17th International Symposium on New Technologies for Urban Safety of Mega Cities in Asia
12-14 December 2018, IIIT Hyderabad__________________________________________________________________________________________
Paper No. 24

A STUDY OF PENETRATION RESISTANCE VALUE AT LOOSE AREA UNDER THE CAVITY:EXPANSION PROCESS OF SUB-SURFACE CAVITY
M.OKAMURA1, M.ABE2, K.KITAMURA3, H.NISHIYAMA4, R.HATAKEYAMA5 and R.KUWANO61Engineer, Disaster Reduction Department, GEO SEARCH Co., LTD., Tokyo, Japan, m-okamura@geosearch.co.jp(corresponding author)
2
Section Chief, Disaster Reduction Department, GEO SEARCH Co., LTD., Tokyo, Japan, m-abe@geosearch.co.jp
3
Associate director, Road and River Department, Fujisawa City, Kanagawa, Japan, fj0-dorokasen-s@city.fujisawa.lg.jp
4Section Chief, Road and River Department, Fujisawa City, Kanagawa, Japan, fj0-doiji@city.fujisawa.lg.jp
5Senior Unit Chief, Road and River Department, Fujisawa City, Kanagawa, Japan, fj-doiji@city.fujisawa.lg.jp
6Professor, ICUS, IIS, the University of Tokyo, Tokyo, Japan, kuwano@iis.u-tokyo.ac.jp
Keywords: road cave-in, sub-surface cavity, expansion, penetration resistance
1. INTRODUCTION
A collaboration among Fujisawa city, IIS Univ. ofTokyo and GEO SEARCH address the countermeasureagainst road cave-in through various approaches inFujisawa city since April 2017. Road cave-in causes
traffic accidents / disturbances, especially in emergency
situations. The purpose of this collaborative study is toinvestigate the mechanism of cavity formation andexpansion. Thus, the study is important in maintaining
road function safety. The study includes the monitoring
surveys of sub-surface cavity using GPR (GroundPenetrating Radar) every 6 months, the detailed
investigation of subsurface cavity, and regional
evaluation based on the result of the surveys. As a part ofthe study, some of surveyed cavities were observed indetail by open-cut method. This paper describes theexpansion process of sub-surface cavities.
2. PURPOSE AND METHODS OF PENETRATIONRESISTANCE TEST OF GROUND BELOW THE
CAVITY
Because there are various mechanisms of cave-in
formation, an identification of the cause of the subsurface cavity is essential for reliable repair. In this study,
the factors of cavity formation and the ground conditions
were investigated in detail at ten cavity-locations.

After removing the pavement, digging up to the top of
the cavity and measuring the size of cavity, the value ofpenetration resistance of the ground below the cavity wasmeasured by a portable cone penetrometer (Fig. 1). Theground condition below the cavity is one of valuable
information that shows the expansion process of thecavity. And the reason for using a portable conepenetrometer is that it is easy to understand the groundconditions accurately. Next, based on the result of thepenetration test, the ground was further dug down to thepoint where the cavity originally generated. During thedigging, the condition of the ground was carefullyobserved. After the causes of cavity were identified,
repair works were carried out. In this study, all of ten
cavities that investigated in detail were located in a sandlayer by washing out due to damaged sewage pipes cracked or broken (Fig.2). However there weredifferences in ground conditions such as the presence ofgroundwater.

Cracked sewer joint

Figure 2: Damaged sewer joint

Portable cone penetrometer

Sub-surface cavity

Figure1: Portable cone penetration test

3. RESULTS AND DISCUSSION
The relevance between the value of looseness below thecavity and the occurrence of cavity is consideredaccording to the ground condition.
Case-A is a typical cavity that caused by washing out
and expanded in a short period in a sand layer. The
washing out of the sand occurred from small damaged
joint of sewage pipe. As shown in Table 1, the result ofpenetration test, the soil below the cavity was uniform
and very loose between the bottom of cavity (GL-0.63m)

17th International Symposium on New Technologies for Urban Safety of Mega Cities in Asia
12-14 December 2018, IIIT Hyderabad__________________________________________________________________________________________
Paper No. 24

and the ground water level (GL-3.5m). The similar
conditions were observed at other three sites in this study.
Table 1: Outline of Case-A

A
Exist
Corrupted Sewage pipe
Cause(GL-2.45m)
Sandy soil
GroundDeep(GL-3.5m)
G.Water
Penetration value and Around cavity (m)CASE
LooseArea

0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
3.1
3.2
3.3
3.4

0.20
0.63

As(Pavement)
Crushedstone

CavitySand(Sub-grade)

2.43

3.50

point of cause

Sewagepipe
φ1650
GroundWater

Case-B is another typical cavity that expanded slowly
in sand and gravel layer with high groundwater level (Fig.
3). As shown in Table 2, the ground water level was GL1.45m. It appeared between the bottom of cavity (GL0.30m) and the damaged point of sewage as the factor ofthe cavity (GL-1.64m). The penetration resistance valueof below the cavity was not uniform. Although it went up
and down, the overall trend becomes larger with thedepth. The similar type of cavities and loosening wereobserved at other two sites in this study.

Figure 3: Sewer pipe and ground water (CASE-B)
Previous researches by laboratory model tests[1]reported two key factors for the expansion of cavity,

which are the fluctuation of water level and type ofground. A cavity in highly permeable model groundexpands with the fluctuation of water level. In actualground, it corresponds to the fluctuation of groundwaterin a sand layer. The process is that a cavity expands
upward while repeating the collapse of upper sand withthe fluctuation of water level. It may be suggested thatthe non-uniform penetration resistance is the sign ofrepeated sand collapse, although it should be noted thatthe non-uniform cone penetration resistance can becaused by the presence of gravels.
Table 2: Outline of Case-B

B
Exist
Corrupted Sewage pipe
Cause
(GL-1.64m)
Sandy soil
GroundShallow(GL-1.45m)
G.Water
Penetration value and Around cavity (m)CASE
LooseArea

00
0.1
0.1
0.2
0.2
0.3
0.3
0.4
0.4
0.5
0.5
0.6
0.6
0.7
0.7
0.8
0.8
0.9
0.9
11
1.1
1.1
1.2
1.2
1.3
1.3
1.4
1.4
1.5
1.5
1.6
1.6
1.7
1.7
1.8
1.8
1.9
1.9
22
2.1
2.1
2.2
2.2
2.3
2.3
2.4
2.4

0.12
0.30

As(Pavement)
Crushed stone(Sub-base)CavitySand and gravell(Sub-grade)
1.45
1.64

point of cause
Sewagepipe
φ800

Ground Water

4. CONCLUSION
The finding of this study based on the detailed
investigation in actual ground is that the penetration
resistance value may reveal a process of cavity
expansion.
It is difficult to investigate the cause of cavity underroad maintenance work in short time in usual way.
Consequently, this consideration based on actual cases
seems to be a lead to great breakthroughs in research into
the causes and mechanisms of cavity.
If the process of expansion of cavity is estimated bysimple method, it helps to solve the issue to maintainroads in optimum approaches.
REFERENCE[1] R. Kuwano, R. Sera, Y. Ohara. 2018. Model tests tosimulate formation and expansion of subsurface cavities.
Proc. of 9th international conference on physicalmodelling in geotechnics, ICPMG2018, pp.1087-1092.