Shaft Grouting - Improving the capacity of bored piles by shaft grouting
Travis_Smith_Work_Term_Report_F2016
1. COOP 3902
Louis W. Bray Construction Ltd.
Subsurface Investigation into the Richmond Forcemain
Repairs and Modifications Project
Travis Smith
7226904
COOP 3902
Submission Date: January 10th, 2016
2. Abstract
This report is an investigation into the subsurface conditions for the Richmond Forcemain
Repairs and Modifications project which had many challenges. The project was situated in the
town of Richmond ON and involved the placement of roughly 1200m of sanitary forcemain
along the Jock River. One of the challenges of this project was the high groundwater table which
required a large amount of dewatering for the trench which extended up to 4.5 mbgl in some
areas. In order to remedy this issue, different dewatering techniques were analyzed to determine
the most advantageous method of dewatering.
Another challenge was the supports for the soil walls in these areas, where the height of the
trench posed a design challenge. This is analyzed in the following report and the proposal is to
stack two (2) 8’ x 16’ trench boxes one on top of the other to provide the required height.
Overall this report provides recommendations into the sizing and cost of the dewatering and soil
support systems required for this project.
Keywords: Subsurface Profile, Trench Dewatering, Soil Stability, Richmond Forcemain
3. TABLE OF CONTENTS
1. Introduction ..…….…………………………………..……………………. 4
2. Subsurface Profile ………………………..……………………………… 6
3. Trench Dewatering ………..……………………………………………... 8
4. Soil Excavation Support …………………………………………………. 15
5. Discussion ……………………………………………………………….. 28
6. Conclusions and Recommendations ……………………………………….. 29
7. References ………………………………………………………………. 31
8. Appendices .……………………………………………………………... 32
List of Tables
1. Table 1: Summary of Proposed and Anticipated Subsurface Elevations ……... 6
2. Table 2: Subsurface Profile Elevations …………………………………. 8
3. Table 3: Parameters for the calculation of radial flow from a water table aquifer 9
4. Table 4: Cost Estimate for the Sump Pumping Scenario …………………….. 13
5. Table 5: Cost Estimate for the Wellpoint System Scenario …………………. 15
6. Table 6: Technical Data for the In Situ Soils at Borehole F1 ……………….. 17
List of Figures
1. Figure 1: Braced cut analysis on trench box supports …………………. 19
2. Figure 2: Analysis of Vertical soil face …………………………………. 20
3. Figure 3: Shear Force and Bending Moment Diagrams for First Cut Section … 21
4. Figure 4: Shear force and Bending Moment Diagrams for Second Cut Section.. 23
4. 1.0 Introduction
The objective of this investigation is to determine the dewatering, bracing, and pumping
requirements for one of the major projects that was tendered during the work term in the Fall of
2016. During the first half of the COOP work term with Louis W. Bray Construction Ltd., a
heavy civil contracting firm, I worked in the Estimating department. During this time, I worked
on various small projects, but spent the majority of my efforts focusing on the Richmond
Forcemain Repairs and Modifications project. It was during this time that I decided to pursue this
topic of subsurface investigation in order to provide my colleagues at the time with additional
technical information in order to put together an improved estimate.
The name of the project was Richmond Forcemain Repairs and Modifications and it
involved the installation of roughly 1200 linear meters of sanitary forcemain from the pumping
station in the town of Richmond, ON to the newly developed Lagoon cell several kilometers
away. The proposed pathway for the sanitary forcemain ran along the Jock River through the
town of Richmond, ON. The cost to construct this forcemain would prove to be difficult to
estimate due to high groundwater table levels in the installation location along the Jock River.
See Appendix B – Subsurface Profile to see the height of the groundwater table relative to the
surface. This high groundwater table could pose many difficulties during construction including
but not limited to: infiltration, basal heaving, soil collapse or failure which all have the potential
to injure the workers on the project. It is for this reason that estimators must know the variables
beforehand in order to prepare for any potential obstacles. In order to safely and effectively
install the sanitary forcemain, the contractor would a) support the soil using trench boxes or
approved equivalent up to the maximum excavation depth of 4.65m below ground level and b)
5. Dewater the trench area to ensure that workers have a safe and stable surface to install the PVC
sanitary forcemain.
The first part of this report deals with the subsurface profile of the soils present in situ.
With the raw data obtained from the Geotechnical investigation provided with the project
(Subsurface Investigation, 2016), the borehole logs can be analyzed and converted into a
subsurface profile drawing. This will aid not only in the parts following, but also for visual
confirmation of the soil stratification.
The second part of the investigation is concerned with the lowering the groundwater table
to the point that work can be done safely within the trench. To perform this, there are three
typical methods used for construction purposes: Sump pumping, wellpoint systems, and deep
well systems. Deep well systems are better suited for small areas and longer term construction
and were not considered in this report. The sump pumping method involves having multiple
large hydraulic submersible pumps which are stationed within the trenches. In this method the
water infiltrates into the trench, where it gathers in the sump area and is pumped out of the trench
into a collection area. The other method is a wellpoint system in which small wells are drilled on
both sides of the trench and water is pumped continuously through a network of pipes connecting
each well point.
