Haddii ciid isku xiran aan ka jarno dhululubo oo aan saarno cadaadis caadi ah P (sl. 1), dhululubada gudaheeda waxaa ka soo bixi doona culeysyo cusub, kuwaas oo ah culeyska toosan ee riixista σ iyo culeyska jiifka ee xiiridda . Haddii aan si tartiib tartiib ah u kordhino cadaadiska P, muddo ka dib dhululubada waxaa ka soo muuqan doona dillaacyo qalloocan, isla markiiba ka dibna dhululubadu way jabaysaa. Xilliga jabitaanka dhululubada waxaa lagu xiiraa saameynta cadaadiska P iyadoo la raacayo dusha xiiridda ee iska caabinta ugu yar N-N.
Saw. 1. Saameynta cadaadiska ku yeeshay dhululubada laga soo bixiyey ciid isku-xiran
Iska caabbinta ciidda ee ay dhululubadu muujisay marka ay ka hortagayso ficilka xoogga P xilliga jabitaanka waxaa la yiraahdaa iska caabbinta goynta ciidda, halka xagasha α ee janjeerka dusha iska caabbinta ugu yar marka loo eego jiifka loo yaqaan xagasha dusha iska caabbinta goynta ciidda ugu yar.
Iska caabbinta xiirista ee ciidda la isku xoojiyey waxay ka kooban tahay laba iska caabbis oo aasaasi ah: sinta gudaha ee u dhexeysa qaybaha adag iyo isku-xirnaanta (cohesion) ee isku haysa qaybaha adag. Adkaysiga xiirista ee ciidda waxaa loo qaataa inuu la mid yahay napniga xiirista ugu weyn marka jabku dhaco, waxaana lagu muujiyaa isla’egta
τf = c + σ tg_φ_
halka τf ay tahay xoogga goynta ciidda [kp/cm2],
c isku-xirnaanta ciidda [kp/cm3] ,
σ walbahaarka caadiga ah ee dhulka [kp/cm3],
φ waa xagasha gudaha ee is jiid-jiidka ciidda.
Qaacidada kore waxaa soo saaray Faransiiska Coulomb 1785. sanadka. Sida ku cad qaacidadan, ciid gaar ah dhexdeeda isku-xirnaanta c waa joogto oo ka madax bannaan cadaadiska toosan, halka iska-caabbinta khilaafka σ tg_ϕ_ ay si toos ah ula siman tahay walbahaarka σ. Xaqiiq ahaan isku-xirnaanta e ma aha mid joogto u ah ciid gaar ah, balse iyaduna ilaa xad bay ku xidhan tahay walbahaarka toosan σ, sababtoo ah saameynta cadaadiskaas dhumucda filimka biyaha ee daboolaya qaybaha adag way yaraataa, sidaasna isku-xirnaantu way kordhaa. Qaacidada Coulomb waxay khusaysaa ciidaha gudbiya biyaha, halkaas oo saameynta cadaadiska toosan biyaha daloollada looga saaro iyada oo aan iska-caabin jirin, taas oo ah xaaladda ciidda jayga ah iyo ciidda cammuudda ah, balse aan ahayn ciidda isku-xiran, oo gudbinteedu yar tahay, sababtaas awgeedna biyaha daloollada si dhib leh ayaa looga saaraya waxayna u hadhaan sidii biyo dalool oo walbahaarsan oo ka dhasha culayska P.
Cadaadisyada wax-ku-oolka ah iyo kuwa dhexdhexaadka ah ee ciidda
Saw. 2. Walbahaarka waxtarka leh iyo kan dhexdhexaadka ah ee carrada
If we place a soil sample, for example dry sand, at the bottom of a vessel and load it with a mass, for example lead shot weighing W, the solid particles of the soil will take up the entire load of this weight. The specific pressure on the surface of the sample p0 = W/A in kp/cm3, where A is the cross-sectional area of the sample, will cause settling of the sample, i.e. a reduction in its porosity, but it will also cause changes in other physical properties of the sample such as an increase in shear strength, an increase in the compressibility modulus, and others. The pressure acting on the contact surfaces between the particles is called effective or intergranular pressure σ’. When consolidation has been achieved, i.e. when the sample has stopped settling under the load p0, the effective pressure is equal to the total specific pressure prevailing at the depth where the soil is observed. In this case, at the depth below the sample surface in fig. 2 σ’ = W/A + hzγ where γ is the unit weight of the sample.
