The highly variable Jurong Formation C F Leung National University of Singapore of Singapore
GEOTECHNICAL SOCIETY OF SINGAPORE (GeoSS) Cordially invites you to
Inaug naugur ura at io ion n of GeoS oSS S Date: Time: Venue:
Tuesday, 29 Jan 2008 6:00 – 8:30pm LT1 Faculty of Enginee Engineerin ring g Natio Na tional nal Universit Universit y of Singapore
GEOTECHNICAL SOCIETY OF SINGAPORE (GeoSS) Cordially invites you to
Inaug naugur ura at io ion n of GeoS oSS S Date: Time: Venue:
Tuesday, 29 Jan 2008 6:00 – 8:30pm LT1 Faculty of Enginee Engineerin ring g Natio Na tional nal Universit Universit y of Singapore
Program 1815 – 1815 – 1900 1 900 1900 – 1900 – 1930 1 930 1930 – 1930 – 2030 2 030
Reception and registration of GeoSS of GeoSS membership Inauguration of GeoSS of GeoSS First GeoSS Lecture
Collapse of of Nicoll Nicoll Highway Highway – – a Global Failure at the Curved Section of a Cut‐and‐Cover Tunnel Construction by Professor KY Yong
Time flies. GeoSS is now in its 10th year. Over the years Membership grows from 100 to 300. Kitty grows from a few thousand dollars to over $200,000.
Events to celebrate GeoSS 10th year • Series of lectures of lectures given by past presidents and other events in 2017. • GeoSS10 Conference (30 Nov to 1 Dec 2017) Theme: A Decade of of Geotechnical Geotechnical Advances Local and overseas experts will be invited to give the state of of practices practices on on the following 10 topics: Topic specific: (1) Deep excavations/retaining structures, (2) Foundations, (3) Tunnelling, (4) Land reclamation and Port, (5) Underground caverns. Cross‐discipline: discipline: (6) (6) Site investigation and geology, (7) Finite element analyses, (8) Ground improvement, (9) design Codes and regulations, (10) Future geotechnical solutions.
• GeoSS 10th AGM to be held on 1 Dec 2017
Outline of this lecture • Introduction to Jurong Formation • Piles socketed in weak sedimentary rocks • The highly variable Jurong Formation (soils and rocks) • Variability • Folding and cavities • Slaking and softening • Seepage and permeability • Compressibility and consolidation
• Concluding remarks
Introduction to Jurong Formation
Jurong Formation occurs in the southern and western part of Singapore New geological map updated by DSTA 2009 Another update soon?
Residual soils of Jurong Formation
These soils are the weathered residue of sedimentary rock. The change from soil to rock is usually gradual.
Weak sedimentary rocks are often highly fractured and can extend down to 100 m depth
Strong sedimentary rock
188 m depth
Unconfined compressive tests (left) are often conducted on rock core (minimum length at least 2 core diameter) to determine Unconfined Compressive Strength qu. As sedimentary rocks are highly fractured, point load tests (top) are often carried out to determine the rock Point Load Index Is(50).
Correlation between qu and Is(50) for weak sedimentary rock
qu = 6 Is(50). Not equal to the commonly adopted qu = 22 Is(50) for strong rocks
After Leung and Radhakrishnan (1990)
Piles socketed in weak sedimentary rock It is generally not possible to found piles on solid sedimentary rock which is usually at great depth (often > 100m). The piles need to be socketed many pile diameters into the weak rock to achieve the desired pile capacity! This was my first research adventure in Jurong Formation.
Alexandria District Park (now part of Maple tree complex)
Alexandra Distripark
Instrumented test piles
Mostly dry holes for bored piling
Load (MN) 0
4
8
12
16
20
0
4 ) m m ( t n e m e l t t e S
8
12
16
Load‐settlement response is reasonably linear up to working load of 10 MN After Radhakrishnan and Leung (1989)
Load (MN) 0
5
10
Unit shaft friction (kPa) 15
20
0
200
400
600
0
Applied load
Fill 1 Marine clay Firm silty clay (N = 12) V. dense clayey silt (N = 130) Weak siltstone (qu=3.5 MPa) Weak siltstone (qu=6.5 MPa) (a)
2
3
4
1: 5 MN 2: 10 MN 3: 15 MN 4: 20 MN
) m 4 ( l e v e l d n u o r 8 g w o l e b h t p e D 12
1
2
3
16
After Radhakrishnan and Leung (1989)
(b)
(c)
Maximum unit shaft 4 friction 600 kPa
Alexandra Distripark
Results show that a large percentage of shaft friction can be mobilised even for piles not tested to ultimate failure. After Radhakrishnan and Leung (1989)
Rock socket adhesion factor • Pile load test results reveal that majority of socket shaft friction can be mobilised even for piles not tested to ultimate failure. • Rock socket adhesion factor = f /q s u
0.6
r o t c a f n 0.4 o i s e h d a t e k c o 0.2 s k c o R
Williams and Pells
Field data in sedimentary rocks
Rowe and Armitage
Rosenberg and Journeaux
Horvath and Kenny
0.0 1
10
100
Unconfined compressive strength q u (MPa)
All piles installed by chiselling. Chiselling affects sockets with qu > 5 MPa
After Leung (1996)
Sedimentary rocks • For qu < 5 MPa (i.e. very weak rock) rock socket adhesion factor is reasonably close to theoretical values. • For qu > 5 MPa (i.e. weak rock & above), value is considerably lower than the theoretical values. This is caused by heavy chiselling that had significantly weakened the rock.
