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European Journal of Environmental and Civil Engineering
ISSN: 1964-8189 (Print) 2116-7214 (Online) Journal homepage: http://www.tandfonline.com/loi/tece20
In situ tests on improvement of collapsible
loess with large thickness by downhole dynamic compaction pile
Yu-chuan Zhang, Yong-guo Yao, An-gang Ma & Chen-lin Liu
To cite this article: Yu-chuan Zhang, Yong-guo Yao, An-gang Ma & Chen-lin Liu
(2017): In situ tests on improvement of collapsible loess with large thickness by downhole dynamic compaction pile, European Journal of Environmental and Civil Engineering, DOI:
10.1080/19648189.2017.1370393
To link to this article: http://dx.doi.org/10.1080/19648189.2017.1370393
Published online: 30 Aug 2017.
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Date: 25 October 2017, At: 02:14
European Journal of Environmental and Civil Engineering, 2017
https://doi.org/10.1080/19648189.2017.1370393
In situ tests on improvement of collapsible loess with large
thickness by down-hole dynamic compaction pile
Yu-chuan Zhanga,b , Yong-guo Yaoa,b, An-gang Mac and Chen-lin Liua,b
School of Civil Engineering and Mechanics, Lanzhou University, Lanzhou, China; bKey Laboratory of Mechanics
on Disaster and Environment in Western China, Ministry of Education, Lanzhou University, Lanzhou, China; cGansu
Zhonglian Construction Engineering Technology Co., LTD, Lanzhou, China
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a
ABSTRACT
This paper presents three in situ tests about down-hole dynamic compaction
pile of 35 m, down-hole dynamic compaction pile of 50 m and pipe sinking
compaction pile. To develop a method on improvement of collapsible
loess foundation with large thickness and investigate improvement
effect and quality control of the method. The tests results showed that
(1) down-hole dynamic compaction pile can improve collapsible loess at
a thickness of 50 m under reasonable design and required construction
quality, improvement effect can be guaranteed; (2) for three tests,
collapsible coefficients of soil between piles are all less than .015 in range
of improvement depth, the collapsibility elimination effect has a downtrend
along depth, whereas the compacting range and extent have no obvious
difference, the depth is not a major factor influencing compacting effect; (3)
the hammer intersection angle can influence improvement effect of downhole dynamic compaction pile (4) a calculation formula of tamping number
is given, which can simplify pre-tamping work because of acquiring tamping
number only through field measurement to meet quality requirement. The
tests results confirm a useful method for improvement project of collapsible
loess foundation with large thickness, and to make those areas become
improved and usable engineering construction site.
ARTICLE HISTORY
Received 26 February 2017
Accepted 16 August 2017
KEYWORD
Large-thickness
collapsible loess; downhole dynamic compaction
pile; improvement depth;
tamping number
Introduction
Loess is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown
dust (Dijkstra, 2001; Lecturer, 1996; Pye, 1995). When soaked by water, some of loess will occur prominent and rapid collapse under the self-weight pressure of overburden soil or self-weight pressure and
additional pressure, which is called collapsible loess (Derbyshire, 2001).The loess is spread widely over
the world, with an area of about 1.3 × 107 km2 (Wang, Zhu, & Huang, 2014). In China, the loess covers
6.4 × 105 km2, the collapsible loess accounts for 4.3 × 105 km2, mainly distributing in central and eastern
Gansu, southern Ningxia, north-west and central Shaanxi or other areas (Figure 1). Among these areas,
in eastern and western Gansu, collapsible loess thickness is between 15 and 40 m (local than 50 m) and
self-weight collapsibility is outstanding, belongs to the large-thickness self-weight collapsible loess
(Zhang, Yang, & Zhang, 2010).
Loess collapse and subsidence can lead to buildings or structures deformation in various degrees,
such as uneven settlement and incline, affecting safety and daily use seriously. Particularly, in self-weight
CONTACT Yu-chuan Zhang zhangshch@lzu.edu.cn
© 2017 Informa UK Limited, trading as Taylor & Francis Group
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2 Y.-C. ZHANG ET AL.
