Question

Describe the following: i. Dewatering behind retaining walls ii. Determination of N-value during a standard Penetration Test (SPT) iii. Compaction of deep beds of loose sands

Answer 1

*i)Dewatering behind retaining wall

this method is usually adopted in offshore structure. so avoid water from creating problem for laying foundation or construction of a strucutre, in this case the water need to be diverted or dewatering need to be done. so, retainin walls are constructed in the form of sheet pile walls or other concrete structure to reatain water its behind. Once presence of water is minimized the constrction process becomes easy. water behind retaining wall can further diverted or in case if its a stangant water it can be removed by pumping or other dewatering techniques.

*ii) Determination of N value

standarard penetration test is done to know the resistance of the soil. this method is usually fallowed in soil to determine the resistance and shear strength parameters. the resistance is determined by the N value and later using this N value the bearing capacity of the soil is determined.

Here N value is the the number of blows given by droping the hammer of 65kg to the sampler through the connecting rods upto penetration of 300mm. Initialy sampler is driver 150mm and then for interval of 150mm penetration the N value is recored and its done for total penetration of 300mm. the number blows obtained for last 300mm are considered as the N value. As per indian standard code on SPT is carried upto 300mm penetration or upto 50 blows whichever is earlier. Because excess of blows would lead to daming the sampler.

higher N value represent the harder the soil strata. the SPT s conducted for every 1.5 or 3m interval.

for the obtained values the correction are applied based on the necessity, such corrections are

DILATANCY CORRECTION, OVER BURDEN PRESSURE CORRECTION.

*iii)Compaction of deep beds of loose sand.

comapction is usually done to densify the sand so that settlement after post construction reduced and the structure built are safe.

for densifying the sand most commonly prefered methods are vibrocompaction, stone cloumns,vibroreplacement etc.

Incase of vibrocompaction comapction upto depth of 65-70m has been achieved. in this method a probe is penetrated into sand beds and its made to vibrate, so due to vibration tha sand bed surrounding the probe gets compacted. this process is repeated to the whole area. this method is prefered if the area to be compacted is large and if whole area is used for construction.

In stone columns method the underground columns consist of gravel or crushed rock rock. the method of installtion is same as that use in vibrocomapction.