The third part of the investigation is the analysis of the soil failure and the required
mitigation measures to support the soils. The most common way to support soils in a trench is
through the use of a P.Eng. certified trench box, which is a pre-manufactured box which prevents
6. the soil from collapse in an excavated area. Trench boxes are normally made of either steel or
aluminum sidewalls which are supported by multiple spreaders which transfer the forces from
one end of the trench to the other without buckling. The thickness and length of the trench box
are very important to ensure that the spreaders or sidewalls do not buckle and cause harm to the
workers within the trench box area. It is for this reason that in this report, the forces, stresses, and
other parameters will be calculated.
2.0 Subsurface Profile
Prior to the tendering of the project, City of Ottawa had a third party engineering firm on
site to do a preliminary subsurface investigation. This investigation included boreholes at various
locations along the proposed pathway which gave information such as the groundwater table
level, soil stratification, and the elevation of bedrock/auger refusal. It was with this information
that the following table was calculated. Table 5.1 of the subsurface investigation was titled
“Summary of Anticipated Excavation Conditions” which provided the borehole ID, Approx.
proposed inverts, and bedrock surface elevation. It was with this information and from that
obtained from the borehole logs that the following values were calculated.
Table 1: Summary of Proposed and Anticipated Subsurface Elevations
Borehole Distance (m) GL GWT
Approx.
Prop.
600mm
Invert
Elev.
Bedrock
Elev.
Depth
of
GWT
(m)
Depth
of
Bedrock
(m)
Depth
of
Exc.
(m)
Excavation
within
GWT
Excavation
within
bedrock
F1 70 93.15 91.80 88.5 87.5 1.35 5.65 4.65 3.30 0.00
F2 80 91.76 91.45 89.2 89.0 0.31 2.76 2.56 2.25 0.00
F3 90 92.25 91.15 89.4 88.7 1.10 3.55 2.85 1.75 0.00
7. F4 130 93.10 91.65 90.2 88.8 1.45 4.30 2.90 1.45 0.00
F5 130 92.88 92.35 90.4 88.5 0.53 4.38 2.48 1.95 0.00
F6 80 93.34 91.20 90.5 90.8 2.14 2.54 2.84 0.70 0.30
F7 280 92.67 N/A 90.2 91.0 - 1.67 2.47 - 0.80
F8 - 90.97 90.80 88.9 89.0 0.17 1.97 2.07 1.90 0.10
Sample Calculations for Table 1:
𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐺𝑊𝑇 (𝑚) = 𝐺𝐿 − 𝐺𝑊𝑇 = 93.15 − 91.80 = 1.35𝑚
𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐵𝑒𝑑𝑟𝑜𝑐𝑘 (𝑚) = 𝐺𝐿 − 𝐵𝑒𝑑𝑟𝑜𝑐𝑘 𝐸𝑙𝑒𝑣. = 93.15 − 87.5 = 5.65𝑚
𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐸𝑥𝑐. (𝑚) = 𝐺𝐿 − 𝐴𝑝𝑝𝑟𝑜𝑥. 𝑃𝑟𝑜𝑝 600𝑚𝑚 𝐼𝑛𝑣𝑒𝑟𝑡 𝐸𝑙𝑒𝑣. = 93.15 − 88.5 = 4.65𝑚
𝐸𝑥𝑐𝑎𝑣𝑎𝑡𝑖𝑜𝑛 𝑤𝑖𝑡ℎ𝑖𝑛 𝐺𝑊𝑇 (𝑚) = 𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐸𝑥𝑐. −𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐺𝑊𝑇 = 4.65 − 1.35 = 3.30𝑚
𝐸𝑥𝑐𝑎𝑣𝑎𝑡𝑖𝑜𝑛 𝑤𝑖𝑡ℎ𝑖𝑛 𝑏𝑒𝑑𝑟𝑜𝑐𝑘 (𝑚) = 𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐸𝑥𝑐. −𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐵𝑒𝑑𝑟𝑜𝑐𝑘 = 4.65 − 5.65
= 0𝑚
With the information gathered from Table 1 above, the estimator will be able to gather an
understanding of the types of excavation required which will increase dramatically based on the
excavation type. For excavation in sandy silty soils, an excavator with the required boom length
would be able to excavate down to the depths required. With the excavation into bedrock though,
a special attachment for the excavator, called a hoe ram, will be required to break up the rock.
Depending on the contract specifications as well, unsuitable material such as limestone rock may
have to be transported off site, with all costs being covered by the contractor. So having a table
like this allows the estimator to provide a better estimate of the anticipated cost for excavating
this trench.
