PVC真空预压发处理软土地基施工方案.docx

Proposed Design Philosophy, Construction, Installation and Monitoring Method
( as per 22.1.7 of section 1.0)

1. Source of borrows pit indicating the location and quantity

We have license given by Vietnam authority to exploit a quantity of 8, 400,000 m3 in Nha Be river and the location is in Nhon Trach district, Dong Nai province.

2. Physical proprieties of proposed fill material

The sampling test of sand proposed to use is carried out according to AASHTO M45-89, C144-88 (see Test Result of Analyses of Sand component). The specific weight of the sand is 2.634, unit weight 1.5g/cm3, organic content 2.87% and modules 2.52.

3. Proposed method of statement and monitoring devices

3.1 Method statement
According to the geotechnical report of the site, the void ratio of the soft silty clay (e) is 2.377, and the permeability coefficient (Kv) 5.5*10-7cm/s. If we adopt “wick drains and pre-loading” method to reach a soil bearing capacity of 50Kpa, 75Kpa, 100Kpa under the site’s conditions, the consolidation period would be far beyond the proposed work period. For the purpose of reducing the consolidation period of foundation soil an overload should be added, though the loading would be 7.30m high to fulfill the bearing capacity of 100Kpa. Obviously, this technique would bear a tremendous number of loading and unloading, which would also mean a high construction cost. The stability of side slope would also be difficult to monitor.

Based on our previous experience of soft soil improvement of China coastal and Hiep Phuoc Power Plant in Nha Be, 15km Southeast of Hochiminh City, the pre-loading method is appropriate only for general area. In process area we suggest to adopted the following method (details see drawings “Proposed Process Area Plan” and “Proposed Process Area Zone” in page 6, 7):

f = 50 Kpa, vacuum precompression method;
f = 75 Kpa, vacuum precompression and filling water (2.0m) on membrane method;
f = 100 Kpa, vacuum precompression compound with loading method.

The hydraulic filling of sea sand should be adopted under the elevation +2.80, which is specified in the tender documents. But based on our site investigation report, we propose to use sweet water sand instead of sea sand for these following reasons:

(1) Fine grade and high mud content of the sea sand;
(2) No local facilities available such as hydraulic filling mud absorption.

The backfill material (sweet water sand) will be delivered by barges to the nearest dock and then carried by trucks to the site.

3.2 Monitoring devices
According to the requirement of the tender documents, settlement gauge should be installed at 50mx50m grid and be measured by leveling instruments.

In accordance with the requirement of the owner, the static penetration test should be carried out at the 10 specified points and the penetration instruments should be buried through the whole depth of the compressible soil layers.

The compaction test of filling materials in site should be carried out with heavy compactor.

4. Design philosophy to achieve the requirement of the specification for soil improvement

4.1 philosophy of vacuum precompression method
The vacuum precompression method is used to consolidate foundation soil. This method consists in laying on the surface of foundation soil a sand bed, and in installing a drainage system with wick drains. The sand bed drain line is buried and connected with vacuum system to form an air exhaust and a dewatering system. A set of airtight seal membranes is laid on the surface of the sand bed with its edge buried in soil. Thus, a negative pressure area within sand bed and foundation soil can be formed from air exhaust and dewaturing pumps. The vacuum method can reach a soil bearing capacity of 80KPa. Under the vacuum condition, soil is in the state of consolidating and pore water is discharged from soil under the pressure difference. The consolidation is completed when pressure of soil to be consolidated is reaching the same pressure of the drainage system.

The philosophy of vacuum recompression method is that under the pressure difference (Pa-Pv), soil can be consolidated; Pa indicates atmospheric pressure and Pv indicates air pressure in the sand bed and the vertical drains. In the calculation of consolidation of the foundation soil, the RENDULIC-TERZAGE Theory and BICOT Theory of Consolidation can be adopted which are the same of those adopted in loading recompression method. The drop of the ground water table may also accelerate the consolidation of foundation soil beside the pressure difference.

The effective stress in the soil increases successively during the period of consolidation of the foundation soil with vacuum recompression method. The horizontal displacement of the soil is toward the center of the consolidation area from the beginning of the period of vacuum recompression, which is different from the loading recompression method. So the stability of the foundation soil will not become a problem, and the required vacuum capacity can immediately be obtained and consequently, the period of vacuum recompression can be curtailed. Details is shown in drawings “Design Philosophy for Vacuum Precompression “ and “Flow Diagram of Vacuum Precompression Method” in page 8, 9.

