Retaining Wall Design With Surcharge Load

[Joy Wida]

Soilzones on both sides of a Retaining Wall typically have some additional external loads applied. Those can be live surcharge loads on the retaining wall such as vehicular traffic, pedestrian traffic, and parking, or permanent loads such as protection systems against slope erosion, and adjacent structures. In this article, we will focus on how to calculate the lateral earth pressure acting on the backface of a Retaining Wall due to three types of superimposed or surcharge loads:

This resultant force from the lateral earth pressure due to a patch or strip load is located at \(\barz\) measured from the bottom of the pressure distribution, \(\barz\) may be estimated using the following expression:

Correctly estimating the lateral earth pressure resultant force due to superimposed loads and its location is a crucial step in the Retaining Wall Design Process. For more information about how this lateral earth pressure is included in the Retaining Wall Design Process, refer to the article here.

SkyCiv offers a free Retaining Wall Calculator that will calculate the lateral earth pressure on the wall, and perform a stability analysis on your retaining walls. The paid version also displays the full calculations, so you can see step by step, how to calculate the stability of a retaining wall against overturning, sliding, and bearing!

Our previous article, Retaining Wall: A Design Approach discusses the principle and concept behind and when and where to consider a retaining wall in our design. We have learned the different checks against the mode of failures in the retaining wall should be considered in the design. To further understand the designed approach, here is a worked example of the design of the retaining wall.

This example is intended to be readily calculated by hand although a lot of structural spreadsheets and software such as Prokon are available. The purpose of this article is for the reader to fully understand the principle behind it.

The next thing to consider is the assumptions that we can make in terms of the geometry of the retaining wall that we are designing. Given the height, H of the retaining wall, we can assume or counter check our initial design considerations should at least according to the following geometric proportions:

Sketches of the retaining wall forces should be considered to properly distinguish the different forces acting on our retaining wall as tackled in the previous article, Retaining Wall: A Design Approach. Based on our example in Figure A.1, we have the forces due to soil pressure, due to water and surcharge load to consider. Figure A.3 below is most likely our analytical model.

Considering the Figure A.3, we can derive the following equation for the active pressures, Pa and passive pressure Pp. Notice that the pressures acting on the wall are equivalent to the area (triangle) of the pressure distribution diagram. Hence,

There are two checks to consider the stability of the retaining wall. One is the check for an overturning moment and the other one is the check for sliding. The weight of the retaining wall including the gravity loads within it plays a vital role in performing the stability check. Refer to Figure A.4 for the mass or weight calculations.

The sliding check should be carried out with reference to the Figure A.4 diagram and considering the summation of vertical forces for resisting force and horizontal forces for sliding force conservatively neglecting the passive pressure, hence:

The foundation bearing capacity usually governs the design of the wall. The soil, particularly under the toe of the foundation, is working very hard to resist the vertical bearing loads, sliding shear, and to provide passive resistance to sliding. The bearing capacity of the soil should be calculated taking into account the effect of simultaneous horizontal loads applied to the foundation from the soil pressure.

For the footing to be safe in soil pressure, the maximum soil pressure under working load shall be less than the allowable soil bearing capacity. The maximum soil bearing pressure under the footing considering 1m strip is:

The presented calculations above are actually too tiring to perform manually especially if you are doing a trial and error design. Thanks to structural design soft wares and spreadsheets, available nowadays, our design life will be easier.

What do you think about this article? Tell us your thoughts! Leave a comment on the section below. Subscribe to our newsletter to be updated with the latest posts or follow us on our social media pages on the below icons.

Thanks for pointing out. We have checked and found out that that is merely a typo error and it has been updated accordingly. We have also double-checked the attached spreadsheet and it is not affecting the results as we conservatively neglect the effect of passive pressure in the calculation.

Also ,would you be able to explain how is the d in critical shear calculated ? number 1.044 is used for similar triangles,however I struggle to find exact theory how you arrived to this number as I get different.

we have to learned the different checks against the mode of failures in the retaining wall should be considered in the design. Here some worked examples of the design of the retaining wall are described.I like the I have also found this resource Rfmasonry.co.nz useful and its related to what you are mentioning.

Hi Ruben, you can actually put your own logo on the space provided. Either editing some option setting on your excel or typing your company name on it. If none of these options are working, do it manually. Once you finish the design, convert the file to pdf and paste your logo from there.

Thanks, Aaron for pointing it out. We have checked and found out that that is merely a typo error and it has been updated accordingly. We have also double-checked the attached spreadsheet and it is not affecting the results in the calculation.

There is also a comment earlier about a typo of MoT = 57.91, the figure was right at 60.02, the weight of section 1 was not added to the equation. Though all of these moments are taken from the top of the toe and not the furthest point, thus moments are not accurate.

sir, Thank you for your valuable information. this is very much useful and one more plea that can we have any examples for considering wings & returns with head wall can be treated as a retaining wall any such kind of examples please post to mail if any thanks in advance

The shear strength is based on an average shear stress on the full effective cross section (bw x d). In a member without shear reinforcement, shear is assumed to be carried by the

concrete web. In a member with shear reinforcement, a portion of the shear strength is assumed to be provided by the concrete and the remainder by the shear reinforcement.

Thank for this detailed design to follow; It has been very helpful. One thing I noticed is that the calculation for the wall stem does not match the value you then indicated for the moment when checking for wall stem flexure. You have listed that Mu=19.40KNm again for tension, but the calculation comes out to the 29.33KNm you used. I believe it was just a typo but it made it a bit confusing to follow then. Thanks again, and God bless.

The main purpose of retaining wall construction is to retain soil; that is why soil lateral earth pressure is a major concern in the design. Sliding soil wedge theory is the basis for most of the theories by which lateral earth pressure is computed.

The wedge theory suggests that a triangular wedge of soil would slide down if the retaining wall were removed suddenly, and the wall has to sustain this wedge soil. Figure 1 shows free body lateral forces acting on retaining walls.

Furthermore, in the case of flat level backfill soil, considering zero friction at soil-wall interface, and soil-sidewall is vertical, the coulomb equation is reduced to the following:

Surcharge loads acting on retaining walls are additional vertical loads that used to the backfill soil above the top of the wall. It can be either dead loads, for example, sloping backfill above the wall height or live load, which could result from the highway or parking lot, paving, or adjacent footing.

Live load surcharge is considered when vehicular actions act on the surface of backfill soil at a distance, which equal or less than the wall height from the wall back face. Active pressure from uniform surcharge is explained in Figure 2.

Point vertical loads on walls are considered to be spread downward in a slope of two vertical to one horizontal. Consequently, there will be rather low compressive stresses at the base of the wall; girder reactions on walls is an example of vertical point load.

Moreover, bearing stresses that directly under girder or beams reactions must be checked in addition to take eccentricity into account with respect to the stem centerline since it influences the stability and design of the stem.

The angle is equal to the backfill slope angle according to Rankine formula and is the same as the soil-stem friction angle according to the coulomb formula. This inclined active pressure has two components that include horizontal and vertical.

Design retaining wall for car bumper might be necessary when the wall extends above grade, and the parking area is close to it. When retaining wall is designed for impact loads, the stem should be checked at equally spaced points along stem length from top to the bottom as impact load spread at the greater length of the stem. Moreover, use the slope of two vertical to one horizontal for spreading impact load.

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Retaining walls are structures that hold back soil or other materials from sliding or eroding. They are commonly used in foundation design to create level areas, support slopes, or protect buildings from earth pressure. In this article, you will learn how to estimate the lateral earth pressure and surcharge on a retaining wall, which are key factors for designing a safe and stable wall.

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