Figure 1. Loess distribution in China (Liu, 1988).
collapsible loess field with large-thickness, serious collapsibility and large thickness bring higher harm
to construction project. So it is necessary to eliminate the collapsibility harm for construction works on
collapsible loess site (GB 50025-2004, 2004; Ma et al., 2014; Qian & Wang, 1985). Changing the physical
and mechanical properties of collapsible soil layer by foundation improvement is the most fundamental
method to eliminate or weaken collapsibility harm, improve the foundation bearing capacity and so
on. Traditionally, loess foundation is improved by cushion, dynamic compaction and compaction (Dai,
2014). Cushion (Zhang, 2011) is suitable for shallow improvement, the improvement depth is about
3 m; the effective reinforcement depth of dynamic compaction (He & Fan, 2007; Mayne, Jones, & Dumas,
1984; Zhao & Xie, 2014) is not more than 10 m; pipe sinking and compacting (Mi & Yang, 2012) is a way
to improve collapsible loess foundation economically and effectively, it also has some advantages to
adjust the foundation non-uniformity and improve impermeability, but the superiority of pipe sinking
and compacting is only reflected within 12 m, when improvement depth greater than 12 m, there will
be difficult to hole and extubation. On the other hand, with the national economic development and
the implementation of western development, a large number of engineering projects need to be carried
out in large-thickness collapsible loess site. Commonly used improvement methods of loess foundation
have been not competent for the demanded improvement depth. It is in extremely urgent to study
effective improvement method of collapsible loess foundation with large thickness.
Down-hole dynamic compaction (DDC) pile (Si & Tang, 1999) is a pile that is created by filling soil
material and tamping strongly in pre-drilled hole, to compact and consolidate filling material in vertical
and soil between piles in lateral at the same time. DDC pile can reach a larger improvement depth and
compaction effect is not engendered in progress of drilling but in progress of compacting filling material (Feng, Shi, Shen, & Li, 2015). So the compaction effect is limited and quality is hard to control, it is
deemed to ‘Have contribution in improving foundation bearing capacity, but little effect in eliminating
collapsibility’. The role in eliminating collapsibility is not underachieving, which brings difficulties to
application in improvement of large-thickness collapsible loess foundation.
Different to above viewpoint, some researchers think as long as the diameter of DDC pile can be
ensured, the ability of eliminating collapsibility will be sure in theory. Moreover, DDC pile has advantage
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EUROPEAN JOURNAL OF ENVIRONMENTAL AND CIVIL ENGINEERING 3
in improvement depth. There have been researches on applying DDC pile in improvement of large-thickness collapsible loess foundation. Based on foundation improvement cases about Ⅲ self-weight collapsible loess site in north-west Shanxi, Chen, Guan, and Liu (2007) mainly focused on working mechanism,
design, construction technology and application of DDC pile. Zhang, Huang, Zhu, and Zeng-Hong (2011)
had done useful works for different length of DDC pile affecting to foundation improvement effect by
soaking load tests. All of literature has proved that the method has some advantages and feasibility in
improving large-thickness collapsible loess foundation. However, there is little information available
in literature about application in larger depth more than 20 m, which makes the method restricted
in the thicker collapsible loess site. Therefore, this paper reports some new tests on improving fully
larger-thickness collapsible loess foundation by DDC pile. To study the feasibility, improvement effect
and quality control about this method, we conducted contrast tests between DDC pile with pipe sinking compaction (PSC) pile (pressing a steel pipe with tip into soil, the soil can be compacted when
forming hole, then filling soil material in the hole to form a pile body). Improvement depth reaches
35 and 50 m, respectively, which is rare in China and around the world. The results have promoted the
further scientific study, test methods and engineering applications in improvement of large-thickness
collapsible loess foundation.
Test site
The in situ tests were conducted in Heping Town of Lanzhou, Gansu Province, China (Figure 2). Test
site is located in transition zone from loess slope terraces to valleys. Strata (from top to bottom) are
loess-like silt, pebble, clay rock or the sand conglomerate. Surface of site is covered continuously by
Late Pleistocene of Quaternary Q3 loess. The loess has light yellow or grey-yellow, macro pores, loose
structure, uniform texture, no vertical joints developed and thickness is approximately 30–70 m.