PSA Building
Pile raft foundation
Foundation layout and instrument plan
Subsurface conditions
The core walls are designed to take the lateral wind load
Construction of bored piles
Construction of 2‐m thick raft
Foundation layout and instrument plan
Foundation monitoring throughout superstructure construction
Very small heave outside column area
Very small settlement under shear core and columns
After Leung et al. (1988)
After Leung et al. (1988)
There is no significant increase in raft pressure, indicating that much of the loads are taken by the piles.
After Leung et al. (1988)
After Leung et al. (1988)
(a) Construction load
Test load
Pile raft cap & creep • Pile behaviour under static load test and under long term working conditions are markedly different. • The presence of raft, pile group effect and possibly rock creep will cause the long load transfer to be less along the pile shaft. I.e. more load is transferred to the pile base.
Edge pile
Distribution of of pile pile load among piles supporting a column
Edge piles take up the most column load
After Leung et al. (1988)
Inner pile
Centre pile
After Leung et al. (1988)
Inner piles take up the least column load
After Leung et al. (1988)
Summary of of findings findings • The raft behaves as a big flexible pile cap • Most of of the the loads taken by piles located under the central core walls and peripheral columns • Little load was taken by pile situated between the core walls and the columns (see next two figures for details)
After Leung et al. (1988)
After Leung et al. (1988)
Precast RC driven piles • Case study on very weak sedimentary rock (qu ~ 1 MPa) [Tanjong Pagar area] • Pile can be designed as driven piles in gravel/sand as the rock is completely fractured during driving • Unit shaft friction can be quite high within the short socket length
Concreting of instrumented pile in casting yard
Joining of pile segments with groove recess for cables of strain gauges
Joining of cables for strain gauges
TP1
(after Leung et al., 1991)
TP2 (after Leung et al., 1991)
(after Leung et al., 1991)
Unit shaft friction can be high within the short socket length but adopting such high unit shaft friction value should be treated with caution (after Leung et al., 1991).
The highly variable Jurong Formation In my early days involving rock socketed piles, I thought Jurong Formation is highly fractured but no complex problems. But I was proven wrong in my subsequent involvement with further projects in Jurong Formation.
20.0
25.0
30.0
The soil profile is extremely variable as the N values between boreholes at 2 m apart can be widely different.
35.0
m , h 40.0 t p e D
45.0
50.0
55.0
Study by Profs J Chu and C F Leung
60.0 0
10
20
30
40
50
SPT N-Values
60
70
80
90
100
This explains why the penetration depth of driven piles can vary greatly within a short distance in Jurong Formation
20.0
In some places, one can clearly identify that the soil layer is inclined.
25.0
30.0
m , h t p e D
35.0
40.0
Boreholes 2 m apart. There is 1 m difference in soil layer elevation.
45.0
50.0 0
10
20
30
40
50
Study by Profs J Chu and C F Leung SPT N-Values
60
70
80
90
100
The soil and rock strata are inclined. Besides the traditional sandstone, siltstone and mudstone of Jurong Formation, take note of presence of tuff and limestone. After Kiso Jiban SI report.
Consistent with observed rock outcrop at NUS
Effects of joint orientation • Unlike granite which is strong where unfavourable orientation of the joints could affect the rock stability, the effects of joint orientation on the rock stability is much less on weak and very weak sedimentary rocks as they are already highly fractured to start with.
Folding and cavities
Folding in Jurong Formation can be so severe that the less competent soils go below competent soils/rocks at some locations.
Singhal & Gupta (2010)
Geology of Singapore (DSTA, 2009)
Limestone cavities • Limestone with cavities have been detected in many locations in Jurong Formation for the past 2 decades • The concrete volume for bored piles is much larger than the bored pile opening volume in some cases • Site investigations so far establish that the cavities are usually slender (but current technology cannot accurately determine the 3‐ dimensioinal cavity extent) • Chemical tests are now often used to confirm the presence of limestone
Tomography to detect limestone cavity. As cavities in Singapore are usually narrow, such technique has great difficulties.
Borehole camera to detect cavities
Weak zones or cavities (void or i nfilled)
Good rock with some fractures at the lower part
Case study 1 • A large number of precast RC piles were installed at a Tuas site • All piles have been driven to set with piles penetrated to hard soil with N value about 70 • Four ultimate pile load tests conducted at site • Two piles passed • Two piles plunged at slightly over 2 times working load
Driven pile plunged at around slightly over 2 times working load during ultimate load test
SPT 0
20
40
60
80
100
120
0 BH32
10
20 ) m ( l e v e l d 30 n u o r g w o l e b 40 h t p e D
50
60
70
Hypothetic profile without underlying weak layer
Borehole made after static load test shows that there are very weak soil (due to folding of Jurong Formation) below pile base. FEM analyses: 1. Confirms pile would plunge due to underlying weak soil. 2. If the underlying weak soil has a minimum N value of over 30, the pile would not plunge. This explains why some test piles did not plunge.
