Formation and Governance of Well Dashen-1 Landslide in Northwest Sichuan

Gas Field

Liu Fuzhen and Liang Faqi

Southwest Petroleum University, Xindu, Sichuan, 610500, China

Keywords: Stability, Landslide, Numerical Fitting, Governance Program.

Abstract: The formation mechanism of Well Dashen-1 landslide in Northwest Sichuan Gas Field was analyzed in the

work, combined with numerical fitting and stability calculation of changes in slope terrain. Thus, the

landslide was determined as an old landslide. Recently, new signs of instability had emerged due to “April

” Lushan Earthquake. Based on actual situation of the slope, reasonable and effective measures of

reinforcement were proposed with landslide characteristics, thus providing references for awareness and

analysis of hazard similar to landslide.

1 PROJECT OVERVIEW

Well Dashen-1, built in 1998, is a major natural gas

well with annual output of more than 130,000 cubic

meters of natural gas. It is located in Group 4,

Xingfu Village, Shunlong, Danleng at Northwest

Sichuan. The landslide is an old soil landslide

located in a slope of southeast side of Shunlong.

There are multiple stairs on slope body, with 140 of

slope gradient. Deformations have existed in the

well site of Well Dashen-1 since its establishment.

Especially after “April 20

” Lushan Earthquake, the

landslide presented new evidence of deformation.

2 LANDSLIDE DEFORMATION

AND NUMERICAL ANALYSIS

According to the survey on Well Dashen-1 landslide

in Northwest Sichuan Gas Field, longitudinal length

of landslide was 240m, lateral width 100m, area 2.40

× 104m

, average thickness 7m, volume 16.8 ×

104m

, landslide sliding direction about 179°, and

slope about 13°. Landslide was chair-like shape,

with gullies on both sides; the gullies intersected

after landslide margin, forming “Two Ditches from

One Source” landscape. Characteristics of Well

Dashen-1 landslide deformation were mainly

reflected on the following aspects: drooping trailing

edge, raised stone retaining wall in central wellsite

of the landslide, forming scarps, snipping highway

in front-central landslide, and slump in notches. New

deformations appeared after "April 20th"

Earthquake, including transverse cracks in wellsite,

settling ground, echelon-type cracks on housing

ground at the right side of wellsite, new drum

deformation in leading edge of stone retaining wall,

and deformed highway at the front of landslide.

Another characteristic of Well Dashen-1 landslide

was that intense deformation existed at the back of

landslide margin near well site, with relatively

weaker deformation at the front and central edge.

2.1 Numerical Fitting Analysis on

Slope Terrain Changes

Profiles were extracted along the slope. Spatial

changes of slope terrain were analyzed through

numerical fitting on elevation h and x-coordinate of

each profile with MATLAB. Based on numerical

fitting relationship between elevation and

coordinates of four profiles along the slope, it could

be seen that there was a linear relationship between

elevation and x coordinate, with about 133° of slope.

The upper part and lower part were both stable

according to the numerical fitting. Through landslide

site as well as fitting relationship analysis, the slope

before and after well site appeared slides, resulting

in mutation of ground elevation. It indicated that

there was slope sliding or human intervention in

wellsite construction, leading to accumulation of

local soil. Thus, the elevation of back wellsite was as

lower as the well site. The elevation of lower-part

wellsite was significantly lower than that of theory

elevation. Further down along the slope, the ground

366

Faqi L. and Fuzhen L.

Formation and Governance of Well Dashen-1 Landslide in Northwest Sichuan Gas Field.

DOI: 10.5220/0006026403660369

In Proceedings of the Information Science and Management Engineering III (ISME 2015), pages 366-369

ISBN: 978-989-758-163-2

elevation exceeded over theory value. However, this

part of slope was in a strong deformation area,

consistent with the site investigation and visiting

situation. Therefore, the slope near well site was

determined as an old landslide.