Laakiin haddii muunadda ku jirta weelka aan ku rarno biyo ilaa dhererka h, iyadoo ay buuxisay dhammaan godadka muunadda, cadaadiska tiirka biyaha ee ka sarreeya muunadda hγw, halka γw ay tahay culayska mugga ee biyaha, ma keeni doono degis muunadda iyo yaraanta daloolkeeda mana kordhin doono adkaysigeeda xiiridda iyo modulus-ka la tuujin karo. Sidaas awgeed cadaadiska hydrostatic-ka hγw waxaa loo yaqaan cadaadis dhexdhexaad. Cadaadiska dhexdhexaadka ah ee lagu calaamadiyo u wuxuu ku gudbaa muunadda dhamaan jihada isagoo leh xoog la mid ah. Cadaadiskani ma keeni doono kordhinta adkaysiga xiiridda, sababtoo ah biyuhu laftoodu wax adkaysi xiirid ah ma laha.
Sidaas awgeed, cadaadiska waxtarka leh σ’ waxaa lagu gudbiyaa iyada oo loo sii marayo dushooyinka taabashada ee u dhexeeya qaybaha adag ee ciidda, halka cadaadiska dhexdhexaadka ah u lagu gudbiyo biyaha ku jira godadka. Haddii qaybta hoose ee weelka lagu buuxiyo muunad ciid ah oo dhererkeeda miisaaneed uu yahay γz, halka korka dusha muunadda ilaa heerka N weelku ka buuxo biyo dherer miisaaneedkoodu yahay γw, markaas cadaadiska guud σ ee meel kasta oo muunadda ka mid ah wuxuu noqonayaa σ = σ’ + u.
Moolka hz, hoosta oogada muunadda, cadaadiska wax-ku-oolka ah wuxuu noqon doonaa σ = σ’ - u
halka σ = h γw + h γz ay tahay
u = (h + hz) γw.
Sidaas darteed waxa ay noqonaysaa σ’ = hz γ’
γ’ waa miisaanka mugga ee ciidda biyo ku hoos jirta.
Isla’egta guud ee xoogga xiirashada ciidda
Haddii aan ku saarno ciid isku xiran culays P, culayskani wuxuu keeni doonaa degista ciidda, waxaana marka hore saamayntiisa ka bixi doona hawada ku jirta daloollada, sidaa awgeed dhowaan daloollada waxa ku hadhi doona oo keliya biyo. Maadaama ciidda isku xiran ay leedahay biyo-maro yar, biyuhu isla markiiba ma bixi karaan, sidaa darteed waxay ku sii jiraan daloollada iyaga oo ah biyo cadaadis ku jira, markii hore waxayna qaadaan culayska dibadda oo dhan P. Waqtiga xiga, saameynta culayska P ayaa biyaha daloollada laga bilaabaa in ay ka baxaan, waxaana dhacda isu-dhowaan walxaha adag, kuwaas oo hadda qaata qayb ka mid ah culayska P. Markaas waxaan leenahay τf =c + (σ - u) tg_ϕ,_ halka σ yahay cadaadiska guud ee ka yimaadda culayska P, u yahay cadaadiska biyaha daloollada.
Cadaadiska biyaha daloolka u waxa uu ku xiran yahay dhowr arrimood, sida socodka waqtiga ee isku-duubnaanta, dhex-mareenka ciidda, baaxadda culayska ciidda iwm. Awoodda xiiridda ciiddu waxay ku xiran tahay cadaadiska biyaha daloolka, waxayna ugu weyn tahay marka u=0, marka isku-duubnaanta ciiddu dhacdo.
