花崗巖剖面土壤崩解特性與初始含水率的關(guān)系

劉丹露,趙媛,2,丁樹(shù)文,鄧羽松,王秋霞,呂國(guó)安?

(1.華中農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院,430070,武漢;2.廣西壯族自治區(qū)亞熱帶作物研究所,530004,南寧)

花崗巖剖面土壤崩解特性與初始含水率的關(guān)系

劉丹露1,趙媛1,2,丁樹(shù)文1,鄧羽松1,王秋霞1,呂國(guó)安1?

(1.華中農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院,430070,武漢;2.廣西壯族自治區(qū)亞熱帶作物研究所,530004,南寧)

土壤的崩解特性是南方紅土區(qū)土壤侵蝕的重要指標(biāo),也是開(kāi)展水土保持工作的重要依據(jù)。作者選擇花崗巖發(fā)育的2個(gè)土壤剖面,分別采集淋溶層、淀積層、過(guò)渡層和母質(zhì)層原狀土壤樣品。利用崩解儀進(jìn)行崩解實(shí)驗(yàn),測(cè)定剖面不同層次土壤崩解特性與初始含水率的關(guān)系。結(jié)果表明:在不同的初始含水率條件下,土壤崩解特性均表現(xiàn)為淋溶層和淀積層土壤崩解緩慢,過(guò)渡層和母質(zhì)層崩解迅速;初始含水率的遞變,對(duì)淋溶層和淀積層土壤達(dá)到崩解完全所需用時(shí)影響較大,對(duì)過(guò)渡層和母質(zhì)層土壤影響較小;隨著初始含水率的減小,淋溶層和淀積層的最大崩解量逐漸增加,風(fēng)干狀態(tài)時(shí)可趨于完全崩解;過(guò)渡層和母質(zhì)層均可全部達(dá)到完全崩解,初始含水率對(duì)其影響較小。研究結(jié)果可為治理南方崩崗水土流失提供依據(jù),也為探索崩崗的發(fā)生機(jī)制提供參考。

花崗巖剖面;崩解特性;初始含水率;崩崗

土壤崩解是土體由于浸水而發(fā)生結(jié)構(gòu)分散的現(xiàn)象,是不可逆的過(guò)程[1]。曲永新等[2]將軟巖的崩解分為泥狀崩解、碎片泥狀、碎片狀和完整不崩解4種類型。鄭敏洲等[3]對(duì)花崗巖殘積土崩解性進(jìn)行室內(nèi)崩解實(shí)驗(yàn)時(shí)發(fā)現(xiàn):當(dāng)土體含水量越大時(shí),崩解速度越快;對(duì)不同顆粒組成的土樣進(jìn)行試驗(yàn),發(fā)現(xiàn)粗顆粒質(zhì)量越多的土體崩解速率越快,崩解量越大;花崗巖殘積土的崩解速度大致分為慢速崩解、快速崩解、再次趨于慢速崩解3個(gè)階段。曾鵬[4]將花崗巖殘積體制成不同含水量、不同壓實(shí)度的擾動(dòng)土樣,進(jìn)行崩解實(shí)驗(yàn)時(shí)發(fā)現(xiàn):隨壓實(shí)度增大,孔隙、裂隙和滲透性逐漸減小,土體的崩解性也發(fā)生相對(duì)減弱;當(dāng)含水量越大時(shí),崩解的速度(完全崩解平均速度)也越快。顏波等[5]通過(guò)對(duì)花崗巖風(fēng)化土崩解特性的研究,認(rèn)為花崗巖風(fēng)化土在雨水作用下能夠發(fā)生崩解,致使土體結(jié)構(gòu)遭到破壞;因此,水在崩解中扮演著重要的角色。從初始含水率的角度,對(duì)花崗巖剖面土壤崩解特性的研究較少,所以本實(shí)驗(yàn)設(shè)置6個(gè)不同的初始含水率,利用崩解儀探究花崗巖剖面土壤的崩解特性。

花崗巖剖面土壤的崩解特性與花崗巖特有的崩崗現(xiàn)象聯(lián)系緊密,同時(shí)龕又為崩崗形成和發(fā)育的重要階段[6-9];所以,研究不同初始含水率下剖面土壤的崩解特性,與崩崗龕形成的關(guān)系,對(duì)于防治崩崗侵蝕及開(kāi)展水土保持工程項(xiàng)目具有一定的科學(xué)意義。

