花岗岩风化岩土体特性的地带性分异及崩岗形成机理研究

A special landform is formed and named as collapsing gully or Benggang in granite areas in tropical and subtropical China. Benggang erosion causes very large soil loss and causes serious adverse effects. Because this erosion is initiated by many factors that interact with each other and this landform is hardly distributed in other countries in the world, the formation mechanisms of Benggang are not yet fully investigated and prevention strategies are not fully developed either. This project is based on the zonal soil development theory and zonal difference of rock-soil phenomenon to investigate the Benggang development properties and its relationship with external environmental factors in five latitude zones. The soil resistance to erosion will be investigated in the zonal Yellow-brown soil, Brown-red soil, Red soil, and Lateritic Red soil, leaning soil depth, physical and chemical properties, extent of hillslope erosion and their relationships with Benggang formation. The granite weathering body underneath the soil will be investigated for the zonal difference of their depth, the skeleton and cemented materials and their combination, and the mechanical stability, which will help to demonstrate the relationship between zonal distribution of weather body and Benggang. The typical landform and hydrology properties in the sample zones will be inverstigated together, discriminating fundamental substance and external environmental factors threshold in Benggang formation. In general, the project is designed to investigate the relationship of different factors and their contribution to Benggang erosion based on rock-soil zonal difference, and also is to develop the theory and the prediction system of Benggang formation mechanisms.

我国热带亚热带花岗岩分布区形成了特殊的崩岗地貌，崩岗区土壤侵蚀强烈，危害极大。崩岗侵蚀影响因素多且作用机制复杂，国际上同类地貌分布少，研究报道不多，其形成机理尚未充分揭示，缺乏有效的防治措施。本项目以我国5个纬度的典型崩岗分布区为样区，基于地带性和土壤形成理论，调查研究崩岗发育状态及其与区域环境要素关系；研究花岗岩发育的黄棕壤、棕红壤、红壤、赤红壤等地带性土壤的抗侵蚀特征，探明土层厚度、理化性质、坡面侵蚀强度等与风化岩体稳定性相关特征的地带性分异；研究花岗岩风化体的风化程度、分层厚度、颗粒和胶结物质组成与结合方式、力学稳定性及其影响因素等地带性差异，揭示风化体稳定的地带性差异与崩岗的关系；结合典型样区的地貌和水文等外部自然特征，辨析崩岗形成的物质基础和外部环境要素阈值。研究以岩土体特性的地带性差异为突破口，理清崩岗侵蚀多种影响因素的关系，阐明崩岗形成机制，开发崩岗形成演化模拟与预测系统。

崩岗是我国南方花岗岩分布区水土流失的重要来源。崩岗侵蚀过程复杂，其形成机理有待进一步研究。本项目的主要研究内容为崩岗分布规律及其环境驱动机制、花岗岩风化成土过程的地带性变化及其对土壤抗蚀性与力学稳定性的影响影响机制、崩岗发育过程对花岗岩风化岩土体稳定性的响应、崩岗发生与演化过程的模拟与预测。通过野外调查、原位监测及室内控制和模拟试验，发现崩岗集中分布于花岗岩风化壳为5.49 ~56.79m，年均气温、降雨侵蚀力、降雨量分别为18.9 ~ 21.2 ℃、5967~15676 MJ·mm·(ha·h·a)-1及> 1701 mm并有一定植被覆盖（FVC > 65%，EVI< 0.61）的低山丘陵区（海拔180 ~ 788 m，坡度15 ~ 35°，坡长0 ~ 50 m）。崩岗侵蚀强度（4.93×104 ~ 6.02×104 t /(km2. yr)）与崩岗形态显著相关（r>0.42, p<0.05），其中崩沿线周长与崩壁高度的积分对崩岗侵蚀强度的贡献最大（r=0.75）；崩岗密度与侵蚀强度与风化壳厚度由北往南增加地带性变化规律一致，而土壤层厚度与崩岗侵蚀强度具有显著的负相关关系（r=-0.63, p<0.05）；崩岗形成的环境驱动力具有明显的空间尺度效应，多年平均降雨侵蚀力和多年平均降雨量是崩岗区域分异规律的主要驱动力，而高程是小流域尺度下崩岗形成的关键；土壤中1:1型黏土矿物、胶结物质的含量及团聚体稳定性由北往南增加的规律与花岗岩风化程度的地带性变化一致，颗粒组成是土壤团聚体稳定性及坡面侵蚀过程的决定因素，而风化岩土体抗蚀性、结构及力学稳定性的剖面分异性是崩岗形成的关键；花岗岩风化岩土体淋溶层和淀积层的结构及力学稳定性对水分变化敏感，当土壤含水率为29.51 ~ 45.42 cm3/cm3时土体处于临界稳定状态，且该层次的脱水收缩促进了径流下切和崩岗快速发育；位于1.3 ~ 34.4m且以粉砂为主的母质层抗蚀性和力学稳定性最低，该区域的非饱和抗剪强度是决定崩岗发育速度的关键；基于花岗岩风化岩土力学稳定性的非线性变化，建立了崩岗应力应变预测方程，基于崩岗形成的关键影响因子，构建了崩岗风险预测模型，综合崩岗发育与侵蚀机制，开发了崩岗发生演化模拟系统。