Firfircoonida kolloidal ee ciidda
Walxaha iska-caabbinta gudaha ee ciidda isku-xidhnaanta iyo sacabka way kala duwanaan karaan, waxayna si weyn ugu xiran yihiin weynaanta qaybo adag ee ciidda iyo sifooyinkeeda plastigga. Si loo ogaado saamaynta isku-xidhnaanta marka la barbar dhigo iska-caabbinta isjiididda gudaha ee ciid isku-xiran, Skempton wuxuu soo bandhigay fikradda firfircoonida colloidal ee ciidda _K_P, taas oo lagu muujiyo xiriirka u dhexeeya index-ka plastigga IP iyo qadarka qaybo ka yar 0,002 mm oo ah % miisaanka guud ee mass-ka. Markaas waxaa la qaatay halbeegga soo socda (sl. 3):
Jaantus. 3. Firfircoonida kolloidalka ee ciidda
KP < 0,75 waa dhoobo aan firfircoonayn,
za KP = 0,75 – 1,25 waa dhoobooyin caadi ah,
ee Kp > 1,25 dhoobada firfircoon.
Marka KP kordho, saameynta isku-xirnaanta marka loo eego khilaafka gudaha ee ciidda isku-xiran ayaa kordha.
Dareenka carrada
If in cohesive soil we take out a cylinder of diameter 3—3,4 cm and height ~ 5 cm and load it with a normal load, at the moment the cylinder fails we will obtain the compressive strength of the soil in its undisturbed state σ__ₙ:
σn = P/A oo ah kp/cm2,
halka A uu yahay iskutallaabta dhululubada.
Kadib marka tijaabada la dhammeeyo, muunadda jabtay waxaan u wareejinnaa oo mar kale ka samaynaa dhululubo isla cabbirrada leh, anagoo adeegsanayna isla caaryadii dhululubada, iyadoo qoyaankeedu ahaado kii ay ku jirtay tijaabadii hore. Kadib isla markiiba marka la sameeyo dhululubada la dib-u-cufay, tijaabada ayaa lagu celinayaa, si loo helo adkaanta cadaadiska ciidda ee xaaladda qasan σ__ₚ.
Dareenka (sensitivitet) S__ₜ waa saamiga xoogga cadaadiska ee xaaladda aan la carqaladayn iyo xoogga cadaadiska ee xaaladda la carqaladeeyey: St = σn/σp,
Sida ku cad hal shuruud, dareenka S__ₜ waxaa loo qiimeeyaa sidan:
- za S__ₜ = 1 — 2 waa dareen-celin hoose oo dhoobada ah,
- marka S__ₜ = 2 — 4 waa dareenka dhexe ee dhoobada,
- marka S__ₜ = 4 — 8 waxaa jira dareen weyn oo dhoobada ah,
- ee S__ₜ > 8 waa dhoobo aad u xasaasi ah.
Dabeecadda waxaa dhacda in saameyn farsamo awgeed, tusaale ahaan gariir, ciidda dhoobada ah ay si lama filaan ah u lumiso adkaysigeeda, ka dibna ay mar kale ku hesho marka ay xaalad deggan tahay. Sifadan waxaa lagu magacaabaa tiksotropiyada ciidda. Dhoobo badan ayaa ilaa heer gaar ah leh astaamo tiksotropi ah. Waxaa la yaqaan xaalado ay jiirada qodista u simbireen sababtoo ah dhoobo tiksotropi ah oo ka soo baxday Norway, Sweden iyo Finland, halkaas oo baaritaan lagu ogaaday in aanu jirin isbeddel ku yimid qoyaanka dhoobada inta hawshan socotay, taas oo ah in qoyaanka dhoobadu uu ahaa mid joogto ah ka hor iyo ka dib soo bixitaanka.