2.2 Affecting Factors of Slope

Deformation

The underground water level of landslide was

relatively shallow. Drains were all near the well site,

without any drainage facilities outside. Therefore,

water from drains and rear catchment of wellsite

directly flew into the lower part of landslide. Due to

open mountain terrain of trailing edge and steep

gradient, it was conducive to collection of surface

water. The poor drainage of slope surface resulted in

seeper of localized low-lying areas. Then, the weight

of landslide was increased with decline of soil shear

strength of slide surface. Meanwhile, the leading

edge of landslide was exposed to air; the slope was

consistent with occurrence of bedrock, thus

providing topography and geological conditions for

slide of landslide.

Special geological structure of wellsite landslide

was another main factor causing instability of

landslides. The first layer of stratum was Quaternary

Holocene artificial filling soil (Q4ml), mainly

composed of gravel, breccia and powder clay, with

sepia color. Dimension stone was mainly composed

of mudstone, siltstone and shale red quartz

sandstone. Dimension stone occupied about 60

percent, with angular shape and poor roundness;

silty clay had high saturated water ratio, occupying

about 40% content. The second layer was landslide

accumulation body (Q4del), mainly composed of

silty clay clip gravel. The content of clay was about

45% with saturated water and hard plastic. The third

layer was mudstone and argillaceous siltstone (K2g).

Mudstone was brown and weathered; argillaceous

siltstone was from gray to brown, weathered.

In addition, influenced by the shake and loading

of "April 20th" Lushan Earthquake, the soil of

landslide began loosening—another factor for

resurrection of landslide.

In conclusion, special geological structure and

impact of earthquake were main factors causing

instability of landslides.

3 STABILITY CALCULATION

The premise of reasonable landslide control was to

correctly determine the stability of landslide and its

development trend. For physical and mechanical

properties of ground in Well Dashen-1 landslide in

Northwest Sichuan Gas Field, calculation

parameters were determined combined with the

inverse analysis of typical section. It was based on

references of related parameters and value of

experience data in other survey reports of Danleng.

according to the results of geological mapping and

drilling profiles, characteristics of Well Dashen-1

landslide deformation were mainly reflected on:

drooping trailing edge, raised stone retaining wall in

central wellsite of the landslide, forming scarps,

snipping highway in front-central landslide, and

slump in notches. New deformations appeared after

"April 20th" Earthquake, including transverse cracks

in wellsite, settling ground, echelon-type cracks on

housing ground at the right side of wellsite, new

drum deformation in leading edge of stone retaining

wall, and deformed highway at the front of

landslide. Intense deformation existed at the back of

landslide margin near well site, with relatively

weaker deformation at the front and central edge.

The most unfavorable slip surface should be broken

line along the base. Calculation of stability was

analyzed mainly according to site geological

judgment.

Factors of rainfall and earthquakes were

considered in the calculation, according to the

features, various possible load cases and their

combinations of disasters. The following three

conditions were selected for calculation of disaster

stability and surplus sliding force:

Condition 1: Natural conditions (weight +

groundwater)

Condition 2: Storm conditions (weight + ground

+ storm)

Condition 3: Earthquake conditions (weight +

ground + earthquake)

The checking computations of landslide stability

was performed based on 50 years, adjusting basic

seismic intensity as  degree, peak ground motion

as 0.15g, response spectra characteristic period of

ground motion as 0.30s. Line Search Method of

Lizheng software was used for calculation of

stability factor and surplus sliding force for the three

conditions. According to results of qualitative

analysis and quantitative calculation, Well Dashen-1

landslide in Northwest Sichuan Gas Field was stable

in natural conditions; the typical profile of whole

landslide was unstable in earthquake conditions,

while the local part was in less stable state; the local

profile was unstable in storm conditions. Evaluation

results were consistent with deformation

characteristics of landslide status quo. On the whole,

Formation and Governance of Well Dashen-1 Landslide in Northwest Sichuan Gas Field

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Formation and Governance of Well Dashen-1 Landslide in Northwest Sichuan Gas Field

367

the landslide stability was poor in storm conditions,

thus measures should be made for prevention and

governance of the landslide. In addition, local

landslide was in basically-stable to less-stable state

in storm conditions, possibly generating new slip

surface.

The calculation results of surplus sliding force

showed that, in Condition 2, the deformation area

had possibility of localized and wide range of sliding

along the interior soft surface of inside soil. Stability

of deformable bodies should be improved to achieve

safety fortification levels of slope, preventing

destruction of secondary slip deformation, ensuring

safety of people and buildings on the landslide.

