风化与土weathering and soil

13 风化与土weathering and soil(中文3900字,英文1900字)
地表状况
地层剖面最上面数米深处一般包括：土、坡积土和风化岩石。其工程性质不同于最底层的基岩。
土 ：是矿物碎屑和植物风化后的混合体，通常小于1米厚，通常被分为上层腐殖质和富含粘土的下层土。
风 化：岩石、坡积土与空气和水接触后，被自然侵蚀和分解的过程。风化深度一般小于10米。
坡积土：泥沙经过运输、沉积在基岩表面上；大多数非固结粘土、沙和粗碎屑碎片;一般属于第四纪，因时间太短不能固结；厚度在0到大于50米之间。

崩积层：边坡碎片，移动的滑坡体主要由重力引起；因此输送泥沙的程度：滑移体>崩积层>土，包括由蠕变和片状冲积层产生的碎片，岩屑堆。丧失植被和地表水增加产生的片状冲积层。

基覆面：被埋坡积土与岩石的界面；普遍存在于软土、坡积土和坚硬岩石之间的显著边界；不容易在风化岩石深剖面作界定；若在坡积土沉积之前形成的侵蚀面，形成的地形可能跟现在的表面完全无关。

工程土：不用劈裂或爆破即可开挖的软弱材料，因此包括：土、坡积土、软岩和弱风化岩石。

13  weathering and soil
GROUND CONDITIONS
Top few metres of the ground profile generally consist of soil ,drift and weathered rock,withengineering properties very different from those of the underlying bedrock.
Soil: mixture of weathered mineral debris and plant material,
usually<1 m thick;many divide into plant-rich topsoil and clay-rich subsoil.
Weathering：the natural decay and break down of rock or drift in contact with air and water;generally < 10m deep.

Drift transported,superficial sediment deposited on top of the bedrock; mostly  unconsolidated clay,sand and coarser clastic debris; generally Quaternary age ,
hence too yang to be consolidated;varies in thickness from 0 to >50m.
Colluvium:slope debris,
moved downslope largely by gravity alone;extent of sediment transport therefore drift>colluvium>soil;
includes debris from creep and sheetwash,also head and
scree.Sheetwash by surface water increases greatly with loss of vegetation.
Rockhead:the buried drift/rock interface;commonly a conspicuous boundary between weak soils and drift and strong rock;may be less well defined in deep profile of weathered rock;formed as erosion surface before drift deposition so its topography may be totally unrelated to modern surface.
Engineering soil:weak material which can be excavated without ripping or blasting,therefore including soil,drift,weak rocks and weathered rock.