By compiling the data from the borehole logs, the data could be shown as a subsurface profile
drawing (Attached in Appendix B). This drawing makes it clear the required excavation depths
8. as well as the soil stratification in each area. For example, in the area of boreholes F6 through F8
the depth of excavation will sink below the bedrock. This 360m stretch of trench will require the
use of a hoe ram or approved equivalent to break around 0.5m of bedrock. The subsurface profile
drawing was made using excel graphing software along with the data shown in Table 2 below;
Table 2: Subsurface Profile Elevations
Well X-Dist Topsoil/FILL Silty Clay Silt Silty Sand Glacial Till Bedrock
(meter
s)
Top Bottom Top Bottom Top Bottom Top Bottom Top
Botto
m
Top
F1 0 93.15 91.02 91.02 89.64 89.64 87.81 - - 87.81 87.48 87.50
F2 70 91.76 90.54 90.54 89.93 - - 89.93 89.02 - - 89.00
F3 150 92.25 91.80 91.80 90.42 90.42 89.20 - - 89.20 88.67 88.70
F4 240 93.10 92.49 - - 92.49 89.59 - - 89.59 88.83 88.80
F5 370 92.88 92.70 - - 92.70 91.05 - - 91.05 88.49 88.50
F6 500 93.34 91.51 - - - - 91.51 90.75 - - 90.80
F7 580 92.67 91.45 - - - - - - 91.45 90.95 91.00
F8 860 90.97 89.60 - - 89.60 89.04 - - - - 89.04
With the drawing shown in Appendix B, the soil stratification can be easily seen and analyzed by
the estimators as they see fit.
3.0 Trench Dewatering
In order to prepare an estimate for trench dewatering, the inflow must be calculated to
determine the pumping rate and sizing requirements. In order to perform this, an analytical
model will be used with several assumptions being made. For the trench proposed for the
Richmond Forcemain project, the trench with the highest cost will be considered in order to
determine the worst case scenario for cost. This means that the area of study will be from
9. borehole F1 through F2, an area of 100.0m. In this section of trench, the excavation depth is
lowest and coupled with a high groundwater level will have the highest cost for dewatering. The
two cases being analyzed are the sump pumping scenario and the wellpoint system. Both of these
methods will have a different kind of pumping requirement, which will be detailed further in the
coming sections.
For the calculation of infiltration into the trench, we will focus on the groundwater
infiltration for the sump pumping scenario. This will be completed using the equation for water
table flow from a line source to a drainage trench. The line source will be assumed roughly 1.5m
away from the trench on both sides. The equation used is shown below in Equation [1]: Equation
for water table flow from a line source to a drainage trench. For the design of the wellpoint
system, the wells will be installed 1.0m below the base of the excavation to prevent basal
heaving. The required drawdown at the point with the lowest excavation of 4.65 mbgl will
require the most pumping to dewater.
𝑄
𝑥
=
𝐾(𝐻2
− ℎ 𝑤
2 )
3.34 ∗ 10−5 𝐿
𝐸𝑞𝑛 [1]
Table 3: Parameters for the calculation of radial flow from a water table aquifer
Name Variable Value
Initial water table elevation from bedrock elevation H 4.3 m
10. Final water table elevation from bedrock elevation h 0.5 m
Radius of influence Ro 11.4 m
Aquifer permeability K 0.001 m/s
Sample Calculations for Table 3:
𝑅0 = 3000(𝐻 − ℎ)√𝐾 = 3000(4.30𝑚 − 0.50𝑚) ∗ √0.001 𝑚/𝑠 = 360.5 𝑚
𝑄 𝑤 (
0.001𝑚
𝑠
∗ (4.3𝑚2
− 0.5𝑚2)
3.34 ∗ 10−5(1.5𝑚)
) ∗ ((4.65𝑚 + 1.0𝑚) ∗ 2 𝑠𝑖𝑑𝑒𝑠)
𝑄 𝑤 = 0.06870
𝑚3
𝑠
= 5,935,680 𝐿/𝑑𝑎𝑦
Therefore the infiltration through the soil will be approximately 5,935,680 liters per day,
meaning that the sump pump will have to handle flows of this magnitude or higher. In order to
ensure that the calculations can be applied to the real life scenario, a factor of safety will be
applied to incorporate the unknown variables that cannot be taken into consideration such as
fissures, fractured bedrock, and pervious soils which may increase the water table flow.
𝑄𝑖𝑛𝑓𝑙𝑜𝑤,𝑤𝑎𝑡𝑒𝑟 𝑡𝑎𝑏𝑙𝑒 𝑓𝑙𝑜𝑤 = 5,935,680
𝐿
𝑑𝑎𝑦
𝑥 1.3 𝐹𝑆 = 7,716,384 𝐿/𝑑𝑎𝑦
With this value of 7,716,384 L/day we can begin to investigate the pumping requirements for the
sump pumping activities. For this type of application, the pumps will be used to draw the water
out of the sump hole and into the collection basin. The type of pump chosen for this purpose is a
submersible hydraulic electric pump. The typical diesel trash pumps were not considered due to
11. the fact that they are not as efficient at pumping vertically out of sump holes. For the submersible
electric pumps, they were also preferred because multiple pumps can be run from one generator,
leading to less maintenance in fueling. Another benefit of the hydraulic pumps is that the oil used
to lubricate the machine parts can be swapped out for vegetable oil in order to be
environmentally conscious. This vegetable oil is greatly advantageous in the case of a priming
error, if the pump is not properly primed, it can backwash hydraulic fluid into the groundwater
potentially causing contaminants to enter the nearby private wells. Since the area in
consideration is near a river and close to residential developments, the submersible pumps will
be used to lessen the environmental impact on the surrounding area.
To determine the pump sizing, we will look at the pumping scenario. With a submersible
hydraulic pump, the pumps will be placed in designated sump holes within the trench itself.