Geotechnically, the loess foundation is evaluated by collapsibility level of loess and collapsibility
grade of site. Evaluation parameter of collapsibility level (collapsibility coefficient) is defined in formula 1.
Figure 2. Sketch map of test site (from geotechnical engineering report of ‘Hengda City’ project in Lanzhou).
4 Y.-C. ZHANG ET AL.
/
�
s = (hp − hp ) h0
(1)
s – Collapsibility coefficient, evaluation parameter of collapsibility, when s < .015, the effect of collapse
settlement to buildings is small and can be neglected, it’s stipulated that loess with s < .015 is non-collapsible loess in GB 50025-2004 (2004); the collapsible level can be divided into three types according
to the value of s, (1) .015 ≤  s ≤ .03, slight collapsibility (2) .03 <  s ≤ .07, medium collapsibility (3)
s > .07, serious collapsibility; the collapsibility coefficient under saturated self-weight pressure is z s
(the self-weight collapsibility coefficient).
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hp Height of specimen under a certain pressure (mm)
′
hp Height of specimen after compressing and soaking stably (mm)
h0 Original height of specimen (mm)
The average collapsibility coefficient of loess in site is .039–.110, the average self-weight collapsibility
coefficient is .007–.059 (Figure 6). The loess belongs to medium or serious collapsible loess.
The collapsibility grade of site is evaluated by entire self-weight collapsibility and total collapsibility,
which are defined in formulas 2 and 3, respectively.
Δzs = 0
n
∑
(2)
zsi hi
i=1
Δzs zsi hi 0 n Entire self-weight collapsibility, evidence to dividing collapsibility grade of site (mm)
Self-weight collapsibility coefficient of layer in number i
Thickness of layer in number i (mm)
Correction coefficient varying with region, getting values according to test data
Number of collapsible loess layers in calculating range, layer with zsi < .015is not accumulation
Δs =
n
∑
(3)
 si hi
i=1
Δs Total collapsibility, evidence to dividing collapsibility grade of site (mm)
si Collapsibility coefficient of layer in number i
 Correction coefficient considering factors, such as lateral extrusion and soaking rate, getting values
according to test data
Based on Δzs and Δs, the test site evaluation is IV (find in Table 1) self-weight collapsible loess, the
collapsibility of site is very serious. So the site needs to be improved as foundation of superstructure.
The stable groundwater level is lower than 70.0 m below the surface, and the influence of which
can’t be considered. The test site was divided into two sections: A and B, respectively. The 35 m DDC
pile tests were carried out in section A, mainly researched improvement effect, design parameters and
Table 1. The collapsibility grade of collapsible loess foundation.
Non-self-weight collapsible site
Total collapsibility (mm)
Δs ≤ 300
300 < Δs ≤ 700
Δs > 700
Δzs ≤ 70 mm
Ⅰ (slight)
Ⅱ (medium)
–
Self-weight collapsible site
70 mm < Δzs ≤ 350 mm
Ⅱ (medium)
Ⅱ or Ⅲ
Ⅲ (serious)
Notes: when Δs > 600 and Δzs ≥ 300, can be judged as Ⅲ, others isⅡ. GB 50025-2004 (2004).
Δzs > 350 mm
–
Ⅲ (serious)
Ⅳ (very serious)
EUROPEAN JOURNAL OF ENVIRONMENTAL AND CIVIL ENGINEERING 5
Table 2. The specific tests design and related parameters.
Partition
Area (m2)
Thickness of loess (m)
Maximum collapsible soil
layer thickness (m)
Water content
Void ratio
s
zs
Test type
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Abbreviation
A
35 × 32
38–42
≥35
B
20 × 20
≥50
≥50
12.2–18.5% (After humidification)
.880–1.014
.011–.044
.005–.053
12 m pipe sinking compaction pile
35 m down-hole dynamic
compaction pile
PSC-12
DDC-35
6.6–11.6%
.774–1.175
–
.025–.076
50 m down-hole dynamic
compaction pile
DDC-50
construction technology and so on, meanwhile carried out contrast tests of PSC pile in section A; the
50 m DDC pile tests were carried out in section B to mainly study the feasibility of construction technology. There have been humidification before tests in section A (in principle, it should be humidified
to construct conveniently for sections A and B, but based on following considerations to section B: (1)
higher improvement depth will lead to larger difficult, longer time and higher cost to humidify; (2) the
water content in original conditions is higher than section A; (3) tests were carried out to only research
the feasibility of construction technology, so there have not been humidification in section B). See Table 2
and Figure 3 for the specific tests design and related parameters.
