。

Expansive Soil Slope Protection treatment
1 Introduction
Expansive Soil Slope Protection treatment is directly related to the safety of railway construction and transport, is a key project measures. The railway has been built in accordance with domestic expansive soil slope cutting experience, expansive soil slope to prevent rain water should first be washed directly, but should reduce the intensity of rain water into the soil caused a substantial attenuation. Therefore, the treatment of expansive soil slope should be "prevention" in the full grasp of expansive soil and development of landslides on the basis of engineering geological conditions, to take effective and reliable engineering measures to ensure the stability of cut slope to stop landslides. Present on the expansive soil slope protection and treatment measures can be summarized as the main surface water protection, slope protection and the protection of retaining the three. Specifically, the following basic principles:
(1) strictly according to the design step by step from the top down slope of the cut slope excavation, excavation of non-scouring the bottom. In case of weak soil structure, surface development, easy-to-strong weathered soil, to rapid construction, and completion of excavation cut slope protection and treatment immediately.
(2) provision of drainage systems. Produced water is swelling and shrinkage of expansive soil deformation and reduce the intensity of the
important factors, so expansive soil slope during the excavation and protection of surface water drainage and other measures to set the cut-off row slope water flow, reduce surface water infiltration of soil body and slope erosion.
(3) a reasonable stack dump. Cutting Excavation spoil pile in the cutting should be avoided at the top of the hill, digging out the general lines should be 2 ~ 5m, because cutting the top of the slope of the load will increase the bulldozers and lateral stress, as well as cutting the top produced a large tensile stress and cracking, opened cracks in the rain infiltration along the cut slope, leading to reduced soil strength.
(4) landslides have occurred on the cutting slope, should be based on the depth and scope of the landslide, location, condition and degree of harm by force to take timely measures to deal with.
(5) If the soil cut slope has entered the place but the overall growth rate of slip deformation stage, it is timely to take, such as load shedding, slowing down the slope, the use of retaining structures and other measures to prevent landslides.
(6) using rubble slope protection, slope protection and other protective measures skeleton good drainage measures to facilitate the rapid slope to drain away water to prevent water infiltration of soil, causing damage. Expansive Soil Slope protection should be based on project size and level, the slope of the stress distribution to different parts of the options, we Jing
railway construction in the long expansive soil slope of taking various forms of protection and treatment, for the future treatment of expansive soil slope protection and the accumulation of data, provide a reference.
2 Project Overview
Chang Jing Han Dan rail line east from the Yangtze River port station, the city should be the days of Solomon, Zaoshi, Jingshan, Zhongxiang to Jingmen Jiaoliu line station, operating full 176.92Km, I take second bid of DK147 +000 ~ DK156 +000 section construction tasks, the line level is: national railway grade Ⅱ, in-line, limiting slope: 6 ‰.
DK147 +000 ~ DK156 +000 Zhongxiang City section of cold water in Jingmen town, hilly terrain, ridge Kong, about 100m above sea level, the line through the area with hills, slopes mainly between reservoirs, rice fields; this paragraph to expansive soil soil-based, basic Capacity 150 ~ 200Kpa, between bedrock is Silurian shale, grayish yellow, gray, weathered extremely serious, the basic capacity from top to bottom for the 120 ~ 400Kpa.
Range of subgrade earthwork 734064m3, which fill 328682 m3, rock fill 7313 m3, excavation 390956 m3, digging rock 7213 m3, the largest part of deepening the cut within 20m, using cutting slope retaining walls, slope protection framework, rubble slope protection , geogrid slope protection works and other forms of protective measures.
3 Slope Protection treatment
We are in the expansive soil slope construction, according to soften the
water swelling of expansive soil, water loss shrinkage cracking, deformation and strength of recurrent decay and structure of surface features such occurrence, the ground cross slope, cut slope height and precipitation to determine the excavation method and slope protection and treatment measures are to expansive soil slope excavation and protective treatment are summarized as follows:
3.1 The road cutting
According to the characteristics of expansive soil in the excavation drainage measures to do well against the first excavation in cutting the top gutter, and the lawn, digging roots, cleared the table work, in order to facilitate the smooth surface water to drain away, and then release cutting excavation line, according to the construction section of the breadth and depth of cut features of the larger, longitudinal channels dug by law, Mechanized Rapid Construction; road cutting, the first excavation along the vertical stratification cut out a vertical channel, and then both sides of the excavation, in accordance with the design layer by layer excavation slope, cut slope excavation is not to design a line along the slope of 30 ~ 50cm thick layer of reserve, to be finished digging when cutting, and then slashing the reserve part of the slope, and slope protection immediately to prevent the collapse occurred at the end of its intensity is too slippery.