1 研究區(qū)概況

湖北省通城縣地跨E 113°36＇～114°4＇,N 29°2＇～29°24＇,位于鄂、湘、贛3省交界處。該區(qū)域?qū)賮啛釒Ъ撅L(fēng)氣候,年均溫16.2℃,最低氣溫為-15.2℃,最高氣溫為39.7℃。年平均降水量1 550 mm,徑流總量9.08億m3。土壤類型以花崗巖發(fā)育的紅壤為主,結(jié)構(gòu)較為松散。地帶性植物以常綠闊葉與落葉闊葉混交林、亞熱帶針葉林為主。通城縣是湖北省有崩崗現(xiàn)象的縣市中,數(shù)量最多、分布最集中的典型地區(qū),也是全國(guó)崩崗防治的重點(diǎn)縣。通城縣的崩崗主要發(fā)生在低丘的花崗巖風(fēng)化殼上,有完整的崩崗剖面發(fā)生層次,具有很強(qiáng)的代表性。

2 材料與方法

2.1分層與采樣

本實(shí)驗(yàn)選取位于通城縣五里鎮(zhèn)的具有南方花崗巖崩崗典型特征的五里崩崗BG1(E 113°46＇28″, N 29°19＇55″)和程鳳崩崗BG2(E 113°46＇30″,N 29° 20＇3″)為研究對(duì)象,進(jìn)行重復(fù)實(shí)驗(yàn)。依據(jù)花崗巖風(fēng)化殼的顏色、質(zhì)地、植物根系等自上而下將剖面土壤劃分為4個(gè)層次,分別為淋溶層(A層)、淀積層(B層)、過(guò)渡層(BC層)和母質(zhì)層(C層),并逐一進(jìn)行采樣。崩崗剖面土壤的基本性質(zhì)見(jiàn)表1。

每層土壤通過(guò)控制不同的風(fēng)干時(shí)間,依次設(shè)置從風(fēng)干到飽和逐漸遞變的6種初始含水率。每一種初始含水率條件下采集3個(gè)土樣,其中取2個(gè)土樣運(yùn)用崩解儀測(cè)定崩解量,1個(gè)土樣用于測(cè)定實(shí)際水分含量。每層土壤采集18個(gè)土樣,所以2個(gè)崩崗一共采集144個(gè)原狀土樣;并且依據(jù)實(shí)驗(yàn)結(jié)果,分別繪制出不同層次剖面土壤崩解量與初始含水率的關(guān)系圖。

表1　崩崗剖面土壤基本性質(zhì)Tab.1 Basic properties of the soil profiles of collapsing gullies

2.2實(shí)驗(yàn)方法

通過(guò)崩解指標(biāo)來(lái)反映土體的崩解特性,常用的崩解量化指標(biāo)為崩解量和崩解速率等。筆者選用崩解量來(lái)反映土樣的崩解特性。

本實(shí)驗(yàn)運(yùn)用崩解儀測(cè)定崩解量。依據(jù)土力學(xué)原理自制實(shí)驗(yàn)所需的崩解儀,其主要由3部分構(gòu)成:浮筒(250mL量筒);網(wǎng)板(10×10 cm2且孔徑1 cm2金屬方格網(wǎng));玻璃水槽(25 cm×25 cm×80 cm)。操作時(shí),首先將土樣放在網(wǎng)板中央,迅速地將土樣浸入裝有自來(lái)水的水槽中,記錄開(kāi)始時(shí)間及浮筒齊水面處刻度的瞬間穩(wěn)定讀數(shù)。根據(jù)崩解的快慢,靈活記錄崩解過(guò)程中不同時(shí)間段內(nèi)浮筒齊水面處的刻度讀數(shù)。當(dāng)試樣完全通過(guò)網(wǎng)格落下,或浮筒齊水面處的刻度讀數(shù)不再有變化時(shí),記下當(dāng)前時(shí)間及刻度讀數(shù),結(jié)束實(shí)驗(yàn)。

張抒等[10]對(duì)崩解量的計(jì)算公式進(jìn)行修正:

式中:At為試樣在時(shí)間t時(shí)的崩解率,%;R0為試驗(yàn)開(kāi)始時(shí)浮筒齊水面處刻度的瞬間穩(wěn)定讀數(shù),cm;Rt為試樣在時(shí)間t時(shí)浮筒齊水面處刻度讀數(shù),cm;Re為完全崩解后浮筒齊水面處刻度讀數(shù),cm。