Foundation improvement design and construction
The design of DDC pile
DDC pile is formed by tamping filling material in pre-drilled hole to compact soil between piles.
Essentially, the compaction process of DDC pile is a process about cylindrical hole expansion (Gong,
1999). Soil between piles is compacted with filling and tamping material, the key to ensure compaction
quality (the range of effective compaction area) is to ensure the tamped expanded body diameter. For
same foundation soil, the diameter of PSC pile determines the effective compaction range, further to
determines pile spacing; unlike PSC pile, the diameter of pre-drilled hole and tamped expanded body
determine the effective compaction range of DDC pile, and then determine pile spacing.
DDC pile is generally distributed as triangle. As shown in Figure 4, reasonable pre-drilled hole diameter is 400–700 mm. To make the compaction coefficient of soil between piles meet design value, the
relationship (formula 4) between tamped expanded body diameter and pile spacing can be derived
from the principle that quality of soil in unit compaction area with unit thickness doesn’t change before
and after compaction.
√
s2 c d max − s2 d + .9d 2 d
(4)
D=
.9c d max
s d D d c ρdmax Pile spacing (m)
The diameter of pre-drilled hole (m)
The diameter of pile body after tamping and expanding (m)
Average dry density of soil between piles before compaction (g/cm3)
Average compaction coefficient of soil between piles
Maximum dry density of soil between piles determined by compaction test (g/cm3)
Generally in engineering, pile spacing need to be ascertained by natural foundation condition and
engineering requirement at first. When the pile spacing is s = 2.0–3.0 d (CECS197:2006, 2006), the DDC
pile can eliminate the collapsibility economically and reasonably. The theoretical tamped expanded
6 Y.-C. ZHANG ET AL.
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Figure 3. The layout of section A with piles and exploratory wells position.
Figure 4. The ketch map of dynamic compaction and expansion.
body diameter can be deduced from the formula 4, of course, the final tamped expanded body diameter
should be ascertained by in situ tests.
Construction of DDC pile
According to the design, drilling machines include power-driven Luoyang pipe and long twist drill of
CFG-40. As shown in Figure 5, firstly, drill with long twist drill (hole diameter is 400 mm, hole depth is
up to 20 m), then expand pre-drilled hole (expand to diameter of 700 mm) until presupposed depth
with Luoyang pipe. Pre-drilling is to improve the speed of creating hole. Mix filling material and fill holes
through loader, tamp and expand filling material with electric self-propelled rammer of JK-B. Hammer is
cone cylinder shape with intersection angle of 40°, weighs 2.78 t, 500 mm diameter and entire lengths
is 2.55 m including hammer tip.
Take 700 mm to pre-drilled diameter and 1.4 m (2.0 d) to pile spacing, then calculate tamped
expanded body diameter by formula 4. Consideration that tamped expanded body diameter directly
affects compaction effect and it is difficult to control in construction, the value of 1.26 g/cm3 (minimum
dry density) is assigned to d in calculation, .97 for c . After these, the tamped expanded body diameter
was calculated as .957 m, taking 960 mm. In order to measure the tamped expanded body diameter
conveniently after opening cut, use 1:9 lime soil as filling material.
To ensure the tamped expanded body diameter, the field pre-tamping was carried out before construction to determine construction parameters, such as falling distance of rammer, height of filling
material and tamping number. When pre-tamping, the sampling tests of dry density can be converted
to measurement of filling material height, so as to get the tamping number required for certain tamping
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EUROPEAN JOURNAL OF ENVIRONMENTAL AND CIVIL ENGINEERING 7
Figure 5. The construction of DDC pile.
quality, which simplifies pre-tamping work and improves efficiency. As shown in Figure 4, once the
filling distance of hammer and the height of filling material are determined, the required diameter of
tamped expanded body and compactness will be met by tamping filling material from h to H, H can
be calculated by formula 5.
