10th Asian Regional Conference of IAEG (2015)

Pore water pressure estimation method at the time of the collapse
using a Soil Strength Probe

Hidemasa OHTA
Ohta Geo-Research Co., Ltd., Japan
E-mail:ohta@ohta-geo.co.jp
Abstract
Process of shallow landslides have not yet been clarified. Slope stabilization
measures have usually been done after the collapse. However, it is desired that they be
carried out before the collapse. Before the collapse，if we understand the structure ofthe strata，the strength of the soil， and the pore water pressure, we will be able to do theappropriate slope stabilization measures. In practice，there are two factors that prevent
them. (1) Difficulties of measuring the strength of the soil in situ. (2) Difficulties ofpore water pressure measurement of the moment of collapse. In places that have not yet
collapse, when c (cohesion), ∅ (internal friction angle), and u (pore water pressure) canbe measured, we can deductively calculate the safety factor (Fs). In the collapsed place，
if we know c and ∅，we can be back-calculated the pore water pressure at the time ofcollapse. New tool that can easily measure the c and ∅ in the field，is a Soil StrengthProbe (SSP,or "DOKENBO" in Japanese). In some cases of the slope collapses, inverse
analysis results of pore water pressure are often pressured hydraulic head.
Keywords: pore water pressure, Soil Strength Probe, DOKENBO, shallow landslidein which

1.

Introduction
Stability of slopes is usually evaluated by thefactor of safety. The factor of safety is defined as
the ratio of the shear strength divided by theshear stress required for equilibrium of the slope.
Limit equilibrium analysis requires information
about the strength of soil, and pore waterpressure. The newly developed soil strength tests,
Soil Strength Probe Test (SSPT) can measure
topsoil thickness and soil strength (cohesion andinternal friction angle) of topsoil. On-site
permeability test of topsoil and seepage analysis
provide the water table in some condition ofprecipitation. Using those coefficients, the safety
factor should be able to be computed, and thereappearance computation of the collapse, too.
However, often, it didn't become so. I think thatthe reasons for the mistake were miss-evaluation
with water pressure.
At this article, I will show the mechanism ofshallow landslides and propose an appropriate
mitigations.
2.

Fs = factor of safetyc ′ = cohesion∅′ = angle of internal frictionu = pore water pressure
σ = normal stress on slip surface, andτ = shear stress required for equilibrium
Both the topsoil thickness and cohesive,frictional components of strength (c ′ and∅′ ) canmeasure by SSPT(fig.1, 2 and 3).
On-site unit weight test using circularcylinder sampling shows in fig.4.
Water table (pore water pressure) is
computed by seepage analysis (FEM) using
on-site permeability test (fig.5 and 6).
Topographical survey using reflectorlesslaser measurement tool shows in fig.7.
Thus all components for calculating Fs canbe measured easily by newly developed on-site
tools. And the safety factor should be able to becomputed deductively.
Therefore by using these methods, it should
be possible to evaluate the stability of the slope
before the collapse.

Factor of safety and on-site tests
The factor of safety is defined by Equation1
and 2.
𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎFs = 𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑓𝑜𝑟 𝑒𝑞𝑢𝑖𝑙𝑖𝑏𝑟𝑖𝑢𝑚 (1)=𝑐 ′ +(𝜎−𝑢)𝑡𝑎𝑛∅′𝜏

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10th Asian Regional Conference of IAEG (2015)

Fig.3 Examples of the SSPT data sheet

Fig.1 On -site Soil Strength Probe Test (SSPT)

Fig.4 Unit weight test using circular cylinder

Fig.2 Structure of Soil Strength Probe (SSP)2

10th Asian Regional Conference of IAEG (2015)
When Fs values were calculated to be lessthan 1, groundwater head was higher than theground surface. In other words, undergroundwater pressure, had become excess pore waterpressure state (fig.8).

Fig.5 On-site permeability test(a) Inverse analysis, Yokosuka 2012

Fig.6 Examples of seepage analysis using
coefficient of on-site permeability(b) Inverse analysis, 30 degree inclined slope
Fig.8 Examples of inverse analysis
Computed water pressure at the time of collapse.
Interestingly, high water pressure slope, had nosmall collapse marks. Apparently, they weresmall risk slopes. Therefore, they had not beenmonitored.

Fig.7 Topographical survey using reflectorlesslaser measurement tool3.

Pore water pressure problem
In several collapse locations, I tried toreproduce the safety factors, and I got someresults that are not expected.
Even when the groundwater level the same as
the ground surface, computed Fs values using all
the components except for the coefficient ofpermeability, were often greater than 1.

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10th Asian Regional Conference of IAEG (2015)

3.1

Evidence of high water pressure at

Therefore, an effective preventive measures is

the time of collapse

a dissipation of high water pressure. For

The traces which the very high waterpressure acted on were left in the scene where theslope collapsed in the heavy rain (fig.9 and 10).
In this way, the excess pore water pressure isrelated to the slope collapse strongly.

example, pipe drain method has its
characteristics (fig.11).

Fig.9 Hishikari, Kagoshima 2003 landslide

Big hole in collapse surface, large rock thatwas blown off by very high water pressure

Fig.10 Shobara, Hiroshima 2010 landslide
5.

Circular collapse, Soil was blown away byvery high water pressure
4.

Effective preventive measures

When the collapse due to heavy rain occurs, I
believe that the main cause of the collapse ishigh groundwater pressure.

4Conclusions(1) The strength test using the SSP, we caneasily calculate the water pressure at thetime of the collapse.
(2) At the time of the collapse, very large
excess pore water pressure occurs.
(3) To prevent collapse, it is effective todissipate high excess pore water pressure.
(For example, pipe drain method)