Regarding the short work period of this project, the vacuum compound with loading recompression method is used in the process area where the required bearing capacity of the foundation soil is 100 Kpa. It has been proved from the theoretical calculation and our previous experience that the effects of vacuum and loading recompression methods on the consolidation can be coaxed. For this purpose, a soil capacity of 150 Kpa had been obtained in one of our previous project in the turbine house of Hiep Phuoc power plant located at Nha Be district in Hochiminh city. During the vacuum process and under the pressure difference (Pa – Pv), the pore water permeates into the vertical drains and the consolidation occurs. On the other hand, when using pre-loading method, the air pressure raises to Pp and the consolidation occurs under the pressure difference (Pp – Pa). Therefore, by using the vacuum compound with pre-loading method, the pressure can be increased to (Pp – PV) and the consolidation effect would be improved as a result.

The process of vacuum compound with pre-loading method is shown in drawing “The Process drawing of Vacuum Precompression combined with Loading Method” in page 10.

4.2 Monitoring method
The loading height and loading period of each step of general area is mainly controlled by the stability of the foundation soil. The settlement monitoring should be mainly concerned in this area.

Because the vacuum precompression method will not cause a soil stability problem, the site measurements have less significance comparable to pre-loading method. According to the requirement of the tender documents, settlement gauges should be installed at 50mx50m grid and be measured by leveling instruments. The total settlement and settlement speed of the foundation in the loading area as well as the settlement or the rising height in the surrounded area must be monitored.

The final settlement S¥ can be calculated from the site monitoring data. The consolidation settlement Sc and the average consolidation ratio are obtained from this value. Moreover, the b value can also be calculated as well as the average consolidation coefficient Cv.

According to the requirement of the owner, the static penetration test should be carried out at the 10 specified points and the penetration instruments should be buried through the whole depth of the compressible soil layers.

4.3 Preliminary geotechnical design for vertical drains and soil discharge
The type SPB plastic drainage plate is adopted as the vertical drains in this project. The vertical drains would be distributed accordingly to an equilateral triangle pattern.

Based on our previous experiences of soft soil improvement in the Mekong delta area and according to the work requirements from the project, we believe that the bearing capacity of processed foundation soil can be reached. However, it is arduous to reach the same requirement for the deformation. It needs a comparatively long period to attain a stable state of foundation soil settlement.

In accordance with the tender document for the general area, the filling materials have to be well compacted to meet the building standard. Still, there are no relevant requirements of bearing capacity, so the inserting depth of the vertical drains should be less than the thickness of the soft soil layers to avoid the occurrence of extreme settlement. In the opposite case, that would imply a large additional quantity of loading materials and a soaring construction cost.

Combining vacuum technique with loading method, the different requirements of bearing capacity of process area can be firmly met. Obviously, the airtight condition during vacuum precompression is a critical factor to ensure a sufficient vacuum level. The depth of vertical drains in the process area is estimated to be 10m to ensure that the end of vertical drains is within the soft soil layer which is about 14m depth. This will avoid any leakage from lower sand layer.

The interval of the vertical drains in general area: L = 1.20m
For process area: f =5 t/m2, L = 1.2m.
f =7.5 t/m2, L = 1.1m.
f =10 t/m2, L = 0.7m.

The initial geotechnical design is shown in two tables: “ Geotechnical Design of Vacuum Precompression Method of Process Area” and “ Geotechnical Design of Pre-loading Method of General Area” in page 11, 12.

5. Proposed vertical drains materials

The SPB0-II type vertical drain is adopted from CHINA. The proprieties of the production meet the requirements of Chinese standard DB3201Q18-87 (details is shown below in table “Quality Standard of SPB Vertical Drains “ ).
uality Standard of SPB Vertical Drains