H=
0 d 2 h
.97D2 d max
(5)
ρ0 Dry density of filling material before dynamic compaction (g/cm3)
ρdmax Maximum dry density of filling material (g/cm3)
h Height of filling material for each time (m)
d The diameter of pre-drilled hole (m)
D Required diameter of tamped expanded body (m)
In the tests, taking falling distance of rammer as 5 m, 2.6 m for h, .98 g/cm3 for ρ0, 1.69 g/cm3 for
ρdmax, H can be obtained from the formula 5 that is .82 m. Through pre-tamping, the tamping number
of tamping filling material from 2.6 m (corresponding d) to .82 m (corresponding D) is 12. Which is
simple and easy to apply, we can also use it in construction process to recheck the quality of tamping.
Evaluation of foundation improvement
Evaluation methodology
Foundation improvement was evaluated by sampling with exploration wells, indoor and outdoor tests.
Excavating exploration wells (layout is shown in Figure 3) and sampling along the improvement depth at
1 m intervals (having removed the loose layer of surface at a thickness of 1 m). Indoor and outdoor tests
mainly include minimum compaction coefficient cmin (defined in formula 6), self-weight collapsibility
coefficient zs, compaction coefficient of pile body c (defined in formula 7) and tamped expanded body
diameter. Sampling of two former tests were conducted at the centroid of triangle consisted with three
neighbouring piles, where compaction effect is weakest. Sampling of latter two were done at distance
of 2/3 hole radius from pile centre (GB 50025-2004, 2004; the tests data at distance of 2/3 hole radius
from pile centre is approximately their average in radius direction). Based on above four parameters,
8 Y.-C. ZHANG ET AL.
Table 3. Evaluation methodology of improvement foundation.
Objects
Parameters
Sampling point
zs
Vertical
Horizontal
Compaction effect of soil
between piles
cmin
1 m intervals
Centroid of triangle consisted with
three neighbouring piles
c
Quality of pile body
Tamped expanded body
diameter
Distance of 2/3 hole radius from pile centre
we can get compactness and collapsibility of soil between piles and pile body quality (reflected in
compaction coefficient and tamped expanded body diameter). See Table 3 for evaluation methodology.
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c min =
d0
d max
(6)
cmin – Minimum compaction coefficient of soil between piles after dynamic compaction representing compactness of soil, it’s stipulated in GB 50025-2004 (2004) that cmin ≥ .88 represents principle
of meeting required compactness and eliminating collapsibility is come true.
d0 Dry density of soil between piles in range of compaction depth (g/cm3)
dmax Maximum dry density of soil between piles (g/cm3)
c =
�d
�
d max
(7)
c Compaction coefficient of pile body, representing compaction quality of pile body, compactness can be met when c ≥ .90
′d Dry density of pile body (g/cm3)
′
dmax Maximum dry density of pile body (g/cm3)
Tests results
Collapsibility of soil between piles
Figure 6 shows the distribution to self-weight collapsibility coefficients of soil between piles before and
after improved with DDC-35, DDC-50 and PSC-12. Figure 6 indicates that the original soil has a serious
collapsibility which characterised by collapsibility coefficients as high as between .045 and .075. By
three improvement tests, the collapsibility coefficients of soil between piles become less than .015,
thereby achieving the purpose of eliminating collapsibility. It also can be known from distribution of
scatter that collapsibility coefficients of three tests are all in a law of smaller top and greater bottom,
the elimination effect of collapsibility is the best within 20 m, the collapsibility coefficients are generally
less than .01 but blow 40 m are about .005–.015.
