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The Twenty-Eighth KKHTCNN Symposium on Civil Engineering
November 16-18, Bangkok, Thailand
Cause of Uplift of Press Pits on Very Low Shear Strength and Sensitive Clay
Wanchai Teparaksa1, Suraparb Keawsawasvong2
1Associate Professor, Department of Civil Engineering, Chulalongkorn University, Thailand
2Geotechnical Engineer, Strategia Engineering Consultants Co., Ltd., Thailand
1wanchai.te@chula.ac.th, 2suraparb@hotmail.com
ABSTRACT
The Samutprakarn subsoil consists of thick sensitive very soft clay which can affect to the piles located around deep
excavation area; for instance, the piles are normally deviated and crack. Therefore, some excavation projects avoid driving the
piles around excavation areas before the excavation work was completed. However, the uplift on excavated structure such as
press pit was found. This paper presents the cause of uplift at the floor of press pit area that developed driving in the surrounding
area.
INTRODUCTION
The underground construction in Samutprakarn province should be thoughtful carefully because the Samutprakarn subsoil
consists of thick layer of very soft clay which has high water content and very low shear strength. This paper presents the case
of uplift for press pit construction at a factory in Samutprakarn province. The causes of uplift of the press pit in this factory are
also discussed. There were four press pits as TRF#1 for 2500 tons, TRF#2 for 2500 tons, BL for 1000 tons and TDM for 5400
tons in this project. The locations of these press pits are shown in Figure 1. The columns and floor slabs as well as the press pits
are supported by deep pile foundations seated in dense sand. The deep piles are I-shape prestress concrete (PC) piles of about
27-28 m. long with section of 300x300mm, 350x350mm. and 400x400mm. The construction sequence started by driven the
PC pile in the floor slab areas which are far from the press pit area. Then, the PC piles are driven in the press pits areas.
Consequently, the construction of press pit was carried out by sheet pile braced excavation method. The excavation depth was
about 8.05 m. from ground surface. The base slab and retaining wall of four press pits was completed before removing the sheet
pile. After the press pit construction was completed, the PC piles of the factory were driven in the areas around the press pits.
After the completion of the pile driving, the uplifts of four press pits were found. After the uplift of press pits were found, the
details of soil investigation were re-investigated as well as measured water pressure by piezometer, and cored the base slab of
press pits. According to general foundation engineering books, such as Bowles (2001) or NAVFAC DM 7.2 (1982), there are
not described about the causes of uplift that uncommonly occurred after the press pit was constructed completely. Therefore, the
aims of this paper are to illustrate the causes of the uplift which occurred in the pit and surrounding area around the pit.
Fig.1. Locations of four press pits
SOIL CONDITIONS
Six bored holes soil investigation was carried out up to 30-40 m. depth below ground surface. The ground surface of the
project is at -1.00m. The soil condition consists of 12 m. thick very soft dark grey clay layer with natural water contents
between 109% to 158%. Below very soft dark grey clay, soft to medium dark gray clay is encountered to about 16.5m. depth.
Stiff to very stiff silty clay layer is found below soft to medium clay to about 27m. depth. The first Samutprakarn dense sand is
encountered below very stiff clay to about 36m. depth then followed by hard silty clay. The summary of soil conditions and its
properties are presented in Table1. The very soft dark grey clay layer is the sensitive clay with very high water contents of
109-158%. This very soft clay layer seems to be the most sensitive clay with highest water content of Bangkok clay. The
ground water table measured at 24 hours after completion of soil boring is at about -1.00m. below the ground surface.
Table 1 The summary of soil condition and property.
Depth
(m.)