通過(guò)式(1)得到不同初始含水率下花崗巖剖面土壤的崩解特性值,并繪制出不同初始含水率下的崩解折線圖。

3 結(jié)果與分析

3.1淋溶層土壤崩解量與初始含水率

淋溶層的崩解性較弱,受初始含水率的影響較大(圖1)。當(dāng)初始含水率較大時(shí),崩解量較小,但當(dāng)初始含水率減小至一定值后,土體在水中的崩解量迅速增加。對(duì)設(shè)定的6個(gè)不同初始含水率下的崩解量進(jìn)行比較發(fā)現(xiàn):當(dāng)BG1在含水率＞12.89%時(shí),崩解量很小,當(dāng)初始含水率下降為6.25%時(shí),BG1的崩解量發(fā)生明顯增加,穩(wěn)定時(shí)的崩解量達(dá)到50%;同樣,BG2在初始含水率＜9.90%時(shí),崩解量發(fā)生明顯增加。當(dāng)初始含水率減小至4.69%時(shí),BG2達(dá)到風(fēng)干狀態(tài),此時(shí)崩解量為12.32%,是飽和含水狀態(tài)下崩解量的6.06倍。

3.2淀積層土壤崩解量與初始含水率

淀積層的崩解情況相似于淋溶層,崩解量與初始含水率成反比(圖2)。BG1在初始含水率為35.59%時(shí),崩解量?jī)H為4.35%;當(dāng)初始含水率降低到2.89%時(shí),崩解量則增至26.32%。同樣BG2在初始含水率為31.93%時(shí),崩解量?jī)H為2.78%;但當(dāng)初始含水率降低到2.20%時(shí),崩解量則能夠達(dá)到100%。總體來(lái)說(shuō),BG1、BG2淀積層的崩解量隨著初始含水率的增加,崩解量逐漸減小;但除初始含水率接近風(fēng)干狀態(tài)時(shí)的崩解量較大外,淀積層在各個(gè)含水率下的崩解量都較小,均不超過(guò)30%。

圖1　BG1、BG2淋溶層在不同初始含水率下的崩解量Fig.1 Disintegration amount of eluvial horizon in different initial soilmoisture at BG1 and BG2

圖2　BG1、BG2淀積層在不同初始含水率下的崩解量Fig.2 Disintegration amount of illuvial horizon in different initial soilmoisture at BG1 and BG2

3.3過(guò)渡層土壤崩解量與初始含水率

由圖3可知,過(guò)渡層遇水后在較短時(shí)間內(nèi)便可以達(dá)到完全崩解;但是由于初始含水率的不同,過(guò)渡層達(dá)到完全崩解狀態(tài)所需的時(shí)間也不同。在6種初始含水率條件下,BG1的過(guò)渡層土體浸水后均在150 s內(nèi)完全崩解,BG2的過(guò)渡層土體則在250 s內(nèi)完全崩解。BG1、BG2達(dá)到完全崩解所需最短時(shí)間都為20 s。當(dāng)BG1、BG2的初始含水率為31.73%和25.81%時(shí),此時(shí)的土樣均已經(jīng)接近飽和狀態(tài),崩解完全達(dá)到第3階段所用時(shí)間都最長(zhǎng)。

3.4母質(zhì)層土壤崩解量與初始含水率

母質(zhì)層的崩解狀況相似于過(guò)渡層,但母質(zhì)層受初始含水率的影響最小(圖4)。除飽和含水率處理的情況之外,其他初始含水率條件下的土體都在40 s內(nèi)達(dá)到完全崩解。當(dāng)初始含水率達(dá)到風(fēng)干狀態(tài)時(shí),其崩解速率最快,極短時(shí)間便可使崩解量達(dá)到100%。初始含水率為1.27%時(shí),BG1在10s內(nèi)可達(dá)到第3階段;初始含水率為1.40%時(shí),BG2完全崩解也只需15 s。

4 討論

由4層土壤的崩解曲線圖,可將崩解過(guò)程分為緩慢崩解、快速崩解、穩(wěn)定階段[3]3個(gè)階段。第1階段耗時(shí)較短,且從淋溶層到母質(zhì)層所需時(shí)間依次大幅遞減;第2階段占整個(gè)崩解實(shí)驗(yàn)的主體,所需時(shí)間近似與初始含水率成正比,實(shí)驗(yàn)進(jìn)行中可明顯觀察到有較大土塊從原土體崩落;第3階段即為實(shí)驗(yàn)結(jié)束。分析3個(gè)階段的崩解過(guò)程,4層土壤的崩解特性差異與其土體結(jié)構(gòu)的物化基本性質(zhì)密切相關(guān)[12]。

圖3　BG1、BG2過(guò)渡層在不同初始含水率下的崩解量Fig.3 Disintegration amount of transitional horizon in different initial soilmoisture at BG1 and BG2

圖4　BG1、BG2母質(zhì)層在不同初始含水率下的崩解量Fig.4 Disintegration amount of parent horizon in different initial soilmoisture at BG1 and BG2