Compaction effect of soil between piles
Figure 7 shows the distribution to minimum compaction coefficients of soil between piles (the average
value of all detection holes) after improved with DDC-35, DDC-50 and PSC-12. Figure 7 indicates that
there is a decreasing trend to minimum compaction coefficients of soil between piles as it changes
with depth (explained in part of ‘evaluation and discussion of improvement method’). The compaction effect is the best in range of 5–15 m. There is no obvious difference on values and distribution of
EUROPEAN JOURNAL OF ENVIRONMENTAL AND CIVIL ENGINEERING 9
Self-weight collapsible coefficients
0.000
0
0.005
0.010
0.015
0.020
0.04
0.05
0.06
0.07
0.08
DDC-35
DDC-50
PSC-12
Before improvement
10
20
Depth/m
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0.03
30
40
50
Figure 6. Distribution chart of self-weight collapsible coefficients of soil between piles.
Minimum compaction coefficients of soil between piles
0
0.86
0.88
0.90
0.92
0.94
0.96
10
20
30
40
DDC-35
DDC-50
PSC-12
50
Figure 7. Distribution chart of the minimum compaction coefficients of soil between piles.
minimum compaction coefficients of soil between piles about three tests, that is, there is no significant
difference on compaction range and extent between three tests. Both DDC-35 and DDC-50 appear low
compaction coefficients of soil between piles at bottom of pile, the phenomenon may be caused by
thick loose soil at bottom of pile, which affects compactness of pile body afterwards affects compaction
effect of soil between piles.
10 Y.-C. ZHANG ET AL.
Compaction effect of pile body
Figure 8 shows distribution to the compaction coefficients of pile body (the average value of all detection holes) about DDC-35, DDC-50 and PSC-12. Figure 8 indicates that the compaction coefficients
of pile body have little change with depth, which are generally between .91 and .96. Where the pile
body compaction coefficients of DDC pile is small corresponds to where the minimum compaction
coefficients of soil between piles is also small, but this phenomenon (explained in part of ‘evaluation
and discussion of improvement method’) doesn’t take place for PSC pile. So we can conclude that the
compaction effect of PSC pile depends on design parameters including pile spacing and so on instead of
the backfill quality of pile body, however, the DDC pile’s mainly depends on backfill quality of pile body.
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Tamped expanded body diameter
The tamped expanded body diameters measured by excavating exploration wells are shown in Figure
9. For 38 cross sections, the maximum is 1190 mm, the minimum is 700 mm and the average is 972 mm,
meeting the required tamped expanded body diameter of 960 mm basically. According to the distribution of tamped expanded body diameters along improvement depth, which varies with depth slightly,
and variation range is between +3 and –2% from required 960 mm. Tamped expanded body diameters
varies with a larger amplitude in the depth about 30–40 m, but returns to normal below that depth.
The change of tamped expanded body diameters may be caused by change of strata or construction
quality, the improvement depth is not main factor affecting tamped expanded body diameters.
Evaluation and discussion of improvement method
The improvement effect of DDC pile in collapsible loess foundation with large thickness is evaluated
by improvement effect of soil between piles and pile body quality, specific evaluation parameters are
showed in Table 4. Combined with the variation of evaluation parameters in part of ‘tests results’, a
similarity in terms of minimum compaction coefficients between DDC pile and PSC pile can be found,
compacting well; collapsibility elimination is ideal except little points at depths of DDC-50; compaction
Compaction coefficients of pile body
0
0.88
0.90
0.92
0.94
0.96
10
20
30
40
50
Figure 8. Distribution chart of compaction coefficients of pile body.
DDC-35
DDC-50
PSC-12
0.98
1.00
EUROPEAN JOURNAL OF ENVIRONMENTAL AND CIVIL ENGINEERING -1500
-1000
-500
0
500
1000
11
1500
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Depth/m
10
20
30
40
50
Figure 9. Tamped expanded body diameters.
Table 4. The tests data statistics of improvement effect.
