Ground Improvement

Final Examination

Teacher: Prof. H. J. Liao 2013/6/17

1. Pervious pavement has been widely adopted as a means for water preservation in urban area. Using the following figure to explain the infiltration problem associated with pervious pavement. (5%) Try to comment on whether we should use the infiltration or capillarity nature of soil to preserve water in urban area. Are there any better ideas than pervious pavement to preserve rain water in urban area? (5%)

ANS:

As shown in the above figure, the permeability of compacted soil varies from 10-5 cm/sec (on dry side) to 10-7 cm/sec (on wet side). Neither can be considered as permeable layer when placed under the pervious pavement. In other words, when rain water infiltrates through the pervious pavement on the surface it will accumulate on the top of compacted layer and then ponding there. In the long run, the softened compacted soil after soaking in water will lose much of the bearing capacity and cause cracking on the pervious pavement.

In the unsaturated zone of soil, the capillarity nature of groundwater can help to transport the water in soil, horizontally and vertically. So if we can create an underground pool for the groundwater (see below), the capillarity of groundwater itself can constantly suck water from the pool and keeps the soil moist even some distance away from the pool. As a result, the groundwater can be preserved in the soil.

2. How does over-compaction occur in the earth fill work? What is the consequence of it? (5%)

3. “If a loading can be placed on a soft clayey soil without bearing capacity failure in the beginning, the loading can stay there for long time.” How can we make use of this concept in geotechnical engineering practice? [For example: placing road embankment on soft ground] (10%)

ANS:

If a loading can be placed on a soft clayey soil without bearing capacity failure in the beginning, the loading can stay there for long time. It implies that if the bearing capacity failure can be prevented in the soft clay then there is a chance to place an embankment on top of soft clay. Usually, this can be achieved by laying down a layer of geo-synthetic material on top of soft clay and then place the soil fill layer by layer upward. The edge of the synthetic material has to be extended some distance beyond the embankment and also properly anchored if necessary.

4. A factory was to be built on a 10m thick sand layer. To prevent liquefaction from happening, it needs to increase the relative density of sand from the initial 40% to the

designed 85% and above.

(1) If the sand compaction pile method is used, design the spacing between sand

compaction pile if a triangular layout is adopted（5％）

Known：diameter of sand pile = 60 cm,

The emax and emin of in-situ sand layer and back-fill sand are equal to 1.1

and 1.4。

(2) If the vibroflotation method is used, design the spacing of vibroflots if a triangular

layout is adopted（5％）

Distance (ft) Relative density (%)

(3) If the dynamic compaction method is used, design the spacing between each compact point if it is layout in square shape and also determine the weight of

compaction hammer.（5％）

Known：1. Max lift up height (H) of the crane is 20m，

Each point was compacted 5 times

When Dr = 85%, the cone resistance qc = 120 kg/cm2。

2. Improved depth (m) = 0.5 WH

ANS:

(1)

For

(2)

Influence factor=18

For triangular layout:

Influence factor for one vibroflot =18/3=6

From figure IF=6

Spacing between vibroflot =6.6 ft Use 6.5 ft first.

(3)

Improved depth

WH=400, H=20 m

For square layout: spacing= =5 m

5. A vertical loading pressure of 5 t/m2 is acting on a clay layer of 10 m thick which is underlain by a sand layer. The properties of clay layer are shown in the figure below:

a. What is the consolidation settlement caused by the loading? How long

does it take to reach 80% consolidation?（5%）

Note： 2

d

vv

H

tCT

b. If preloading method is adopted, what loading pressure is needed to

complete 80% settlement of (a) in two year?（5%）

%)100log(933.0781.1%,60 UTUFor v

c. If PVD drainage method is used, what spacing is needed to complete 80%

settlement of (a) in two year?（5%）[ =0.8]

d. With the loading of (b), what is the spacing needed for PVD to complete

80% settlement of (a) in one year? （5%）[ =0.8]

Note： 2

4UT

Dimension of PVD = 10 cm × 0.5 cm，

Diameter of equivalent circle,

)(2 btdw

= 0.8，

Use triangular layout pattern, de = 1.05S ANS:

(a)

T=14.2 yrs.

(b)

for

(c)

Assume n=20

Assume n=25

Assume

Assume

can meet the requirement

For triangular layout

(d)

For (b), it needs to reach

Assume

Assume

6. The grouted zones for the arrival pit and the departure pit of shield tunneling project are different from each other in length. What are the typical lengths of grout zone for departure and arrival pits? Why is this difference resulted? (5%)

7. The gelling process of sodium silicate (SS) grout is a matter of neutralization. The pH value of SS grout is decreased by adding acid reagent to it. Why is SS grout only used for temporary purpose? How to improve the durability of SS grout? (5%)

ANS:

• When mixed with reactants, the pH of sodium silicate (SS) grout is dropping from 12 (alkaline) to 6~8 (neutral). Then grout starts to gel. But there are some sodium ions remained in the gel structure formed from the mixing of SS grout and organic or inorganic reactants. The existence of these sodium ions will reverse the gelling process (back to alkaline) and cause gel structure to decompose later. As a result, the gel structure formed by SS grout and organic or inorganic reactants is considered only good for temporary use.

• Colloidal silica (PERMA ROCK) is made by extracting alkali from sodium silicate with ion-exchange resin in the factory. This process forms colloids with a diameter of 5 - 20 nm. When react with some inorganic salt reactant, a structure like a pile of spherical colloids connected to each other will be formed. Since there is no sodium ion remained in the gel structure, this gel structure can remain stable over long time.

•

8. Jet grouting has been commonly used in the construction of subway system. What efforts have been made in the past years to increase the diameter of jet grouting pile (e.g. grouting technique) and also to improve the quality of jet grouting pile (e.g. the uniformity of pile diameter). (5%)

ANS:

9. Use the following information to determine the center-to-center spacing of deep mixing piles to control the settlement within 20 cm. (10%)

s = mvH/ [(n－1)ap＋1]

ap = Ap / (Ap+Ac)

Piles are layout in triangular pattern. Diameter and length of pile = 1 m & 10 m

n = p/c = 5 Thickness of clay layer = 10 m

mv = 0.01 m2/t Structure loading pressure, = 10 t/m2

ANS:

10. A pumping test result of a underground station (20m × 200m in rectangular shape) is

shown in the figure below. Three observation wells IW1, IW2, and IW3 were located 1m,

3m, and 5m from the pumping well respectively. The depth of wells is about GL-50m,

and screen of wells is placed at 3m from bottom of wells. The pumping rate of pumping

well was about 2 m3/min, water level was at 23 m below ground surface, and duration

time was 3 days. The gradient (slope) Δ s of drawdown can be determined from figure

below and its influence range r0=36.5m.

ANS:

◎ Find T and S

◎ Find

Design the needed total pumping rate Qt and numbers of pumping wells

which were installed around the periphery of the excavation and through the

diaphragm wall. It is required to keep the groundwater head in sand layer to

maintain at GL-22m (the original groundwater level was at GL 5m ) for 1 year

(= 525600 min) during the water pumping period. (15%)

◎ Find number of wells

※ Reduction factor

200 m

Water level in the well