Therefore the suction head will be zero and the only consideration is the discharge head from the
sump to the collection basin. In order to be consistent, we will assume the length of discharge is
5.0m vertically, and 30.0m horizontally to the collection basin. To calculate the required pump
head, the energy equation is applied.
Assumptions:
Assume that losses to are deemed negligible.
Velocity at point 1 is zero within the sump hole.
Velocity at point 2 is also zero within the collection basin.
‘Head loss in the 4” discharge hose was determined by assuming that the discharge would
be roughly 900 GPM and using the pump curve relating the head loss per 100ft section of
12. discharge hose. For 900 GPM, the estimate for head loss is roughly 10.5 ft loss per 100ft
section of pipe.’ (Water Flow through Hoses – Pressure Loss, 2016)
𝑃1
𝜌𝑔
+
𝑉1
2
2𝑔
+ 𝑧1 + ℎ 𝑝 =
𝑃2
𝜌𝑔
+
𝑉2
2
2𝑔
+ 𝑧2 + ℎ 𝑡 + ℎ𝑙
ℎ 𝑝 = (𝑧1 − 𝑧2) + ℎ𝑙
ℎ 𝑝 = (−5.0𝑚 − 0.0𝑚) + (
10.5𝑓𝑡
100𝑓𝑡
∗ 100𝑓𝑡 ∗
0.3048𝑚
𝑓𝑡
)
ℎ 𝑝 = 8.2 𝑚 = 26.9 𝑓𝑡
For our initial calculations, we will assume that the pump has a diameter of 4”. ‘The pump that
will be used is a Thompson 4” Hydraulic Submersible Pump Heads (HST) pump. With this
pump and the required pump head of 26.9 feet, we can refer to the pump curve to determine the
discharge. Referring to the pump curve for the 40 HST pump, the discharge will be 1150 GPM
or 6,268,636 L/day. Assuming that the pump operates at roughly 80% efficiency, the expected
discharge for the 4” submersible pump will be 920 GPM or 5,015,909 L/day.’ (Hydraulic
Submersible Pump Heads (HST), 2016)
In order to dewater the trench, the pumps will need to not only handle the daily infiltration of
groundwater, they will need to drawdown the water table in order to allow for the construction of
the forcemain. It is for this reason that I proposed placing two (2) 4” hydraulic submersible
pumps spaced one at 1/3 and one at 2/3 of the total length of the trench.
To place an estimate on the cost of the sump pumping, we will consider the time frame of one
month. In order to simplify the estimate, some elements such as mobilization/demobilization,
security, and additional engineering costs were not included.
13. Table 4: Cost Estimate for the Sump Pumping Scenario
Item Qty Unit Unit Price Unit Cost ($/month)
4" Hydraulic Submersible Electric Pump 2 ea $2,300.00 $4,600.00
4" Discharge Hose 60 m $3.33 $200.04
600V Generator 1 ea $2,400.00 $2,400.00
Fuel for generator 3800 L/month $0.80 $3,040.00
Enviro-tank Collection Basin 1 ea $20,000.00 $20,000.00
Labour for fueling 80 hr/month $35.00 $2,800.00
Total $33,040.04
Therefore, the cost estimate for the pumping equipment alone will be approximately $33,040 per
month, not including the aforementioned items above.
In the second scenario, the inflow will be considered as radial flow from a water table aquifer.
The formula used to find the flow into the trench will be found using Eqn [2]: Equation for radial
flow from a water table aquifer.
𝑄 𝑤 =
𝜋𝐾(𝐻2
− ℎ 𝑤
2
)
ln(
𝑅 𝑜
𝑟𝑤
)
𝐸𝑞𝑛 [2]
Sample Calculations:
𝑅0 = 3000(𝐻 − ℎ)√𝐾 = 3000(5.30𝑚 − 0.50𝑚) ∗ √0.001
𝑚
𝑠
= 455.4 𝑚
14. 𝑟𝑤 = 4 𝑖𝑛 = 0.1016 𝑚
𝑄 𝑤 =
𝜋 (
0.001𝑚
𝑠
) ((5.30𝑚)2
− (0.50𝑚)2)
ln (
455.4𝑚
0.1016𝑚
)
𝑄 𝑊 = 0.01040
𝑚3
𝑠
= 898,764.5 𝐿/𝑑𝑎𝑦
Therefore the infiltration through the soil will be approximately 898,764 liters per day, meaning
that the well point system will have to handle flows of this magnitude or higher. In order to
ensure that the calculations can be applied to the real life scenario, a factor of safety will be
applied to incorporate the unknown variables that cannot be taken into consideration such as
fissures, fractured bedrock, and pervious soils which may increase the water table flow.
𝑄𝑖𝑛𝑓𝑙𝑜𝑤,𝑤𝑎𝑡𝑒𝑟 𝑡𝑎𝑏𝑙𝑒 𝑓𝑙𝑜𝑤 = 898,764.5
𝐿
𝑑𝑎𝑦
𝑥 1.3 𝐹𝑆 = 1,168,393 𝐿/𝑑𝑎𝑦
‘From the quote received attached as Appendix C, from one of the dewatering companies
bidding on this project, we have the following empirical information. The configuration of
wellpoints is different from that of regular pumps. Wellpoints typically have many drilled well
holes which are interconnected by tubing and eventually to pumps which are spread evenly
among the drilled well points. The wellpoints are placed 1.0m apart and are drilled 5.0m in
depth. For the proposed wellpoint system, the wellpoints will be placed along a 1000m stretch on
both sides, totaling 2000m of dewatering. Each pump will dewater roughly 250m of pipe.