淋溶層砂粒質(zhì)量分?jǐn)?shù)較大,BG1和BG2分別為36.28%、38.28%;有機(jī)質(zhì)質(zhì)量分?jǐn)?shù)也較高,分別為17.32和19.50 g/kg;土壤總孔隙度分別為51.70%和53.21%:所以淋溶層的結(jié)構(gòu)較松散,呈團(tuán)粒結(jié)構(gòu)。淀積層的崩解量小于淋溶層,是因?yàn)榈矸e層的黏粒質(zhì)量分?jǐn)?shù)較大,分別為40.73%和38.85%。淋溶層的覆蓋使得淀積層結(jié)構(gòu)更為緊實(shí),總孔隙度為4層當(dāng)中最小,分別為47.55%、46.79%。淋溶層土體浸水時(shí),水迅速浸入大孔隙,土體內(nèi)原有空氣無(wú)法排出,空氣張力增大使得土體的黏聚力減小。當(dāng)張力大于黏聚力,土體向外剝落,水的楔力作用使土體沿裂隙崩落[9]。同時(shí)淋溶層中的膠結(jié)物在水中發(fā)生溶解,使其有少量土粒散落。崩解釋放被擠壓的氣體,土體內(nèi)部應(yīng)力逐漸穩(wěn)定,崩解速度也逐漸變緩[10-13],趨于第3階段。淀積層在崩解的第1階段,空氣被擠壓而產(chǎn)生的張力對(duì)土體影響較小,水分入滲相對(duì)均勻,所以土體崩解的程度較小;第2階段時(shí),當(dāng)初始含水率＜3%,表面才產(chǎn)生細(xì)小的裂隙,使外部土體崩落;部分膠結(jié)物溶解使崩解趨于第3階段。BG2淀積層的崩解狀況有所差異,與其土體本身性質(zhì)不均一有關(guān)[14]。土體中有紅、黃斑塊,使其在風(fēng)干時(shí)收縮不一,發(fā)育有較大的裂隙,所以當(dāng)水分由裂隙浸入土體,土塊易發(fā)生大量崩塌[15];但是,總體來(lái)說(shuō)BG1、BG2淀積層的崩解量小于淋溶層。

過(guò)渡層和母質(zhì)層遇水后短時(shí)間內(nèi)崩解量可達(dá)100%,其崩解完成速度較淋溶層、淀積層快了約30倍。過(guò)渡層的黏粒質(zhì)量分?jǐn)?shù)較前兩層相比,下降至16.53%、8.06%,并且其黏聚力幾乎為0,而且土壤總孔隙度較淀積層比分別增加至49.43%、49.06%。母質(zhì)層的黏粒質(zhì)量分?jǐn)?shù)分別只占3.68%、4.04%,而且顆粒較粗,孔隙度最大,分別為54.34%,55.47%;所以母質(zhì)層結(jié)構(gòu)較脆弱,基本上為松散堆砌。過(guò)渡層和母質(zhì)層一旦遇水,可迅速軟化崩解。初始含水率接近土壤飽和狀態(tài)時(shí),水分浸入土體速率發(fā)生減慢,所以出現(xiàn)短暫的穩(wěn)定期[4];但隨著水的逐步浸入,土體受到外力作用,其結(jié)構(gòu)便瞬間遭到破壞[3]。土體初始含水率越低,水越容易進(jìn)入土體發(fā)生崩解;所以當(dāng)初始含水率為非飽和狀態(tài)時(shí),土體浸入水中,水分快速浸入土體,崩解迅速達(dá)到第3階段[16-18]。過(guò)渡層和母質(zhì)層遇水軟化性極強(qiáng),無(wú)論是提供可侵蝕的物質(zhì)量,還是崩解的強(qiáng)度都大幅高于淋溶層和淀積層,這也是崩崗龕出現(xiàn)在這2個(gè)層次的關(guān)鍵;因此,只要水分能夠進(jìn)入花崗巖崩崗的下層土體,逐漸侵蝕形成龕后,崩崗侵蝕的速度和侵蝕量就會(huì)大大增加。

5 結(jié)論

1)在6種初始含水率條件下,花崗巖土壤剖面的崩解特性均表現(xiàn)為淋溶層和淀積層土壤崩解速率較為緩慢,過(guò)渡層和母質(zhì)層崩解速率相對(duì)迅速。

2)在不同的初始含水率條件下,淋溶層和淀積層土壤崩解完全所需要的時(shí)間受其影響較大,過(guò)渡層和母質(zhì)層崩解完全所需要的時(shí)間受初始含水率的影響較小。