Test results statistics
zs
cmin
c
Tamped expanded body diameter (mm)
Sample number
Maximum
Minimum
Average
Percent (zs < .015)
Sample number
Maximum
Minimum
Average
Percent (cmin ≥ .88)
Sample number
Maximum
Minimum
Average
Percent (c ≥ .90)
Sample number
Maximum
Minimum
Average
Percent (D ≥ 960)
PSC-12
78
.004
0
.0005
100%
78
.99
.83
.91
80.77%
78
.99
.80
.92
65.38%
–
DDC-35
244
.013
0
.002
100%
244
.99
.81
.89
77.46%
244
.99
.79
.93
80.74%
28
1100
850
986
64.3%
DDC-50
300
.019
0
.005
97.30%
300
.99
.82
.89
72.70%
300
.99
.81
.92
80%
38
1190
700
970
63.15%
coefficients of pile body for DDC pile meet the requirement basically, DDC pile is significantly higher than
PSC pile in percentage of c ≥ .90, however compaction coefficients of pile body for DDC pile are still
small when compared with the limit .93 in JGJ 79-2002, 2002, compaction quality is not good enough;
the tamped expanded body diameters reach design requirements. So, for DDC pile, we can conclude
that foundation improvement achieves desired results, the method have feasibility and advantages
in improvement of collapsible loess foundation with large thickness. Meanwhile, we found three key
problems about DDC pile in tests, doing following discussions at now.
12 Y.-C. ZHANG ET AL.
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Discussion one
Analysing collapsibility coefficients, minimum compaction coefficients, compaction coefficients of pile
body and tamped expanded body diameters at same depth, we find an obvious corresponding relationship exist in them. When the tamped expanded body diameters meets design requirement, the
corresponding compaction coefficients of pile body, collapsibility coefficients and minimum compaction coefficients of soil between piles can also meet requirements. At the improvement depth about
33 m of DDC-50, self-weight collapsibility coefficient of soil between piles is greater than .015,where
the minimum compaction coefficient is less than .88, the compaction coefficient of pile body is less
than .90, and the tamped expanded body diameter is less than 900 mm. That is, the quality of pile body
(compaction coefficient and diameter) and the compaction effect of soil between piles (compaction
coefficient and collapsibility coefficient) are all not satisfied at this point. Essentially, because of the
poor backfill quality of pile body at some positions, the compaction coefficients of pile body are low,
further resulting the tamped expanded body diameters are not up to design requirements, then both
compaction range and extent of soil between piles reduce, without doubt the compaction coefficients
will be small and collapsibility is not eliminated completely. So, we can get conclusions:
(1) The backfill quality of tamped expanded body is the key to ensure eliminate collapsibility for
loess foundation improved by DDC pile. It is not accurate to use a single parameter to evaluate
improvement effect, because the difference of foundation soil and the heterogeneity of backfill
soil can all lead to some test parameters too large or too small. Only when the quality of pile
body (compaction coefficient and diameter) and the compaction effect of soil between piles
(compaction coefficient and collapsibility coefficient) have a corresponding relationship can
we judge construction quality or foundation improvement effect.
(2) When the designed pile spacing (or tamped expanded body diameter) is reasonable and the
quality of pile body meets requirement, it is feasible to improve loess foundation at a thickness
of 50 m with DDC pile, improvement effect can be ensured.
Discussion two
We put forward that the compaction coefficients of pile body are not ideal in previous section. To
reason, we had done tests and analyses, turned out it is mainly related to hammer intersection angle.
Analysing the mechanics principle of foundation soil, the tamped expanded compaction effect is formed
by hammer tamping and extruding backfill soil in pre-drilled hole, further to compact soil between
piles. Stress process of backfill soil has two kinds. First is that hammer compacts the filling soil under its
bottom, making these soils move downward as a soil plug. Because the pressure of soil under hammer
exceeds soil strength, soil structure is destroyed, softened and lateral pressure coefficient increases.
Soil is compacted not only vertically but also laterally, resulting increase of reinforced zone width in
horizontal direction. So the stress distribution in reinforced zone presents a horizontal and wide apple
shape that is different from elliptical distribution of static load. This stress process is similar to that of
the soil under dynamic compaction (Zhou, Wang, & Zhang, 2006), the hammer bottom of which is relatively flat. Second is the situation that hammer bottom is relatively sharp. At this situation, hammer tip
spikes into soil, the soil suffers splitting failure then is pushed around along vertical direction of hammer
bevel, compacting soil at horizontal direction to reinforce lateral soil, however the reinforcement effect
to soil under the hammer is very small or not. This stress process is similar to that of PSC pile (Han &
Huang, 1999). So we can conclude that the different hammer bottom shape makes stress mechanism
of foundation soil different, foundation improvement effect is also different. The hammer intersection
angle of previous in situ tests is 40°, which is relatively sharp, therefore it is beneficial to the compaction
effect of soil around pile, but the compaction effect of pile body is limited. For supporting this analysis,
the model test was also carried out. Figure 10 is the test results about compaction coefficient of pile
body and soil between piles changing with the hammer intersection angle. Figure 10 indicates that it
EUROPEAN JOURNAL OF ENVIRONMENTAL AND CIVIL ENGINEERING 13
is disadvantageous to compact pile body when the hammer intersection angle is less than 50 degrees,
which proves well why the compaction coefficients of pile body are not ideal in in situ tests.
