SOIL PHASE RELATIONSHIPS, INDEX PROPERTIES AND CLASSIFICATION
Case 4: When the soil is submerged
3.12 DETERMINATIO N O F ATTERBERG LIMIT S Liquid Limi t
The apparatu s shown in Fig. 3.11 is the Casagrande Liquid Limit Device use d for determining th e liquid limits of soils. Figure 3.12 shows a hand-operated liquid limit device. The device contains a brass cup which can be raised and allowed to fall on a hard rubber base by turning the handle. The cup is raised by one cm. The limits are determined on that portion of soil finer than a No. 40 sieve (ASTM Test Designation D-4318). About 100 g of soil is mixed thoroughly with distilled water into a uniform paste. A portion o f the past e i s placed i n th e cu p an d levele d t o a maximu m dept h o f 1 0 mm. A channel o f th e dimension s of 1 1 m m widt h and 8 mm dept h i s cu t throug h th e sampl e alon g th e
Brass cup Sample ^ ,
Liquid limit device Hard stee l
Casagrandes groovin g tool AST M groovin g tool
Figure 3.1 1 Casagrande' s liqui d limit apparatu s
symmetrical axi s of the cup. The grooving tool should always be held normal to the cup at the point of contact. The handle is turned at a rate of about two revolutions per second an d the number of blows necessary t o close th e groove alon g th e bottom fo r a distance o f 12. 5 mm i s counted. Th e groov e should be closed b y a flow o f the soil and not by slippag e betwee n the soil an d th e cup. The water content o f the soil in the cup is altered an d the tests repeated. At least four tests should be carried ou t by adjusting th e water contents in such a way that the number of blows required t o close the groov e may fall within the range of 5 to 40. A plot of water content against the log of blows is made as shown in Fig. 3.13. Within the range of 5 to 40 blows, the plotted point s li e almost o n a straight line . The curve so obtained is known as a 'flow curve'. The water content corresponding t o 25 blows i s termed the liquid limit. The equation of the flow curv e can be written as
= -If\ogN+C (3.37)
where, w = wate r content
/, = slop e of the flow curve , termed a s flow index N = numbe r of blows
C = a constant.
Liquid Limi t b y One-Poin t Metho d
The determination o f liquid limi t as explained earlier require s a considerable amoun t of time and labor. W e can us e wha t is termed th e 'one-poin t method ' i f an approximat e valu e o f th e limi t is required. The formula used for this purpose is
(N (3.38)
Soil Phas e Relationships , Inde x Propertie s an d Soil Classificatio n 4 9
Figure 3.12 Hand-operate d liquid limit device (Courtesy : Soiltest, USA ) where w is the water content corresponding to the number of blows N, and n, an index whose value has been foun d t o var y from 0.06 8 t o 0.121 . An averag e valu e of 0.10 4 ma y b e usefu l fo r al l practica l purposes. It is, however, a good practice to check this method with the conventional method as and when possible.
Liquid Limi t b y th e Us e o f Fal l Con e Penetromete r
Figure 3.1 4 show s th e arrangemen t o f th e apparatus . Th e soi l whos e liqui d limi t i s t o b e determined i s mixe d wel l int o a sof t consistenc y an d pressed int o the cylindrical mol d o f 5 cm diameter an d 5 cm high. The cone which has a central angle of 31° and a total mass of 14 8 g will be kept free on the surfac e of the soil . The depth of penetration 3; of the cone is measured i n mm on the graduated scale afte r 3 0 sec of penetration. The liqui d limit wl may be computed by using the formula ,
Wf = wy + 0.01(25 - y)(wy +15) (3.39 )
where w i s the water content corresponding t o the penetration y .
The procedure i s based o n the assumption that the penetration lies between 2 0 and 30 mm.
Even this method has to be used with caution.
Plastic Limi t
About 1 5 g of soil , passing through a No. 40 sieve , is mixed thoroughly . The soi l i s rolled o n a glass plate with the hand, until it is about 3 mm in diameter. This procedure of mixing and rolling is repeated til l the soil shows signs of crumbling. The water content of the crumbled portion of the thread is determined. This is called the plastic limit.
ON o
^
co 4^ o
s
oNJ o
Liquid limit
C
3 4 6 8 1 0 2 0 2 5
Log numbe r of blows N 40 6 0 100 Figure 3.13 Determinatio n o f liqui d limit
Shrinkage Limi t
The shrinkage limit of a soil can be determined b y either of the following methods : 1. Determinatio n of vvs, when the specifi c gravity of the solids Gs is unknown.
2. Determinatio n of vvv, when the specific gravity of the solids, Gs is known.
Figure 3.1 4 Liqui d limit b y the us e of th e fal l con e penetrometer : (a ) a schemati c diagram, an d (b ) a photograph (Courtesy : Soiltest , USA)
Soil Phas e Relationships , Inde x Propertie s and Soil Classification 51 Method I When G5f i s Unknown
Three block diagrams of a sample of soil having the same mass of solids Ms, are given in Fig. 3.15.
Block diagram (a) represents a specimen in the plastic state, which just fills a container of known volume, Vo. The mass of the specimen is Mo. The specimen is then dried gradually, and as it reaches the shrinkag e limit , the specime n i s represente d b y bloc k diagra m (b) . Th e specime n remain s saturated up to this limit but reaches a constant volume Vd. When the specimen is completely dried, its mass will be Ms wherea s its volume remains as Vd.
These different state s are represented in Fig. 3.10. The shrinkage limit can be written as w = M
M, where, M = M
(3.40) Ms- (Vo - Vd) pw
Therefore w = M x 100% (3.41)
The volum e of the dry specimen can be determined either by the displacement o f mercury method or wax method. Many prefer the wax method because wax is non-toxic. The wax method is particularly recommended i n an academic environment.
Determination o f Dr y Volum e Vd of Sampl e b y Displacemen t i n Mercur y Place a small dish filled with mercury up to the top in a big dish. Cover the dish with a glass plate containing three metal prongs in such a way that the plate is entrapped. Remove the mercury spilt over into the big dish and take out the cover plate from th e small dish. Place the soil sampl e on the mercury. Submerge th e sample wit h the pronged glas s plate and make th e glass plate flus h with the top of the dish. Weigh the mercury that is spilt over due to displacement. Th e volume of the sample is obtained by dividing the weight of the mercury by its specific gravity which may be taken as 13.6 . Figure 3.16 show s the apparatus used for the determination of dry volume.
Method I I When Go0 is Known
M 100 where, M w =(Vd-Vs)pw = M
T
M(a) (b ) (c )
Figure 3.15 Determinatio n of shrinkag e limit
Glass plate
Figure 3.16 Determinatio n o f dr y volum e b y mercury displacemen t metho d
Therefore, v v = •
M -xlOO = xlOO (3.42)
or w = -^ --- - xlO O (3.43)
where, p = 1 for all practical purposes .