Question

Two undisturbed clay samples were taken from the middle of the overconsolidated and normally consolidated clay layers in the soil profile shown. The water table is at the top of the overconsolidated clay layer. Consolidation tests were carried out on the two samples and the results are summarized in the table below. Assume that the bulk, dry, and saturated unit weights of the sand are 16 kN/m³, 16 kN/m³ and 19 kN/m³, respectively. The specific gravity of the clay soil grains is 2.70. A 2-m high compacted fill with a unit weight of 20 kN/m³ is placed at the ground level. Ignore settlement of the sand layer.

a. What would be the pore water pressures and effective stresses at the middle of the clay layers after one month?

b. Plot the combined consolidation settlement of the two clay layers against different times

c. determine the time for a settlement of 200 mm (combined settlements of the NC and OC clay layers) to occur?

GL 1.0 m Sand 2.0 m Oc clay Sand 1.5 m 3.0 m NC clay Impervious stiff stratum N.С. Clay 29.0 65.0 0.60 0.07 2.5 О.С. Clay 20.0 50.0 0.55 0.06 13.0 Natural water content(%) Preconsolidation pressure (kPa) Compression index Recompression index Coefficient of consolidation (m2/year)

Answer 1

clay layer Effe ctive sFrees ap the Center be fore placiny the ful rc. 2lana"z wnd. "t.(AIA7-2ae.)误cl°1 q:/。 water o san cia G te. 0.5y '. From the above

gilu 20x2. 26 83 o Givem For he. above conation aitle sment awen ty, lte。 Tueknessclay la t 51 26. 83 -ラ S, 0.021. C. 2.

C6. 83 so 0.54 011m 110 mm o C Gla Normally Conso LidateA cley Effec Hve- verhaal stress a he-cenhor of clay 9.81 ak- 1O 83 19 6 KN/m 2

65-47十40 NC. 1oS.yt ot 82 0. 209 m . ナノNL day ore- water ress ure after 1 mantthj Dete.r mination 12 C-v ㅈ 1. σ g3 m.//month. here, oc la is wnå er double drainane Condihnon, s Pet meabie sanå retent at bop am& bortom.

4 Now U모 ui where., ial pore pressure at the Center cl er WI 9.81 1 Pore pressure. ahan y hime 't, ut 뼘

foom tne above eation 9.81 A fterr one. monh, pere pressure at the center s oc clay layer 5 m Siilarhy NC clay laes: C2. 5 m/yr ereN C Single-drainage, ,'. corna,'Hon рте v anes 3 2 0 0 23 Lef U <60% lo o U72- D O

I nitial pone pressure at the center NC clay layeris ui-u ui q.oS . Pore rssure after one month at the center xN/ m -To pl。トsettlement vs Hrne , we need to find- s ett le.memr ak various hime, he iu ass be cal cul atea L e 2 month: U ForOC Clay harmey. 2- ニ2.16G

T 81.933x too -u 1781--"Tv aloove equat von 1:81-2-166 o 933 → U2 99.61ズ 0o 9G xILO 0 O → S2. 109.57 mm. For NC- cLay : 3 2. O D X LOD 下 兀

NC CIay じ0 2.2 2 D9 2 So to mmm other,time values SeHlern enr can be cal culateA which are shoun in the abe belo w emce for muia cornsponding tU6 0 canonly be applica → For NC elay first afply fer mula for U < 60 then appbyo U 60

OC clay layer					NC clay layer
Time (months)	Tv	U (%)	St (mm)		Time (months)	Tv	U (assuming <60%)	U (assuming >60%)	Final U (%)	S (mm)
1	1.083	94.401	103.841		1	0.023	17.154	-	17.154	35.852
2	2.166	99.613	109.575		2	0.046	24.259	-	24.259	50.702
3	3.249	99.973	109.971		3	0.069	29.712	-	29.712	62.097
4	4.332	99.998	109.998		4	0.092	34.308	-	34.308	71.704
5	5.415	100.000	110.000		5	0.116	38.358	-	38.358	80.167
6	6.498	100.000	110.000		6	0.139	42.019	-	42.019	87.819
7	7.581	100.000	110.000		7	0.162	45.385	-	45.385	94.855
8	8.664	100.000	110.000		8	0.185	48.519	-	48.519	101.404
9	9.747	100.000	110.000		9	0.208	51.462	-	51.462	107.556
10	10.83	100.000	110.000		10	0.231	54.246	-	54.246	113.374
11	11.913	100.000	110.000		11	0.254	56.893	-	56.893	118.907
12	12.996	100.000	110.000		12	0.277	59.423	-	59.423	124.195
13	14.079	100.000	110.000		13	0.300	61.850	61.374	61.374	128.272
14	15.162	100.000	110.000		14	0.324		63.516	63.516	132.748
15	16.245	100.000	110.000		15	0.347		65.538	65.538	136.975
					20	0.462		74.089	74.089	154.846
					25	0.578		80.518	80.518	168.283
					30	0.693		85.352	85.352	178.386
					35	0.809		88.987	88.987	185.982
					40	0.924		91.719	91.719	191.693
					45	1.040		93.774	93.774	195.988
					50	1.156		95.319	95.319	199.216
					60	1.387		97.354	97.354	203.469
					70	1.618		98.504	98.504	205.873
					80	1.849		99.154	99.154	207.232
					90	2.080		99.522	99.522	208.001
					100	2.311		99.730	99.730	208.435
					120	2.773		99.914	99.914	208.819
					140	3.236		99.972	99.972	208.942
					160	3.698		99.991	99.991	208.982
					180	4.160		99.997	99.997	208.994
					200	4.622		99.999	99.999	208.998
					220	5.084		100.000	100.000	208.999

Settlement vs time 111.000 110.000 109.000 E 108.000 OC clay 106.000 105.000 im 103.000 10 12 1A 16 (time (months)

250.000 200.000 E 150.000 E 100.000 50.000 0.000 50 100 150 200 250 (time (months)

(c)

Time required for combined settlement of 200 mm.

From the table above it is clear that OC layer undergoes complete 110 mm settlement by five months.

Another required 90 mm settlement for NC layer takes 7 months to occur.

Hence total time required for 200 mm combined settlement is 7 months.