Therefore the number of pumps required to dewater will be eight (8) pumps.
15. Table 5: Cost Estimate for the Wellpoint System Scenario
Item Qty Unit Unit Price Unit Cost ($/month)
Wellpoint pump (Fuel included) 8 ea 4400 35200
Envirotank 1 ea 2200 2200
Total 37400
Therefore comparing the costs of the two scenarios, the cost will be higher on a monthly basis
for the wellpoint scenario. This is due to the fact that each well must be drilled individually
which requires more time and material costs. The advantage to the wellpoint system is that it is
able to draw down the water table to the point that the soil becomes unsaturated. This
unsaturated soil causes the soil to retain more of its internal shear strength and applies less active
pressure stress on the excavated soil face, leading to less support required. This will be
investigated further in the next section.
4.0 Soil Excavation Support
In the Richmond Forcemain project, the health and safety will be the largest concern and
focus for the workers on site. The open cut excavation proposed for the sanitary forcemain will
be of utmost importance. It is for this reason that the stability of the soil and the type of bracing
will be analyzed to ensure they can safely handle the loads imposed by the soil. For this
application of open cut excavation, the types of soil failure possible would be toppling due to the
granular nature of the fill and sandy materials. Another possible source of failure could be sliding
which is due to the high groundwater table causing the soil to be saturated and prone to sliding.
For the design significance of excavating and backfilling, the factor of safety applied will
be 1.3, a typical value for non-critical structures. In order to apply this factor of safety, the trench
16. boxes will be designed to ensure they meet and exceed the forces and stresses exerted by the cut
soil.
For this stability analysis, due to the nature of the work, it will be considered short-term
stability analysis. This means that total stress analysis will be applied, and pore water pressure
will be assumed zero as it does not have time to develop and will be constantly changing.
In order to analyze the soil, several assumptions will need to be made in order to apply
equations to the real scenario. The analysis will be conducted on the deepest part of the trench,
which is the 4.65m deep open cut excavation. According to the borehole log for this point, we
have the soil stratification and groundwater level which will allow us to analyze the probable
forces and stresses. The analysis and design will focus on one of the possible cases, where the
groundwater table is at 1.16m below ground level as per the borehole logs and the soils present
are as per the borehole logs as well. The groundwater table was raised 0.24m from the reported
value since the work will be done in the springtime and groundwater levels will be higher than
when the measurements were taken.
‘Now the parameters for the soils ca be determined. Since certain values that are required
were not found through the borehole logs or subsequent analysis, assumptions and
generalizations will need to be made in order to quantify and apply values to the soils. A
commonly used website titled geotechinfo.data was used to correlate the values and type of soils
found to an approximate value for the unit weight and internal friction angle of the soils. The unit
weights for the soil are empirical values determined using the tables of an online resource. For
17. example, referring to the table titled “Typical Values of Soil Index Properties” we can determine
that for a stiff silty clay the dry unit weight would be approximately 140 lb/ft3 and the saturated
unit weight would be 80 lb/ft3.’ (Soil Unit Weight, 2017).
‘For the angle of internal friction, a table was also referred to “Typical values of soil
friction angle for different soils according to USCS” in order to get an estimate of the typical
friction angles for the soils present.’ (Soil friction angle, 2017)
Table 6 below shows the technical data compiled for the soils in situ in the location of
borehole F1.
Table 6: Technical Data for the In Situ Soils at Borehole F1
Soil Type
Start
Elevation
(m)
Final
Elevation
(m)
Thickness
(m)
Dry Unit
Weight
(kg/m^3)
Saturated
Unit
Weight
(kg/m^3)
Friction
angle,
φ (°)
Undrained
Cohesion, Cu
(kN/m^3)
Granular Fill 93.15 91.02 2.13 1842 849 30 0
Stiff, grey brown Silty Clay 91.02 89.64 1.38 2243 1281 27 35
Very loose, grey Silt 89.64 88.5 1.14 1297 817 25 0
‘For the soil analysis, the technique used to calculate the required supports for braced
cuts was used. This allows for the calculation of the forces on the struts or spreaders of the trench
boxes. This method also allows for the calculation of the maximum bending moment which will
be exerted on the trench box walls or panels. Since the method of analyzing braced cuts usually
involves trenches that are 6m deep with no groundwater table, the analysis will be exceeding
what has been previously been analyzed using this method. This method was chosen due to the
information it provides which will be critical for the design of the trench box.