Soil Description
wn
(%)
t
(t/m3)
Su
(t/m2)
N-value
(Blows/ft)
0 - 12.0
Very Soft dark gray Clay
109 - 158
1.3 - 1.4
0.39 – 0.99
-
12.0 - 16.5
Soft to Medium dark gray Clay
70 - 90
1.5 - 1.6
1.2 - 2.3
-
16.5 - 22.0
Stiff Silty Clay
41 - 45
1.7 – 1.9
-
10 - 18
22.0 – 27.0
Very Stiff Silty Clay
23 - 25
2.0
-
16 - 31
27.0 – 36.0
Dense Silty sand
-
2.0
-
27 - 39
36.0 - 40.0
Hard Silty Clay
16 -19
2.0
-
> 30
Note: wn = Natural water content (%), Su = Undrained shear strength (t/m2), t = Total unit weight (t/m3), N-value = SPT N-value (Blow/ft)
CONSTRUCTION SEQUENCE AND OBSERVED UPLIFT ON PRESS PITS
The building in this factory project was designed by using I-shape driven prestress concrete pile as the pile foundation to
support the column, floor slab and press pits. The pile capacity and layout was designed based on soil investigation. There are a
lot of driven piles in the project. As the soil conditions consists of thick very sensitive soft clay of about 12 m, before starting the
construction of press pit, the trail open excavation was carried out on April 2014 as shown in Figure 2. There was no ground
water during excavation. The result of excavation showed the collapse of excavation slope and the soil was flow as also shown
in Figure 2.
Fig.2. Trial excavation work
Due to the collapse as shown in Figure 2, it can be seen that the trial open excavation is very dangerous to construct the
press pit. Therefore, the sheet pile braced excavation method was carried out to be the soil protection system. Generally, during
excavation by sheet pile braced cut system in the very soft sensitive clay, the pile deviation and pile crack are usually occurred
both inside the pit and outside at the surrounding area of the pit. As a result, the contractor proposes the excavated method by
sheet pile bracing system without pile installation outside of the sheet pile wall as shown in Figure 3. In addition, the contractor
proposed the pile driven sequence into 3 phases as well as construction of press pit with detailed as follows:
Zone1: Piles driving was started on outside of the areas that the press pit would be constructed by waived the surrounding
area of the press pit. The piles surrounding the press pit area were not driven because the piles outside the excavation pit are
normally deviated and crack during deep excavation in the very soft clay area. So, the area of zone1 should be far away from
press pit areas. Some piles in zone1 were driven at the same time with the excavation of the press pit or the construction of the
press pit slab and retaining wall because zone1 is huge area. In zone1, the piles are used to support the floor slab and column of
the factory.
Zone2: Pile driving was started only inside the press pits area by waived the surrounding area of the press pit. After
completion of pile driving, the sheet pile braced excavation was carried out in order to construct the press pits. This is the
method modified from trial excavation work. Three bracing layers were performed for excavation up to -8.05m. depth. During
excavation, ground water and soil heaving were not found in the pit, and also pile crack inside the pit was not found as well.
The sheet pile was removed after the base slab and retaining wall of all press pits was constructed completely. Any disturbances
to the surrounding pile were not found because the surrounding piles were not yet driven.
Zone3: After completion of press pit structure and piling in zone 1, the piles surrounding the press pits were driven as
shown in Figure 4. The piling in zone3 was carried out between November to December 2014. After completion of piling in
zone3, the uplifts of press pits were observed. However, the pile driving sequence and excavation of press pit by sheet pile
braced excavation system are the best method based on trial excavation work. The uplift of the press pit is the unforeseen
behavior.
Fig. 3. Sheet pile braced excavation system Fig. 4. The pile driven in the surrounding area of the press pit
The uplift of press pits were observed and measured on 14 December 2014 as presented in Figure 1 as summarized in the
Table 2.
Table 2 Summary of uplift at four press pits
Press Pit Area
Uplift (mm.)
2500 tons TRF#1
-70 to -110
2500 tons TRF#2
-120 to -130
1000 tons BL
0 to -65
5400 tons TDM
-30 to -99
After uplift of press pits were observed, concrete coring on base slab of the press pits were carried out at 3 locations as
shown in Figure 1 as location1 at press pit 2500 tons TRF#2 on 16 December 2014, location2 at press pit 1000 tons BL on 16
December 2014, and location3 at 5400 tons TDM on 19 December 2014.