Appropriate preventive measures should be taken to

deal with this issue.

4 CONTROL MEASURES OF

LANDSLIDE

Based on qualitative analysis of stability and causes

of the landslide, heavy rainfall was the main factor

inducing landslide deformation. Well Dashen-1

landslide had a whole slide 30 years ago, since when

the landslide had been in a steady state. However,

new deformation emerged in the landslide after

“April 20th” Lushan Earthquake, with resurrection

of old landslide, road deformation of front landslide,

settlement of well site field, as well as formation of

multiple fractures. These were signs that the

landslide had revived in the earthquake—it would

slide under rainfall. Especially at the well site, the

landslide was prone to generating secondary

landslide due to higher artificial filling soil with an

air face. The slide area was about 6000m2, average

moving thickness about 6m, and earthwork of about

3.6 × 104m3.

In accordance with characteristics and dangers of

geological disasters, harm degree of booster stations

was second level, and the complexity of geological

conditions in work area was simple type. Therefore,

the level of disaster prevention project was

comprehensively determined as Level II.

The seismic intensity within region was Degree

VII, and ground acceleration of design peak was

0.15g, according to "China Seismic Zonation Map"

(China Earthquake Administration, 1990) and

"Seismic Design of Buildings" (GB50011-2001).

Loads and combination design should be

considered in the control project. For loads in the

design, the dead weight of landslide under natural

state was taken into account; surface construction

load was calculated with 15KN / m2 * building

floors* building density; road vehicle load was

computed by 20KN/m uniform load. Under storm

conditions, saturated shear strength, 1/2 soil height

and 1/3 soil height above stable water level were

used to calculate permeability of landslide and

deformation, and impact degree of storm on rise of

stable groundwater levels, respectively.

The work analyzed landslide mechanism and

stability calculation, describing the development

trend of structure, terrain and topography of Well

Dashen-1 landslide. Therefore, reinforcement

measure of “anti-slide pile + cut-off drains” was

proposed to control Well Dashen-1 landslide, with

consideration of technical soundness, safety,

reliability, feasibility and economy. A row of 18

anti-slide piles were built in the walled side of Well

Dashen-1 based on stability and landslide thrust.

They were distributed into A-type pile, B-type pile

and C-type pile according to depth of sliding

surface. A-type pile: length 12.0m, pile center

distance 5.0m, pile-section 1 × 1.5m. Four A-type

piles were arranged in right-most landslide while

three in left-most landslide. The section buried into

bedrock should not be less than 5.0 m. B-type pile:

length 16.0m, pile center distance 5.0m, pile-section

1 × 1.5m. There were 4 B-type piles, 2 piles were

arranged in left and right position, respectively. The

anti-slide pile embedded in bedrock should not be

less than 6.0m. C-type pile: length 18.0m, pile center

distance 5.0m, pile-section 1 × 1.5m. Seven piles

were set in central slope, and anti-slide pile

embedded in bedrock was not less than 6.0m. C-type

piles in wellsite and seven arranged anti-slide piles

were combined into double rows of plum-shaped

anti-slide piles for retaining the landslide. Anti-slide

piles were arranged in stone retaining wall of well

site. In case of pipeline in excavation, the position of

anti-slide piles could be adjusted. Retaining plates

should be set among the piles, with thickness of

0.3m, height 2.0m, and total length of 82m.

Measures for interception of water should be

adopted for further improvement of landslide

stability factor, building anti-slide piles to increase

stabilizing force

5 CONCLUSIONS

Landslide disaster is an important issue of mountain

area construction. Landslide in mining areas can

cause heavy losses including destroying mine

facilities with staff casualties, destruction of plants,

and downtime of mine production. Only with full

understanding and analysis of formation mechanism

ISME 2015 - Information Science and Management Engineering III

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and predisposing factors of landslides could they

obtain better governance. The work

comprehensively analyzed the special structure,

formation mechanism and predisposing factors as

well as deformation of Well Dashen-1 landslide in

Northwest Sichuan Gas Field. Simulation of its

stability was conducted, proposing targeted

measures of landslide, thus providing references for

the same type of landslide analysis.

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