18. To begin with the design of the supports for the open cut excavation, the cost of the setup
needs to be taken into consideration. The use of soldier beams or sheet piles are very expensive
for short term construction such as underground sewer work and are not considered for long
stretches of trench. The preferred method for construction is the use of trench boxes which are
dragged along by the excavator and provide a moving work area where the forces and stresses of
the soil are supported by the trench box. For this application, the requirement is to support the
soil to a depth of 4.65m, which is very challenging for a trench box setup. Manufacturers do not
provide trench boxes with heights greater than 10’ or 3.05m due to economical and functional
constraints. It is for this reason that I have proposed to have a setup of two (2) 8’ or 2.438m high
trench boxes stacked one on top of the other for this difficult portion of the trench. This will
provide a 4.8m soil support height and will allow for the re-use of the trench boxes in the other
areas of the open cut trench along the proposed forcemain pathway. This will provide savings in
that the trench box provider will not be charging for multiple mobilization and demobilization
costs and it will also be easier for the contractor to track the costs of the boxes under one
grouping. ‘(Chapter 7: Sheet Pile Walls and Braced Cuts, 2016)
‘As previously discussed, the height for the trench boxes will be 2.438m (8’) in height, but the
other dimensions can be interchanged in order to incorporate for the conditions present in situ.
The variables that will be determined are the length of the trench box (16” to 28”), the thickness
of the panels or walls (4” to 8”), and the inside width of the spreaders or struts (24” to 144”).’
(Trench Protection, 2017)
19. Figure 1 - Braced cut analysis on trench box supports
‘As seen above in Figure 1 above, the parameters have been given and the analysis can begin. In
order to determine the active earth pressure exerted on the trench panels, we must assign an
apparent lateral earth pressure diagram. This diagram is dependent on the type of soil, for the
stiff silty clay layer;
𝛾𝐻
𝑐 𝑢
=
(22.43
𝑘𝑔
𝑚3)(1.38𝑚)
35 𝑘𝑁/𝑚3
= 0.88 < 4 => 𝑆𝑇𝐼𝐹𝐹 𝐶𝐿𝐴𝑌
Since the granular fill and loose silt have trace clay components, the entire soil envelope will be
treated as a stiff clay for the apparent lateral earth pressure envelope in order to simplify
calculations. For this type of soil envelope, the active earth pressure can have a factor between
20. 0.2 and 0.4, the 0.4 value was chosen for design purposes in order to provide a better support
system.
Now the analysis of the section can begin, this is initiated by rotating the vertical subsurface into
something that can be analyzed like a beam. This was performed and is shown in Figure 2 below.
Figure 2 - Analysis of Vertical soil face
Analysis from the surface to point B
𝑅1 =
1
2
∗ 1.16𝑚 ∗ 0.4 ∗ 18.42
𝑘𝑔
𝑚3
∗ 4.65𝑚 = 19.87
𝑘𝑁
𝑚
𝑅2 = 0.865𝑚 ∗ 0.4 ∗ 8.49
𝑘𝑔
𝑚3
∗ 4.65𝑚 = 13.66
𝑘𝑁
𝑚
𝑅3 =
1
2
∗ 1000
𝑘𝑔
𝑚3
∗ 9.81
𝑚
𝑠2
∗ (0.865𝑚)2
= 3.67
𝑘𝑁
𝑚
23. Figure 4: Shear force and Bending Moment Diagrams for Second Cut Section
Struts/Spreaders:
Let the length of the spreaders, s, be 84” or 2.13m
𝑃(𝐴) = 𝐴 ∗ 𝑠 = 18.00
𝑘𝑁
𝑚
∗ 2.13𝑚 = 38.34 𝑘𝑁
𝑃(𝐵) = (𝐵1 + 𝐵2) ∗ 𝑠 = (19.00 − 42.56) ∗ 2.13𝑚 = −50.18 𝑘𝑁
𝑃(𝐶) = 𝐶 ∗ 𝑠 = 182.47
𝑘𝑁
𝑚
∗ 2.13𝑚 = 388.66 𝑘𝑁
Walls/Panels:
From the bending moment diagram, we can determine the maximum positive and negative
bending moments.
𝑀 𝑚𝑎𝑥,−′ 𝑣𝑒 = −58.36
𝑘𝑁
𝑚2
24. 𝑀 𝑚𝑎𝑥,+′ 𝑣𝑒 = 21.88
𝑘𝑁
𝑚2
Now we can begin to design the trench box, but first the factor of safety must be applied to all of
the calculated values.