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Discussion three
For the phenomenon that the minimum compaction coefficients of soil between piles has a decreasing
trend along depth, it can be deemed that lateral pressure in deep strata is larger than shallow. But the
tamping number is got from shallow test, thus, it is necessary to add tamping number appropriately
for ensuring compaction effect when improvement depth is more than 20 m.
In a word, the paper works verifies feasibility about large-thickness collapsible loess foundation
improved by DDC pile, studies construction technology and quality control, explores and explains key
issues. There is the first time to reach 50 m for improvement of collapsible loess foundation. This provides
theoretical guidance and practical approach to solve the improvement challenge of large-thickness
collapsible loess foundation, having significance to construction development in loess region. But there
also exists following deficiencies:
(1) The tamped expanded body diameter of DDC pile is difficult to control, affected by human
factors and strata conditions easily, dispersing highly; the replacement rate of DDC pile is
larger than that of PSC pile, which leads to high cost of foundation improvement. While the
compaction effect of PSC pile depends on design, having lighter human affect, and quality is
easier to ensure. So when two compaction methods can all be used, priority should be adopted
to PSC pile.
(2) The tamping number of construction control parameters is affected by depth, the improvement effect will decrease with depth when adopting same tamping number. There is a need
to research the changing rule of tamping number with depth.
Conclusions
The following conclusions can be drawn based on in situ tests on the improvement of large-thickness
collapsible loess foundation.
(1) Down-hole dynamic compaction pile is feasible to improve 50 m collapsible loess foundation when the pile spacing of design (or tamped expanded body diameter) is reasonable and
Figure 10. The curve of compaction coefficient changing with intersection angle of rammer.
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14 Y.-C. ZHANG ET AL.
the quality of pile body meets requirement, improvement effect can be ensured; the focus
and difficulty to construction of down-hole dynamic compaction pile are ensuring tamped
expanded body diameter. According to the formula 5, we can obtain tamping number meeting
the requirement of tamping quality by field measurement, which simplifies pre-tamping work
and improves efficiency.
(2) Three in situ tests can all make collapsibility coefficients of soil between piles less than .015, and
collapsibility coefficients are both in a law of smaller top and greater bottom after improvement. Collapsibility elimination effect is the best within 20 m, collapsibility coefficients are
generally less than .01. Compaction coefficients of soil between piles change as same law of
collapsibility coefficients, there is no significant difference on compaction range and extent
between three tests at same depth.
(3) If hammer intersection angle is small, the compaction effect of pile body will be not ideal, but
a larger hammer intersection angle can affect compaction range. For general collapsible loess,
when hammer intersection angle is between 50 and 60 degrees, the compaction of pile body
and soil around pile can be both taken into account.
(4) It is not accurate to use a single parameter to evaluate the improvement effect, because of
the difference of foundation soil and the heterogeneity of backfill soil, some test parameters
will be too large or too small. Only when the quality of pile body (compaction coefficient and
diameter) and the compaction effect of soil between piles (compaction coefficient and collapsibility coefficient) have a corresponding relationship can we judge construction quality or
foundation improvement effect.
Acknowledgements
Special acknowledgements are due to my colleagues, and in particular to the Prof. Zhang Huyuan, of the Key Laboratory
of Mechanics on Western Disaster and Environment (Lanzhou University, Lanzhou, China), for providing advice on writing
and modifying of paper.
Disclosure statement
No potential conflict of interest was reported by the authors.
ORCID
Yu-chuan Zhang http://orcid.org/0000-0003-3454-2345
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