3)對(duì)4層土壤崩解特性曲線圖進(jìn)行比較,發(fā)現(xiàn)崩解量隨時(shí)間均呈增加趨勢(shì),而過(guò)渡層和母質(zhì)層的崩解變化更為明顯,近似于“S”形。當(dāng)初始含水率逐次遞減,淋溶層和淀積層的最大崩解量逐次增加,風(fēng)干狀態(tài)時(shí)接近完全崩解;但過(guò)渡層和母質(zhì)層受初始含水率影響較小,均可全部達(dá)到完全崩解。

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Correlation between initial soilmoisture content and the characteristics of collapsing gullies in granite regions

Liu Danlu1,Zhao Yuan1,2,Ding Shuwen1,Deng Yusong1,Wang Qiuxia1,Lyu Guoan1
(1.College of Resources and Environment,Huazhong Agricultural University,430070,Wuhan,China; 2.Guangxi Subtropical Crops Research Institute,530004,Nanning,China)

[Background]The soil disintegrating characteristics is an important indicator of the soil erosion of red soil in southern China,and also the critical basis for conducting the soil conservation. Tongcheng county is one of the typical granite regions in southeastern Hubei Province,with the largest quantity and the most centralized distribution of collapsing gullies.Samples selected in Tongcheng are representative in the subject.[M ethods]The influences of the correlations between the initial soil moisture contentand the disintegrating characteristics of different soil layers on the occurrencemechanism of the collapsing gullies were analyzed.Soil samples of two 3-meter-TVD(true vertical depth)soil profiles were collected in Tongcheng.According to different criteria such as color and texture of weathering crust of granite,the soil profiles can be divided into 4 different layers from top to bottom, including the eluvial horizon,illuvial horizon,transitional horizon,and parenthorizon.To each horizon, this study set6 experimental groups of 6 different levels of initial soilmoisture from dry to saturation,by controlling the length of air-drying duration,and using disintegration tester to conduct disintegrateexperimentwith the soil samples.[Results]The results proved that at different levels of the initial soil moisture content,soil of eluvial horizon and illuvial horizon disintegrated in a slow progress and the soil of transitional horizon and parent horizon did fast.The initial soilmoisture content had a considerable effect on the speed to complete the disintegration of the eluvial horizon and illuvial horizon,while little impact on the easily-disintegrated soil of the other two layers.Themaximum disintegration also was affected by the initial soil moisture content,the maximum disintegration of the eluvial and illuvial horizon significantly increased with the decrease of the initial moisture content,and the disintegration was approximately complete under air-dried condition.The disintegration of the transitional and parenthorizon was fully completed at any level of initial soilmoisture content,so the maximum disintegration was less affected by the initial soil moisture content.Drawing and analyzing the graphical sheets of the disintegration amount of different horizons in different soil moisture,the graphs showed that the disintegration amount presented an increasing trend with the time varying;the influence of disintegrating time on the disintegration amount of the transitional and parent horizon at different levels of initial soil moisture content appeared S curve.[Conclusions]Hence,it can be inferred that in a granite profile, the soil of the transitional horizon and parent horizon disintegrated more easily and formed the niche, which resulted in the head erosion of the collapsing gully.This provided a basis for further research on the collapsing of granite and laid a foundation for the study of the occurrencemechanism of the collapsing gullies.

granite soil profile;disintegration characteristics;initialmoisture content;collapsing gully

S157.1

A

1672-3007(2016)02-0017-06

10.16843/j.sswc.2016.02.003

2015-09-22

2016-02-20

項(xiàng)目名稱:國(guó)家自然科學(xué)基金“花崗巖紅壤優(yōu)先流及其與崩崗侵蝕發(fā)育的關(guān)系”(41571258);華中農(nóng)業(yè)大學(xué)國(guó)家級(jí)大學(xué)生創(chuàng)新創(chuàng)業(yè)訓(xùn)練計(jì)劃“花崗巖崩崗不同層次土壤可蝕性與抗沖性對(duì)龕形成的影響”(201510504021);國(guó)家科技支撐計(jì)劃“紅壤崩崗侵蝕區(qū)農(nóng)田質(zhì)量保護(hù)與崩崗治理技術(shù)與示范”(2011BAD31B04)

劉丹露(1994—),女,本科生。主要研究方向:水土保持。E-mail:liudanlu@webmail.hzau.edu.cn

簡(jiǎn)介:呂國(guó)安(1957—),男,教授,博士生導(dǎo)師。主要研究方向:農(nóng)業(yè)水資源利用與水土保持。E-mail:glu@mail. hzau.edu.cn