𝑃𝐴,𝑎𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 = 38.34 𝑘𝑁 ∗ 1.13 = 43.3 𝑘𝑁
𝑃𝐵,𝑎𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 = −50.18 𝑘𝑁 ∗ 1.13 = −56.7 𝑘𝑁
𝑃𝐶,𝑎𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 = 388.36 𝑘𝑁 ∗ 1.13 = 438.8 𝑘𝑁
𝑀 𝑚𝑎𝑥,−′ 𝑣𝑒,𝑎𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 = −58.36
𝑘𝑁
𝑚2
∗ 1.13 = −65.9 𝑘𝑁/𝑚2
𝑀 𝑚𝑎𝑥,+′ 𝑣𝑒,𝑎𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 = 21.88
𝑘𝑁
𝑚2
∗ 1.13 = 24.7 𝑘𝑁/𝑚2
Looking at the maximum bending moments of -65.9 kN/m2 and 24.7 kN/m2, the
thickness of the wall will be determined which can sustain these factored values. ‘In order to
determine the require thickness of the steel panel or wall, we will apply an analysis called elastic
moment resistance. This formula was chosen in place of the yield moment resistance since we
want the designed wall to be able to sustain the loads and moments without bending or yielding
which may compromise the integrity and safety of the structure.’ (Flexural Members (Beams),
2016)
Elastic moment resistance:
𝜎 𝑚𝑎𝑥 =
𝑀𝑥 ∗ 𝑦 𝑚𝑎𝑥
𝐼 𝑥
𝐼 𝑥 =
𝑏ℎ3
12
=
(𝑡)(2.625𝑚)3
12
= 1.5075 ∗ 𝑡 𝑚3
25. 𝑦 𝑚𝑎𝑥 =
2.625𝑚
2
= 1.3125𝑚
𝜎 𝑚𝑎𝑥 = 400𝑥103
𝑘𝑁
𝑚2
=
(−65.9
𝑘𝑁
𝑚2) (1.3125𝑚)
1.5075 𝑡 𝑚3
603𝑥103
𝑘𝑁𝑚 ∗ 𝑡 = 86.49 𝑘𝑁/𝑚
𝑡 = 143𝑥10−6
𝑚 = 0.143 𝑚𝑚
Since the required value for the thickness of the wall is quite low, the recommendation is to rent
a 6” thick double walled panel. This is due to the fact that often calculations are not enough to
model the in situ soil. In order to keep safety the priority during construction, a 6” thick panel
would be preferred.
The final step in the design of the trench box will be to determine the required area of the
spreader required to support the axial forces which are transferred from the walls to the
spreaders. In order to calculate this, the maximum axial force of 438.8 kN will be used for the
calculations. ‘The method of analysis will be to investigate the possible modes of failure for ideal
compression members. This means that we will determine the required pipe width for three (3)
cases of failure: Yielding, Local Buckling, and Global Buckling. The inside width or inside
diameter of the spreader will 0.2187m (8”) as per the suppliers limitations. For walls that are 6”
or 8” thick, the spreaders required have a minimum thickness of 0.2187m (8”).’(Compression
Members, 2016)
Yielding:
𝐶 = 𝐴 ∗ 𝐹𝑦
26. 𝐶 = 438.8 𝑘𝑁 = 𝐴 ∗ 400 𝑀𝑃𝑎
𝐴 = 1.097 ∗ 10−3
𝑚2
Global Buckling:
𝑃𝑐𝑟 =
𝜋2
𝐸𝐼
𝑙2
𝑃𝑐𝑟 = 438.8 𝑘𝑁 =
𝜋2
∗ (200 ∗ 109 𝑁
𝑚2)(
𝜋
64
∗ (𝐷4
− 𝑑4))
(2.13𝑚)2
1.99𝑥103
𝑘𝑁 ∗ 𝑚2
= 96.895𝑥109
𝑁/𝑚2(𝐷4
− 𝑑4)
𝐷4
− 𝑑4
=
1.99𝑥103
𝑘𝑁 ∗ 𝑚2
96.895𝑥106 𝑘𝑁
𝑚2
= 20.537𝑥10−6
𝑚4
Local Buckling:
For circular hollow sections:
𝐷
𝑡
<
23,000
𝐹𝑦
𝐷
𝑡
<
23,000
400 𝑀𝑃𝑎
= 57.5
With these restricting equations, we can determine the required thickness and width of the
spreader that meets the requirement for possible modes of failure. The process is iterative and
involves the use of the suppliers provided sizes, i.e 24”, 30”, 36”, etc… for the inside width of
the spreader. The first value chosen was an inside width of 48”, a value roughly in the middle of
the choices.
Case 1: Spreader with 0.2187m (8”) thickness and 1.312m (48”) inside diameter
27. Yielding Check
𝐴 𝑝𝑟𝑜𝑣 =
𝜋
4
(𝐷2
− 𝑑2) =
𝜋
4
((1.5307𝑚)2
− (1.312𝑚)2) = 0.488 𝑚2
𝐴 𝑝𝑟𝑜𝑣 = 488.2 𝑥10−3
𝑚2
> 𝐴 𝑟𝑒𝑞 = 1.097𝑥10−3
𝑚2
𝐺𝑂𝑂𝐷.
Global Buckling Check
𝐷4
− 𝑑4
> 20.537𝑥10−6
𝑚4
(1.5307𝑚)4
− (1.312𝑚)4
= 2.52 𝑚4
> 20.537𝑥10−6
𝑚4
𝐺𝑂𝑂𝐷.
Local Buckling Check
𝐷
𝑡
=
1.5307𝑚
0.2187𝑚
= 6.99 < 57.5 𝐺𝑂𝑂𝐷.
This case meets all of the required elements, but in order to minimize the cost of the system, we
will attempt to provide a system that is within reasonable cost but with a smaller size which will
have a lower rental cost.
Case 2: Spreader with 0.2187m (8”) thickness and 0.656m (24”) inside diameter
Yielding Check
𝐴 𝑝𝑟𝑜𝑣 =
𝜋
4
(𝐷2
− 𝑑2) =
𝜋
4
((0.8748𝑚)2
− (0.656𝑚)2) = 263.0𝑥10−3
𝑚2
𝐴 𝑝𝑟𝑜𝑣 = 263.0 𝑥10−3
𝑚2
> 𝐴 𝑟𝑒𝑞 = 1.097𝑥10−3
𝑚2
𝐺𝑂𝑂𝐷.