Parameter

Unit
Type

Remarks
SPB-1
SPB-2
SPB-3

Materials

Plastic strip core coated with a membrane filter

Section
dimensions
Width
mm
100±2
100±2
100±2

Thickness
mm
>3.5
>4.0
>4.5

Longitudinal drain capacity
m3/s
15*10-6
25*10-6

Side pressure 350KN/m2
Tension strength
KN/10cm
>1.0
>1.3

Elongation is 100%
Elongation

<10
<10

Permeability coefficient
cm/s
5*10-4
5*10-4

Capacity of insulating soil
mm
<75
<75

Tension strength of
membrane
Dry
N/cm
>15
>30

Elongation is 10%
Wet
N/cm
>10
>20

Elongation is 15%
Linear weight
g/m
90-100
90-100

Unit length
m
200
200

Site clearing
Geotextile laying
Preparation of wick drains
inserting equipment
Site office and
control center
Vacuum pipes install
Wick drains insertion
Second sand bedding
First sand bedding
Membrane laying
Air extracting equipment
Vacuum gauge and relay install
Air extraction test
Precompression by air extraction
Vacuum pressure and settlement readings
in the process area
Equipment removal of the precompression method
Refill and leveling
Weir construc-tion
Control and operation center
Electrical and light circuit
Recheck the vacuum pumps
Temporary water pumps installation
Vacuum pipe preparation
Membrane preparation
Settlement gauges
Initial reading of settlement and relay
Resilience reading of ground settlement
Leakage repair of membrane
Results verification

Flow Diagram of Vacuum Precompression Method

Geotechnical design of vacuum precompression methods of process area

Process Area
(m2)

Average depth of soft soil
(m)

Bearing Capacity of foundation soil (t/m2)

Interval of vertical drains
(m)

Depth of vertical drains
(m)

Calculated
Final consolida-tion settlement
(m)

Depth of vertical drains
(m)

Calculated
Final consolida-tion settlement (m)

Estimated Settlement
(m)

Depth of vertical drains
(m)

Calculated
Final consolida-tion settlement (m)

1

28254 *
__________
25286

14.41
5
1.2
13.4
2.82
10.0
2.10
1.60
8.0
1.69
7.5
1.1
13.4
2.95
10.0
2.30
1.80
8.0
1.90
10
0.8
13.4
3.60
10.0
2.69
2.40
8.0
2.35

2

41428.00 *
____________
37450.00

13.32
5
1.2
12.3
2.59
10.0
2.10
1.60
8.0
1.69
7.5
1.1
12.3
2.72
10.0
2.30
1.80
8.0
1.90
10
0.8
12.3
3.30
10.0
2.69
2.40
8.0
2.15

3

54313.00 *
____________
49603.00

13.05
5
1.02
12.0
2.53
10.0
2.10
1.60
8.0
1.69
7.5
1.1
12.0
2.65
10.0
2.30
1.80
8.0
1.90
10
0.8
12.0
3.23
10.0
2.69
2.40
8.0
2.15

Note: The “ *” shows that the numerator means the proposed process area, which is extended from the area specified in the tender documents (denominator) by extending edge to 4.0 m.
When f = 7.5t/m2, the settlement (1.8) is calculated by using the average settlement after vacuum precompression of the main building of Hiep Phouc power plant (1.55) added settlement occurred in consolidation period (90days) which is (90* 3)mm.

Geotechnical design of vacuum precompression method in process area

Scheme

Process Area
(m2)

Average depth of soft soil
(m)

Bearing Capacity of foundation soil (t/m2)

Interval of vertical drains
(m)

Treatment Depth
(m)

Estimated Settlement
(m)

1

28254 *
_______________
25286

14.41
5
1.2
10.0
1.60
7.5
1.1
10.0
1.80
10
0.7
10.0
2.40

2

41428*
_______________
37450

13.32
5
1.2
10.0
1.60
7.5
1.1
10.0
1.80
10
0.7
10.0
2.40

3

54313 *
________________
49603

13.05
5
1.2
10.0
1.60
7.5
1.1
10.0
1.80
10
0.7
10.0
2.40

Note: 1) “ * ” : the numerator indicates the proposed process area, which is extended from the area specified in the tender documents
(denominator). The edge of the specified area is extended by 4.0 m.
2) The length of vertical drains = treatment depth + sand bed thickness ( 2.0m) + end margin (0.20m) = 12.20m.

Geotechnical design of pre-loading method in general area

Scheme

general Area
(m2)

Average depth of soft soil
(m)

Interval of vertical drains
(m)

Single films drainage
Treatment Depth
(m)
Estimated settlement (m)
1
110934
14.63
1.20
10.0
1.0
2
98770
15.22
1.20
10.0
1.0
3
88617
15.71
1.20
10.0
1.0

Note: The length o