3.2 Structure of protective gear
In order to prevent the collapse of slope instability, to ensure slope
stability, retaining wall basis with down slope. According to a high degree of slope and soil pressure to choose the height and thickness of the retaining wall to take advantage of passive earth pressure to reduce the overturning moment, increasing the stability of retaining wall, retaining wall basis of the following sliding surface depth of about 1.0m, the wall behind the set filter layer to facilitate drainage, retaining walls in the foundation excavation and construction process to segment jump to dig construction, to ensure slope stability; every 10 ~ 15m to set an expansion joint, and make joints and draining holes. Retaining walls generally used in combination with other slope protection, retaining wall height above the slope, if the end to use of more natural slope, the slope of the slope between the 1:1.5 ~ 1:1.75; retaining wall height above the slope greater than 6m slope used rubble skeleton, skeleton grass planted to prevent water erosion, protection of slope stability; when cutting close to a pond when the puzzle pieces by dry stone slope protection, dry stone puzzle piece located below gravel bedding, and make drainage measures to prevent the occurrence of expansive soil soaked by the water deformation of slope stability.
3.3 Geogrid Slope
Geosynthetic embankment reinforcement at home and abroad such as the application is very broad, and with the soil, stone, concrete and other combination of covered in the slope or river, constitutes anti-erosion slope. Jing railway construction in the long, we in some sections of the slope using
geogrid reinforcement, the first in accordance with the design requirements for dredged along the slope of the thickness of 2m, and 50cm for each additional geogrid layer compaction, at last, the slope gradient brush, as required for seeding grass slope to reduce slope rainfall. This method can make rates lower slope of expansive soil slope, thus saving land occupation, and can effectively increase the slope stability.
3.4 Lime curing of expansive soil
In recent years, railway and water conservancy construction was the use of lime, cement and other materials, chemical curing, made a lot of experience. Lime curing effect is due to base exchange, cementation of clay particles and the interaction of lime apparent; Lime's main role is to reduce the swelling soil liquid limit and increase strength. Jing We are a long slope on the part of railway construction in the section tested, protective works well. First, in accordance with the requirements of the design along the slope gradient of dredged 1m, and then based on past experiences of soil crushed 6% to 8% by adding lime mix ratio for each 30cm layer of compacted treatment, cutting the top of the gutter extension of the coverage, so formed on the surface of expansive soil dust cover, which effectively reduces the swelling of expansive soils and improve the strength, with good anti-erosion ability to enhance the stability of the slope.
4 Conclusion
Expansive Soil Slope protection and treatment of a variety of
engineering measures, the key is to grasp the characteristics of expansive soils, rational use of local resources, local conditions to take effective measures to implement the project. To the above advantages and disadvantages in terms of several protective measures, protection for the retaining structure protective effect of expansive soil slope is more obvious and can effectively control the stability of the slope, but the material into large, relatively rich in suitable stone area used; protection geogrids and lime can reduce curing of expansive soil slope rate, saving land, and improve the expansive soil slope stability and increase the slope of the anti-erosion ability, but its construction affected by the weather more severe, must seize the favorable opportunity to speed construction.
In addition, the Railway Protection expansive soil slope and carefully observe the treatment process to verify the geological conditions, pay attention to geological changes, prevent expansive soil cut slope is, and is followed by another near the soil (weathering of rocks or other hard of soil), so that the boundary between the cracks easily, resulting in rain water into the expansive soil slope deformation, so that the landslide slope instability, the construction of the railway transport safety and security for the future to stay hidden. In case such a situation to deal with the interface in time to prevent rain water poured into the cracks caused harm.。