Global Buckling Check
𝐷4
− 𝑑4
> 20.537𝑥10−6
𝑚4
(0.8748𝑚)4
− (0.656𝑚)4
= 400.4𝑥10−3
𝑚4
> 20.537𝑥10−6
𝑚4
𝐺𝑂𝑂𝐷.
Local Buckling Check
28. 𝐷
𝑡
=
0.656𝑚
0.2187𝑚
= 2.999 < 57.5 𝐺𝑂𝑂𝐷.
This size of spreader meets the requirements and has the lowest price. Therefore for this design,
the spreaders chosen will be 0.2187m (8”) in thickness, and will have an inside diameter of
0.656m (24”).
5.0 Discussion
The analysis conducted went into great detail into the theoretical and practical sides of
construction. In Section 2.0: Subsurface Profile, the borehole logs and geotechnical investigation
performed by a third party were investigated to determine and produce a cross section of the soil
stratification. It is worth noting that after the soil profile was completed, the was an addendum to
the tender that raised the invert elevation of the forcemain in certain sections to reduce the
amount of excavation in the bedrock. Without the information extracted from the borehole logs,
the owners would not have noticed the extra cost of excavating and removing rock in these areas.
Aside from that, the depths and thicknesses of the layers of soils were also determined and
shown in Table 2: Subsurface Profile Elevations and were used in the other sections of this
report.
In the second part of the report, Section 3.0: Trench Dewatering the methods of trench
dewatering were analyzed. It was determined that cost of the well point dewatering system
would exceed that of the sump pumping, yet the decision for which system would be chosen was
not clear. In my professional opinion, the use of wellpoints for dewatering would provide a more
dependable option. One reason for this method would be that it would be easier to increase the
29. pumping rate by adding additional pumps or larger pumps if there are unforeseen circumstances
such as fissures or porous rock which increase infiltration. The wellpoint system as well will
provide a more stable working surface where the groundwater table will be lowered below the
trench. With a groundwater table below the trench, there will be less concern of soil failure due
to saturation and also less concern with basal heaving at the bottom of the trench. It is the
recommendation of this report to subcontract the dewatering portion of the contract to a
dewatering company who specializes in the use of wellpoints and will be able to provide the
required equipment.
In the final part of the report, Section 4.0: Soil Support System, the excavated soil face
was analyzed in order to determine the required supports. The proposed support system for the
trench, due to the short term of construction, was recommended to be trench boxes. Trench boxes
provide a temporary support which can be moved along with the excavation as it progresses.
During the analysis the forces and stresses were calculated leading to the following
recommendations. For the trench box walls or panels, it was recommended that the contractor
rent a 6” double wall panel. For the spreaders bars or struts, the recommendation is to rent
spreaders with an 0.2187m (8”) thickness and 0.656m (24”) inside width. With these
components, the excavator would be able to move the trench boxes along as the forcemain is
installed, reducing the cost compared to other support methods.
6.0 Conclusions
Despite the many challenges that are present on this project, it is my opinion that with this
additional information the estimators will be able to provide a more accurate estimate. With this
30. report in combination with the subsurface investigation provided by Houle Chevrier Engineering
Ltd., the estimator can request directly for the information on the required equipment and
materials rather than having to pay for separate analysis.
31. 7.0 References
1. “Subsurface Investigation”, Richmond Forcemain Repairs and Modifications, Ottawa,
Ontario. Houle Chevrier Engineering Ltd. April 1, 2016.
2. Draft Permit to Take Water 5028-AAVHUN
3. “Category 3 Permit to Take Water Application Package”, Richmond Forcemain
Upgrades, Ottawa, Ontario. Houle Chevrier Engineering Ltd. April 8, 2016.
4. “Hydraulic Submersible Pump Heads (HST).” Thompson Pump, December 27, 2016,
https://www.thompsonpump.com/Hydraulic-Submersible-Pump-Heads--HST--10-181.html.
5. “Water Flow through Hoses – Pressure Loss” The Engineering ToolBox, December 28,
2016, http://www.engineeringtoolbox.com/water-pressure-loss-hose-d_1525.html.
6. “Soil Unit Weight” Geotechnical Info.com, January 2, 2017,
http://www.geotechnicalinfo.com/soil_unit_weight.html.
7. “Soil friction angle” Geotechnical Info.com, January 2, 2017,
http://www.geotechdata.info/parameter/angle-of-friction.html.
8. Dimitrova, Rozalina. “Chapter 7: Sheet Pile Walls and Braced Cuts” University of
Ottawa, Summer 2016.
9. “Trench Protection” CAT The Rental Store, January 3, 2017,
http://www.catrents.ca/Products/Equipment/Rental_Equipment/_downloads/2014_Trencher.pdf.
10. “Flexural Members (Beams)” CVG3147-Structural Steel Design I, 2016
11. “Compression Members” CVG3147-Structural Steel Design I, 2016
32. 8.0 Appendices
Appendix A – Conceptual Model for Groundwater Lowering System
Appendix B – Subsurface Profile