Pile group stress zone
The stress zone at the base of a large pile group may extend deeper. If there is a weaker soil layer within the stress zone, the pile group base resistance may be reduced and/or severe settlement of the pile group.
Hence one must also check against pile group effect!
Side view
Case study 2 • Gravity caisson wharf structure for container ports
Discussions • What if there are weak zones or cavities in the in‐situ hard stratum? • Say at shallow depth below the sand key • At mid‐depth below the sand key • At great depth below the sand key
• The present state of technology could not provide a more precise size and extent of cavities which are typically slender in Singapore.
Slaking and softening Rocks and soils of Jurong Formation
Photo from Internet
Slake durability test is one of the methods to evaluate soil and rock slaking
Minimal slaking
Slight slaking rock
rock
Wide range of slaking
Soils will definitely slake with time
After Leung and Radhakrishnan (1990)
Degree of saturation
Unsaturated soil sample on the left slakes very fast. Saturated soil sample on the right takes a much longer time to slake. Study by Profs J Chu and C F Leung
Dry unit weight • Soil with low dry unit weight (e.g. < 14 kN/m3) is found to be susceptible to slaking and softening. • This is logical as a low dry unit weight implies that the soil has nothing much within its solid. Study by Profs J Chu and C F Leung
Soil type • Although some soils have higher tendency to slake, it is not definite that these soils will slake. • Although some soils have relatively low tendency to slake, it is not definite that these soils will not slake. • Thus the parameters of degree of saturation and low dry unit weight are better indicators. Study by Profs J Chu and C F Leung
Seepage and Permeability
Laboratory permeability tests usually produce too low coefficient of permeability as the best part (core) of the rock/soil is tested. Hence results are generally not reliable.
Field permeability tests are preferred. However, due to high variability of Jurong Formation: 1. More tests are needed. 2. Tests should be conducted in locations with more fractures.
Illustration from Internet
The variable coefficient of permeability explains why tunneling in Jurong Formation can be erratic. 3.5
3.0
HBF SB Drive HBF NB Drive
) 2.5 % ( s s o l e 2.0 m u l o V1.5 e v i t a l e R 1.0
CNT SB Drive CNT NB Drive
Performance of tunneling is more erratic in Jurong Formation
(after Nick Shirlaw)
0.5
0.0 0
0.2
0.4
0.6
0.8
Face Pressure/Overburden
1
1.2
1.4
After Xu et al. (2015)
Study by NTU on Jurong Island underground cavern reveals large variation of permeability of Jurong Formation
After Xu et al. (2015)
Deep excavation and shaft construction in Jurong Formation • Water knows how to find its way through the weakest parts of Jurong Formation with the highest permeability. • It is hence advisable to have the retaining wall penetrating beyond the highly fractured rocks. Once severe water inflow occurs, it is very hard to stop it and to take remedial action. • The drainage path through Jurong Formation is faster though not reaching double drainage state but could be considerably faster than one way drainage state normally adopted in design involving consolidation settlement analyses.
Compressibility and consolidation
Scenario 1: Weak Soils below Hard Soils The weak soil cannot be improved by PVD which cannot penetrate through the top hard layer. Stiff/hard soils to weak rocks
Weak soil with N value about 2
Stiff/hard soils to weak rocks
1. How do we improve the weak soil? 2. What would be ultimate settlement and rate of settlement of the weak soil under loading? Does the load reach the weak layer? Above all, we need to know he compressibility and consolidation characteristics of the weak soil.
Scenario 2: Deep hard residual soils (can be down to 100 m depth)
Stiff soil (N < 50)
In some projects, there is a limiting remaining ground settlement requirement under working condition after construction is completed.
1. Would the hard soils settle? 2. If yes, how long would it take? Hard soil (N = 50)
Very hard soil (N = 100)
Weak soil
Again, one needs to know the compressibility and consolidation characteristics of the hard soils. Wesley (2016) proposed that for residual soils, it may be better to employ the e‐P curve rather than the traditional e‐log P curve.
Concluding remarks • Jurong formation is highly variable with rapid changes in all directions within a short distance. This is due to the complex geology. • Piles socketed in Jurong formation requires long socket length due to higher fracture state till great depth. • Weak soils may be present below stiff soils due to folding. This can pose great challenges to foundation and gravity caisson wharf design and construction. • Better technology is required to evaluate the size and extent of limestone cavity. • The coefficient of permeability can vary greatly and the effective coefficient of permeability can be high. As such water seepage can be problematic. Retaining wall and shaft need to penetrate beyond the highly fractured layer.