At location1: After coring of base slab, water flow up was found and pile head was separated from base slab. The water
flow up was stopped after 30 minute as shown in Figure 5.
At location2: The concrete coring was push up about 30cm. by uplift pressure and water flow was stopped after 10 minute.
The pile head was not separated from base slab as shown in Figure6.
At location 3: After coring concrete of base slab, no water flow was found and pile head was not separated from base slab
as shown in Figure7.
The water flow at location1 was strong and high pressure because it was the first coring hole and uplift pressure was still
very high. For location2 the water flow was less than location1 because the uplift pressure was relief. At location 3, the coring
was about 1day later than location1 and location2, there was no uplift water pressure.
Fig.5. Concrete coring at location1
The piles were driven in the surrounding area around the
press pits after the press pit were constructed completely
Press pit
Fig.6. Concrete coring at location 2 Fig.7. Concrete coring at location 3
CAUSE OF UPLIFT ON PRESS PIT
The press pit construction was completed by sheet pile braced excavation system. During excavation, there was no ground
water and soil heaving at the final excavated depth (-8.05 m.). Also, there was no pile crack inside of the press pit. After base
slab and retaining wall of press pit were casted in October 2014, and sheet pile was removed in November 2014, there was no
uplift on press pits. During these times, the piling work was carried out on the surrounding area of the press pit or zone3 as
shown in Figure 4. The uplift of press pit was found on 14 December 2014. That means the piling works at surrounding the
press pit created the uplift on the press pits. In addition, only piling work in zone 3 was carried out near the press pit area.
The piling work induces the excess pore pressure in the very soft clay. The 12 m. thick very soft clay in this project is very
sensitive clay with very high water content of 109-158%. Driving of pile strongly compressed soil, and lead to a buildup of
excess pore water pressure. The soil pressure from piling work was also induced soil heaving. The very soft clay is the very
sensitive clay, the vibration due to pile driving surrounding area of press pit (zone 3) is create the disturbance to very soft clay
and lead to reduce or loss of the shear strength of very soft clay. The detail about the vibration due to pile driving can be learned
more in Design of Structures and Foundation for Vibration Machine by Arya et. al. (1979). The excess pore pressure, loss of
shear strength as well as soil heaving creates uplift at the press pit area because the overburden pressure at press pit area was
lower than the surrounding area as presented in Figure 8.
After uplift occurred at the press pit area, the additional soil investigation with 3 bored holes were carried out during January
2015 with field vane shear test and installation of piezometer at 10m. and 18m. depth. The results show that the very soft clay is
about 16.5m. thick with very high natural water content. The sensitivity of the very soft clay from field vane shear test is about
3-6, but it is about 10 based on unconfined compression test on undisturbed and remold sample. This very soft clay layer can be
classified as extra sensitive clay (St > 8) (Bowles, 2001). Therefore, based on the re-soil investigation, it can be concluded that
the pilling work in extra sensitive clay can easily disturb to the underground structure if a pile is driven in close area.
Fig.8. The uplift and soil heaving at press pit area
CONCLUSIONS
This paper presents the cause of uplift at the floor of press pit that occurs while a factory project in Samutprakarn province
was constructed. The soil condition consists of 12m. thick very soft dark grey clay with high water content between and very
low shear strength. The pile driven was separated into 3 zones which were the press pit zone, the very far from press pit zone,
and the surrounding press pit zone. Based on the observation, after piles in the surrounding press pit zone were driven, the uplift
occurred on the floor of the press pit. It can be concluded that the disturbance from the pilling work on the very soft and very
sensitive clay create the excess pore pressure, loss of shear strength of soft clay and induced the uplift on the press pit structure.
REFERENCES
J. E. Bowles. (2001), “Foundation Analysis and Design 5th ed”, McGraw-Hill. USA.
NAVFAC DM 7.2 (1982) “Foundation and Earth Structure, Design Manual 7.2”, Department of the NAVY, USA.
S. Arya, M. Oneil, and G. Pincus (1979) “Design of Structures and Foundation for Vibration Machine,” Gulf Publishing Co. Ltd, USA.